Groundwater assessment and modelling for Tasmania
Harrington GA, Crosbie R, Marvanek S, McCallum J, Currie D, Richardson S, Waclawik V, Anders L, Georgiou J, Middlemis H and Bond K
A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Project
December 2009
Contributors
Project Management: David Post, Tom Hatton, Mac Kirby, Therese McGillion and Linda Merrin Report Production: Frances Marston, Susan Cuddy, Maryam Ahmad, William Francis, Becky Schmidt, Siobhan Duffy, Heinz Buettikofer, Alex Dyce, Simon Gallant, Chris Maguire and Ben Wurcker
Project Team: CSIRO: Francis Chiew, Neil Viney, Glenn Harrington, Jin Teng, Ang Yang, Glen Walker, Jack Katzfey, John McGregor, Kim Nguyen, Russell Crosbie, Steve Marvanek, Dewi Kirono, Ian Smith, James McCallum, Mick Hartcher, Freddie Mpelasoka, Jai Vaze, Andrew Freebairn, Janice Bathols, Randal Donohue, Li Lingtao, Tim McVicar and David Kent
Tasmanian Department of Bryce Graham, Ludovic Schmidt, John Gooderham, Shivaraj Gurung, Primary Industries, Parks, Miladin Latinovic, Chris Bobbi, Scott Hardie, Tom Krasnicki, Danielle Hardie and Water and Environment: Don Rockliff
Hydro Tasmania Consulting: Fiona Ling, Mark Willis, James Bennett, Vila Gupta, Kim Robinson, Kiran Paudel and Keiran Jacka
Sinclair Knight Merz: Stuart Richardson, Dougal Currie, Louise Anders and Vic Waclavik
Aquaterra Consulting: Hugh Middlemis, Joel Georgiou and Katharine Bond
Tasmania Sustainable Yields Project acknowledgments Prepared by CSIRO for the Australian Government under the Water for the Future Plan of the Australian Government Department of the Environment, Water, Heritage and the Arts. Important aspects of the work were undertaken by the Tasmanian Department of Primary Industries, Parks, Water and Environment; Hydro Tasmania Consulting; Sinclair Knight Merz; and Aquaterra Consulting. Project guidance was provided by the Steering Committee: Australian Government Department of the Environment, Water, Heritage and the Arts; Tasmanian Department of Primary Industries, Parks, Water and Environment; CSIRO Water for a Healthy Country Flagship; and the Bureau of Meteorology. Scientific rigour for this report was ensured by external reviewer, Don Armstrong. Valuable input was provided by the Sustainable Yields Technical Reference Panel: CSIRO Land and Water; Australian Government Department of the Environment, Water, Heritage and the Arts; Tasmanian Department of Primary Industries, Parks, Water, and Environment; Western Australian Department of Water; and the National Water Commission. We acknowledge input from the following individuals: Richard McLoughlin, Alan Harradine, Louise Minty, Ian Prosser, Patricia Please, Martin Read, Rod Oliver, Dugald Black, Ian Loh, Albert Van Dijk, Geoff Podger, Scott Keyworth, Helen Beringen, Mary Mulcahy, Paul Jupp, Amanda Sutton, Josie Grayson, Melanie Jose, Ali Wood, Peter Fitch, Wenju Cai, Ken Currie, Eric Lam, Imogen Fullagar, Nathan Bindoff, Stuart Corney, Mike Pook and Richard Davis.
Tasmania Sustainable Yields Project disclaimers Derived from or contains data and/or software provided by the Organisations. The Organisations give no warranty in relation to the data and/or software they provided (including accuracy, reliability, completeness, currency or suitability) and accept no liability (including without limitation, liability in negligence) for any loss, damage or costs (including consequential damage) relating to any use or reliance on the data or software including any material derived from that data or software. Data must not be used for direct marketing or be used in breach of the privacy laws. Organisations include: the Tasmanian Department of Primary Industries, Parks, Water, and Environment; Hydro Tasmania Consulting; Sinclair Knight Merz; Aquaterra Consulting; Antarctic Climate and Ecosystems CRC; Tasmanian Irrigation Development Board; Private Forests Tasmania; and the Queensland Department of Environment and Resource Management.
Data on proposed irrigation developments were supplied by the Tasmanian Irrigation Development Board in June 2009. Data on projected increases in commercial forest plantations were provided by Private Forests Tasmania in February 2009.
CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. Data are assumed to be correct as received from the Organisations.
Citation Harrington GA, Crosbie R, Marvanek S, McCallum J, Currie D, Richardson S, Waclawik V, Anders L, Georgiou J, Middlemis H and Bond K (2009) Groundwater assessment and modelling for Tasmania. A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Water for a Healthy Country Flagship, Australia.
Publication Details Published by CSIRO © 2009 all rights reserved. This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from CSIRO.
ISSN 1835-095X Photo on cover: Irrigated field near Moriarty (CSIRO) Director’s foreword
Following the November 2006 Summit on the southern Murray-Darling Basin (MDB), the then Prime Minister and MDB state Premiers commissioned CSIRO to undertake an assessment of sustainable yields of surface and groundwater systems within the MDB. The project set an international benchmark for rigorous and detailed basin-scale assessment of the anticipated impacts of climate change, catchment development and increasing groundwater extraction on the availability and use of water resources.
On 26 March 2008, the Council of Australian Governments (COAG) agreed to expand the CSIRO assessments of sustainable yield so that, for the first time, Australia would have a comprehensive scientific assessment of water yield in all major water systems across the country. This would allow a consistent analytical framework for water policy decisions across the nation. The Tasmania Sustainable Yields Project, together with allied projects for northern Australia and south-west Western Australia, will provide a nation-wide expansion of the assessments.
The CSIRO Tasmania Sustainable Yields Project is providing critical information on current and likely future water availability. This information will help governments, industry and communities consider the environmental, social and economic aspects of the sustainable use and management of the precious water assets of Tasmania.
The projects are the first rigorous attempt for the regions to estimate the impacts of catchment development, changing groundwater extraction, climate variability and anticipated climate change, on water resources at a whole-of-region-scale, explicitly considering the connectivity of surface and groundwater systems. To do this, we are undertaking the most comprehensive hydrological modelling ever attempted for the region, using rainfall-runoff models, groundwater recharge models, river system models and groundwater models, and considering all upstream-downstream and surface- subsurface connections.
To deliver on the projects CSIRO is drawing on the scientific leadership and technical expertise of national and state government agencies in Queensland, Tasmania, the Northern Territory and Western Australia, as well as Australia’s leading industry consultants. The projects are dependent on the cooperative participation of over 50 government and private sector organisations. The projects have established a comprehensive but efficient process of internal and external quality assurance on all the work performed and all the results delivered, including advice from senior academic, industry and government experts.
The projects are led by the Water for a Healthy Country Flagship, a CSIRO-led research initiative established to deliver the science required for sustainable management of water resources in Australia. By building the capacity and capability required to deliver on this ambitious goal, the Flagship is ideally positioned to accept the challenge presented by this complex integrative project.
CSIRO has given the Sustainable Yields Projects its highest priority. It is in that context that I am very pleased and proud to commend this report to the Australian Government.
Dr Tom Hatton
Director, Water for a Healthy Country
National Research Flagships
CSIRO
Executive summary
This report presents the results of the groundwater assessment and modelling components of the CSIRO Tasmania Sustainable Yields Project. All assessments were performed at the scale of the major aquifer systems within five reporting regions: Arthur-Inglis-Cam, Mersey-Forth, Pipers-Ringarooma, South Esk and Derwent-South East. The level of technical assessment varied depending on the availability of existing data, knowledge and numerical groundwater flow models. Assessments were performed for four climate and development scenarios. The four scenarios are:
Scenario A – historical climate (1 January 1924 to 31 December 2007) and current development Scenario B – recent climate (data from 1 January 1997 to 31 December 2007 were concatenated to make an 84-year sequence) and current development Scenario C – future climate (84-year sequence scaled for ~2030 conditions) and current development Scenario D – future climate (84-year sequence scaled for ~2030 conditions) and future development.
Generally, there is a lack of fundamental groundwater data for Tasmania and this has led to large uncertainties in the groundwater recharge and flow modelling. Nevertheless, this report provides a valuable starting point for water resources planning in Tasmania, and will inform decision makers of the need for investment in both data collection and refined groundwater modelling.
Across the project area, modelled mean annual diffuse groundwater recharge varies by more than two orders of magnitude. For all modelled areas, annual recharge declines noticeably over the 84-year historical period (1924 to 2007). The climate over the recent (1997 to 2007) period results in the lowest groundwater recharge rates of the historical period, assuming land use did not change over this period. Recharge rates under the future climate are likely to be within the range of rates experienced during the historical period.
Total groundwater extraction is estimated to be around 38 GL/year, with almost 90 percent of this extraction occurring in the Smithton Syncline groundwater assessment area (16 GL/year) and the Mersey-Forth region (17.4 GL/year). The areas of most concentrated groundwater extraction reflect the high-yielding nature of the dolomite and basalt aquifers (respectively) in these areas and, in the case of the Mersey-Forth region, the extremely fertile soils associated with the underlying basalt geology.
The ratio of groundwater extraction (E) to diffuse recharge (R) for the majority of investigated aquifers is very low under all climate and development scenarios. This suggests that there are opportunities for future groundwater development, providing estimates of sustainable extraction limits can be made in the interim. Most of these opportunities lie in basalt aquifers that have already started to be developed; however, in each case the scale of possible future development is likely to be less than 10 GL/year. In cases where E/R is already in the range 0.3 to 0.7, such as occurs during drought periods in the Mella area in the Arthur-Inglis-Cam region and the Wesley Vale area in the Mersey-Forth region, there is a high risk that further groundwater development could lead to declining groundwater levels, which in turn could reduce the groundwater contribution to many rivers.
Detailed numerical modelling of three different aquifer systems in the north of Tasmania revealed that under the recent climate (Scenario B), groundwater levels are likely to either remain similar to current levels, or to decline gradually through until 2030. Under the future climate (Scenario C), groundwater levels fluctuate within the range experienced under the historical climate (Scenario A). In the Mella and Togari groundwater assessment areas of the Arthur-Inglis-Cam region, groundwater levels rise from current conditions, even under future groundwater development at E/R of 0.25. Similar results were obtained for the Wesley Vale groundwater assessment area within the Mersey-Forth region, although localised areas of intensive groundwater extraction could expect declines of up to 10 m if future development approached an E/R of 0.25. Likewise in the Scottsdale groundwater assessment area of the Pipers-Ringarooma region, groundwater levels under future development (Scenario D) (E/R ~0.11) decline by up to 2 m in some areas where extraction, or area of forestry plantations, was increased.
The middle and lower reaches of most rivers exhibit a high degree of connectivity with groundwater in the adjacent aquifers. This connectivity is vital for maintaining streamflow in summer months to support instream and riparian ecosystems. Rivers that deeply incise basalt aquifers along the north coast are particularly reliant on inputs from groundwater, and thus extraction near these rivers may have detrimental impacts on streamflow. Groundwater modelling has indicated that, under the future climate (Scenario C), there is likely to be little change to surface–groundwater
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ i interaction fluxes. Under future development (Scenario D), the changes to fluxes are also small. The most notable change from historical conditions is that variably gaining/losing stream reaches revert to losing reaches as a result of increased groundwater extraction lowering watertables adjacent to the rivers.
Most of Tasmania suffers from a lack of long-term, high-quality groundwater monitoring data. The greatest limitation of the existing numerical models, both in terms of the certainty of results from this project and for future determinations of sustainable extraction limits, is the absence of reliable groundwater extraction data. Long-term regional monitoring of groundwater levels and salinity would also greatly facilitate future assessments. A better understanding of surface–groundwater interactions is warranted in many catchments, particularly in the basalt catchments along the north coast where rivers are deeply incised in the major aquifers. Greater knowledge of the nature of interactions between the karstic Smithton dolomite and the Duck and Montagu rivers is required before further groundwater development occurs in the Mella and Togari groundwater assessment areas.
ii ▪ Groundwater assessment and modelling for Tasmania © CSIRO 2009 Table of contents
1 Introduction ...... 1 1.1 CSIRO Tasmania Sustainable Yields Project ...... 1 1.2 Groundwater assessment and modelling...... 1 2 Methods...... 4 2.1 Prioritisation of groundwater assessment areas ...... 4 2.2 Groundwater assessments ...... 4 2.3 Recharge estimation and scenario definition ...... 5 2.4 Groundwater modelling and assumptions...... 7 Recharge time series for groundwater models ...... 8 Mella-Togari model ...... 9 Wesley Vale model ...... 10 Scottsdale model...... 11 3 Knowledge gaps, limitations and uncertainty ...... 12 3.1 Monitoring data gaps...... 12 3.2 Surface–groundwater interactions ...... 12 3.3 Numerical model knowledge gaps, limitations and uncertainty...... 12 4 The Arthur-Inglis-Cam region...... 14 4.1 Contextual information...... 14 4.1.1 Hydrogeology ...... 14 Arthur-Inglis-Cam ...... 15 Flinders Island...... 16 King Island ...... 16 4.1.2 Surface–groundwater interactions ...... 16 Arthur-Inglis-Cam ...... 16 Flinders Island...... 16 King Island ...... 17 4.1.3 Groundwater extraction...... 17 Arthur-Inglis-Cam ...... 17 Flinders Island...... 17 King Island ...... 17 4.1.4 Groundwater resource protection and management issues ...... 17 4.1.5 Previous estimates of recharge and discharge...... 18 4.1.6 Groundwater level trends ...... 20 Arthur-Inglis-Cam ...... 20 Flinders Island...... 20 King Island ...... 21 4.2 Groundwater system assessment...... 21 4.2.1 Recharge/discharge ...... 21 4.2.2 Surface–groundwater interactions ...... 21 4.2.3 Conceptual model ...... 22 4.3 Scenario assessment ...... 23 4.3.1 Recharge impacts ...... 23 4.3.2 Modelled impacts to groundwater levels and fluxes in the Mella and Togari groundwater assessment areas...... 26 Under historical climate (Scenario A)...... 28 Under recent climate (Scenario B)...... 32 Under future climate (Scenario C)...... 35 Under future development (Scenario D) ...... 38 Water balance under scenarios A, B, C and D ...... 41 4.3.3 Reporting metrics ...... 43 Extraction relative to recharge ...... 43 Extraction relative to baseflow ...... 44 4.4 Impacts of use...... 45 4.4.1 Management risks...... 45 4.4.2 Waterlogging and salt accession ...... 45 5 The Mersey-Forth region...... 46 5.1 Contextual information...... 46 5.1.1 Hydrogeology ...... 46 5.1.2 Surface–groundwater interactions ...... 47 5.1.3 Groundwater extraction...... 48 5.1.4 Groundwater resource protection and management...... 48 5.1.5 Previous estimates of recharge and discharge...... 48 5.1.6 Groundwater level and salinity trends ...... 50 5.2 Groundwater system assessment...... 51 5.2.1 Recharge/discharge ...... 51
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ iii 5.2.2 Surface–groundwater interactions ...... 52 5.2.3 Conceptual model ...... 53 5.3 Scenario assessment ...... 55 5.3.1 Recharge impacts ...... 55 5.3.2 Modelled impacts to groundwater levels and fluxes in the Wesley Vale groundwater assessment area ...... 58 Under historical climte (Scenario A)...... 59 Under recent climate (Scenario B)...... 64 Under future climate (Scenario C)...... 68 Under future development (Scenario D) ...... 72 Water balance under scenarios A, B, C and D ...... 76 5.3.3 Reporting metrics ...... 77 Extraction relative to recharge ...... 77 Extraction relative to baseflow ...... 78 5.4 Impacts of use...... 79 5.4.1 Management risks...... 79 5.4.2 Waterlogging and salt accession ...... 79 6 The Pipers-Ringarooma region ...... 80 6.1 Contextual information...... 80 6.1.1 Hydrogeology ...... 80 6.1.2 Surface–groundwater interactions ...... 81 6.1.3 Groundwater extraction...... 82 6.1.4 Groundwater resource protection and management...... 82 6.1.5 Previous estimates of recharge and discharge...... 82 6.1.6 Groundwater level and salinity trends ...... 84 6.2 Groundwater system assessment...... 85 6.2.1 Recharge/discharge ...... 85 6.2.2 Surface–groundwater interactions ...... 86 6.2.3 Conceptual model ...... 86 6.3 Scenario assessment ...... 88 6.3.1 Recharge impacts ...... 88 6.3.2 Modelled impacts to groundwater levels and fluxes in the Scottsdale groundwater assessment area ...... 90 Under historical climate (Scenario A)...... 92 Under recent climate (Scenario B)...... 96 Under future climate (Scenario C)...... 99 Under future development (Scenario D) ...... 102 Water balance under scenarios A, B, C and D ...... 105 6.3.3 Reporting metrics ...... 106 Extraction relative to recharge ...... 106 Extraction relative to baseflow ...... 107 6.4 Impacts of use...... 108 6.4.1 Management risks...... 108 6.4.2 Waterlogging and salt accession ...... 108 7 The South Esk region ...... 109 7.1 Contextual information...... 109 7.1.1 Hydrogeology ...... 109 7.1.2 Surface–groundwater interactions ...... 110 7.1.3 Groundwater extraction...... 110 7.1.4 Groundwater resource protection and management...... 111 7.1.5 Previous estimates of recharge and discharge...... 111 7.1.6 Groundwater level and salinity trends ...... 111 7.2 Groundwater system assessment...... 112 7.2.1 Recharge/discharge ...... 112 7.2.2 Surface–groundwater interactions ...... 113 7.2.3 Conceptual model ...... 114 7.3 Scenario assessment ...... 115 7.3.1 Recharge impacts ...... 115 7.3.2 Reporting metrics ...... 117 7.4 Impacts of use...... 118 7.4.1 Management risks...... 118 7.4.2 Waterlogging and salt accession ...... 118 8 The Derwent-South East region ...... 119 8.1 Contextual information...... 119 8.1.1 Hydrogeology ...... 119 8.1.2 Surface–groundwater interactions ...... 120 8.1.3 Groundwater extraction...... 121 iv ▪ Groundwater assessment and modelling for Tasmania © CSIRO 2009 8.1.4 Groundwater resource protection and management...... 121 8.1.5 Previous estimates of recharge and discharge...... 121 8.1.6 Groundwater level and salinity trends ...... 122 8.2 Groundwater system assessment...... 123 8.2.1 Recharge/discharge ...... 123 8.2.2 Surface–groundwater interactions ...... 124 8.2.3 Conceptual model ...... 125 8.3 Scenario assessment ...... 126 8.3.1 Recharge impacts ...... 126 8.3.2 Reporting metrics ...... 129 8.4 Impacts of use...... 130 8.4.1 Management risks...... 130 8.4.2 Waterlogging and salt accession ...... 130 9 Conclusions ...... 131 10 References...... 132 11 Appendices...... 134 Appendix A: Groundwater model benchmarking...... 134 A.1 Introduction ...... 134 A.2 Methods ...... 134 A.3 Results ...... 135 Benchmarking Model with WAVES version 1 recharge ...... 135 Benchmarking Model with WAVES version 2 recharge ...... 136 Benchmarking Model with WAVES version 3 recharge ...... 136 A.4 Conclusion ...... 137 Appendix B: Surface–groundwater interaction maps for selected catchments ...... 150 Appendix C: Groundwater map of the Coal River groundwater assessment area ...... 164 Appendix D: Surface water catchments of the CSIRO Tasmania Sustainable Yields Project area 165
Tables
Table 1. Groundwater assessment areas in each region, sorted by tier of assessment ...... 4 Table 2. Summary of the main tasks performed for each groundwater assessment area ...... 6 Table 3. Previous estimates of groundwater fluxes for the Arthur-Inglis-Cam region...... 19 Table 4. Water balances from steady-state numerical models for the Arthur-Inglis-Cam region ...... 19 Table 5. Estimated diffuse recharge, discharge to streams and extraction for groundwater assessment areas in the Arthur-Inglis-Cam region...... 21 Table 6. Aggregated recharge scaling factors for groundwater assessment areas in the Arthur-Inglis-Cam region under scenarios A, B, C and D ...... 25 Table 7. Scaled mean annual recharge for groundwater assessment areas in the Arthur-Inglis-Cam region under scenarios A, B, C and D ...... 26 Table 8. Mean annual water balance for Mella and Togari under scenarios A, B, C and D ...... 43 Table 9. Extraction relative to recharge (E/R) for groundwater assessment areas in the Arthur-Inglis-Cam region under scenarios A, B, C and D ...... 44 Table 10. Modelled mean annual baseflow volume for Mella and Togari groundwater assessment areas under scenarios A, B, C and D ...... 44 Table 11. Mean 24-year extraction relative to baseflow (E/B) for Mella and Togari groundwater assessment areas under scenarios A, B, C and D ...... 45 Table 12. Groundwater statistics for the Mersey-Forth region including annual recharge, extraction and discharge details...... 49 Table 13. Modelled water balance results for the Wesley Vale groundwater assessment areas...... 50 Table 14. Estimated diffuse recharge, discharge to streams and extraction for the Arthur-Inglis-Cam region...... 52 Table 15. Aggregated recharge scaling factors for groundwater assessment areas in the Mersey-Forth region under scenarios A, B, C and D...... 57 Table 16. Scaled mean annual recharge for groundwater assessment areas in the Mersey-Forth region under scenarios A, B, C and D ...... 57 Table 17. Mean annual water balance for Wesley Vale groundwater assessment area under scenarios A, B, C and D ...... 77 Table 18. Extraction relative to recharge (E/R) for groundwater assessment areas in the Mersey-Forth region under scenarios A, B, C and D...... 78
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ v Table 19. Modelled mean annual baseflow volume for Wesley Vale groundwater assessment area under scenarios A, B, C and D ...... 78 Table 20. Modelled extraction relative to baseflow (E/B) for Wesley Vale groundwater assessment area under scenarios A, B, C and D ...... 79 Table 21. Previous estimates of groundwater fluxes for the Pipers-Ringarooma region...... 83 Table 22. Water balances from steady-state numerical models for groundwater assessment areas in the Pipers-Ringarooma region ...... 84 Table 23. Estimated historical recharge, discharge to streams and current and future extraction for groundwater assessment areas in the Pipers-Ringarooma region ...... 86 Table 24. Aggregated recharge scaling factors for groundwater assessment areas in the Pipers-Ringarooma region under scenarios A, B, C and D...... 90 Table 25. Scaled mean annual recharge for groundwater assessment areas in the Pipers-Ringarooma region under scenarios A, B, C and D...... 90 Table 26. Mean annual water balance for the Scottsdale groundwater assessment area under scenarios A, B, C and D...... 106 Table 27. Extraction relative to recharge (E/R) for groundwater assessment areas in the Pipers-Ringarooma region under scenarios A, B, C and D...... 107 Table 28. Mean annual baseflow volume for Scottsdale groundwater assessment area under scenarios A, B, C and D ...... 107 Table 29. Mean 24-year extraction relative to baseflow (E/B) for Scottsdale groundwater assessment area under scenarios A, B, C and D ...... 107 Table 30. Previous estimates of groundwater fluxes for the South Esk region ...... 111 Table 31. Estimated diffuse recharge, discharge to streams and extraction for the Longford groundwater assessment area in the South Esk region...... 113 Table 32. Aggregated recharge scaling factors for the Longford groundwater assessment area in the South Esk region under scenarios A, B, C and D...... 116 Table 33. Scaled mean annual recharge for the Longford groundwater assessment area in the South Esk region under scenarios A, B, C and D ...... 117 Table 34. Extraction relative to recharge (E/R) for groundwater assessment areas in the South Esk region under scenarios A, B, C and D ...... 117 Table 35. Previous estimates of groundwater fluxes in the Derwent-South East region ...... 122 Table 36. Estimated diffuse recharge and extraction for the groundwater assessment areas in the Derwent-South East region..124 Table 37. Aggregated recharge scaling factors for groundwater assessment areas in the Derwent-South East region under scenarios A, B, C and D...... 128 Table 38. Scaled mean annual recharge for groundwater assessment areas in the Derwent-South East region under scenarios A, B, C and D ...... 129 Table 39. Extraction relative to recharge (E/R) for groundwater assessment areas in the Derwent-South East region under scenarios A, B, C and D...... 130 Table 40. Range of acceptable aquifer parameters for the Wesley Vale groundwater assessment areas ...... 134 Table 41. Simulated mean annual water balance volumes during the benchmarking period for the DPIPWE model and the benchmarking models with WAVES recharge ...... 143 Table 42. Surface geology code index for Figure 67 to Figure 77...... 161
Figures
Figure 1. Location of groundwater assessment areas (and their DPIPWE model class) relative to regions...... 2 Figure 2. Simplified geology of Tasmania...... 3 Figure 3. Example of 84-year Scenario A recharge time series from WAVES. Red lines show 23-year periods for scenarios Awet, Amid and Adry ...... 6 Figure 4. Distribution of assumed future plantation forests for Scenario D ...... 7 Figure 5. Location of current and future irrigation areas, current plantation forests and extraction wells in the Mella-Togari model..9 Figure 6. Location of current and future irrigation areas, current and future plantation forests and extraction wells in the Wesley Vale model ...... 10 Figure 7. Location of current and future irrigation areas, current and future plantation forests and extraction wells in the Scottsdale model ...... 11 Figure 8. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Arthur-Inglis-Cam region...... 14 Figure 9. Hydrographs for the (a) Togari and (b) Hampshire monitoring wells, showing the water level (in metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall ...... 20 Figure 10. Conceptual hydrogeological model for the Arthur-Inglis-Cam region ...... 23 Figure 11. Spatial distribution of recharge scaling factors in the Arthur-Inglis-Cam region for scenarios Awet, Amid, Adry and B relative to Scenario A...... 24 Figure 12. Spatial distribution of recharge scaling factors in the Arthur-Inglis-Cam region for scenarios C and D relative to Scenario A ...... 25 Figure 13. Conceptual groundwater model for the Mella and Togari groundwater assessment areas ...... 27 vi ▪ Groundwater assessment and modelling for Tasmania © CSIRO 2009 Figure 14. Location of the Mella-Togari model extent and reporting sites...... 28 Figure 15. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2...... 30 Figure 16. Simulated gaining and losing river reaches under Scenario Amid ...... 32 Figure 17. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2...... 33 Figure 18. Simulated gaining and losing river reaches under Scenario B ...... 35 Figure 19. Groundwater levels for the DPIPWE model calibration period and Scenario C at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2...... 36 Figure 20. Simulated gaining and losing river reaches under Scenario Cmid ...... 38 Figure 21. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2...... 39 Figure 22. Simulated gaining and losing river reaches under Scenario Dmid ...... 41 Figure 23. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Mersey-Forth region...... 46 Figure 24. Hydrographs for the (a) Barrington and (b) Lloyd’s well 3 monitoring wells, showing the water level (in metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall ...... 51 Figure 25. Conceptual hydrogeological model for the Mersey-Forth region...... 54 Figure 26. Spatial distribution of recharge scaling factors in the Mersey-Forth region for the 23-year Scenario A and the 11-year Scenario B relative to the 84-year historical modelled period...... 55 Figure 27. Spatial distribution of recharge scaling factors in the Mersey-Forth region for the 84-year scenarios C and D relative to the 84-year historical modelled period...... 56 Figure 28. Conceptual groundwater model for the Wesley Vale groundwater assessment areas ...... 58 Figure 29. Location of the Wesley Vale model extent and reporting sites...... 59 Figure 30. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 ...... 61 Figure 31. Simulated gaining and losing river reaches under Scenario Amid ...... 64 Figure 32. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 ...... 65 Figure 33. Simulated gaining and losing river reaches under Scenario B ...... 68 Figure 34. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 ...... 69 Figure 35. Simulated gaining and losing river reaches under Scenario Cmid ...... 72 Figure 36. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 ...... 73 Figure 37. Simulated gaining and losing river reaches under Scenario Dmid ...... 76 Figure 38. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Pipers-Ringarooma region ...... 80 Figure 39. Hydrographs for the (a) Jetsonville and (b) Winnaleah monitoring wells, showing the water level (metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall ...... 85 Figure 40. Conceptual hydrogeological model for the Pipers-Ringarooma region ...... 87 Figure 41. Spatial distribution of recharge scaling factors in the Pipers-Ringarooma region for the 23-year Scenario A and the 11-year Scenario B relative to the 84-year historical modelled period ...... 88 Figure 42. Spatial distribution of recharge scaling factors in the Pipers-Ringarooma region for the 84-year scenarios C and D relative to the 84-year historical modelled period ...... 89 Figure 43. Conceptual groundwater model for the Scottsdale groundwater assessment area ...... 91 Figure 44. Location of the Scottsdale model extent and reporting sites ...... 92 Figure 45. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3...... 94 Figure 46. Simulated gaining and losing river reaches under Scenario Amid ...... 96 Figure 47. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3...... 97 Figure 48. Simulated gaining and losing river reaches under Scenario B ...... 99 Figure 49. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3...... 100 Figure 50. Simulated gaining and losing river reaches under Scenario Cmid ...... 102 Figure 51. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3...... 103 Figure 52. Simulated gaining and losing river reaches under Scenario Dmid ...... 105 Figure 53. Groundwater assessment areas, salinity of groundwater wells and surface–groundwater interactions in the South Esk region ...... 109 Figure 54. Hydrographs for the (a) Hagley and (b) Cressy monitoring wells, showing the water level (metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall...... 112 Figure 55. Conceptual hydrogeological model for the South Esk region...... 114
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ vii Figure 56. Spatial distribution of recharge scaling factors in the South Esk region for the 23-year Scenario A and the 11-year Scenario B relative to the 84-year historical modelled period...... 115 Figure 57. Spatial distribution of recharge scaling factors in the South Esk region for scenarios C and D relative to Scenario A..116 Figure 58. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Derwent-South East region...... 119 Figure 59. Hydrographs for the (a) Pawleena Road and (b) Tunnack monitoring wells, showing the water level (in metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall ...... 123 Figure 60. Conceptual hydrogeological model for the Derwent-South East region ...... 126 Figure 61. Spatial distribution of recharge scaling factors in the Derwent-South East region for the 23 year Scenario A and the 11-year Scenario B relative to the 84 year historical modelled period...... 127 Figure 62. Spatial distribution of recharge scaling factors in the Derwent-South East region for the 84-year scenarios C and D relative to the 84-year historical modelled period ...... 128 Figure 63. Location of modelled wells used in the benchmarking exercise (Wesley Vale groundwater assessment area)...... 137 Figure 64. Simulated results with WAVES recharge, and observed groundwater levels for the benchmarking period at the 20 monitoring wells (see Figure 63 for locations) ...... 138 Figure 65. Simulated total recharge for (a) DPIPWE Modflow model (b) WAVES version 1 (c) WAVES version 2 and (d) WAVES version 3 ...... 144 Figure 66. Simulated model results for DPIPWE model and benchmarking model with WAVES version 3 recharge, and observed groundwater levels for the DPIPWE model calibration period at the 20 monitoring wells ...... 145 Figure 67. Surface–groundwater interactions map for the Duck catchment showing surface geology and location of available groundwater level data...... 150 Figure 68. Surface–groundwater interactions map for the Montagu catchment showing surface geology and location of available groundwater level data...... 151 Figure 69. Surface–groundwater interactions map for the Inglis-Flowerdale catchment showing surface geology and location of available groundwater level data ...... 152 Figure 70. Surface–groundwater interactions map for the Cam, Emu and Blythe catchments showing surface geology and location of available groundwater level data ...... 153 Figure 71. Surface–groundwater interactions map for Flinders Island showing surface geology and location of available groundwater level data...... 154 Figure 72. Surface–groundwater interactions map for the Leven and Forth-Wilmot catchments showing surface geology and location of available groundwater level data...... 155 Figure 73. Surface–groundwater interactions map for the Rubicon catchment showing surface geology and location of available groundwater level data...... 156 Figure 74. Surface–groundwater interactions map for the Great Forester-Brid catchment showing surface geology and location of available groundwater level data ...... 157 Figure 75. Surface–groundwater interactions map for the Ringarooma catchment showing surface geology and location of available groundwater level data ...... 158 Figure 76. Surface–groundwater interactions map for the Longford groundwater assessment area showing surface geology and location of available groundwater level data...... 159 Figure 77. Surface–groundwater interactions map for the Coal River groundwater assessment area showing surface geology and location of available groundwater level data...... 160 Figure 78. Groundwater elevation contours for the Coal River groundwater assessment area ...... 164 Figure 79. Surface water catchments of the CSIRO Tasmania Sustainable Yields Project area...... 165
viii ▪ Groundwater assessment and modelling for Tasmania © CSIRO 2009
1 Introduction
1.1 CSIRO Tasmania Sustainable Yields Project
This report is one in a series of technical reports from the CSIRO Tasmania Sustainable Yields Project. The terms of reference for this project are to estimate current and future water availability in each catchment and aquifer in Tasmania (considering climate change, forestry, groundwater and irrigation development) and to compare the estimated current and future water availability with the amount of water required to meet the current levels of extractive use.
The purpose of this report is to describe the current and potential future groundwater resource availability for key aquifers within the five regions across Tasmania as follows (Figure 1):
Arthur-Inglis-Cam (including Flinders and King islands) Mersey-Forth Pipers-Ringarooma South Esk Derwent-South East.
1.2 Groundwater assessment and modelling
The groundwater assessment and modelling component of this project involved collating existing data and knowledge to report on the occurrence, status and possible future condition of groundwater resources across the five regions. The assessments are reported at the regional scale (sections 4 to 8), with explicit detail and assessment at the scale of the main aquifer units where background data are available. The geology of Tasmania is extremely complex and it is not uncommon to have Cainozoic sedimentary aquifers adjacent to Jurassic igneous rocks, Permo-Triassic sedimentary aquifers and Precambrian fractured rock aquifers (see Figure 2).
Parts of regions that were already represented with an existing, transient numerical groundwater flow model (prior to this project) were assessed quantitatively for the impacts of current and future climate and development by modelling them under scenarios A, B, C and D. This modelling has enabled detailed assessments of changes in recharge and associated changes in groundwater levels and surface–groundwater interactions. For parts of regions without groundwater models, the potential impacts of the four climate and development scenarios have only been assessed quantitatively in terms of changes in groundwater recharge.
The Department of Primary Industries, Parks, Water and Environment (DPIPWE) in Tasmania recently completed a two-year project titled ‘Development of Models for Tasmanian Groundwater Resources’ (hereafter referred to simply as the DPIPWE project). It was funded by the Tasmanian Government and the National Water Commission under the Raising National Water Standards Program. The DPIPWE project collated background information and built new groundwater models for 19 areas located mostly in the north of Tasmania. These new models ranged in complexity from simple conceptual models with first order water balances (termed Class A models) through to calibrated, transient numerical groundwater flow models (termed Class C models). All 19 modelled areas from the DPIPWE project fall within the Tasmania Sustainable Yields Project area, so it made sense to utilise these models for this project. The only areas in the five regions that contain significant groundwater resources not covered by the DPIPWE project are the Longford (in the South Esk region) and Coal River (in the Derwent-South East region). Hence, a total of 21 groundwater assessment areas (GAAs) are the focus for this report (Figure 1).
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 1
Figure 1. Location of groundwater assessment areas (and their DPIPWE model class) relative to regions
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Figure 2. Simplified geology of Tasmania
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2 Methods
2.1 Prioritisation of groundwater assessment areas
The level of assessment that should be undertaken for each region was determined using the same model prioritisation scheme used previously by DPIPWE, at least where there was overlap. The different classes of DPIPWE models (A, B and C) were originally determined in 2007 using local knowledge and assessment of data availability and either current or potential future risk of stressed groundwater resources. Accordingly, a tiered approach was adopted for this project, whereby Tier 1 assessments captured the most detailed (Class C) groundwater model areas, Tier 2 assessments captured an intermediate level of detail and sometimes numerical (Class B) models, and Tier 3 assessments captured the most basic (Class A) models and those areas with very limited historical data or understanding of the groundwater resources (see Table 1).
Parts of the project area containing significant groundwater resources that were not covered by the DPIPWE project were assigned either a Tier 2 (Longford) or Tier 3 (Coal River) level assessment (see Table 1).
Table 1. Groundwater assessment areas in each region, sorted by tier of assessment
Region Groundwater assessment area (DPIPWE Model Class) Surface water catchment* Tier 1 Arthur-Inglis-Cam Mella (C), Togari (B) Duck, Montagu Mersey-Forth Wesley Vale (C) Mersey, Rubicon Pipers-Ringarooma Scottsdale (C) Great Forester-Brid Tier 2 Arthur-Inglis-Cam Inglis-Cam (B), Cam-Emu-Blythe (B) Inglis-Flowerdale, Cam, Emu, Blythe Mersey-Forth Leven-Forth-Wilmot (B) Forth-Wilmot, Leven Pipers-Ringarooma Ringarooma (B) Ringarooma, North Esk South Esk Longford (no DPIPWE model) Macquarie, South Esk Derwent-South East Swansea-Nine Mile Beach (B) Swan-Apsley Tier 3 Arthur-Inglis-Cam Flinders Island (A) Flinders Island, Welcome, King Island, Arthur, Smithton Syncline (A) Black-Detention, Emu, Blythe King Island (A) remaining areas (no DPIPWE model) Mersey-Forth Partial overlap with Mole Creek (A), Spreyton (A), Mersey, Leven, Forth-Wilmot, Rubicon, Tamar Sheffield-Barrington (A) and Kimberley-Deloraine (A) Estuary remaining areas (no DPIPWE model) Pipers-Ringarooma remaining areas (no DPIPWE model) Musselroe-Ansons, George, Scamander-Douglas, North Esk, Pipers, Little Forester, Boobyalla-Tomahawk South Esk Partial overlap with Mole Creek (A) and Kimberley- Meander, Great Lake, Brumbys, Macquarie, South Deloraine (A) Esk remaining areas (no DPIPWE model) Derwent-South East Mt Wellington-Huonville (A) and Cygnet-Cradoc (A) Huon, Swan-Apsley, Little Swanport, Prosser, remaining areas (no DPIPWE model), Sorell Tertiary Carlton-Tasman Peninsula, Coal-Pitt Water, Jordan, Basalt (A) Clyde, Ouse, Upper Derwent, Lower Derwent, Derwent Estuary * Surface water catchments are shown in Appendix D.
2.2 Groundwater assessments
The level of technical assessment undertaken in each tier 1, 2 or 3 area within a region has reflected the quality and quantity of data, knowledge and models available to this project. A summary of the key tasks performed in each region is provided in Table 2, and detailed explanations of the methods employed for each task are provided in Description of Project Methods (CSIRO, 2008). The table shows where contextual information and assessment tasks have already
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been undertaken for the DPIPWE project. In these instances, the pertinent information and results were synthesised for this project.
2.3 Recharge estimation and scenario definition
The method used to estimate the changes in diffuse recharge for this project is based upon the method used for the Murray-Darling Basin Sustainable Yields Project (Crosbie et al., 2008), which utilises the one-dimensional, soil-vegetation-atmosphere-transfer model WAVES (Zhang and Dawes, 1998). This model was chosen because of its balance in complexity between modelling plant physiology, soil physics and water balance. One of its advantages is the ability to simulate plant growth. WAVES can model the impact that changes in climate might have upon recharge via changes in different elements of the water balance. These include transpiration and the interception of rainfall on the plant canopy. WAVES requires three data sets: climate, soils and vegetation.
The 84-year historical climate sequence (1924 to 2007) was extracted from SILO
The results of WAVES modelling at the 20 control points were used to create regression equations between mean annual rainfall and mean annual recharge for each combination of soil and vegetation type. This allowed recharge to be upscaled using a raster coverage of soils, vegetation and mean annual rainfall using a grid spacing of 0.05 x 0.05 degrees. In contrast to surface water assessments, groundwater systems do not reset each year, but respond to longer period changes in rainfall. For this reason, representative sequences from the historical record were chosen to estimate groundwater responses for the next 23 years (to ~2030). Consecutive 23-year sequences from the 84-year historical modelled recharge sequence were analysed and ranked to generate three separate 23-year scenarios for the historical climate scenario (Scenario A) for further assessment (see Figure 3):
Scenario Awet is the wettest 90th percentile 23-year period from within the 84-year modelled record Scenario Amid is the wettest 50th percentile 23-year period from within the 84-year modelled record Scenario Adry is the wettest 10th percentile 23-year period from within the 84-year modelled record.
Throughout the WAVES modelling there is a downward trend evident in the recharge time series (see Figure 3). This trend generally sees the period selected for Scenario Awet early in the 84-year sequence and the period selected for Scenario Adry late in the 84-year sequence. This trend and the skewed nature of the distribution of annual recharge are responsible for the median (Scenario Amid) being greater than the mean in most cases. This causes Scenario Amid recharge scaling factors to be greater than one. This trend in the recharge time series is explored further in Crosbie et al. (2009).
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Figure 3. Example of 84-year Scenario A recharge time series from WAVES. Red lines show 23-year periods for scenarios Awet, Amid and Adry
Table 2. Summary of the main tasks performed for each groundwater assessment area
Groundwater SW-GW Contextual GW GW level Conceptual Management Change in Extraction/ GW Extraction/ assessment area* interaction information extraction mapping model issues recharge recharge conditions baseflow (Tier) mapping Arthur-Inglis-Cam
Mella and Togari (1) synthesised synthesised synthesised produced** synthesised modelled estimated modelled estimated
Inglis-Cam (2) synthesised synthesised synthesised produced synthesised modelled estimated NA NA
Flinders Island (3) synthesised synthesised NR produced synthesised modelled estimated NA NA identified Welcome (3) synthesised synthesised NR NR synthesised modelled estimated NA NA
King Island (3) synthesised synthesised NR NR synthesised modelled estimated NA NA
Other (3) collated NR NR NR produced modelled NR NA NA
Mersey-Forth
Wesley Vale (1) synthesised synthesised synthesised produced** synthesised modelled estimated modelled estimated
Cam-Emu-Blythe (2) synthesised synthesised synthesised produced synthesised modelled estimated NA NA identified Leven-Forth-Wilmot synthesised synthesised synthesised produced synthesised modelled estimated NA NA (2) Other (3) collated NR NR NR produced modelled NR NA NA
Pipers-Ringarooma
Scottsdale (1) synthesised synthesised synthesised produced** synthesised modelled estimated modelled estimated
Ringarooma (2) synthesised synthesised synthesised produced synthesised identified modelled estimated NA NA
Other (3) collated NR NR NR produced modelled NR NA NA
South Esk
Longford (2) collated estimated synthesised produced produced modelled estimated NA NA identified Other (3) collated NR NR NR produced modelled NR NA NA
Derwent-South East
Coal River (2) collated estimated produced produced produced modelled estimated NA NA Mt Wellington- Huonville and synthesised synthesised NR NR synthesised identified modelled estimated NA NA Cygnet-Cradoc (3) Other (3) collated NR NR NR produced modelled NR NA NA * Note that where groundwater assessment areas identified in Table 1 only partially overlap regions and surface water catchments, they have been grouped together in this table under the ‘other’ area category. ** Map constrained using both hydraulics and hydrochemistry NA – not available NR – not reported
At the 20 control points, Scenario B was clipped from the Scenario A modelled recharge time series, specifically for the last 11 years (1 January 1997 to 31 December 2007). Relationships were established between mean annual rainfall and mean annual recharge from the recent (1997 to 2007) years of modelling, enabling a raster of Scenario B recharge to be
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constructed. Dividing this new raster by the Scenario A raster produced a raster of recharge scaling factors (RSFs) used in further analysis.
Under Scenario C, the climate sequences extracted from SILO were scaled to account for a changed climate as projected by 15 different GCMs for three global warming scenarios (Post et al., 2009). The 45 climate scenarios were modelled using WAVES at the 20 control points for every combination of soil and vegetation types. Regression equations were developed between mean annual rainfall and mean annual recharge for the 45 future climate scenarios and the 84-year historical base case. These regression equations enabled upscaling of the results to produce 45 rasters of RSFs in the same manner as Scenario B. The mean RSF was aggregated to a region level and the different GCMs were ranked:
Scenario Cwet is the wettest 90th percentile (rank 2 of 15 rasters) for the high global warming scenario Scenario Cmid is the wettest 50th percentile (rank 8 of 15 rasters) for the medium global warming scenario Scenario Cdry is the wettest 10th percentile (rank 14 of 15 rasters) for the high global warming scenario.
The same WAVES modelling as Scenario C was used for Scenario D. The assumed future commercial plantation forest areas from Viney et al. (2009) (Figure 4) were incorporated into the vegetation raster for upscaling and then 45 recharge rasters were created in the same manner as Scenario C. The same GCMs were used in each region as Scenario C to create RSF rasters for scenarios Dwet, Dmid and Ddry.
Figure 4. Distribution of assumed future plantation forests for Scenario D
2.4 Groundwater modelling and assumptions
Three calibrated, transient groundwater flow models were available to this project following recent completion of the DPIPWE Development of Groundwater models for Tasmania project (Aquaterra/REM, 2009a; c; e). These models cover the Mella and Togari groundwater assessment areas (GAAs) in the Arthur-Inglis-Cam region, the Wesley Vale GAA in the Mersey-Forth region, and the Scottsdale GAA in the Pipers-Ringarooma region (see Figure 1). All three models were designed and calibrated in accordance with the MDBC Groundwater Modelling Guidelines (MDBC, 2001).
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Recharge time series for groundwater models
The groundwater models utilised for this project were calibrated as part of a previous project (REM/Aquaterra, 2008a–s) and so the WAVES recharge time series could not be used directly in the models (see model benchmarking exercise reported in Appendix A). The existing calibrated recharge was modified through the use of RSFs. WAVES calculates a time series of recharge only at the control points, hence, the new recharge time series for the groundwater model recharge zones were calculated using the shape of the time series from the nearest control point as well as the average from the existing recharge calibrated in the model multiplied by the RSF for that scenario.
RWS, i RMSi, ..RRSF MC S (1) RWS where RMS,i is the new recharge to go into the model for a given scenario at stress period i, RWS,i is the recharge from
WAVES for the nearest control point at stress period i, RWS is the average recharge from WAVES for the scenario from the nearest control point, is the average recharge from the model calibration for the model recharge zone and RMC
RSFS is the average RSF for the scenario calculated for the model zone.
For scenarios Awet, Amid and Adry, the recharge time series from the nearest control point was used in Equation (1) for the appropriate 23-year period along with the RSF. For Scenario B the 11-year time series was looped 2.1 times to create a 23-year time series.
For scenarios C and D, the 23-year time period used for the wet, mid and dry scenarios was the same as that used for Scenario Amid. This means that for the numerically modelled areas, the average recharge for the model zone derived through model calibration was multiplied by the RSF for Scenario Amid and the RSF for the scenario under investigation e.g. RMCAmidCRSF RSF . The results of the numerical modelling for scenarios C and D cannot be compared to the 84-year historical period but are directly comparable to Scenario Amid. Because scenarios C and D in the models are built from Amid, a situation can arise where the recharge implemented in the numerical models for Cdry is greater than the historical average even though the RSF for Scenario Cdry is below one.
In addition to the potential future expansion of plantation forests, groundwater modelling under Scenario D also considered increased groundwater extraction for irrigation and enhanced recharge beneath crops that are irrigated using either surface water or groundwater.
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Mella-Togari model
While the WAVES-scaled results (above) were used as input for diffuse recharge in the numerical models, all other input parameters and assumptions were as reported in Aquaterra/REM (2009a; b). Both the irrigation area and number of extraction wells were increased for Scenario D so that the total extraction (E) would be capped at 25 percent of diffuse recharge (R). The rationale for increasing E/R to 0.25 for Scenario D was that this value, whilst arbitrary, represents a level of extraction that could be considered a precautionary upper limit until a more rigorous assessment of sustainable yield can be undertaken. The expanded irrigation area for groundwater was calculated based on the currently assumed irrigation application rate (ML/ha/year) and the total extraction volume (E in ML), and this was represented in the model as additional recharge due to root-zone drainage at a rate of 15 percent of irrigation applications. The expanded irrigation area and the related additional extraction wells required to meet the E/R of 0.25 were placed away from rivers in cleared land that was neither classified as remnant vegetation, nor that assumed for future commercial forest plantations. No expansion of surface water irrigation or forestry was considered in either the Mella or Togari GAAs (Ling et al. 2009a). The modelled current plantation forest areas, current and future irrigation area and well locations are shown in Figure 5.
Figure 5. Location of current and future irrigation areas, current plantation forests and extraction wells in the Mella-Togari model
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Wesley Vale model
The WAVES-scaled results (above) were used as input for diffuse recharge in the numerical models. Modelled extraction was also increased from that achieved in the original DPIPWE model to better match the estimated extraction for the area. All other input parameters and assumptions were as reported in Aquaterra/REM (2009c), including the addition of root-zone drainage (recharge) under irrigation areas at 15 percent of irrigation applications from surface water or groundwater sources. Both the irrigation area and number of extraction wells were increased for Scenario D so that the total extraction (E) would be capped at 25 percent of diffuse recharge (R). The rationale for increasing E/R to 0.25 for Scenario D was that this value, whilst arbitrary, represents a level of extraction that could be considered a precautionary upper limit until a more rigorous assessment of sustainable yield can be undertaken. The existing irrigation area in the model was based on land use data from BRS (2002). New irrigation diversion volumes sourced from surface water are projected to amount to 7.2 GL/year (Ling et al., 2009b). This was represented in the model by applying 15 percent of that volume as additional root-zone drainage over the existing irrigation area (i.e. the existing area was not expanded but the recharge volume was increased). The expanded irrigation area for groundwater was then added, with the area calculated based on the total extraction volume (E in ML), divided by the assumed irrigation application rate (ML/ha/year). The future expanded irrigation area and the related additional extraction wells required to meet the E/R of 0.25 were placed away from rivers in cleared land that was not classified as remnant vegetation, nor assumed for future plantation forests. The modelled current and future commercial forest plantations, irrigation areas and well locations are shown in Figure 6.
Figure 6. Location of current and future irrigation areas, current and future plantation forests and extraction wells in the Wesley Vale model
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Scottsdale model
While the WAVES-scaled results were used as input for diffuse recharge in the numerical models, all other input parameters and assumptions were as reported in Aquaterra/REM (2009e. Both the irrigation area and number of extraction wells were increased for Scenario D so that the total extraction (E) would be capped at 25 percent of diffuse recharge (R). The rationale for increasing E/R to 0.25 for Scenario D was that this value, whilst arbitrary, represents a level of extraction that could be considered a precautionary upper limit until a more rigorous assessment of sustainable yield can be undertaken. The existing irrigation area in the model was based on land use data from BRS (2002). New irrigation diversion volumes sourced from surface water are projected to amount to 35.3 GL/year (Ling et al., 2009c). This was represented in the model by applying 15 percent of that volume as additional root-zone drainage over the existing irrigation area (i.e. the existing area was not expanded but the recharge volume was increased). The expanded irrigation area for groundwater was then added, with the area calculated based on the total extraction volume (E in ML), divided by the assumed irrigation application rate (ML/ha/year). The future expanded irrigation area and the related additional extraction wells required to meet the E/R of 0.25 were placed away from rivers in cleared land that was not classified as remnant vegetation, nor assumed for future plantation forests. The modelled existing and future commercial forest plantations, irrigation areas and well locations are shown in Figure 7.
Figure 7. Location of current and future irrigation areas, current and future plantation forests and extraction wells in the Scottsdale model
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3 Knowledge gaps, limitations and uncertainty
3.1 Monitoring data gaps
Most of Tasmania suffers from a lack of long-term, high-quality groundwater monitoring data. The greatest limitation of the existing groundwater models, both in terms of uncertainty in results from this project and future determinations of sustainable extraction limits, is the absence of reliable groundwater extraction and water level monitoring data. Until this data has been acquired and used in the calibration process, the models should only be considered as preliminary regional-scale assessment tools. Furthermore, the modelled hydrographs presented here should be viewed in terms of the general trends in groundwater levels and not necessarily the absolute values.
3.2 Surface–groundwater interactions
Greater knowledge is required of the interactions between groundwater and surface water. In particular, knowledge of the flow paths and residence times of groundwater discharging to the rivers incising dolomite or basalt aquifers would be very useful for informing future management decisions. Additionally, there is a need to quantify groundwater recharge from losing rivers in those groundwater systems targeted for future development, especially those where the ratio of extraction to recharge is likely to approach or exceed 0.25.
3.3 Numerical model knowledge gaps, limitations and uncertainty
The existing transient, numerical groundwater flow models provide a preliminary assessment tool for investigating the potential effects of climate change at a regional scale. The models allow detailed quantitative investigation of the groundwater responses and interactions with surface water features under climate and development scenarios. While this is very useful to guide risk-based approaches to resources management, it is not possible at this time to be definitive about the level of predictive uncertainty, and care should be taken not to rely too heavily upon the absolute magnitude of the groundwater resource condition at any particular time. Interpretation of the results should be informed by understanding the following uncertainties that affect the three numerical models:
Aquifer parameters were assumed to be uniform throughout the catchment (for each aquifer unit) when it is likely that they may vary considerably over short spatial scales. Thus, while the models provide a realistic interpretation of regional flow processes, sub-regional and local results may be less reliable. This can sometimes affect groundwater pumping scenarios where the model may not be able to sustain the assumed pumping, but in reality the aquifer conditions can support high pumping rates and growth. To inform stream-aquifer interaction, there is a lack of detailed information available on the river bed levels, and the highly dynamic distribution of river water levels with space and time, and so the models were established during the DPIPWE project by: o assuming a river/stream bed level at 2 m below the topographic digital elevation model o assuming a permanent river/stream water level at 0.2 m above the adopted bed level for the Scottsdale and Wesley Vale models (i.e. all river/stream features can be variably gaining/losing, but the minor streams in the Scottsdale and Wesley Vale models, which are located in the upper parts of the catchment, are generally losing in the model due to the deep watertable in these areas, which is consistent with the surface–groundwater interaction understanding described in this report; see also Appendix B) o assuming a permanent river/stream water level at 0.2 m above the adopted bed level only for major rivers in the Mella-Togari model, and at the adopted bed level for minor streams (i.e. minor streams can only ever act as gaining streams in the Mella-Togari model, while major rivers can be variably gaining and/or losing; this was due to more detail being available for the stream network in this catchment) o validating the model calibration/history match by considering the match between the baseflow contributions calculated by the groundwater models and the available baseflow data.
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Under conditions of falling groundwater levels due to reduced recharge and/or additional groundwater pumping, the above river feature model assumptions can result in the modelled variably gaining/losing streams/rivers providing induced leakage volumes to groundwater, which can partly sustain groundwater level responses. Further, the maximum flow rate for induced leakage is constrained by the relatively low maximum driving head of 0.2 m (i.e. the specified water level in the model for rivers/streams) and the river/stream bed conductance term. It is also worth pointing out that, those streams that are identified as always losing (most minor headwater streams in the Scottsdale and Wesley Vale models) would not be subject to change under such conditions, and those minor headwater streams in the Mella-Togari model that are modelled as only gaining would also not change in character. For the development scenarios, assumptions were made about the area of irrigation and the distribution of pumping wells (see Section 2.4 for details). As this project scope did not include optimisation of the scale or spatial distribution of the future development, the irrigation application rates and the well pumping rates for growth scenarios were assumed based on known/existing application/pumping rates, which are affected by significant uncertainty. For example, there is evidence in some cases of pumping effects in observation wells for which nearby extraction well data has not been captured, and so is not input to the model. For one or two specific scenarios, certain models may not have been able to sustain pumping from certain wells, and so the target extraction volume in the numerical models may not have been quite achieved. For the Scottsdale model, this problem was exacerbated by the thin aquifer conditions in many areas, and so some model refinements of well configurations were undertaken to match the development scenarios as closely as possible.
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4 The Arthur-Inglis-Cam region
4.1 Contextual information
4.1.1 Hydrogeology
The location of groundwater assessment areas (GAAs) in the Arthur-Inglis-Cam region is shown in Figure 8. Note that the Leven-Forth-Wilmot GAA falls mainly in the adjoining Mersey-Forth region and thus is discussed in Chapter 5. Groundwater salinities are shown (where reported), which also gives some indication of where groundwater extraction occurs. Surface–groundwater interactions have also been mapped (see Section 4.2.3 for further discussion). The following summary of the region’s hydrogeology discusses Flinders Island and King Island separately from the remainder of the region.
Figure 8. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Arthur-Inglis-Cam region
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Arthur-Inglis-Cam
The geology of the Arthur-Inglis-Cam region is complex (see Figure 2, Figure 67 and Figure 68). In the west of the region, basement is comprised of Meso-Proterozoic siltstone which outcrops over much of the central and southern parts of the region. These sediments were intruded by dykes with a north-east south-west orientation throughout much of the Arthur and Black-Detention catchments and are present in the north of the Mella GAA. The basement is overlain by a complex sequence of schist, phyllite, mudstone and dolomite of Neoproterozoic age. These deposits are overlain by Late Cambrian conglomerates, siltstones and mudstones in parts of the Mella and Togari GAAs. Tertiary basalts occur along the eastern and western edges of the Smithton Syncline GAA, but are generally absent from the Mella and Togari GAAs. The Quaternary sequence is dominated by extensive deposits of coastal sands and gravels.
In the east of the region, basement is comprised of mid- to late-Cambrian sedimentary sequences that were intruded by granites during the Devonian, which outcrop in Cam-Emu-Blythe. In Inglis-Cam, basement is overlain extensively by Permian sequences of tillite, mudstone, siltstone and sandstone. Tertiary basalts overlie much of the area and are widespread in the Inglis-Cam and Cam-Emu-Blythe GAAs. The basalts are comprised of numerous layers of basalt with intervening sediments containing extensive quartz gravels and clays. The Quaternary sequence is predominantly alluvial sand and gravel, infilling valleys and hillslopes.
Two principal aquifers occur across the region. The Tertiary basalt aquifer (which includes interbedded Tertiary sediments) occurs extensively across the Inglis-Cam, and Cam-Emu-Blythe GAAs (REM/Aquaterra, 2008d; e) and in smaller areas throughout the remainder of the region. The basalt is present at or near the surface and extends to depths of around 200 m with maximum recorded depth of up to 300 m in the Waratah basalt plateau area. Storage within this aquifer system occurs in the fracture network and vesicles formed in the basalt when it cooled after extrusion (a vesicle is a small cavity in an igneous rock formed by the expansion of a bubble of gas during the solidification of the rock). There is also storage within the pore spaces of interbedded sediments which were deposited in between the basaltic lava flows during successive volcanic eruptions. The system is considered to be a dual porosity aquifer (that is, having primary and secondary porosity) with movement of groundwater occurring through the pore spaces of fluvial sediments and interconnected vesicles and also through the fracture network. The variability in porosity and fracture network results in the aquifer being heterogeneous with a broad range of measured well yields. In general, yields are less than 5 L/second and water quality (where reported) is generally very good with salinities less than 500 mg/L.
The other major aquifer of the region is the Neoproterozoic Smithton dolomite, which occurs throughout the Mella, Togari and Smithton Syncline GAAs (REM/Aquaterra, 2008a; b; f). The dolomite is present near the surface, typically underlying a thin veneer of Quaternary sediments, and extends to a depth of 2000 to 3000 m (although the main transmissive zone occurs in the upper 100 to 200 m). This aquifer is extremely heterogeneous, with a broad range of aquifer transmissivities and can be high yielding in places (>20 L/second). Reported salinities are less than 1000 mg/L. It is considered to be karstic in the top 100 m, but the inter-granular porosity is low. This aquifer may be locally confined by overlying Quaternary sediments and by the occurrence of crystallised dolomite at the top of the Precambrian sequence. It is regionally unconfined. In the southern part of the Welcome River catchment, groundwater is sourced from a dolomite aquifer which is in geological terms a correlate of the Smithton dolomite. Yields are variable with some higher yielding wells supporting irrigation near Redpa in the Welcome catchment.
The Cambrian siltstones of the Scopus Formation can also supply reasonable well yields; however, groundwater storage and flow in these siltstones occurs primarily within joints and bedding plains. Consequently, well yields are highly variable, but generally less than 5 L/second and reported salinities are less than 1000 mg/L. The degree of hydraulic connection between the Scopus Formation and the Smithton dolomite aquifer is unknown.
Shallow Quaternary deposits can also host localised aquifers where impervious layers lead to perched watertables. These groundwater resources are considered to be volumetrically insignificant, but may play an important role in recharging the underlying dolomite aquifer and supporting groundwater dependant ecosystems.
Fractured-rock aquifers, associated with basement geologies, occur throughout the region, particularly outside of the GAAs. They generally possess moderate aquifer properties with few fractures leading to low yields. They are not significantly utilised.
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Flinders Island
The geology of Flinders Island is dominated in the southwest corner of the island by the Mathinna Beds (Ordovician to Devonian meta-sedimentary rocks) and Devonian granites; and in the north and east by Tertiary marine limestone and Tertiary basalts covered by a layer of Quaternary beach dune deposits. Quaternary and Tertiary sediments and Tertiary basalts are considered the significant aquifers on the island with town water supplied from Tertiary basalts.
Groundwater salinity on Flinders Island ranges between 240 to 2700 mg/L and well yields range from 0.07 up to 37 L/second (REM/Aquaterra, 2008g). King Island
The surface geology of King Island is an eroded plateau of Precambrian sedimentary and metamorphic rocks with intrusions of granite and basic igneous rocks overlain by a thin veneer of alluvium and coastal sands. In the southeast of the island, metamorphic fractured-rock aquifers have yields in a range from 0.07 up to 2.5 L/second and their recorded groundwater salinity is usually below 500 mg/L. Quaternary coastal sands form the main aquifer on the island providing the town water supply of Currie. This unconfined aquifer has reliable supplies of very good quality water but is low yielding. Groundwater salinity ranges from 100 to 4000 mg/L and well yields are low throughout and range from 0.03 up to 2.5 L/second. (REM/Aquaterra, 2008c).
4.1.2 Surface–groundwater interactions
Arthur-Inglis-Cam
Streams throughout the Arthur-Inglis-Cam region are thought to be predominantly gaining (REM/Aquaterra, 2008a–g). Groundwater levels, albeit sparsely recorded, are higher than the adjacent surface water levels throughout the region with few exceptions. Surface water modelling conducted for DPIPWE indicated groundwater discharge to streams represented 50 percent or more of the total flow in streams (REM/Aquaterra, 2008a–g).
In the basaltic terrain of the Inglis-Cam and Cam-Emu-Blythe GAAs, groundwater discharge to streams via springs and seeps is common. This can occur where structural benches of unweathered or unfractured basalt impede movement of groundwater down-slope. Springs and seeps can also occur along the geological contact between the Tertiary basalt and basement rocks. The rivers in these GAAs are also deeply incised with the adjacent groundwater levels rising away from the river channel, suggesting groundwater discharge to streams. Groundwater recharge via leakage from any of the main rivers is most likely to occur in the upper portion of the catchments where surface water elevations are greater than groundwater elevations.
Within the dolomitic terrain of the Mella GAA, hydrochemical sampling indicated active groundwater discharge to the Duck River and its tributaries (REM/Aquaterra, 2008a). Groundwater elevations are also typically greater than the adjacent surface water levels. In the upper reaches of these catchments, small ephemeral streams may be losing when they periodically flow and they are elevated above the watertable. It is possible that some complex surface–groundwater interactions occur where the dolomite is karstic as the direction and magnitude of flow between the stream and aquifer may change significantly over short distances. In contrast there may be little flux between surface water and groundwater where the dolomite has silicified (and hence has negligible permeability) or a low permeability clay layer separates the stream from the aquifer.
Similar processes are thought to occur in the Togari GAA and throughout the remainder of Smithton Syncline GAA. Flinders Island
Streams on Flinders Island that flow west (e.g. Pats River, South Pats River and Hays Creek) are mostly short ephemeral streams flowing quickly to the ocean from steep terrain. In the east, coastal lagoons have formed due to sand dunes and rising Granitic basement blocking drainage. In the south and west of the island, hydrogeochemical sampling indicated that groundwater is discharging to the rivers and is sourced from different aquifer types as the rivers pass across different lithological terrain (REM/Aquaterra, 2008g). Groundwater level elevations are mostly above the water levels of streams flowing across the coastal sedimentary plains, indicating gaining streams. It is likely that losing streams
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occur in the steep slopes of the upper reaches of the catchment where surface water levels are higher than groundwater levels (REM/Aquaterra, 2008g). King Island
The main river systems on King Island are the Sea Elephant River in the central north-east region and the Ettrick River in the far southwest. There is insufficient data to compare surface water and groundwater levels but, based on known responses of streams in similar settings, it is likely that both streams are losing streams in their upper reaches and gaining streams in their lower reaches. Groundwater commonly discharges to Bass Strait from coastal springs emanating from the Quaternary dune systems overlying the fractured bedrock (Ezzy, 2003).
4.1.3 Groundwater extraction
Groundwater extraction in Tasmania is not metered and as such there are no historical records. Estimates of groundwater extraction for irrigation have been made based on the difference between the estimated water requirements for irrigated areas and surface water allocations. Groundwater-use surveys were also carried out in 2008 by REM to help inform these estimates. Previous estimates are summarised in Table 3. Arthur-Inglis-Cam
Within the Mella and Togari GAAs, groundwater is largely extracted from the Smithton dolomite and the Cambrian siltstones of the Scopus Formation with some extraction from the Black River dolomite in the southern end of the Smithton Syncline GAA.
In the Inglis-Cam and Cam-Emu-Blythe GAAs, groundwater is predominantly extracted from the Tertiary basalt and to a lesser degree from the Precambrian metasediments.
Total groundwater extraction for irrigation, stock supplies and industrial purposes in the Smithton Syncline, including Mella and Togari GAAs is estimated to be around 11,500 ML/year (REM/Aquaterra, 2008f). Extraction in the Inglis-Cam GAA has been estimated at less than 1,000 ML/year (REM/Aquaterra, 2008d), whilst groundwater extraction in the Cam-Emu-Blythe GAA has been estimated to total 1,500 ML/year (REM/Aquaterra, 2008e). Flinders Island
Groundwater extraction on Flinders Island is largely sourced from either Quaternary or Tertiary sedimentary aquifers throughout the island, the Tertiary basalt in the south or from the Mathinna Beds in the south-west. The only known groundwater users are for town water supply (sourced from shallow spear point wells and a deeper production well completed in Tertiary basalt), and approximately 50 farmers that use groundwater for stock and domestic purposes (M Sherriff (Flinders Council), 2008, pers. comm.). Total groundwater extraction is estimated to be less than 1000 ML/year. King Island
On King Island, groundwater is mainly used for town water supply and is estimated at less than 1000 ML/year. This supply is sourced from a nest of shallow spear point wells completed in the Quaternary coastal sand aquifer. Other known groundwater users are the local abattoir and King Island Dairy. Groundwater usage for the King Island Dairy is estimated at 100 ML/year and for the abattoir it is estimated at approximately 57 ML/year. Groundwater is also commonly used as wash down water for over 25 dairies across the island.
4.1.4 Groundwater resource protection and management issues
Regulation of the water well drilling industry has recently been introduced into Tasmania. Drilling contractors are required to hold a Tasmanian Well Drillers Licence. A permit to drill system is in place that requires all landowners to obtain a Well Works Permit prior to the commencement of drilling. Whilst drillers have always been required to return information relating to the construction of wells, there has been no regulation or controls on well operation, such as the collection of extraction data and long-term monitoring of groundwater levels or salinity. The state-wide groundwater monitoring
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network is too sparse to adequately monitor groundwater conditions. In lieu of these management and data gaps, DPIPWE is making progress on the development of groundwater management plans and has expanded the monitoring network.
Localised land salinisation has been identified on both Flinders Island and King Island. Above-ground diesel storage tanks on farms have also been known to cause small hydrocarbon plumes. Dairies are very common in north-western Tasmania and on King Island. Wash down water from milking sheds can cause local nutrient plumes on farms. Diffuse nutrient enrichment from cattle waste has been detected in the Togari GAA. The unlined Smithton sewage lagoons, in the Mella GAA, are the largest in Tasmania and a nutrient plume has been detected emanating from the site which is monitored by the local Council. The town water supply on King Island relies on groundwater for a drinking water supply. The town water supply extraction spear array is down gradient of the town cemetery and storm water is also directly recharged into the aquifer. Additional point sources of groundwater contamination in this region include activities such as historical landfills, underground storage tanks and fuel depots, plus food processing (meat and vegetables) plant disposal sites. Acid discharge from acid sulphate soils is known to occur in the Mella and King Island GAAs and is associated with drainage of the watertable (Gurung, 2001).
In north-western Tasmania, groundwater is mainly extracted for crop irrigation by pivot irrigators and for dairy farms. Over extraction during summer months has caused cross property impacts in the Mella GAA due to well drawdown interference. The long-term State Government monitoring well at Montagu indicates that the local aquifer at least surrounding this well does not fully recover during winter months. More targeted monitoring is recommended in this region to sustainably manage the groundwater resource.
4.1.5 Previous estimates of recharge and discharge
REM/Aquaterra (2008a–g) performed a series of groundwater assessments throughout the Arthur-Inglis-Cam region for the recent DPIPWE groundwater modelling project. These assessments covered the Mella, Togari, Smithton Syncline, Inglis-Cam and Cam-Emu-Blythe GAAs as well as King Island and Flinders Island. Key components of the water balance were estimated as part of these studies
Diffuse groundwater recharge was estimated via several methods including the steady state Chloride Mass Balance method, Water Table Fluctuation method, and an empirical relationship for estimating evapotranspiration derived by Zhang et al. (1999, 2001). From the results of these methods a best estimate for recharge was determined (REM/Aquaterra, 2008a–s).
A second estimate of recharge was defined during the DPIPWE project for regions where a numerical groundwater flow model was constructed and calibrated. In the Arthur-Inglis-Cam region, models were constructed for the Mella and Togari GAAs (modelled together) (Aquaterra/REM, 2009a), and the Inglis-Cam and Cam-Emu-Blythe GAAs (modelled together) (Aquaterra/REM, 2009b). Recharge rates were defined across the model domains according to rainfall, land use and surface geology and were refined during the calibration process.
In addition, as part of the National Land and Water Audit 2000, SKM (2000a) estimated recharge to determine sustainable yields for groundwater catchments. The boundaries to these catchments were based on aquifer boundaries and covered areas similar to the Welcome and Montagu catchments, and the Inglis-Flowerdale, Cam, Emu and Blythe catchments. The REM/Aquaterra (2008a–s) catchments coincide with GAAs defined in this project, but not with the SKM (2000a) catchments.
Groundwater discharge estimates were also conducted for the DPIPWE groundwater modelling project (REM/Aquaterra, 2008a–s). Groundwater discharge to streams has been estimated for a number of catchments according to surface water modelling. Estimates of lateral throughflow (predominantly groundwater discharge to the ocean) were based on flow-net analysis. Groundwater losses through evapotranspiration were estimated for the Mella, Togari, Inglis-Cam and Cam-Emu-Blythe GAAs from numerical models (REM/Aquaterra, 2008d; e).
A summary of estimates of recharge, discharge and extraction for selected catchments is provided in Table 3.
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Table 3. Previous estimates of groundwater fluxes for the Arthur-Inglis-Cam region
Area Diffuse recharge Extraction Total discharge * Surface water catchment GL/y Mella – conceptual model (REM/Aquaterra, 51.0 4.5 111.3 Duck 2008a) Togari – conceptual model (REM/Aquaterra, 30.0 3.5 63.4 Montagu 2008b) Smithton Syncline – conceptual model 310.0 11.5 210.0 Welcome, Montagu, Duck (REM/Aquaterra, 2008f) Mella-Togari – numerical model 58.0 7.6 97.5 Montagu, Duck (Aquaterra/REM, 2009a) Smithton (SKM, 2000a) 60.0 0.8 NA Welcome, Montagu, Duck Cam-Emu-Blythe – conceptual model 170.0 1.5 150.0 Cam, Emu, Blythe (REM/Aquaterra, 2008e) Inglis-Cam – conceptual model 220.0 1.0 160.0 Inglis-Flowerdale (REM/Aquaterra, 2008d) Inglis-Blythe (Aquaterra/REM, 2009e) 162.0 1.8 185.1 Inglis-Flowerdale, Cam, Emu, Blythe Burnie (SKM, 2000a) 140.0 2.2 NA Cam, Emu, Blythe, Inglis- Flowerdale (Leven, Forth- Wilmot, Rubicon, Mersey) Flinders Island – conceptual model 26.0 1.0 4.3 Flinders Island (REM/Aquaterra, 2008g) Flinders Island (SKM, 2000a) 38.0 0.1 NA Flinders Island King Island – conceptual model 29.0 1.0 15.0 King Island (REM/Aquaterra, 2008h) * Discharge volumes are derived from estimates of groundwater discharge to streams, groundwater extraction and lateral discharge (REM/Aquaterra, 2008a–s). Losses to evapotranspiration are included for modelled areas (Aquaterra/REM, 2009d; e) but not elsewhere. NA – not available
Steady-state water balances for the numerical models developed in the DPIPWE groundwater modelling project are shown in Table 4. A steady-state water balance is representative of average annual fluxes under historical climate. The modelled groundwater extraction for the Togari and Inglis-Blythe models was less than that estimated (Aquaterra/REM, 2009d; e). This is because the simulated network of pumping wells was not optimised; the actual pumping rates were not available and had to be assumed. Where the assumed rate was greater than what the model could support for a particular well the pumping ceased, causing a reduction in the total modelled extraction. This should not be interpreted as a better estimate of extraction.
Table 4. Water balances from steady-state numerical models for the Arthur-Inglis-Cam region
Component Mella Togari Inglis-Cam and Cam-Emu-Blythe Groundwater Groundwater Groundwater Groundwater Groundwater Groundwater inflows outflows inflows outflows inflows outflows GL/y Diffuse recharge 29.4 - 28.3 - 162.0 - Constant head 4.2 3.6 4.3 3.6 0.3 6.8 Rivers 15.0 17.4 9.8 7.4 6.6 34.3 Drains - - - 2.7 - - Evapotranspiration - 26.4 - 22.5 - 125.4 Extraction - 4.5 - 3.1 - 1.8 Lateral flux 4.8 1.6 1.6 4.8 - - Storage - - - - 16.2 16.8 Total 53.4 53.4 44.0 44.1 185.0 185.1
The various assessments indicate that groundwater extraction rates from the various catchments are between one and three orders of magnitude less than estimates of recharge indicating that there is currently a low level of threat from over-use.
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4.1.6 Groundwater level trends
Arthur-Inglis-Cam
Time series of groundwater level and salinity data within the region are rare. State observation wells that have sufficient data tend to reflect localised conditions more so than regional trends. Although the monitoring wells are completed in the major aquifers, a meaningful analysis of trends between wells is generally not possible on a regional scale because of the low well density. Whilst recognising these limitations, hydrographs for two monitoring wells are presented in Figure 9 (see Figure 8 for their locations). The two major aquifers from the region are represented: the Togari monitoring well (Figure 9a) is completed in Smithton dolomite; the Hampshire monitoring well (located in the Inglis-Cam GAA) is completed in Tertiary basalt (Figure 9b). Local rainfall data is presented alongside the groundwater level data. In both hydrographs, there has been a reduction in recovery levels since 1996 during drier than average years. However, a significant declining trend in water levels is not noted. Some local pumping influences can be seen in the Togari monitoring well since 2001.
(a) Togari
0 400
2 300
4 200
6 100 (mm)mean
Waterlevel (m BGL) Water level (m BGL) 8 0 Cumulative deviation from mean rainfall (mm) Cumulative deviation from the
10 -100 1991 1993 1996 1999 2001 2004
(a) Hampshire
4 800
6 600
8 400
10 200 (mm)mean
Water level(m BGL) Water level (m BGL) 12 0 Cumulative deviation from mean rainfall (mm) Cumulative deviation from the
14 -200 1991 1993 1996 1999 2001 2004
Figure 9. Hydrographs for the (a) Togari and (b) Hampshire monitoring wells, showing the water level (in metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall
Flinders Island
A network of shallow piezometers was installed on Flinders Island in 2002 to monitor groundwater levels as part of dryland salinity investigations. Many of these piezometers subsequently dried out as they were installed during a particularly wet period, and there is limited data available to inform any regional trends in groundwater levels.
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King Island
The only groundwater monitoring known to have occurred on King Island was limited to the coastal sand aquifer along the west coast. Records could not be obtained for this project.
4.2 Groundwater system assessment
4.2.1 Recharge/discharge
Table 5 presents a summary of the key components of the groundwater balance for GAAs in the Arthur-Inglis-Cam region. These estimates are carried through for the scenario analysis in the following section.
The diffuse recharge rates represent an historical annual mean. For the Mella and Togari GAAs, where further modelling was conducted for this project, the recharge rates were derived from the mean annual rates during the DPIPWE model calibration period. For the Inglis-Cam and Cam-Emu-Blythe GAAs, the recharge rate was equivalent to the rates used for the calibrated numerical models (steady state only) used in the DPIPWE groundwater modelling project (REM/Aquaterra, 2009d; e). In GAAs where no numerical model exists, rates from the ‘preliminary conceptual water balance’ of the DPIPWE groundwater modelling project (REM/Aquaterra, 2008c; f–g) have been used.
Current extraction is based on estimates derived during the DPIPWE groundwater modelling project (REM/Aquaterra, 2008a–s). There is a large degree of uncertainty associated with these estimates because they were not based on metered data. It is possible that actual values may be significantly higher or lower. Furthermore, there is no available information to guide estimates of future groundwater extraction at 2030. Hence, a precautionary approach was taken. In Mella and Togari, where scenario modelling is being conducted, future extraction was assumed to be capped at 25 percent of recharge under Scenario Dmid. In the remaining GAAs, where a greater degree of uncertainty exists, there was assumed to be no increase in extraction by 2030.
For Mella and Togari, groundwater discharge to streams has been calculated from the numerical model and represents the mean annual rate from the DPIPWE model calibration period. This flux is not available for other GAAs.
Recharge via stream leakage and groundwater losses due to evapotranspiration are significant components of the groundwater balance but have not been determined in this section, as they are difficult to quantify.
Table 5. Estimated diffuse recharge, discharge to streams and extraction for groundwater assessment areas in the Arthur-Inglis-Cam region
Groundwater assessment area Diffuse Current Future Discharge to recharge extraction extraction 2030 streams 2007/08 GL/y Mella 29.9 4.5 8.6 26.0 Togari 29.1 3.5 8.5 10.6 Smithton Syncline (including 313.7 11.5 20.6 na Mella and Togari) Inglis-Cam 87.3 1.0 1.0 na Cam-Emu-Blythe 74.6 1.5 1.5 na King Island 29.3 1.0 1.0 na Flinders Island 26.2 1.0 1.0 na Total 531.1 16.0 25.1 na na – not available
4.2.2 Surface–groundwater interactions
Surface–groundwater interactions account for a significant component of the groundwater balance. The likely nature of these interactions has been mapped for the Arthur-Inglis-Cam region (see Figure 8) in the Duck, Montagu,
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Inglis-Flowerdale, Cam-Emu-Blythe, and Flinders Island surface water catchments. More detailed, catchment-scale maps are shown in Appendix B.
In the Duck River catchment (Mella GAA), the major watercourses have been classified as predominantly gaining based on the available data. However, it is likely that surface–groundwater interactions are more varied than indicated by Figure 8. Only large streams have been mapped (stream order ≥3) and many of the smaller ephemeral streams in the upper catchment are not shown. Such streams are likely to be losing when they periodically flow. Furthermore, on the Mella plains where the Duck River passes directly through Smithton dolomite, some significant data gaps exist. So while the available data suggests the river is entirely gaining, it is possible that some reaches are losing or variably gaining/losing.
The upper reaches of the Montagu River (in the Togari GAA) pass through Smithton dolomite that is variably silicified and karstic. In this region, groundwater levels are both above and below the adjacent surface water elevations and the river can stop flowing in late summer. Hence, it is likely the Montagu River is gaining when the watertable is high, and losing when the watertable drops below the surface water level. In the Britton’s Swamp region, the watertable is approximately 20 m below surface water elevations. The streams in this region are classified as losing, but the magnitude of flux between the streams and groundwater may be small, given the presence of a low permeability clay layer (up to 20 m thick) underlying the streams. Further downstream, where the Montagu River flows permanently, the river is assumed to be gaining (as consistent with the Duck River).
All watercourses in the Inglis-Flowerdale (Inglis-Cam GAA) and Cam-Emu-Blythe (Cam-Emu-Blythe GAA) catchments have been classified as gaining. The classification was based on the conceptualisation of the major Tertiary basalt/sediments aquifer occurring on broad ridges and plateaus between deeply incised river valleys where the Tertiary sequence has been eroded. Spring flow is common on valley sides, particularly at the contact of the Tertiary aquifer and underlying basement geologies. Available groundwater elevations were also higher than the adjacent surface water elevations. In summary, the available data suggests streams are most likely to be gaining. It is likely that some exceptions may occur throughout the catchment. For example, small ephemeral streams in the catchment headwaters may be losing when they periodically flow. Other reaches may be variably gaining/losing but cannot be identified.
4.2.3 Conceptual model
Detailed conceptual models have previously been developed for all GAAs in the region (REM/Aquaterra, 2008a–s). Figure 10 summarises these models from the perspective of Bass Strait, looking south. King Island and Flinders Island are not presented.
In the Mella and Togari GAAs, the principal aquifer occurs in Precambrian dolomite that underlies a thin veneer of Quaternary sediments. The two GAAs are separated by a line of hills comprised of Cambrian siltstone. The dolomite is karstic in the top 100 m and is locally confined in places by silicified dolomite or clayey sediments. Recharge occurs throughout the catchment, including from the surrounding fractured rock Cambrian siltstones that outcrop at the edges of the GAA. The flow system is considered to be sub-regional in scale, extending from the recharge areas in surrounding hills through to river discharge and to the coast, where discharge occurs as throughflow.
In the Inglis-Cam and Cam-Emu-Blythe GAAs, broad basalt ridges tending north-east to south-west are separated by deeply incised river valleys where basement units outcrop. The basalt units are comprised of numerous basalt layers with intervening sediments and form the principal aquifer in these GAAs. The deep weathering and multiple lava flows in the basalt have resulted in complex aquifer systems of both local and intermediate scale. Local flow systems occur in upper, heavily weathered sections of the basalt, with flow direction largely controlled by surface topography and inferred to be in the direction of the main surface water drainage features. Intermediate systems occur in the interlayered and fractured horizons within the basaltic sequence. These aquifers can be separated and confined by relatively thick sequences of massive, poorly fractured basalt. Groundwater flow direction within the intermediate systems is inferred to be controlled by deep leads (ancient river valleys that have been filled with Tertiary basalt sediments) with discharge at the coast as throughflow. Groundwater springs are also common throughout the catchment.
The areas of the region outside of the GAAs are typified by local groundwater flow systems associated with low-yielding fractured rock aquifers. The primary exception to this is for the area near Waratah, where Tertiary basalts extend from
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the Inglis-Cam and Cam-Emu-Blythe GAAs. The groundwater flow systems that occur here are thought to be similar to those of Inglis-Cam and Cam-Emu-Blythe.
On King Island, the primary aquifer is associated with aeolian coastal sands and extends 20 to 30 km along the east and west coasts. Secondary aquifers include the fractured Proterozoic igneous and metamorphic rocks that underlie most of the island.
On Flinders Island, the primary aquifers are associated with the Quaternary and Tertiary sediments in the east and north of the island, and the Tertiary basalt in the south. Secondary aquifers include the Mathinna Beds in the south-west. The groundwater flow systems within each aquifer type on the islands are subcatchment in scale, with diffuse recharge to generally unconfined watertables and subsequent discharge to shallow, incised streams on the plains and to the coast.
Figure 10. Conceptual hydrogeological model for the Arthur-Inglis-Cam region
4.3 Scenario assessment 4.3.1 Recharge impacts
The WAVES model (Zhang and Dawes, 1998) was used to estimate the change in groundwater recharge across the Arthur-Inglis-Cam region under a range of different climate scenarios (Section 2.3). The historical (1924 to 2007) modelled recharge record was assessed to establish any difference between wet and dry periods of recharge. A 23-year period was used, which allows the projection of recharge estimates to 2030I – in other words, to estimate recharge in 2030 assuming future climate is similar to historical climate (Scenario A). Under scenarios Awet, Amid and Adry, the recharge changes for a 23-year period compared to the recharge under the entire period of the historical climate (see Figure 11). For the recharge that is exceeded in 10 percent of 23-year periods (Scenario Awet), recharge is on average 61 percent greater that the historical average (that is, a recharge scaling factor (RSF) of 1.61). For the recharge that is exceeded in 50 percent of 23-year periods (Scenario Amid), recharge is on average 14 percent greater than the historical average (RSF=1.14). For the recharge that is exceeded in 90 percent of 23-year periods (Scenario Adry), recharge is on average 54 percent lower than the historical average (RSF=0.46) (Table 6).
The recent climate (1996 to 2007) in the Arthur-Inglis-Cam region has been drier than the historical mean (1924 to 2007) and consequently the calculated recharge decreases 68 percent under Scenario B relative to Scenario A (see Table 6).
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Figure 11. Spatial distribution of recharge scaling factors in the Arthur-Inglis-Cam region for scenarios Awet, Amid, Adry and B relative to Scenario A
Under Scenario Cwet, recharge increases by 10 percent for the region as a whole, but this is not spatially uniform with greater increases in the south and in Flinders Island (see Figure 12, Table 6). Under Scenario Cmid, recharge increases by 5 percent for the region as a whole with the same pattern as under Scenario Cwet. Under Scenario Cdry, recharge decreases overall by 3 percent.
The difference in diffuse recharge between Scenario C and Scenario D is due to the impact of future increases in commercial forest cover upon recharge. The greatest differences are in the east of the region (see Figure 12), where future forest cover is assumed to increase (see Figure 4). The impact is a small reduction in recharge under scenarios Dwet, Dmid and Ddry relative to Scenario C (see Table 6).
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Figure 12. Spatial distribution of recharge scaling factors in the Arthur-Inglis-Cam region for scenarios C and D relative to Scenario A
To calculate the change in recharge under each scenario, RSFs were derived from WAVES modelling for each GAA (see Table 6). For non-modelled GAAs the RSFs were then multiplied by the historical mean recharge rates (see Table 5) to calculate scaled recharge under each scenario (Table 7). For modelled GAAs (i.e. Mella and Togari), the RSFs from WAVES were passed through individual recharge zones of the numerical groundwater model to calculate a scaled recharge volume for the GAA under each scenario (see Section 2.4).
Table 6. Aggregated recharge scaling factors for groundwater assessment areas in the Arthur-Inglis-Cam region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Mella 1.54 1.19 0.47 0.27 0.96 0.98 0.95 0.96 0.98 0.95 Togari 1.56 1.15 0.50 0.42 1.01 1.01 0.94 1.01 1.01 0.94 Smithton Syncline 1.63 1.16 0.44 0.31 1.03 1.01 0.93 1.03 1.01 0.93 Inglis-Cam 1.51 1.14 0.52 0.32 0.96 0.97 0.94 0.89 0.89 0.86 Cam-Emu-Blythe 1.52 1.14 0.51 0.31 0.96 0.98 0.96 0.84 0.86 0.83 King Island 1.46 1.18 0.57 0.39 1.02 1.07 1.07 1.02 1.07 1.07 Flinders Island 1.43 1.16 0.63 0.46 1.17 1.13 1.13 1.17 1.13 1.13 Arthur-Inglis-Cam region 1.61 1.14 0.46 0.32 1.10 1.05 0.97 1.08 1.04 0.95
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 25
Table 7. Scaled mean annual recharge for groundwater assessment areas in the Arthur-Inglis-Cam region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Mella 46 34 15 10 34 34 33 34 34 33 Togari 45 34 14 10 34 34 32 34 34 32 Smithton Syncline 511 364 138 97 323 317 292 323 317 292 Inglis-Cam 132 100 45 28 84 85 82 78 78 75 Cam-Emu-Blythe 113 85 38 23 72 73 72 63 64 62 King Island 43 35 17 11 30 31 31 30 31 31 Flinders Island 37 30 17 12 31 30 30 31 30 30
In terms of recharge impacts, a significant amount of variability is evident under Scenario A across the different GAAs, with recharge under Scenario Awet to approximately three times greater than Scenario Adry. The greatest amount of variability is evident in the Smithton Syncline GAA, although it is still comparable to the other GAAs. Recharge under Scenario B is significantly less than the historical 23-year median recharge (Scenario Amid). The difference is particularly acute in the Mella, Smithton Syncline, Inglis-Cam and Cam-Emu-Blythe GAAs. There is a reduction in recharge of typically 10 to 20 percent under Scenario Cmid, although little change is evident at Mella, Togari and Flinders Island. There is less variability evident under Scenario C with comparison to historical conditions; however, this is due to the method adopted where scenarios Cwet, Cmid and Cdry are based on the same 23-year period as Scenario Amid. There is little difference in recharge under scenarios C and D for most GAAs. This suggests that the impact of land use change may be minor. The exception occurs in Inglis-Cam and Cam-Emu-Blythe where a forecast expansion in forestry causes roughly a 10 percent decrease in recharge.
4.3.2 Modelled impacts to groundwater levels and fluxes in the Mella and Togari groundwater assessment areas
The existing numerical groundwater flow model for the Mella and Togari groundwater assessment areas is that developed by Aquaterra/REM (2009b). Figure 13 summarises the conceptual groundwater model for these GAAs. The numerical model follows this conceptualisation, consisting of two layers (Quaternary Sediments and basement outcrops for layer 1 and a combination of Precambrian dolomite, mudstones/siltstones and basement geologies for layer 2) with features representing inflows from diffuse recharge, irrigation and river leakage. The outflow components are represented by coastal, evapotranspiration, irrigation drainage and extraction well features. The historical recharge time series of the existing model was altered for the current project using the WAVES model record, as detailed in Section 2.4.
Figure 14 shows the location of three monitoring wells in the Mella and Togari GAAs which contain historical water level measurements. The map also shows a fictitious reporting well that has been placed in the model at the mound spring location which has been deemed a key reporting site for environmental purposes in this area. Modelled groundwater levels at these location sites were assessed under the four climate and development scenarios. The modelled river gaining and losing regimes have also been assessed and the results are discussed below.
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Figure 13. Conceptual groundwater model for the Mella and Togari groundwater assessment areas
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 27
Figure 14. Location of the Mella-Togari model extent and reporting sites
Under historical climate (Scenario A)
Figure 15a–f shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and scenarios Awet, Amid and Adry.
The model results were analysed to identify the areas of deepest drawdown due to extraction, and fictitious monitoring wells were specified in the model at these points (identified with a ‘DD’ prefix) to allow reporting of the time series of maximum drawdown effects. The results are presented in Figure 15, Figure 17, Figure 19 and Figure 21, showing no significant difference in terms of trend compared to the real monitoring points, but with additional drawdown that reflects their location within the area of most concentrated extraction. The results indicate that these aquifer systems appear to be quite robust, in that they:
respond rapidly to pumping and establish a new, dynamic hydrological equilibrium show dynamic water level changes in response to climatic and pumping effects (e.g. lower groundwater levels in centres of pumping; increased groundwater levels in response to recharge) show no apparent long-term trend of increasing or decreasing levels.
The Montagu well located in the Mella GAA, which is completed in the Cambrian Scopus Formation, shows large seasonal variations (difference between simulated water levels in winter to simulated water levels in the following summer period) of up to 10 m under Scenario Awet. There is a slight upward adjustment to the water levels during the prediction simulation, with a final simulated water level at 2030 being similar to the historical simulated water levels during the mid-1970s.
The Montagu well under Scenario Amid has smaller seasonal variations in water levels relative to Scenario Awet of approximately 2 to 4 m. The water level trend also shows a slight recovery over the prediction period, showing a similar trend to the historical warm up before the mid-1990s (i.e. pre-drought conditions). The final water level under Scenario Amid is less than 2 m lower for the Montagu well than it is under Scenario Awet at 2030.
With a maximum range of only 2 m, the Montagu well under Scenario Adry is simulated to have smaller seasonal amplitude than it does under scenarios Awet and Amid. The water level time series pattern is stable under Scenario Adry
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until the modelled year 2025, where a decline of ~1.5 m is simulated between the year 2025 and 2030. The final water level under Scenario Adry at 2030 is approximately 1 m lower than the simulated 2007 water level and is approximately 2 and 4 m below the water levels under scenarios Amid and Awet respectively.
The Trowutta well which is located in the Mella GAA, shows similar trends to the Montagu modelled water levels described above. The main difference is the larger simulated seasonal variability which shows variations of up to 20 m, 10 m and 5 m under scenarios Awet, Amid and Adry respectively. This is due to the Trowutta well being completed in the karstifed Black River dolomite aquifer which tends to have variable storage and thus show larger changes in water levels in response to a particular stress, such as diffuse recharge in this instance.
The mound spring reporting well has been included in the model for the Mella GAA as this location is categorised as a groundwater dependent ecosystem in the Conservation of Freshwater Ecosystem Values (CFEV) database (CFEV database, v1.0, 2005). The simulated hydrographs under scenarios Awet and Amid show no long term downward trend from 2007 to 2030 with seasonal variations of up to 6 m under Scenario Awet and over 3 m under Scenario Amid. However, the modelled trend under Scenario Adry shows water levels declining from ~10 m AHD at the year 2025, to less than 8 m AHD at 2030. Although Scenario Adry shows a downward trend post-2025, the final water level at 2030 is only about 1 m lower than the water level simulated at the end of the historical period.
The simulated water levels between 2007 and 2030 for the Togari well, drilled in Smithton dolomite aquifer, remain consistent at approximately 28 m AHD under both scenarios Awet and Amid. As simulated in the other wells in the Mella area, there is a downward trend from ~2025 under Scenario Adrywith the modelled water levels dropping from approximately 27 m AHD at 2025 to around 25 m AHD at 2030. The final simulated water levels at 2030 are marginally lower than the water levels simulated at the end of the historical period. This is true for all water levels under Scenario Adry simulated in the Mella and Togari GAAs.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 29
(a) Montagu
16 14 A Awet 12 Amid 10 Adry observed 8 6
Waterlevel (m AHD) 4 2 1966 1975 1985 1995 2005 2015 2025
(b) Trowutta
215 A Awet 205 Amid Adry observed 195
185 Waterlevel (m AHD)
175 1966 1975 1985 1995 2005 2015 2025
(c) Mound spring
20 A 18 Awet 16 Amid Adry 14 12 10
Water level (m AHD) 8 6 1966 1975 1985 1995 2005 2015 2025
(d) Togari
34 32 30 28 26 A Awet 24 Amid
Water level (m22 AHD) Adry observed 20 1966 1975 1985 1995 2005 2015 2025
Figure 15. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2
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(e) DD1
30
28
26
24 A Awet 22 Amid
Water level (m AHD) Adry 20 1966 1975 1985 1995 2005 2015 2025
(f) DD2
28 A 25 Awet 22 Amid Adry 19
16
Water level (m13 AHD)
10 1966 1975 1985 1995 2005 2015 2025
Figure 15. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2 (continued)
Figure 16 shows the simulated gaining, losing and variably gaining/losing river reaches under Scenario Amid between 2007 and 2030. As consistent with the conceptualisation of surface–groundwater interactions (see Figure 8), much of the Duck River is gaining. There are, however, some differences. A number of mid- to upper-reaches of the catchment are classified as losing or variably gaining/losing by the model when hydrochemical analyses indicated they were gaining (REM/Aquaterra, 2008a). This may suggest that modelled groundwater levels may be too low in this portion of the catchment, yet there is little available groundwater level data to check whether this is correct. In the main irrigation zone in the north of the GAA, the model suggests that streams are variably gaining/losing as opposed to being conceptualised as gaining. Hydrochemical analysis was not carried out in this district. Where Figure 14 shows a detailed network of streams, the minor streams in Mella and Togari, as well as the irrigation drainage network, are configured in the model as potential gaining only (i.e. cannot be losing). Hence, all the plots of model results in terms of gaining/losing streams (e.g. Figure 16) do not display these minor streams and irrigation drains, because the groundwater levels are always too low for these potentially gaining features to be activated.
The Montagu River in the Togari GAA is either losing or variably gaining/losing upstream from the Togari drainage network. The Togari drainage network is located within the irrigation area of Togari in the vicinity of the Togari observation well shown in Figure 14. The Togari drainage network is always gaining as it is represented by drain features in the model and can only extract water from the groundwater system. However, the Montagu River in this zone is classified by the model as being variably gaining/losing, as consistent with the conceptualisation (see Figure 8). The Montagu River is predominantly gaining downstream from the Togari drainage network with the tributaries tending to be either losing or variably gaining/losing in areas of higher topography.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 31
Figure 16. Simulated gaining and losing river reaches under Scenario Amid
Under recent climate (Scenario B)
Figure 17 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario B. All four Scenario B simulated water levels show a steady trend throughout the prediction period with the simulated water levels and season variability resembling the water levels simulated during the 11 years that Scenario B was based on (1997 to 2007).
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(a) Montagu
16
14 A 12 B observed 10 8 6
Waterlevel (m AHD) 4 2 1966 1975 1985 1995 2005 2015 2025
(b) Trowutta
215
A 205 B observed 195
185 Water level (m AHD)
175 1966 1975 1985 1995 2005 2015 2025
(c) Mound spring
20 18 A 16 B 14 12 10
Waterlevel (m AHD) 8 6 1966 1975 1985 1995 2005 2015 2025
(d) Togari
34 32 30 28 26 A 24 B
Water level (m AHD) 22 observed 20 1966 1975 1985 1995 2005 2015 2025
Figure 17. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 33
(e) DD1
30
28
26
24 A 22 Water level (m AHD) B
20 1966 1975 1985 1995 2005 2015 2025
(f) DD2
28
25 A 22 B 19
16
Water level (m AHD) 13
10 1966 1975 1985 1995 2005 2015 2025
Figure 17. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2 (continued)
Figure 18 shows the simulated gaining, losing and variably gaining/losing river reaches under Scenario B between 2007 and 2030. The major difference when compared to the regime under Scenario Amid is that the Togari drainage network removes no water from the groundwater system. The Montagu River upstream of the Togari drainage network becomes all losing with the river sections showing variable gaining/losing characteristics under Scenario Amid disappearing. The tributaries to the Duck and Montagu rivers also exhibit increased reaches of losing conditions under Scenario B relative to Scenario Amid.
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Figure 18. Simulated gaining and losing river reaches under Scenario B
Under future climate (Scenario C)
Figure 19 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario C. There are no discernable differences between the simulated water levels under scenarios Cwet, Cmid and Cdry. This is due to the recharge scaling factors showing insignificant change between scenarios Cwet, Cmid and Cdry, as tabulated in Table 6. All Scenario C modelled hydrographs under Scenario C show very similar responses to those under Scenario Amid. This shows that the impact of climate change on groundwater levels to 2030 is likely to be small compared with the impacts of climate variability (as represented by scenarios Awet, Amid and Adry).
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 35
(a) Montagu
16 A 14 Cwet 12 Cmid Cdry 10 observed 8 6
Waterlevel (m AHD) 4 2 1966 1975 1985 1995 2005 2015 2025
(b) Trowutta
215 A Cwet 205 Cmid Cdry observed 195
185 Water level (m AHD)
175 1966 1975 1985 1995 2005 2015 2025
(c) Mound spring
20 A 18 Cwet 16 Cmid Cdry 14 12 10
Water level (m AHD) 8 6 1966 1975 1985 1995 2005 2015 2025
(d) Togari site
34 32 30 28 26 A Cwet 24 Cmid Cdry Water level (m AHD) 22 observed 20 1966 1975 1985 1995 2005 2015 2025
Figure 19. Groundwater levels for the DPIPWE model calibration period and Scenario C at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2
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(e) DD1
30
28
26
24 A Cwet Cmid 22
Water level (m AHD) Cdry
20 1966 1975 1985 1995 2005 2015 2025
(f) DD2
28 A 25 Cwet 22 Cmid Cdry 19
16
13 Water level (m AHD)
10 1966 1975 1985 1995 2005 2015 2025
Figure 19. Groundwater levels for the DPIPWE model calibration period and Scenario C at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2 (continued)
Figure 20 shows the simulated gaining, losing and variably gaining/losing river reaches under Scenario Cmid between 2007 and 2030. There is no significant difference to the losing and gaining regime under scenarios Cwet, Cmid or Cdry relative to Scenario Amid.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 37
Figure 20. Simulated gaining and losing river reaches under Scenario Cmid
Under future development (Scenario D)
Figure 21 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario D. There are no discernable differences between the simulated water levels under scenarios Dwet, Dmid and Ddry. This is due to the recharge scaling factors showing no differences between scenarios Dwet, Dmid and Ddry as tabulated in Table 6. The modelled water levels for all wells, including the mound spring show insignificant change to the modelled water levels under Scenario Cmid shown in Figure 19. The Togari well shows a simulated drawdown of up to ~1 m during the 23-year prediction period but tends to recover long term relative to Scenario C.
Figure 21 shows no significant difference at the DD bores, compared to Scenario C, in terms of trend compared to the real monitoring points, but with additional drawdown in the order of 2 to 3 m, which reflects their location within the area of most concentrated extraction under Scenario D.
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(a) Montagu
16 A 14 Dwet Dmid 12 Ddry 10 observed 8 6
Water level (m AHD) 4 2 1966 1975 1985 1995 2005 2015 2025
(b) Trowutta
215 A Dwet 205 Dmid Ddry observed 195
185 Water level (m AHD)
175 1966 1975 1985 1995 2005 2015 2025
(c) Mound spring
20 A 18 Dwet 16 Dmid Ddry 14 12 10
Water level(m AHD) 8 6 1966 1975 1985 1995 2005 2015 2025
(d) Togari
34 32 30 28
26 A Dwet 24 Dmid
Water level (m AHD) 22 Ddry observed 20 1966 1975 1985 1995 2005 2015 2025
Figure 21. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 39
(e) DD1
30
28
26
24 A Dwet 22 Dmid Water level (m AHD) Ddry 20 1966 1975 1985 1995 2005 2015 2025
(f) DD1
28 A 25 Dwet Dmid 22 Ddry 19
16
Water level (m AHD) 13
10 1966 1975 1985 1995 2005 2015 2025
Figure 21. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) Montagu (b) Trowutta (c) Mound spring (d) Togari (e) DD1 and (f) DD2 (continued)
Figure 22 shows the simulated gaining, losing and variably gaining/losing river reaches under Scenario Dmid between 2007 and 2030. There is no significant difference to the losing and gaining regime under scenarios Dwet, Dmid or Ddry relative to scenarios Cmid or Amid.
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Figure 22. Simulated gaining and losing river reaches under Scenario Dmid
Water balance under scenarios A, B, C and D
Table 8 presents the modelled mean annual water balances under scenarios A, B, C and D for the Mella and Togari GAAs. For the Mella area, diffuse recharge is the greatest source of inflow under scenarios Awet (45.9 GL/year), Amid (34.5 GL/year) and scenarios Cwet, Cmid, Cdry, Dwet, Dmid and Ddry (32.7 to 34.5 GL/year). The diffuse recharge inflow under scenarios Adry and B is only 14.5 and 10.5 GL/year respectively. River leakage into the model (river losing conditions) remains relatively consistent under scenarios Awet, Amid, Adry, B, Cwet, Cmid and Cdry with a mean annual range of 14.6 to 19.0 GL/year; which is the largest inflow component under scenarios Adry and B. Simulated baseflow to rivers (river gaining conditions) is the highest outflow component for all scenarios in the Mella area with Scenario Awet exhibiting the largest baseflow of 35.2 GL/year under Scenario Awet. The smallest simulated baseflows of 17.8 and 15.7 GL/year occur under scenarios Adry and B respectively. As explained in Section 2.4, simulating groundwater under Scenario D involves:
1. increasing the E/R value to 0.25 by increasing the number of extraction wells to enable an increase in the total extraction volume
2. increasing the irrigation area and associated irrigation deep drainage volume to reflect additional surface application from both groundwater and surface water
3. including changes to recharge due to potential future commercial forest plantations as projected by WAVES (although no expansion in commercial forest plantations is planned for the Mella and Togari GAAs).
This results in the modelled mean annual irrigation drainage (termed irrigation drainage in Table 8) increasing from 1.0 GL/year to 2.0 GL/year under Scenario D, equivalent to an increase of 100 percent. The groundwater extraction volume increases from the current estimated rate of 4.5 GL/year to 8.4 (under Scenario Ddry) to 8.9 GL/year (under scenarios Dwet and Dmid), an increase of between 84 percent and 97 percent. The impact of Scenario D on the water balance can be assessed by comparing the river leakage, baseflow, evapotranspiration and storage losses with the results under Scenario C and the modelled observations. Results are as follows:
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 41
Modelled mean annual leakage from rivers increases by ~2 to 3 percent. This slight increase to river leakage under Scenario D is caused by the additional future development wells increasing stress on the aquifer system which induces more leakage from the modelled river cells. Model mean annual baseflow (river gaining conditions) decreases by ~5 percent. Decrease to modelled baseflow under Scenario D is due to the future development wells capturing more groundwater which leaves less available to report to river as baseflow. Evapotranspiration increases by ~8 percent. Model mean annual storage loss increases by ~20 percent (although this only represents a small part of the overall groundwater balance). Groundwater is lost from storage to help support the increase in extraction due to Scenario D’s development wells.
For the Togari GAA, diffuse recharge is the greatest source of inflow under scenarios Awet (45.2GL/year), Amid (34.0 GL/year) and scenarios Cwet, Cmid, Cdry, Dwet, Dmid and Ddry (32.2 to 34.0 GL/year). The diffuse recharge under scenarios Adry and B is only 14.3 and 10.3 GL/year respectively. River leakage into the model (river losing conditions) remains relatively consistent between scenarios A, B and C with a mean annual range of 9.2 to 11.9 GL/year, which is the largest inflow component under Scenario B. The combination of baseflow and losses to Togari’s drainage network collectively make up the majority of the outflow for all scenarios, except under scenarios Amid, Adry and B where evapotranspiration is the largest outflow component of 16.4, 11.8 and 10.3 GL/year respectively. Modelled extraction varies under scenarios A, B and C and is less than that estimated for the Togari GAA (3.5 GL). This is associated with a limitation of the model and indicates that extraction is not yet fully optimised under these scenarios.
Under Scenario D, the Togari GAA modelled mean annual irrigation drainage increases from 0.4 GL/year to 1.3 GL/year; an increase of 225 percent. The groundwater extraction volume increases from the current modelled rate of 3.3 GL/year to 8.2 to 8.6 GL/year under the scenarios Dwet, Dmid and Ddry, an increase of ~155 percent (current extraction is estimated at 3.5 GL/year (Table 5), however, only 3.3 GL/year could be implemented in the model (refer to Section 3.3)). The impact of Scenario D on the water balance can be assessed by comparing the river leakage, baseflow, evapotranspiration and storage losses with results under Scenario C and the modelled observations are as follows:
model mean annual river leakage (river losing conditions) increases by ~4 percent model mean annual baseflow (river gaining conditions) decreases by ~5 percent evapotranspiration increases by ~8 percent model mean annual storage loss increases by ~25 percent.
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Table 8. Mean annual water balance for Mella and Togari under scenarios A, B, C and D
Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Mella components River leakage 14.6 15.6 18.2 19.0 15.6 15.6 15.8 16.0 16.0 16.2 Coastal inflow 1.4 1.4 1.6 1.6 1.4 1.4 1.4 1.4 1.4 1.4 Storage loss 13.1 11.2 6.5 5.5 11.2 11.2 10.9 13.4 13.4 12.9 Diffuse recharge 45.9 34.5 14.5 10.5 34.5 34.5 32.8 34.5 34.5 32.7 Irrigation drainage 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 2.0 2.0 Total In 75.9 63.7 41.8 37.5 63.7 63.7 61.9 67.3 67.3 65.2 Extraction 4.5 4.5 4.5 4.5 4.5 4.5 4.5 8.9 8.9 8.4 Baseflow 35.2 27.9 17.8 15.7 27.9 27.9 27.0 26.5 26.5 25.7 Coastal discharge 3.7 2.8 1.4 1.0 2.8 2.8 2.7 2.8 2.8 2.7 Storage out 15.6 13.8 6.9 5.8 13.8 13.8 13.4 15.4 15.4 14.8 Evapotranspiration 20.0 17.6 13.6 12.7 17.6 17.6 17.3 16.3 16.3 16.1 Total Out 79.0 66.7 44.2 39.7 66.6 66.6 64.8 69.8 69.8 67.7 Discrepancy -3.1 -3.0 -2.4 -2.2 -3.0 -2.9 -2.9 -2.5 -2.5 -2.5 Togari components River leakage 9.2 9.9 11.6 11.9 9.9 9.9 10.1 10.3 10.3 10.4 Coastal inflow 1.1 1.2 1.5 1.6 1.2 1.2 1.2 1.2 1.2 1.2 Storage loss 13.8 12.3 7.3 5.9 12.3 12.3 12.1 15.4 15.4 14.9 Diffuse recharge 45.2 34.0 14.3 10.3 34.0 34.0 32.3 34.0 34.0 32.2 Irrigation drainage 0.4 0.4 0.4 0.4 0.4 0.4 0.4 1.3 1.3 1.3 Total In 69.7 57.8 35.1 30.1 57.8 57.8 56.0 62.1 62.2 60.1 Extraction 3.3 3.3 3.3 3.3 3.3 3.3 3.3 8.6 8.6 8.2 Baseflow 14.8 11.7 6.5 5.3 11.7 11.7 11.2 11.0 11.0 10.6 Drainage 13.6 8.5 2.3 1.9 8.5 8.5 7.8 6.6 6.7 5.9 Coastal discharge 2.6 2.0 0.8 0.5 2.0 2.0 1.9 2.0 2.0 1.9 Storage out 13.9 13.0 7.9 6.5 13.0 13.0 12.9 16.0 16.0 15.9 Evapotranspiration 18.4 16.4 11.8 10.3 16.4 16.4 16.1 15.5 15.5 15.2 Total Out 66.6 54.9 32.7 27.8 54.8 54.8 53.1 59.6 59.6 57.6 Discrepancy 3.1 3.0 2.4 2.2 3.0 3.0 2.9 2.5 2.5 2.5
4.3.3 Reporting metrics
Extraction relative to recharge
Ratios of modelled extraction relative to recharge (E/R) are presented in Table 9. The E/R values presented for scenarios A, B and C in Table 9 were derived using the best estimates of current extraction (see Table 5) in all GAAs (including Togari). The reason for using estimates of current extraction from Table 5 for Togari rather than those implemented in the numerical model is because the model, when configured with the current distribution of extraction wells, was unable to extract the current estimated rate of 3.5 GL/year. Thus, a more realistic measure of aquifer stress is obtained with the value from Table 5. The future extraction used to calculate E/R for Scenario D was derived from Table 5 for all GAAs other than Mella and Togari, in which case the extraction implemented in the model (Table 8) was used to ensure E/R values could be assessed against the modelled hydrographs and water balances. The recharge rates used for the E/R calculations were all taken from Table 7, which is consistent with the numerical models.
The E/R ratio is commonly used to assess the potential level of stress within aquifers. Where the ratio is greater than 1.0, the groundwater resources are being extracted at a rate greater than diffuse recharge is able to replenish the groundwater. For the purposes of this report, levels of development are defined as:
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 43
low, E/R zero to 0.3 medium, E/R 0.3 to 0.7 high, E/R 0.7 to 1.0 very high, E/R>1.0.
Extraction is very low compared to recharge in Inglis-Cam, Cam-Emu-Blythe, King Island, Flinders Island and in the region of Smithton Syncline outside of Togari and Mella. For these GAAs, there is little impact on E/R due to climate change and development and it remains low under all scenarios. The level of development is more pronounced in Mella and Togari where there is a greater intensity of extraction. However, E/R indicates a low level of development under all scenarios in Mella and Togari with the exception of Scenario Adry for Mella and Scenario B for Mella and Togari, where a medium level of development is registered. This indicates that a continuation of the current drought conditions may place some pressure on groundwater resources in Mella.
It is noted that the low E/Rs are achieved partly because no expansion in future extraction is forecast in non-modelled GAAs, and only limited expansion is forecast in Togari and Mella. Furthermore, a low E/R does not necessarily mean that extraction rates are sustainable. For instance, concentrated extraction may lead to localised drawdown impacts (such as the dewatering of a significant wetland). Such impacts may not be reflected in a regional E/R metric, yet extraction within the catchment could not be considered sustainable.
Table 9. Extraction relative to recharge (E/R) for groundwater assessment areas in the Arthur-Inglis-Cam region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Mella 0.10 0.13 0.31 0.43 0.13 0.13 0.14 0.25 0.25 0.26 Togari 0.08 0.10 0.24 0.34 0.10 0.10 0.11 0.25 0.25 0.26 Smithton Syncline 0.02 0.03 0.08 0.12 0.04 0.04 0.04 0.06 0.07 0.07 Inglis-Cam 0.01 0.01 0.02 0.04 0.01 0.01 0.01 0.01 0.01 0.01 Cam-Emu-Blythe 0.01 0.02 0.04 0.06 0.02 0.02 0.02 0.02 0.02 0.02 King Island 0.02 0.03 0.06 0.09 0.03 0.03 0.03 0.03 0.03 0.03 Flinders Island 0.03 0.03 0.06 0.08 0.03 0.03 0.03 0.03 0.03 0.03
Extraction relative to baseflow
Table 10 shows the modelled mean annual baseflow volume under scenarios A, B, C and D. For the Mella GAA, the modelled mean annual baseflows are between 18 percent and 23 percent lower than the modelled mean annual diffuse recharge under scenarios Awet, Amid, Cwet, Cmid, Cdry, Dwet, Dmid and Ddry. Modelled mean annual baseflows under scenarios Adry and B are higher than the modelled mean annual diffuse recharge by 23 percent and 50 percent respectively. This implies that under the current model arrangement (i.e. river stages remain constant throughout the model); baseflows are less sensitive to climate than recharge. However, given that these rivers are conceptualised as predominantly gaining, the actual sensitivity is likely to be far greater than that modelled.
For the Togari GAA, modelled mean annual baseflows are lower than for Mella, and are between 48 percent and 68 percent lower than the modelled mean annual diffuse recharge for all scenarios.
Table 10. Modelled mean annual baseflow volume for Mella and Togari groundwater assessment areas under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Mella 35.2 27.9 17.8 15.7 27.9 27.9 27.0 26.5 26.5 25.7 Togari 14.8 11.7 6.5 5.3 11.7 11.7 11.2 11.0 11.0 10.6
Table 11 shows the simulated mean annual extraction relative to baseflow under scenarios A, B, C and D. The E/B values under scenarios A, B and C in the Mella GAA are all under 0.3 and are classified as low development/stress
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conditions as defined above. The E/B values under scenarios Dwet, Dmid and Ddry are marginally over the 0.3 threshold and are thus classified as medium development conditions. This differs from the classification calculated from the E/R definition which classifies groundwater under Scenario B in the Mella GAA as medium development/stressed with an E/R value of 0.43.
Scenarios Adry and B for the Togari GAA, have E/B values of 0.54 and 0.66 respectively and are classified as medium development/stressed. Scenarios Dwet, Dmid and Ddry all have E/B values that are larger than 0.7 and are thus classified as high development scenarios. This differs from the E/R method which only classifies Scenario B as a medium development/stress condition with a smaller E/R value of 0.34.
Table 11. Mean 24-year extraction relative to baseflow (E/B) for Mella and Togari groundwater assessment areas under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Mella 0.13 0.16 0.25 0.29 0.16 0.16 0.17 0.34 0.34 0.33 Togari 0.24 0.30 0.54 0.66 0.30 0.30 0.31 0.78 0.78 0.77
4.4 Impacts of use 4.4.1 Management risks
Whilst the current level of groundwater extraction is thought to be low in much of the region, there is significant uncertainty regarding actual extraction rates and only limited ability to monitor the resource condition. In the case of a continued unregulated groundwater management framework, there is a risk that unsustainable levels of extraction may develop, particularly at the local scale. This would cause a number of adverse impacts (e.g. well drawdown interference, reduced groundwater discharge to streams or groundwater dependent ecosystems). The recently introduced Tasmanian well construction permit system and an enhanced monitoring network will help to mitigate these risks. Particular effort should be directed towards capturing more data in the Mella GAA, where the groundwater resource may be stressed under the current drought conditions. 4.4.2 Waterlogging and salt accession
A return to wetter conditions or an influx of irrigation water has the potential to cause waterlogging and/or land salinisation, particularly in low-lying shallow watertable areas. This risk is most pronounced for King Island and Flinders Island, where salinity has previously been identified. For the remainder of the Arthur-Inglis-Cam region, where groundwater salinity is typically low, the risk of salinisation is less significant.
Under scenarios C and D the Mella and Togari GAAs show water levels that tend to rise in 2007. This occurs because scenarios Cdry, Cmid and Cwet are all based on Scenario Amid which is wetter than the recent climate (1997 to 2007). Investigating the modelled depth to watertable reveals the following observations:
Depth to watertable over most of the Mella and Togari GAAs varies by less than a couple of metres between scenarios C and D. Depth to watertable is generally shallow over the Mella (~1 m) and Togari (~2 m) irrigation regions due to their relatively low topography. Depth to watertable is greater in areas that have higher local topography and/or a higher concentration of extraction wells. For example, the section of Togari irrigation area located in the Christmas Hills region has a local topography of ~50 m AHD and a modelled depth to watertable of ~20 m. Depth to watertable is marginally greater under Scenario D than Scenario C due to additional groundwater extraction. The difference is small due to the additional extraction wells in the development scenario pumping from the Smithton dolomite (layer 2) which has large transmissivities in the current model arrangement between 200 and 500 m2/day.
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5 The Mersey-Forth region
5.1 Contextual information
5.1.1 Hydrogeology
The location of groundwater assessment areas (GAAs) in the Mersey-Forth region is shown in Figure 23. Note that the Cam-Emu-Blythe GAA falls mainly in the adjoining Arthur-Inglis-Cam region and thus has been discussed in Chapter 4. Groundwater salinities are shown (where reported), which also gives some indication as to where groundwater extraction occurs. Surface–groundwater interactions have also been mapped (see Section 5.2.2 for further discussion).
Figure 23. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Mersey-Forth region
The geology of the Mersey-Forth region is complex (see Figure 2 and maps in Appendix B). The oldest rocks apart from the Cambrian basement rocks are Ordovician in age and are found in the Mole Creek GAA. These rocks consist of limestone that display karstic (cave) features (see Figure 23). Mudstone, siltstone and sandstone, minor limestone and conglomerate of Permian age are found in the eastern parts of the region. These rocks are fractured and faulted but some units still retain intergranular porosity. Jurassic doleritic intrusive rocks occur across the region but are most prominent in the north-east and south-east of the region. Tertiary basalt overlays the basement rocks extensively along the coast. These basalts are comprised of a number of discrete volcanic flows stacked on one another. In some places, sediments have been preserved between the flows. The basalt flows are fractured along the contacts of individual flows
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as they cooled and also exhibit areas where vesicular voids are present. Quaternary deposits are formed as dune and coastal sediments, as colluvial talus deposits in elevated areas and as alluvial valley fills along rivers and streams.
The principal aquifer across the region is the Tertiary basalt aquifer, which includes interbedded Tertiary sediments. This aquifer occurs extensively in the Wesley Vale, Leven-Forth-Wilmot, Sheffield-Barrington and Kimberley-Deloraine GAAs (REM/Aquaterra, 2008h–k; Cromer, 1993). Storage within this aquifer system occurs in the fracture network and vesicles (a vesicle is a small cavity in an igneous rock formed by the expansion of a bubble of gas during the solidification of the rock). There is also storage within the interbedded sediments that separate lava flows from successive volcanic eruptions. The system is considered to be a dual porosity aquifer with movement of groundwater occurring through the fluvial sediments and interconnected vesicles and also through the jointing and fracture network. The variability in porosity and fracture network results in the aquifer being heterogeneous with a broad range of measured well yields and aquifer transmissivities. Yields are mostly less than 5 L/second, but can be greater than 10 L/second in localised zones. Groundwater salinity is generally less than 500 mg/L. These aquifers support the majority of groundwater extraction in Wesley Vale, which is the most concentrated zone of groundwater development in the region.
Fine-grained Tertiary sediments at the northern end of the Tamar River potentially hold significant volumes of groundwater but water quality is usually poor in the Tamar area. These sediments comprise alternating beds of clay, silty clay, sandy clay, sand, gravel and conglomerate.
Permian sedimentary aquifers occur throughout much of the region and are used to supply groundwater within the Spreyton GAA (REM/Aquaterra, 2008l), the eastern half of the Mole Creek GAA (REM/Aquaterra, 2008m), and to a limited extent in Wesley Vale. They are composed of mudstone, siltstone and sandstone, minor limestone and conglomerate. They are generally viewed as fractured rock aquifers although there is a component of inter-granular storage and flow in coarser grained sediments. Aquifers are commonly confined to semi-confined with an area near Spreyton displaying artesian properties from gravel/tillite at the base of the Permian (Bacon and Latinovic, 2003). Typically, yields are less than 5 L/second and salinities are less than 1000 mg/L.
Within the western half of the Mole Creek GAA, groundwater is sourced from Ordovician karstic limestone. This limestone comprises karst flow paths and cave systems. The aquifer is extremely heterogeneous with a broad range of measured well yields ranging from 1 L/second to more than 20 L/second.
Fractured rock aquifers occur in Jurassic dolerite outside of GAAs in the east of the region. Storage within the dolerite varies with respect to joint density and they are not often used within this region for groundwater supply.
Quaternary deposits can also host localised aquifers where impervious layers lead to perched watertables. These local groundwater systems are considered to be volumetrically insignificant, but may play an important role in recharging underlying aquifers and supporting groundwater dependant ecosystems.
5.1.2 Surface–groundwater interactions
Streams throughout the Mersey-Forth region are thought to be predominantly gaining (REM/Aquaterra, 2008h–m). Groundwater levels, albeit sparsely recorded, are higher than the adjacent surface water levels throughout the region with few exceptions. Surface water modelling conducted for DPIPWE indicated groundwater discharge to streams represented 50 percent or more of the total flow in streams (REM/Aquaterra, 2008h–m). Hydrochemical analysis of surface water and groundwater in the Wesley Vale GAA also indicated gaining streams (REM/Aquaterra, 2008h).
In the basaltic terrain of the Wesley Vale, Leven Forth Wilmot, Sheffield-Barrington and Kimberley-Deloraine GAAs, groundwater discharge to streams via springs and seeps is common. This can occur where structural benches of unweathered or unfractured basalt impede movement of groundwater down-slope. Springs and seeps can also occur along the geological contact between the Tertiary basalt and basement rocks. The rivers in the Leven Forth Wilmot and Sheffield-Barrington GAAs are also deeply incised with the adjacent groundwater levels rising away from the river channel, suggesting groundwater discharge to streams. Groundwater recharge via leakage from any of the main rivers is most likely to occur in the upper portion of the catchments where surface water elevations are higher than groundwater elevations.
In the Mole Creek GAA, the Mersey River and a significant number of other permanent surface water features occur in a region characterised by karstic limestone. In general, karstic systems have highly interconnected surface and
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groundwater. Groundwater will discharge into surface water systems when higher than surface water elevations – typically during low flow periods. Groundwater recharge via stream leakage will occur during high flow periods when surface water levels are higher than the watertable. Spring discharge, which has declined in recent years due to drought and land use change, is also a feature of this region (Hunter et al., 2009).
5.1.3 Groundwater extraction
Groundwater extraction in Tasmania is not metered and as such there are no historical records. Estimates of groundwater extraction within the Leven-Forth-Wilmot and Kimberley-Deloraine GAAs have been made based on industry standard application rates for irrigated areas (REM/Aquaterra, 2008i; k), while estimates within Wesley Vale, Sheffield-Barrington, Spreyton and Mole Creek GAAs are based on up-scaled results from groundwater use surveys (REM/Aquaterra, 2008h; j; l; m). This up-scaling involves determining the number of production wells within the GAA and using the average extraction rate and irrigation schedule from each of the GAAs to determine an estimate for the absolute groundwater extraction volume. This can vary considerably between GAAs depending upon geology, well yields, land use and other factors.
Groundwater extraction for irrigation, stock supplies and industrial purposes in the Leven-Forth-Wilmot GAA is estimated to total around 1500 ML/year. Extraction in the Mole Creek GAA has been estimated at 1000 ML/year and for Kimberley-Deloraine GAA 6000 ML/year, whilst groundwater extraction in the Wesley Vale GAA has been estimated to total 4800 ML/year; Spreyton GAA, 750 ML/year and Sheffield-Barrington GAA, 3300 ML/year.
5.1.4 Groundwater resource protection and management
Regulation of the water well drilling industry has recently been introduced into Tasmania. Drilling contractors are required to hold a Tasmanian Well Drillers Licence. A permit to drill system is in place that requires all landowners to obtain a Well Works Permit prior to the commencement of drilling. Whilst drillers have always been required to return information relating to the construction of wells, there is no regulation or controls on well operation, such as the collection of extraction data and long-term monitoring of groundwater levels or salinity. The state-wide groundwater monitoring network is too sparse to adequately monitor groundwater conditions. In lieu of these management and data gaps, DPIPWE is making progress on the development of groundwater management plans and has expanded the monitoring network.
Within this region other management issues include point sources of pollution which are generally localised and include activities such as sewage lagoons, historical landfills, underground storage tanks and fuel depots. No significant areas of salinity (not associated with coastal activity) are known to exist.
Groundwater is predominately used for the irrigation of pasture, orchards, vegetable and poppy crops. Significant irrigation occurs in the Wesley Vale GAA where high yielding basalt aquifers are used to support crops grown in the highly fertile basaltic soils. Localised property sized impacts have been reported to State Government since 1980. These occur when one property owner extracts large volumes during the irrigation season and the cone of depression influences the yields of neighbouring wells.
5.1.5 Previous estimates of recharge and discharge
REM/Aquaterra (2008h–m) performed a series of groundwater assessments throughout the Mersey-Forth region. The assessments focused on deriving estimates of diffuse recharge amongst other components of the water balance.
Diffuse groundwater recharge from rainfall varies spatially and temporally according to rainfall intensity and distribution, soil type, geology, land-use and topography. Groundwater recharge rates from diffuse rainfall sources were estimated using a variety of methods including the steady state Chloride Mass Balance method, the Water Table Fluctuation method and the empirical relationship for estimating evapotranspiration derived by Zhang et al. (1999; 2001). A best estimate for diffuse recharge was determined based on these analyses (REM/Aquaterra, 2008h–m).
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A second estimate of recharge was defined during the DPIPWE project for regions where a numerical groundwater flow model was constructed and calibrated. In the Mersey-Forth region, models were constructed for the Wesley Vale and Sheffield-Barrington GAAs (Aquaterra/REM, 2009c; d). Recharge rates were defined across the model domains according to rainfall, land use and surface geology and were refined during the calibration process.
In addition, as part of the National Land and Water Audit 2000, SKM (2000b) conducted an estimate of recharge based on an assumed rainfall percentage factor. In the case of Wesley Vale this approach yielded a recharge rate that was considerably lower than that estimated by the DPIPWE project. These catchments were based on aquifer boundaries and covered areas similar to the Leven, Forth-Wilmot and Mersey catchments. The REM/Aquaterra (2008h–m) catchments coincide with GAAs defined in this project, but not with the SKM (2000b) catchments.
Groundwater discharge estimates were also conducted for the DPIPWE groundwater modelling project (REM/Aquaterra, 2008h–m). Groundwater discharge to streams was estimated for a number of catchments according to surface water modelling. Estimates of lateral throughflow (predominantly groundwater discharge to the ocean) were based on flow-net analysis. Groundwater losses through evapotranspiration were estimated for Wesley Vale and Sheffield-Barrington from numerical models (Aquaterra/REM 2009c; d).
A summary of estimates of recharge, discharge and extraction for selected catchments is provided in Table 12.
Table 12. Groundwater statistics for the Mersey-Forth region including annual recharge, extraction and discharge details
Area Diffuse recharge Extraction volume Total discharge Surface water catchment estimate estimate volume estimate GL/y GL/y* Leven-Forth-Wilmot – conceptual model 150.0 1.5 160 Leven, Forth-Wilmot (REM/Aquaterra, 2008i) Sheffield-Barrington – conceptual model 82.0 3.3 30 Mersey, Forth-Wilmot (REM/Aquaterra, 2008j) Sheffield-Barrington – numerical model 40.0 3.3 46 Mersey (Aquaterra/REM, 2009d) Kimberley-Deloraine – conceptual model 72.0 6.0 120 Mersey (Meander) (REM/Aquaterra, 2008k) Mole Creek– conceptual model 170.0 1.0 180 Mersey (Meander) (REM/Aquaterra, 2008m) Spreyton – conceptual model 71.0 0.8 62 Rubicon, Mersey (REM/Aquaterra, 2008l) Spreyton (SKM, 2000b) 2.5 0.2 NA Mersey Wesley Vale – conceptual model 16.0 4.8 11 Rubicon (REM/Aquaterra, 2008h) Wesley Vale – numerical model 25.0 3.2 39 Rubicon (Aquaterra/REM, 2009c) Wesley Vale (SKM, 2000b) 4.8 2.6 NA Rubicon Burnie (SKM, 2000b) 140.0 2.2 NA Leven, Forth-Wilmot, Rubicon, Mersey (Cam, Emu, Blythe, Inglis- Flowerdale) * Discharge volumes are derived from estimates of groundwater discharge to streams, groundwater extraction and lateral discharge (REM/Aquaterra, 2008h–m). Losses to evapotranspiration are included for modelled areas (Aquaterra/REM, 2009c; d) but not elsewhere. NA – not available
Steady state water balances for the numerical models developed in the DPIPWE groundwater modelling project are shown in Table 13. A steady state water balance is representative of average annual fluxes for an historical climate. It is noted that the modelled groundwater extraction (3.2 GL/year) was less than that estimated (4.8 GL/year). This was due to a limitation of the modelling approach and should not be interpreted as a refined estimate of the extraction (Aquaterra/REM, 2009c; d). This is because the simulated network of pumping wells was not optimised, that is, the actual pumping rates were not available and had to be assumed; where the assumed rate was greater than what the model could support for a particular well, the pumping ceased causing a reduction in the total modelled extraction.
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Table 13. Modelled water balance results for the Wesley Vale groundwater assessment areas
Component Wesley Vale Sheffield-Barrington Groundwater inflows Groundwater outflows Groundwater inflows Groundwater outflows GL/y Diffuse recharge 25.4 - 39.9 - Constant head 0.2 4.8 0.1 2.7 Rivers 13.4 22.2 2.7 10.6 Evapotranspiration - 8.7 - 29.3 Extraction - 3.2 - 3.3 Total 39.0 39.0 42.8 46.0
5.1.6 Groundwater level and salinity trends
Some detailed groundwater level monitoring has occurred in the Wesley Vale GAA since the 1980s. A network of 12 regional monitoring wells was established, and observations were also made in a network of 11 irrigation wells. Neither a long-term rise nor decline in groundwater levels is evident in either of these two monitoring networks. An example is shown in Figure 24b, where little response in water levels is seen in Lloyd’s well 3 despite varying rainfall.
Time-series groundwater level and salinity data within the remainder of the region are rare (four in all). All wells are completed in different aquifer systems and a meaningful analysis of trends between wells is generally not possible on a regional scale. Whilst recognising these limitations, a hydrograph for the Barrington monitoring well (Sheffield-Barrington GAA) is presented in Figure 24a. The well is completed in the major aquifer of the region, Tertiary basalt. A small downward trend (~2 m) in groundwater levels is evident during the drier than average period from 1996.
No long-term trend in salinity levels is evident across the region (Ezzy, 2004).
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(a) Barrington 0 400
300 2 200 4 100
6 (mm)mean 0
Water level (m BGL) Water level (m BGL) 8
Cumulative deviation from mean rainfall (mm) -100 Cumulative deviation from the
10 -200 1991 1993 1996 1999 2001 2004 (b) Lloyd’s well 3 0 400
2 200 4
6 0 mean (mm) 8 Water level (m BGL) Water level (m BGL) -200 10 Cumulative deviation from mean rainfall (mm) from deviation Cumulative the
12 -400 1991 1993 1996 1999 2001 2004
Figure 24. Hydrographs for the (a) Barrington and (b) Lloyd’s well 3 monitoring wells, showing the water level (in metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall
5.2 Groundwater system assessment
5.2.1 Recharge/discharge
Table 14 presents a summary of the key components of the groundwater balance for GAAs within the Mersey-Forth region. These estimates are carried through for the scenario analysis in the following section.
The diffuse recharge rates represent an historical annual average. For the Wesley Vale GAA, where further modelling analysis was conducted for the current project, the recharge rate was derived from the average annual rates during the DPIPWE model calibration period of the numerical model. For the Sheffield-Barrington GAA, the recharge rate was equivalent to that used for the calibrated numerical model (steady state only) developed for the DPIPWE groundwater modelling project (Aquaterra/REM 2009d). In GAAs where no numerical model exists, rates from the ‘preliminary conceptual water balance’ of the DPIPWE groundwater modelling project (REM/Aquaterra, 2008i–m) were used.
Current extraction is based on estimates derived during the DPIPWE groundwater modelling project (REM/Aquaterra, 2008h–m). There is a large degree of uncertainty associated with these estimates because they were not based on metered data. It is possible that actual values may be significantly higher or lower. Furthermore, there is no available information to guide estimates of future groundwater extraction at 2030. Hence, a precautionary approach was taken. In Wesley Vale, where scenario modelling is being conducted, future extraction was assumed to be capped at 25 percent of diffuse recharge under Scenario Dmid. In the remaining GAAs, where a greater degree of uncertainty exists, there was assumed to be no increase in extraction by 2030.
For Wesley Vale, groundwater discharge to streams has been calculated from the numerical model and represents the average annual rate from the DPIPWE model calibration period. This flux is not available for other GAAs in the region.
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Recharge via stream leakage and groundwater losses due to evapotranspiration are significant components of the groundwater balance but have not been determined in this section as they are difficult to quantify. An analysis of these fluxes will be presented in Section 5.3.2 as part of the numerical modelling assessments.
Table 14. Estimated diffuse recharge, discharge to streams and extraction for the Mersey-Forth region
Groundwater assessment area Diffuse Current extraction Future extraction Discharge to recharge 2007/08 2030 streams GL/y Wesley Vale 23.4 4.8 7.1 26.0 Leven-Forth-Wilmot 153.6 1.5 1.5 na Sheffield-Barrington 39.9 3.3 3.3 na Mole Creek 170.1 1.0 1.0 na Spreyton 70.5 0.8 0.8 na Kimberley-Deloraine 71.8 6.0 6.0 na Total 529.3 17.4 19.7 na na – not available
5.2.2 Surface–groundwater interactions
Interactions between groundwater and surface water account for a significant component of the groundwater balance. The likely nature of these interactions has been mapped for the Rubicon (which includes the Wesley Vale GAA), Leven, Forth and Wilmot surface water catchments (see Figure 23).
There was limited data to inform the surface–groundwater interaction mapping, but where available it suggested that streams within the Wesley Vale GAA were gaining. Groundwater elevations were predominantly higher than the adjacent surface water elevations. Hydrochemical data (REM/Aquaterra, 2008h) suggested groundwater was actively contributing to streamflow at the points sampled.
In the remainder of the Rubicon catchment, there is very little data available to inform the likely nature of surface–groundwater interactions. Most streams are assumed to be gaining because they are perennial and occur in deeply incised valleys. The only exception to this gaining classification was for a small region in the Spreyton catchment, near Parkham, where groundwater levels were both above and below and surface water elevations. Variably gaining/losing conditions were inferred for this region.
Most watercourses in the Leven, Forth and Wilmot catchments (which includes the Leven-Forth-Wilmot GAA) have been classified as gaining. The classification was based on the conceptualisation of the major Tertiary basalt aquifer occurring on broad plateaus between deeply incised river valleys where the Tertiary sequence has been eroded. Spring flow is common on valley sides, particularly at the contact of the Tertiary aquifer and underlying basement geologies. Available groundwater elevations were also higher than the adjacent surface water elevations. In summary, the available data suggests streams are most likely to be gaining. The exception was Claytons Rivulet where groundwater elevations were below surface water levels and the stream was inferred to be losing.
Due to the paucity of available data, there is a low level of confidence in the classifications made in Figure 23. More complex and varied surface–groundwater interactions are likely. For example, small ephemeral streams in catchment headwaters may be losing when they periodically flow and the watertable is below surface water levels. Other reaches, identified as gaining, may be more appropriately classified as variably gaining/losing. However, there is no data to support such distinctions.
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5.2.3 Conceptual model
Detailed conceptual models have previously been developed for each GAA within the Mersey-Forth region (REM/Aquaterra, 2008h–m). Figure 25 summarises these models and depicts the hydrogeological interaction between adjoining GAAs and the major surface water features. The region is depicted from the perspective of the Bass Strait, looking south towards the central highlands and Great Western Tiers.
In Wesley Vale, the major productive aquifers occur in Tertiary age basalt and sediments that were deposited as separate layers in a basin that is several hundred metres thick in places and extends below sea level. Recharge occurs across the catchment, but is lower where heavily weathered basalt is present at the surface, higher where sediment layers or less weathered / more fractured basalt outcrops. Local flow systems occur in upper, heavily weathered sections of the basalt, with flow direction largely controlled by surface topography and inferred to be in the direction of the main surface water drainage features. Intermediate systems occur in the inter-layered and fractured horizons within the basaltic sequence. These aquifers can be separated and confined by relatively thick sequences of massive, poorly fractured basalt. Groundwater is used intensively within Wesley Vale, compared to the other GAAs in the region. It is often pumped into storage dams from which irrigation occurs. Many of the production wells installed in Wesley Vale are screened across separate basalt and sediment layers.
Three GAAs are present to the south of Wesley Vale in the Mersey River catchment. Mole Creek sits at the base of the Great Western Tiers and the major aquifers occur within Ordovician limestone (western portion of GAA) and Permian units (eastern portion of GAA). The aquifer is recharged from rainfall, and possibly from surface water features where hydraulic gradients permit. The aquifer may also receive lateral inflows from the Great Western Tiers. Local flow paths are likely to be complex in karstic areas. Groundwater discharge occurs to the Mersey River and laterally to the Kimberley-Deloraine GAA.
The Kimberley-Deloraine GAA covers an area where the main aquifers occur within Tertiary basalt and sediments. Surface water accounts for most irrigation needs and groundwater discharge occur towards surface water features and laterally across adjoining GAA boundaries.
The Spreyton GAA covers an area where the main aquifers occur in Permian sandstone and mudstone. Groundwater use is restricted to a few high yielding zones. The majority of the lateral groundwater flow discharge from the Spreyton GAA occurs to the Mersey and Rubicon Rivers.
The Tertiary basalt/sediment aquifers to the west of the Mersey River (in the Sheffield-Barrington and Leven Forth Wilmot GAAs) are different to the Wesley Vale Basin in that they occur as plateaus between deeply incised streams where the Tertiary sequence has been eroded. Groundwater flow occurs towards the main surface water drainage features or follows deep leads (ancient river valleys that have been filled with Tertiary basalt/sediments) to discharge at the coast. Springs are common, and occur where structural benches of unweathered or unfractured basalt impede movement of groundwater down-slope on the sides of deeply incised valleys, and along the geological contact between the Tertiary basalt and basement rocks.
Maps of groundwater elevation contours have been produced all GAAs within the region (REM/Aquaterra, 2008h–m), which provide a more detailed representation of groundwater flow paths. In most cases, the flow direction is largely controlled by surface topography and inferred to be in the direction of the main surface water drainage features.
The areas of the region outside of the GAAs are typified by local groundwater flow systems associated with low-yielding fractured rock aquifers. An exception to this occurs in the east of the region on the flanks of the Tamar Estuary where Tertiary sediment aquifers are likely to form intermediate flow systems which discharge into the Tamar Estuary.
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Figure 25. Conceptual hydrogeological model for the Mersey-Forth region
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5.3 Scenario assessment
5.3.1 Recharge impacts
The WAVES model (Zhang and Dawes, 1998) was used to estimate the change in groundwater recharge across the Mersey-Forth region under a range of different climate scenarios (section 2.3). The historical (1924 to 2007) modelled recharge was assessed to establish any difference between wet and dry periods of recharge. A 23-year period was used, which allows the projection of recharge estimates to 2030. In other words, to estimate recharge in 2030 assuming Scenario C is similar to Scenario A. Under scenarios Awet, Amid and Adry, the recharge does change for a 23-year period compared to the recharge under the entire period of the historical climate (Figure 26). For the recharge that is exceeded in 10 percent of 23-year periods (Scenario Awet), recharge is on average 54 percent greater that the historical mean (that is, a recharge scaling factor (RSF) of 1.54). For the recharge that is exceeded in 50 percent of 23-year periods (Scenario Amid), recharge is on average 12 percent greater than the historical average (RSF=1.12). For the recharge that is exceeded in 90 percent of 23-year periods (Scenario Adry), recharge is on average 50 percent lower than the historical mean (RSF=0.50) (Table 15).
The recent (1997 to 2007) climate in the Mersey-Forth region has been drier than the historical (1924 to 2007) mean and consequently the calculated recharge decreases 52 percent under Scenario B relative to Scenario A (Table 15).
Figure 26. Spatial distribution of recharge scaling factors in the Mersey-Forth region for the 23-year Scenario A and the 11-year Scenario B relative to the 84-year historical modelled period
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Under Scenario Cwet, recharge increases 11 percent for the region as a whole, but this is not spatially uniform with greater increases in the south-west and north-east and decreases in recharge in the centre of the region (see Figure 27, Table 15). Under Scenario Cmid, recharge increases 6 percent for the region as a whole with the same pattern as Cwet. Under Scenario Cdry, recharge decreases overall 2 percent with the same pattern as scenarios Cwet and Cmid.
The difference in diffuse recharge between Scenario C and Scenario D is due to the impact of future forestry upon recharge. The greatest differences are in the centre and north of the region (see Figure 27) where future forestry is assumed to increase (Figure 4). The impact is a reduction in recharge for scenario Dwet, Dmid and Ddry of about 5 percent greater than scenarios Cwet, Cmid and Cdry respectively (see Table 15).
Figure 27. Spatial distribution of recharge scaling factors in the Mersey-Forth region for the 84-year scenarios C and D relative to the 84-year historical modelled period
To calculate recharge under each scenario, RSFs were derived from WAVES modelling for each GAA (Table 15). For non-modelled areas, the RSFs were then multiplied by the historical average recharge rates (Table 14) to calculate scaled recharge under each scenario (Table 6). A slightly different approach was taken for the Wesley Vale GAA – the RSFs from WAVES were passed through individual recharge zones of the numerical groundwater model to calculate a scaled recharge rate for the GAA under each scenario.
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Table 15. Aggregated recharge scaling factors for groundwater assessment areas in the Mersey-Forth region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Wesley Vale 1.60 1.22 0.37 0.23 1.01 1.10 1.04 0.84 0.90 0.85 Leven-Forth-Wilmot 1.57 1.16 0.43 0.24 0.95 0.99 0.90 0.82 0.85 0.78 Sheffield-Barrington 1.57 1.18 0.44 0.24 0.96 0.99 0.91 0.80 0.83 0.76 Mole Creek 1.54 1.14 0.53 0.48 0.99 0.95 0.88 0.90 0.85 0.78 Spreyton 1.62 1.20 0.36 0.24 1.02 1.09 1.00 0.92 0.98 0.89 Kimberley-Deloraine 1.60 1.20 0.41 0.25 1.04 1.03 0.95 0.95 0.93 0.86 Mersey-Forth Region 1.54 1.12 0.50 0.48 1.11 1.06 0.98 1.05 1.01 0.93
Table 16. Scaled mean annual recharge for groundwater assessment areas in the Mersey-Forth region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Wesley Vale 36 26 8 5 27 29 27 26 28 27 Leven-Forth-Wilmot 241 178 66 37 146 152 138 126 131 120 Sheffield-Barrington 63 47 18 10 38 40 36 32 33 30 Mole Creek 262 194 90 82 168 162 150 153 145 133 Spreyton 114 85 25 17 72 77 71 65 69 63 Kimberley-Deloraine 115 86 29 18 75 74 68 68 67 62
In terms of recharge impacts, a significant amount of variability is evident under Scenario A across the different GAAs, with the wettest years from history (Scenario Awet) leading to significantly more recharge than the driest years (Scenario Adry). Recharge under Scenario B is significantly less than that under Scenario Amid. It is noted that the impact of drought appears to be less severe in the Mole Creek GAA.
For non-modelled GAAs, there is a reduction in recharge of typically 10 to 20 percent under Scenario Cmid. There is less variability evident under Scenario C by comparison with historical conditions; however, this is due to the method adopted where scenarios Cwet, Cmid and Cdry are based on Scenario Amid. There is a forecast expansion in commercial forest cover throughout much of the region and the combined impact of climate change and forest expansion (Scenario Dmid) represents a recharge reduction of 20 to 30 percent from the historical 23-year median (Scenario Amid). There is a recharge reduction of 10 to 20 percent associated with the forestry expansion, which is evident when recharge under scenarios C and D are compared.
For the Wesley Vale GAA, there is a small increase in recharge (from Scenario Amid) due to climate change (Scenario Cmid). Recharge under Scenario Dmid remains higher than the historical 23-year median (Scenario Amid).
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 57
5.3.2 Modelled impacts to groundwater levels and fluxes in the Wesley Vale groundwater assessment area
The existing numerical groundwater flow model for the Wesley Vale groundwater assessment area is that developed by Aquaterra/REM (2009c). Figure 28 summarises the conceptual groundwater model for the Wesley Vale GAA. The numerical model follows this conceptualisation, consisting of three layers (predominantly Moriarty basalt for layer 1, Wesley Vale Sands for layer 2 and Thirlstane basalt for layer 3) with features representing inflows from diffuse recharge, irrigation and river leakage. The outflow components are represented by coastal, evapotranspiration, and extraction well features. The historical recharge time series of the existing model was altered for the current project using the WAVES model record, as detailed in Section 2.4.
Figure 29 shows the location of five historical groundwater level monitoring wells in the Wesley Vale assessment area near special value wetland and river sites as identified in the CFEV database. The map also shows six fictitious reporting wells (SV1, SV2, SV3, DD1, DD2 and DD3) at key reporting / special value sites where no real observation wells exist. Modelled groundwater levels at these locations were assessed under all of the climate and development scenarios. The modelled river gaining and losing regimes have also been assessed and the results are discussed in the following section.
Figure 28. Conceptual groundwater model for the Wesley Vale groundwater assessment areas
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Figure 29. Location of the Wesley Vale model extent and reporting sites Under historical climte (Scenario A)
Figure 30a–k shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario A for the real monitoring wells. Figure 30f–h show the simulated water levels for the DPIPWE model calibration period and under scenarios Awet, Amid and Adry for the fictitious reporting wells.
The model results were analysed to identify the areas of deepest drawdown due to extraction, and fictitious monitoring wells were specified in the model at these points (identified with a ‘DD’ prefix – Figure 30i–k) to allow reporting of the time series of maximum drawdown effects. The results are presented in Figure 30, Figure 32, Figure 34 and Figure 36, showing no significant difference in terms of trend compared to the real monitoring points, but with additional drawdown that reflects their location within the area of most concentrated extraction. The results indicate that these aquifer systems appear to be quite robust, in that they:
respond rapidly to pumping and establish a new, dynamic hydrological equilibrium show dynamic water level changes in response to climatic and pumping effects (e.g. lower groundwater levels in centres of pumping; increased groundwater levels in response to recharge) show no apparent long term trend of increasing or decreasing levels.
Lloyd’s well 3 (labelled 3L, Figure 9a) is located in the upper section of the catchment, adjacent to a river special value site. None of the scenarios show any evidence of long term trends with the final simulated water levels at 2030 resembling the simulated water levels at 2007 under scenarios Awet, Amid and Adry. Under Scenario Awet the simulated seasonal water level varies up to ~ 2.5 m relative to under scenarios Amid and Adry which show variations of up to 1.5 m and less than 0.5 m respectively.
The modelled responses under all scenarios show a smaller variation in water levels than the observed data (up to 6 m). As explained in Section 2.4, the lack of modelled variation in simulated wells close to river systems has contributed to the assumptions invoked on the river boundary conditions. The river boundary conditions maintain a constant stage elevation with time and thus do not vary during high and low flow events and therefore do not influence the aquifer water levels in the vicinity. The modelled groundwater level variations are due to rainfall changes, irrigation periods and groundwater extraction only.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 59
Figure 30b and c show the observed and simulated water levels for Lloyd’s well seven (7L) and Lloyd’s well twelve (12L) respectively. Both wells are located in the central region of the Wesley Vale assessment area and even though the magnitude of groundwater levels at 7L are larger due to its higher topography, the modelled groundwater responses are similar. Both wells show no long-term trends in the simulated water levels with water levels at 2030 under all scenarios similar to the simulated water levels at 2007.
Figure 30d shows the observed and simulated water levels for the ROB1 well which is located in the central region of the Wesley Vale assessment area. ROB1 is located in an intensive groundwater extraction area, which is reflected in both the observed and modelled water levels during the historical warm up period, with groundwater levels varying over 10 m in response to the surrounding extraction regime. Under scenarios Awet, Amid and Adry there is a seasonal sinusoidal response in groundwater levels which is caused by a combination of seasonal variations in rainfall and summer extraction. Simulated seasonal variability in groundwater elevations typically vary by over ~10 under scenarios Awet and Amid and by ~7 m under Scenario Adry. The long-term drawdown under all scenarios is insignificant with simulated water levels under Scenario Adry of only a couple of metres lower than under Scenario Awet and Scenario Adry at 2030. On a side note, the observed water levels during the later periods of the DPIPWE model calibration period show large drawdown influences caused by the intense groundwater extraction that exists in this area; however, a lack of continual monitored pumping records during this time has not allowed for a close water level match at this well location. However, the prediction models assume all wells that have pumped at some time during the historical warm up period are activated in the prediction period and thus explains why the simulated water levels show a decline in water levels under all scenarios (A, B, C and D) when compared to the historical simulated water levels. For more information on model calibration, refer to Aquaterra/REM (2009c).
Figure 30e shows the observed and simulated water levels for the DOB2 well which is located in the northern region of the Wesley Vale assessment area and is completed in the high yielding Moriarty basalt unit. Under scenarios Awet, Amid and Adry there is a seasonal sinusoidal response in groundwater levels which is caused by a combination of the seasonal variations in rainfall and summer extraction. Simulated seasonal variability in groundwater elevations typically vary between 5 and ~20 m under scenarios Awet and Amid and by ~3 m under Scenario Adry. Due to some pumping wells going dry under scenarios Amid and Adry, the modelled water levels tend to rise slightly over the 23-year projection period. This also happens under Scenario Awet but not until approximately 2020. Before 2020, the modelled water levels under Scenario Awet was actually lower than under scenarios Adry and Amid due to the higher diffuse recharge rate allowing the neighbouring wells to pump longer and thus keeping the local simulated water levels down. There is also no long-term downward trend observed with the final simulated water levels for scenarios Awet, Amid and Adry being at a similar level at 2030 compared to the simulated water level at the beginning of the prediction period. On a side note, the observed water levels during the DPIPWE model calibration period show large drawdown influences caused by the intense groundwater extraction that exists in this area; however, a lack of continual monitored pumping records has not allowed for a close water level match at this well location. For more information on model calibration, refer to Aquaterra/REM (2009c).
Figure 30f–h shows the simulated water levels for reporting wells SV1, SV2 and SV3 respectively. None of the three wells show any long-term trends in the simulated water levels with scenarios Awet, Amid and Adry showing similar water levels at 2030 under scenarios Awet, Amid and Adry similar to the simulated water levels at 2007. The seasonal amplitude of watertable fluctuation for scenarios Awet and Amid in well SV1 is typically around 4 m, with some wetter years showing an excess of up to 12 m for Scenario Awet and 6 m under Scenario Amid. The seasonal amplitude under scenarios Awet and Amid for well SV2 is typically around 1 m with some wetter years showing an excess of 7 m for Scenario Awet and 3 m under Scenario Amid. Under Scenario Adry, both the SV1 and SV2 wells show very little seasonal variability in simulated water levels. Well SV3, shows very little difference between scenarios Awet, Amid and Adry which could be due to a number of reasons including:
close proximity to river boundary cells that tend to constrain water levels in neighbouring aquifer cells (as discussed in sections 2.4 and 3) wells located in low topography areas where evapotranspiration is more active, can have a buffering effect on simulated water level fluctuations modelled water levels close to the coast are going to be more constrained by the assumptions placed on the coastal boundary features.
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(a) 3L
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A 84 Awet Amid 82 Adry Water level (m AHD) observed 80 1966 1975 1985 1995 2005 2015 2025
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80 A 76 Awet Amid Adry 72 observed 68
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(c) 12L
30 A Awet 26 Amid Adry observed 22
18 Water level (m AHD)
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(d) ROB1
94 A 84 Awet Amid Adry 74 observed
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54 Water level (m AHD)
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Figure 30. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3
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(e) DOB2
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Waterlevel (m AHD) Adry 15 observed
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(f) SV1
86 A Awet 82 Amid Adry
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74 A Awet 71 Amid Adry
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(h) SV3
16 A 14 Awet Amid Adry 12
10
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Figure 30. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
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(i) DD1
80 A Awet 65 Amid Adry
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(k) DD3
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Figure 30. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
Figure 31 shows the simulated gaining, losing, and variably gaining/losing river reaches under Scenario Amid between 2007 and 2030. The main river channels of the Panatana system and the river system in the west are always gaining which is consistent with the conceptualisation discussed in Section 5.2.2. However, the majority of tributaries to the main river channels show variably gaining/losing conditions. The most up-stream section of the Pardoe Creek, near Northdown, is always losing due to this area having high topography. Part of the modelled river system close to the coast also shows losing conditions; however, this may be an artefact of the assumptions place on the modelled feature representing the coastal boundary.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 63
Figure 31. Simulated gaining and losing river reaches under Scenario Amid
Under recent climate (Scenario B)
Figure 32 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario B. All hydrographs show a similar response to the Scenario Adry simulation under Scenario Adry from 2007 to 2030 with the seasonal variation in water levels being quite small. The simulated water levels over the prediction period reflect the water levels simulated during the DPIPWE model calibration period during the last 11 years (1997 to 2007). Any downward trends anticipated under Scenario B modelled water levels may be buffered by the increase in modelled river leakage (refer to Section 5.3.3). The modelled river features assume a constant stage for all time, and thus can always supply water to the groundwater system depending on the prevailing hydraulic gradients. Model assumptions and limitation are discussed further in sections 2.4 and 3.
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(a) 3L
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84 A 82 Water level (m AHD) B observed 80 1966 1975 1985 1995 2005 2015 2025
(b) 7L
80 A B 76 observed
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30 A B 26 observed
22
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(d) ROB1
94 A 84 B observed 74
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Water level (m AHD) 54
44 1966 1975 1985 1995 2005 2015 2025
Figure 32. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3
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(e) DOB2
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30 A Waterlevel (m AHD) 15 B observed 0 1966 1975 1985 1995 2005 2015 2025
(f) SV1
86 A B 82
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74 A B 71
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(h) SV3
16 A B 14
12
10
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Figure 32. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
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(i) DD1
80 A B 65
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95 A 90 B
85
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Figure 32. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
Figure 33 shows the simulated gaining, losing, and variably gaining/losing river reaches under Scenario B between 2007 and 2030. There is a dramatic increase to the simulated losing reaches when compared with Scenario Amid, with the majority of the variably gaining/losing reaches becoming all losing under Scenario B. The increased pattern of modelled losing reaches is attributed to the modelled water levels reducing in both overall magnitude and seasonal variability due to decrease in diffuse recharge. This caused modelled baseflow to decrease and river leakage (i.e. river losses) to increase. Further discussion on modelled water balances are discussed in Section 5.3.3.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 67
Figure 33. Simulated gaining and losing river reaches under Scenario B
Under future climate (Scenario C)
Figure 34 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario C. There are no discernable differences between the simulated water levels under scenarios Cwet, Cmid and Cdry. This is due to the recharge scaling factors for Scenario C showing insignificant change between scenarios Cwet, Cmid and Cdry as tabulated in Table 15. All Scenario C modelled hydrographs under Scenario C show very similar responses to under Scenario Amid. This shows that the impact of climate change on groundwater levels to 2030 is likely to be small compared with the impacts of climate variability (as represented by scenarios Awet, Amid and Adry).
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(a) 3L
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80 1966 1975 1985 1995 2005 2015 2025
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80 A Cwet 76 Cmid Cdry 72 observed
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30 A Cwet 26 Cmid Cdry observed 22
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(d) ROB1
94 A Cwet 84 Cmid Cdry 74 observed
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Water level (m AHD) 54
44 1966 1975 1985 1995 2005 2015 2025
Figure 34. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 69
(e) DOB2
100 A Cwet 80 Cmid Cdry 60 observed
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0 1966 1975 1985 1995 2005 2015 2025
(f) SV1
86 A Cwet 82 Cmid Cdry
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(g) SV2
74 A Cwet 71 Cmid Cdry
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62 1966 1975 1985 1995 2005 2015 2025
(h) SV3
16 A 14 Cwet Cmid Cdry 12
10
8 Waterlevel (m AHD)
6 1966 1975 1985 1995 2005 2015 2025
Figure 34. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
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(i) DD1
80 A Cwet 65 Cmid Cdry
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Water level(m AHD) Cmid Cdry 40 1966 1975 1985 1995 2005 2015 2025
(k) DD3
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Water level(m AHD) 70 Cmid Cdry 65 1966 1975 1985 1995 2005 2015 2025
Figure 34. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
Figure 35 shows the simulated gaining, losing, and variably gaining/losing river reaches under Scenario Cmid between 2007 and 2030. There is no significant difference to the losing and gaining regime under scenarios Cwet, Cmid and Cdry relative to Scenario Amid.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 71
Figure 35. Simulated gaining and losing river reaches under Scenario Cmid
Under future development (Scenario D)
Figure 36 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and Scenario D. There are no discernable differences between the simulated water levels for scenarios Dwet, Dmid and Ddry. This is due to the recharge scaling factors for Scenario D showing insignificant change between scenarios Dwet, Dmid and Ddry (Table 15). The modelled water levels for all wells except ROB1, DOB2 and SV2 show insignificant change to the modelled Cmid water levels shown in Figure 34. Scenario D shows a simulated drawdown of ~10 m for well ROB1, ~4 m for well DOB2 and ~1 m for well SV2 relative to Scenario C. Referring to Section 2.4, these wells are located in areas that have potential for future irrigation expansion and thus are in close proximity to the modelled future extraction wells required to meet the E/R of 0.25. The assumed future extraction from groundwater results in small local drawdown impacts in this area.
Figure 36 shows no significant difference at the DD bores, compared to Scenario C, in terms of trend compared to the real monitoring points, but with additional drawdown in the order of 5 to 15 m, which reflects their location within the area of most concentrated extraction for Scenario D.
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(a) 3L
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80 A Dwet 76 Dmid Ddry 72 observed
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(c) 12L
30 A Dwet 26 Dmid Ddry observed 22
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(d) ROB1
94 A 84 Dwet Dmid 74 Ddry observed 64
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Figure 36. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3
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(e) DOB2
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86 A Dwet 82 Dmid Ddry
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74 A Dwet 71 Dmid Ddry
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(h) SV3
16 A 14 Dwet Dmid Ddry 12
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6 1966 1975 1985 1995 2005 2015 2025
Figure 36. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
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(i) DD1
80 A Dwet 65 Dmid Ddry
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(k) DD3
95
90
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Water level(m AHD) 70 Dmid Ddry 65 1966 1975 1985 1995 2005 2015 2025
Figure 36. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) 3L (b) 7L (c) 12L (d) ROB1 (e) DOB2 (f) SV1 (g) SV2 (h) SV3 (i) DD1 (j) DD2 and (k) DD3 (continued)
Figure 37 shows the simulated gaining, losing, and variably gaining/losing river reaches for Scenario Dmid between 2007 and 2030. There is a slight difference to the losing and gaining regime of Scenario Dmid compared to Scenario Cmid in the central area of the catchment. Due to the modelled future irrigation expansion in this area, the addition of Scenario D extraction wells have caused very minor sections of the river reaches to change from gaining to variably gaining/losing. The most upstream section of the western river system located south-west of well DOB2 has also changed with some modelled river cells changing from variably gaining/losing under Scenario Cmid to always losing under Scenario Dmid. However, these impacts may be regarded as insignificant compared to the amount of development simulated under Scenario D. Further discussion on losing and gaining river reaches are discussed in Section 5.3.3.
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 75
Figure 37. Simulated gaining and losing river reaches under Scenario Dmid
Water balance under scenarios A, B, C and D
Table 17 presents the modelled mean annual water balance under scenarios A, B, C and D for the Wesley Vale GAA. The diffuse recharge is the greatest source of inflow under Scenario Awet (35.9 GL/year), Scenario Amid (26.2 GL/year) and scenarios Cwet, Cmid, Cdry, Dwet, Dmid and Ddry (26.1 to 28.8 GL/year). The recharge inflow under scenarios Adry and B is only 8.0 and 5.0 GL/year respectively. River leakage into the model (river losing conditions) remains relatively consistent between scenarios Awet, Amid, Adry, B, Cwet, Cmid and Cdry with a mean annual range of 12.1 to 18.2 GL/year; which is the largest inflow component under scenarios Adry (17.3 GL/year) and B (18.2 GL/year). Simulated baseflow to rivers (river gaining conditions) is the highest outflow component for all scenarios with Scenario Awet exhibiting the largest baseflow of 34.6 GL/year and Scenario B exhibiting the smallest baseflow of 16.4 GL/year.
Modelled extraction varies under scenarios A, B and C and is less than that estimated for the GAA (4.8 GL). This is associated with a limitation of the model and indicates that extraction is not yet fully optimised under these scenarios.
As explained in Section 2.4, simulating Scenario D involves:
1. increasing the E/R to 0.25 by increasing the number of extraction wells to enable an increase in the total extraction volume
2. increasing the irrigation area and associated irrigation deep drainage volume to reflect additional surface application from both groundwater and surface water
3. including changes to recharge due to potential future commercial forest plantations as projected by WAVES.
This results in the modelled mean annual irrigation drainage (termed irrigation drainage in Table 17) increasing from 3.5 GL/year to 5 GL/year; equivalent to an increase of ~40 percent. The groundwater extraction volume increases from the current modelled rate of 4.3 GL/year (under Scenario Cmid) to 7.1 GL/year, equivalent to an increase of 65 percent (current extraction is estimated at 4.8 GL/year (Table 14); however, only 4.3 GL/year could be sustained by the model (refer to Section 3)).The impact of Scenario D on the water balance can be assessed by comparing the river leakage,
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baseflow, evapotranspiration and storage losses with the Scenario C results and the modelled observations. Results are as follows:
Model mean annual river leakage (river losing conditions) increases by ~3 percent. This increase to river leakage under Scenario D is caused by the additional future development wells inflicting increased stress on the aquifer system which induces more leakage from the modelled river cells. Model mean annual baseflow (river gaining conditions) decreases by ~ 3 percent. Decrease to modelled baseflow in Scenario D is due to the future development wells capturing more groundwater which leaves less available to report to river as baseflow. Model mean annual storage loss increases by ~16 percent (although this represents only a small part of the overall groundwater balance). Groundwater is lost from storage to help support the increase in extraction due to Scenario D’s development wells.
Table 17. Mean annual water balance for Wesley Vale groundwater assessment area under scenarios A, B, C and D
Wesley Vale components Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y River leakage 12.1 13.4 17.3 18.2 13.3 13.0 13.2 13.8 13.4 13.7 Coastal inflow 0.1 0.1 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 Storage loss 4.4 3.5 2.1 1.7 3.5 3.7 3.6 4.2 4.3 4.2 Diffuse recharge 35.9 26.2 8.0 5.0 26.6 28.8 27.1 26.1 28.3 26.7 Irrigation drainage 3.5 3.5 3.5 3.5 3.5 3.5 3.5 5.0 5.0 5.0 Total In 56.0 46.7 31.1 28.6 47.0 49.0 47.5 49.3 51.2 49.7 Extraction 4.5 4.2 4.1 4.0 4.2 4.3 4.3 7.1 7.1 7.1 Baseflow 34.6 27.9 17.8 16.4 28.2 29.6 28.5 27.3 28.6 27.6 Coastal discharge 6.4 5.4 3.6 3.4 5.5 5.7 5.5 5.5 5.7 5.5 Storage out 4.0 3.6 2.0 1.6 3.6 3.7 3.6 4.1 4.2 4.2 Evapotranspiration 6.6 5.6 3.6 3.4 5.6 5.8 5.7 5.4 5.7 5.5 Total Out 56.0 46.7 31.1 28.6 47.1 49.0 47.6 49.4 51.3 49.9 Discrepancy 0.0 0.0 0.0 0.0 -0.1 0.0 0.0 -0.1 -0.1 -0.1
5.3.3 Reporting metrics
Extraction relative to recharge
The ratio of extraction relative to recharge (E/R) for each GAA is shown in Table 18. The ratio is commonly used to assess the potential level of stress within aquifers. Where the ratio is greater than 1.0, the groundwater resources are being extracted at a rate greater than diffuse recharge is able to replenish the groundwater. For the purposes of this report, levels of development are defined as:
low, E/R zero to 0.3 medium, E/R 0.3 to 0.7 high, E/R 0.7 to 1.0 very high, E/R >1.0.
Extraction is very low compared to recharge in Leven-Forth-Wilmot, Mole Creek and Spreyton. For these GAAs, there is little impact on E/R due to varying climate and development, and it remains low under all scenarios. The impact of climate and land use is more pronounced in Sheffield-Barrington and Kimberley-Deloraine, and particularly so in Wesley Vale where there is a greater intensity of extraction. A continuation of the recent drought conditions (Scenario B) results in a high level of development in Wesley Vale and may place pressure on groundwater resources. A medium level of development is registered for Sheffield-Barrington and Kimberley-Deloraine under this scenario.
E/R remains low under scenarios C and D for all GAAs as the recharge reduction of 10 to 30 percent associated with climate change and development does not translate to significantly greater E/R.
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It is noted that the low E/R ratios are achieved under Scenario D partly because no expansion in future extraction is forecast in non-modelled GAAs, and only limited expansion is forecast in Wesley Vale. Furthermore, a low E/R value does not necessarily mean that extraction rates are sustainable. For instance, concentrated extraction may lead to localised drawdown impacts (such as the dewatering of a significant wetland). Such impacts may not be reflected in a regional E/R metric, yet extraction within the catchment could not be considered as sustainable.
Table 18. Extraction relative to recharge (E/R) for groundwater assessment areas in the Mersey-Forth region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Wesley Vale 0.13 0.18 0.60 0.95 0.18 0.17 0.18 0.27 0.25 0.27 Leven-Forth-Wilmot 0.01 0.01 0.02 0.04 0.01 0.01 0.01 0.01 0.01 0.01 Sheffield-Barrington 0.05 0.07 0.19 0.34 0.09 0.08 0.09 0.10 0.10 0.11 Mole Creek 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Kimberley-Deloraine 0.05 0.07 0.20 0.33 0.08 0.08 0.09 0.09 0.09 0.10
The E/R values under scenarios A, B and C in Table 18 were derived using the best estimates of current extraction (Table 14) in all GAAs (including Wesley Vale). The reason for using estimates of current extraction from Table 14 for Wesley Vale rather than those implemented in the numerical model is because the model, when configured with the current distribution of extraction wells, was unable to extract the current estimated rate of 4.8 GL/year. Thus, a more realistic measure of aquifer stress is obtained with the value from Table 14. The future extraction used to calculate E/R under Scenario D was derived from Table 14 for all GAAs other than Wesley Vale, in which case the extraction implemented in the model (Table 17) was used to ensure E/R values could be assessed against the modelled hydrographs and water balances. The recharge rates used for the E/R calculations were all taken from the model for Wesley Vale, or from Table 14 for remaining GAAs. Extraction relative to baseflow
Table 19 shows the modelled mean annual baseflow volume under scenarios A, B, C and D. When these results are compared to modelled diffuse recharge (Table 17), baseflow under every scenario except Scenario Awet exceeds diffuse recharge. Simulated baseflows are also less sensitive to changes in climate than recharge under the current model arrangement due to the constant river stage approach invoked in the model (refer to sections 2.4 and 3 for further information on assumptions and limitations). However, given these rivers are conceptualised as predominantly gaining, the actual sensitivity is likely to be far greater than that modelled.
Table 19. Modelled mean annual baseflow volume for Wesley Vale groundwater assessment area under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Wesley Vale 34.6 27.9 17.8 16.4 28.2 29.6 28.5 27.1 28.5 27.4
Table 20 shows the ratio of extraction relative to modelled baseflow under scenarios A, B, C and D (with values of E derived as per the E/R calculations above). The E/B ratio is considered to be a useful metric for comparing scenarios in deeply-incised geologies such as the Wesley Vale GAA because baseflow is a key discharge feature that needs to be managed for environmental purposes. Analogous to the concept of E/R ratios above, an E/B>1 is likely to reflect current or potential future stress, both to groundwater and surface water resources.
The E/B values under all scenarios are below 0.3 and are classified as low development/stress conditions as defined above for E/R. Scenarios Adry, B, Dwet, Dmid and Dry come close to being classified as medium development with E/B values ranging between 0.25 to 0.29. This differs slightly from the classification calculated from the E/R definition which classifies Scenario Adry as a medium development with an E/R value of 0.6 and Scenario B as high development with an E/R value of 0.95.
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Table 20. Modelled extraction relative to baseflow (E/B) for Wesley Vale groundwater assessment area under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Wesley Vale 0.14 0.17 0.27 0.29 0.17 0.16 0.17 0.26 0.25 0.26
5.4 Impacts of use
5.4.1 Management risks
Whilst the current level of groundwater extraction is thought to be low in much of the region, there is significant uncertainty regarding actual extraction rates and only limited ability to monitor the resource condition. In the case of a continued unregulated groundwater management framework, there is a risk that unsustainable levels of extraction may develop, particularly at the local scale. This would cause a number of adverse impacts (e.g. well drawdown interference, reduced groundwater discharge to streams or groundwater dependent ecosystems). The recently introduced Tasmanian well construction permit system and an enhanced monitoring network will help to mitigate these risks. Particular effort should be directed towards the Wesley Vale GAA, where the groundwater resource may be stressed under the current drought conditions.
5.4.2 Waterlogging and salt accession
A return to wetter conditions or an influx of irrigation water has the potential to cause waterlogging and/or land salinisation, particularly in shallow watertable areas. There is forecast to be a significant influx of irrigation water (7 GL from surface water alone) in the Wesley Vale GAA, and the added recharge may cause the watertable to rise. Whilst groundwater salinity is typically low in this area and the risk of salinisation is less than other parts of Tasmania, there is still potential for waterlogging events to occur. Investigating the modelled depth to watertable results under scenarios C and D reveals the following observations:
Depth to watertable over most of the Wesley Vale GAA varies by up to a few metres between scenarios C and D Depth to watertable is greater in areas that have higher local topography and/or a higher concentration of extraction wells. For example, areas in the vicinity of monitoring wells ROB1 and DOB2, which have modelled depth to watertable ranging from 10 m to over 30 m under Scenario C Depth to watertable is generally greater under Scenario D than it is under Scenario C due to additional groundwater extraction. The difference is generally within a few metres in areas identified for development.
The modelled depth to watertable is generally increased under the development scenario as a result of increasing extraction to achieve an E/R value of 0.25. However, if future development was to solely use imported surface water, then the increase in irrigation drainage without additional groundwater extraction may cause watertable levels to rise.
These risks are also relevant to other parts of the region where a significant expansion in irrigation activities is forecast.
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6 The Pipers-Ringarooma region
6.1 Contextual information
6.1.1 Hydrogeology
The location of groundwater assessment areas (GAAs) in the Pipers-Ringarooma region is shown in Figure 38. Groundwater salinities are shown (where reported), which also gives some indication as to where groundwater extraction occurs (predominantly in the Scottsdale and Ringarooma GAAs, but also within the Pipers catchment in the west of the region). Surface–groundwater interactions have also been mapped (see Section 6.2.2 for further discussion).
Figure 38. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Pipers-Ringarooma region
The basement geology is comprised of Ordovician-Devonian metasediments (Mathinna Group), which were intruded by granite during the Devonian (see Figure 2 and maps in Appendix B). The Mathinna Group and Devonian granite outcrop over a large portion of the region. Permian and Triassic units consist of mudstone, sandstone and limestone, and are limited in extent. Extensive sheets of Jurassic dolerite occur in the southern and western margin of the region. Thick sequences of sand, gravel, silt and clay were deposited during the Tertiary. The sediments filled ancient river valleys to form what is referred to as Tertiary deep leads, and are present over much of the region. They are capped, in places, by
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Tertiary basalt. Recent sedimentation during the Quaternary has resulted in sand and gravel deposits in coastal areas, alluvial deposits of sand, gravel and mud in river valleys, and talus deposited near eroding highland areas.
The most significant aquifers in the region are the Tertiary sediment and Tertiary basalt aquifers. The basalt and sediments occur as two separate layers in contrast to many of the Tertiary basalt/sediment sequences throughout northern Tasmania, which are inter-layered.
In the Tertiary sediment aquifers, groundwater is stored in the pore spaces of well sorted, unconsolidated sediments. The aquifers occur extensively across the region and supply most of the groundwater resources in the Scottsdale GAA (REM/Aquaterra, 2008n) and in the George catchment. Tertiary sediment aquifers are also used for water supply in the Ringarooma GAA (REM/Aquaterra, 2008o). In general, well yields from the Tertiary sediment aquifers are less than 5 L/second, but some higher yielding zones occur in deep leads (>10 L/second). Groundwater salinities, where reported, are generally less than 500 mg/L.
Tertiary basalt (where present) caps the Tertiary sediments. In the Scottsdale GAA, the basalt is highly weathered, clay-rich and is not significantly used for groundwater supplies (REM/Aquaterra, 2008n). In contrast, it is less weathered in the Ringarooma GAA and groundwater is stored in a network of fractures and vesicles to form a locally significant aquifer (a vesicle is a small cavity in an igneous rock formed by the expansion of a bubble of gas during the solidification of the rock). In general, well yields are less than 5 L/second and salinities are less than 500 mg/L. The watertable in these units is typically perched and semi-confined. Springs occur where thick clay is present at the base of the basalt and impedes the downward movement of water. Tertiary basalt aquifers are also utilised in the Pipers catchment.
Aquifers associated with the Mathinna Group can supply reliable groundwater resources, but have not been highly developed across the region. These are generally fractured rock aquifers although there is a component of inter-granular storage in coarser grained sediments. They are utilised in the Pipers catchment and in parts of the Scottsdale and Ringarooma GAAs. In general, well yields are less than 5 L/second and salinities are less than 500 mg/L (REM/Aquaterra, 2008n; o). Well yields can be significantly higher in heavily fractured zones. For instance, the water supply for the township of Ringarooma is from wells installed in the Mathinna Group.
Local groundwater flow systems in Quaternary coastal sand, coastal plain, alluvium and talus deposits supply groundwater at a number of locations about the coastal fringe of the region. Some limited extraction also occurs from Permian sediment aquifers, which are not widespread in the region.
Devonian granites dominate the geology in the north-east corner of Tasmania and strongly influence the hydrogeology of the region. But these rocks are sparsely jointed and, while some wells produce water, yields are low and water quality variable (Bacon and Latinovic, 2003). They are generally considered to be unproductive aquifers.
6.1.2 Surface–groundwater interactions
Streams throughout the Scottsdale and Ringarooma GAAs are thought to be predominantly gaining (REM/Aquaterra, 2008n; o). Groundwater levels, albeit sparsely recorded, are higher than the adjacent surface water levels throughout the GAAs with few exceptions. Surface water modelling conducted for DPIPWE indicated groundwater discharge to streams represented 50 percent or more of the total flow in streams (REM/Aquaterra, 2008n; o). Hydrochemical analysis of surface water and groundwater in the Scottsdale GAA is consistent with gaining streams (REM/Aquaterra, 2008n). Spring discharge is also a feature of the region and can occur at the base of Tertiary basalt unit where thick clay layers impedes the downward movement of water.
A hydrological study of the McKerrows Marsh, a significant wetland in the Scottsdale GAA, suggests this portion of the Great Forester River is variably gaining/losing (Bobbi and Gurung, 2006). That is, the wetland receives groundwater discharge during low flow periods, and recharges the groundwater via stream leakage during high flow periods. Other variably gaining/losing streams may occur elsewhere on the flat, sedimentary plains of coastal zones throughout the region.
Losing streams are most likely to occur in the steep, high relief, portion of the upper catchments where surface water elevations are greater than groundwater elevations.
No previous studies of surface–groundwater interactions have occurred in the region outside of the Scottsdale and Ringarooma GAAs.
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6.1.3 Groundwater extraction
Groundwater extraction in Tasmania has not been metered and as such there are no historical records. However, groundwater extraction for the region is thought to be very low.
For the Scottsdale GAA, current extraction rates are thought to be negligible (REM/Aquaterra, 2008n). This was supported by a groundwater use survey and confirmed earlier estimates of minimal groundwater use by the National Land and Water Audit (SKM, 2000b). For the Ringarooma GAA, groundwater extraction was estimated at 1000 ML/year based on a groundwater use survey (REM/Aquaterra, 2008o). Earlier estimates for part of this GAA, also suggested extraction was low – a total of 129 ML/year for the Ringarooma, Legerwood, Winnaleah and Tomahawk subcatchments (SKM, 2000b). For the remainder of the region, extraction was estimated to be 498 ML/year (SKM, 2000b). A summary of these estimates is listed in Table 21.
6.1.4 Groundwater resource protection and management
Regulation of the water well drilling industry has recently been introduced into Tasmania. Drilling contractors are required to hold a Tasmanian Well Drillers Licence. A permit to drill system is in place that requires all landowners to obtain a Well Works Permit prior to the commencement of drilling. Whilst drillers have always been required to return information relating to the construction of wells, there is no regulation or controls on well operation, such as the collection of extraction data and long-term monitoring of groundwater levels or salinity. The state-wide groundwater monitoring network is too sparse to adequately monitor groundwater conditions. In lieu of these management and data gaps, DPIPWE is making progress on the development of groundwater management plans and has expanded the monitoring network.
Groundwater quality is mainly at risk in this region from point sources of pollution. The Jetsonville deep lead aquifer in Tertiary sediments north of Scottsdale has an unlined and uncapped landfill in its recharge area. The Scottsdale waste water treatment plant (which historically received large volumes of waste water from a food processing plant) discharges to the recharge area of the Jetsonville aquifer. High levels of groundwater salinity are known to occur in the Waterhouse area where farm dams normally have a turkey nest design to avoid interaction with shallow saline groundwater.
Groundwater extraction for irrigation, stock, domestic or industrial purposes has localised affects on reserves. For instance, a water bottling plant near Winnaleah historically pumped water from the local aquifer, causing the township water supply well to go dry.
An increased demand in groundwater use is anticipated for the Scottsdale GAA due to restrictions associated with further surface water development (REM/Aquaterra, 2008n).
6.1.5 Previous estimates of recharge and discharge
REM/Aquaterra (2008n; o) performed a series of groundwater assessments for the Scottsdale and Ringarooma GAAs for the recent DPIPWE groundwater modelling project. Key components of the water balance were estimated as part of these studies.
Diffuse groundwater recharge was estimated using a variety of methods including the steady state Chloride Mass Balance method, Water Table Fluctuation method and the empirical relationship for estimating evapotranspiration derived by Zhang et al. (1999; 2001). From the results of these methods a best estimate for recharge was determined (REM/Aquaterra, 2008n; o).
A second estimate of recharge was defined during the DPIPWE project for regions where a numerical groundwater flow model was constructed and calibrated. In the Pipers-Ringarooma region, models were constructed for the Scottsdale (Aquaterra/REM, 2009e), and Ringarooma GAAs (Aquaterra/REM, 2009f). Recharge rates were defined across the model domains according to rainfall, land use and surface geology and were refined during the calibration process.
In addition, as part of the National Land and Water Audit 2000, SKM (2000b) conducted an estimate of recharge to determine sustainable yields for groundwater catchments based on a percentage of rainfall infiltration. These catchments
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were based on aquifer boundaries and covered an area similar to the Scottsdale GAA and isolated sections of the Ringarooma GAA surrounding certain towns.
The REM/Aquaterra (2008a–s) catchments coincide with GAAs defined in this project, but not with the SKM (2000b) catchments. For instance the Ringarooma ‘catchment’ studied by SKM (2000b) is not the same study area as the DPIPWE models for Ringarooma (REM/Aquaterra, 2008o; Aquaterra/REM, 2009f). It is considerably smaller. Hence the much lower recharge rate.
Groundwater discharge estimates were also conducted for the DPIPWE groundwater modelling project (REM/Aquaterra, 2008n; o). Groundwater discharge to streams was estimated according to surface water modelling. Estimates of lateral throughflow (predominantly groundwater discharge to the ocean) were based on flow-net analysis. Groundwater losses through evapotranspiration were estimated for from numerical models (Aquaterra/REM, 2009e; f).
A summary of estimates of recharge, discharge and extraction for selected catchments is provided in Table 21.
Table 21. Previous estimates of groundwater fluxes for the Pipers-Ringarooma region
Area Diffuse Extraction Total Surface water catchment recharge discharge* GL/y Scottsdale – conceptual model 150 negligible 100 Great Forester-Brid (REM/Aquaterra, 2008n) Scottsdale – numerical model 81 0 110 Great Forester-Brid (Aquaterra/REM, 2009e) Scottsdale (SKM, 2000b) 1 0.056 NA Great Forester-Brid Ringarooma – conceptual model 180 1 160 Ringarooma (REM/Aquaterra, 2008o) Ringarooma – numerical model 160 0 200 Ringarooma (Aquaterra/REM, 2009f) Ringarooma** (SKM, 2000b) 1 0.06 NA Ringarooma Legerwood (SKM, 2000b) 1 0.029 NA na Winnaleah (SKM, 2000b) 1 0.035 NA na Tomahawk (SKM, 2000b) 38 0.005 NA na Unincorporated Area North East (SKM, 180 0.5 NA Musselroe-Ansons, George, Scamander-Douglas, 2000b) North Esk, Pipers, Little Forester * Discharge volumes are derived from estimates of groundwater discharge to streams, groundwater extraction and lateral discharge (REM/Aquaterra, 2008n; o). Losses to evapotranspiration are included for modelled areas (Aquaterra/REM, 2009e; f) but not elsewhere. ** The Ringarooma ‘catchment’ studied by SKM (2000b) is not the same study area as for the DPIPWE models (REM/Aquaterra, 2008o; Aquaterra/REM, 2009f). It is considerably smaller. Hence the much lower recharge rate NA – not available na – not applicable
Steady-state water balances for the numerical models developed in the DPIPWE groundwater modelling project are shown in Table 22. A steady-state water balance is representative of mean annual fluxes under an historical climate. Note that groundwater extraction was not a component of either model.
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Table 22. Water balances from steady-state numerical models for groundwater assessment areas in the Pipers-Ringarooma region
Component Scottsdale Ringarooma
Groundwater Groundwater Groundwater Groundwater inflows outflows inflows outflows
GL/y Diffuse recharge 80.7 - 159.1 - Constant head 0.1 7.2 0.1 8.8 Rivers 29.9 74.1 41.6 148.2 Evapotranspiration - 30.3 - 43.9 Extraction - 0.0 - 0.0 Storage - 0.1 - - Total 110.7 111.7 200.8 200.9
6.1.6 Groundwater level and salinity trends
Time-series groundwater level and salinity data within the region are rare. State observation wells that have sufficient data tend to reflect localised conditions more so than regional trends. Although the monitoring wells are completed in the major aquifers, a meaningful analysis of trends between wells is generally not possible on a regional scale because of the low well density. Whilst recognising these limitations, hydrographs for two monitoring wells are presented in Figure 39. The two major aquifers from the region are represented – the Jetsonville monitoring well (located in the Scottsdale GAA) (Figure 39a) is completed in Tertiary sediments; the Winnaleah monitoring well (located in the Ringarooma GAA) is completed in Tertiary basalt (Figure 39b). Local rainfall data is presented alongside the groundwater level data.
The Jetsonville monitoring well hydrograph (Figure 39a) has sporadic rises of more than 5m between March and September of 1996, 1997 and 2005, thought to occur during flood events (the well is located on a flood plain). Groundwater levels have declined by more than 4 m over 15 years reflecting reduced recharge due to below-average rainfall. Water levels in the Winnaleah monitoring well (Figure 39b) have remained stable, despite below average rainfall during the monitoring period.
In the other monitoring wells of the region, there is no significant trend in groundwater levels in the Branxholm and Waterhouse wells, and a slight rising trend evident in the Pipers River well (Ezzy, 2006).
In general, groundwater salinity levels have remained stable over the monitoring period (since 1991) in the monitoring wells across the region.
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(a) Jetsonville 0 1600
4 1200
8 800 mean (mm)mean
Water level (m12 BGL) Water level (m BGL) 400 Cumulative deviation from mean rainfall (mm) Cumulative deviation from the
16 0 1991 1993 1996 1999 2001 2004 (b) Winnaleah 42 1500
43 1000
44 500 mean (mm)mean
Water level (m BGL)45 Water level (m BGL) 0
Cumulative deviation from mean rainfall (mm) Cumulative deviation from the 46 -500 1991 1993 1996 1999 2001 2004 Figure 39. Hydrographs for the (a) Jetsonville and (b) Winnaleah monitoring wells, showing the water level (metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall
6.2 Groundwater system assessment
6.2.1 Recharge/discharge
Table 23 presents a summary of the key components of the groundwater balance for GAAs within the Pipers-Ringarooma region. These estimates are carried through for the scenario analysis in the following section.
The diffuse recharge rates represent an historical annual average. For the Scottsdale GAA, where further modelling analysis was conducted for this project, the recharge rate was derived from the average annual rates during the DPIPWE model calibration period of the numerical model. For the Ringarooma GAA, the recharge rate was equivalent to that used for the calibrated numerical model (steady state only) developed for the DPIPWE groundwater modelling project (Aquaterra/REM, 2009f).
Current extraction was based on estimates derived during the DPIPWE groundwater modelling project (REM/Aquaterra, 2008n; o). There was no information to guide estimates of future groundwater extraction at 2030. Hence, a precautionary approach was taken. In Scottsdale, where scenario modelling is being conducted, future extraction was assumed to be capped at 25 percent of recharge under Scenario Dmid. In Ringarooma, where a greater degree of uncertainty exists, there was assumed to be no increase in extraction by 2030.
For Scottsdale, groundwater discharge to streams has been calculated from the numerical model and represents the average annual rate from the DPIPWE model calibration period. This flux was not available for Ringarooma.
Recharge via stream leakage and groundwater losses due to evapotranspiration are significant components of the groundwater balance but have not been determined in this section as they are difficult to quantify. An analysis of these fluxes will be presented in Section 6.3.2 as part of the numerical modelling assessments.
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Table 23. Estimated historical recharge, discharge to streams and current and future extraction for groundwater assessment areas in the Pipers-Ringarooma region
Groundwater Diffuse Current extraction Future extraction Discharge to assessment area recharge 2007/08 2030 streams GL/y Scottsdale 86.2 0.0 24.3 91.8 Ringarooma 159.1 1.0 1.0 na Total 245.3 1.0 25.3 na
6.2.2 Surface–groundwater interactions
Interactions between groundwater and surface water account for a significant component of the groundwater balance. The likely nature of these interactions has been mapped for the Great Forrester-Brid surface water catchment (which roughly aligns with Scottsdale GAA) and Ringarooma surface water catchment (identical to the GAA) (Figure 38). More detailed, catchment-scale maps are shown in Appendix B.
There were limited data to inform the surface–groundwater interaction mapping, but where available data suggested that streams within the Great Forester-Brid catchment were mostly gaining. Groundwater elevations were higher than the adjacent surface water elevations. Hydrochemical data (REM/Aquaterra, 2008n) suggested groundwater was actively contributing to streamflow at the points sampled.
The only exception to the gaining classification was for the coastal sedimentary plains, where streams were classified as variably gaining/losing. This was based on a hydrological study of McKerrows Marsh (Bobbi and Gurung, 2006) where an analysis of surface water and groundwater levels over time demonstrated that the direction of flux between the wetland and aquifer varied. The wetland receives groundwater discharge during low flow periods, and recharges the groundwater via stream leakage during high flow periods. Similar surface–groundwater interactions are assumed to occur elsewhere on the sedimentary plain.
Surface–groundwater interactions in the Ringarooma GAA are thought to be similar to the Scottsdale GAA because the hydrogeological conditions are alike. Groundwater elevations are higher than the adjacent surface water elevations, and a similarly high baseflow component in streamflow was identified by surface water modelling. The major rivers were therefore classified as gaining, with the exception of the lowermost reach of the Ringarooma River that was assumed to be variably gaining/losing, where it spills onto a sedimentary plain forming a wetland much like the McKerrows Marsh.
Due to the paucity of available data, there is a low level of confidence in the classifications made in Figure 38. More complex and varied surface–groundwater interactions are likely. For example, streams in the steep, high relief areas of the catchment headwaters may be losing. However, there is no data to determine the location of such reaches.
6.2.3 Conceptual model
Detailed conceptual models have previously been developed for the Scottsdale and Ringarooma GAAs (Aquaterra/REM, 2009e; f; Moore, 1992). Figure 40 summarises these models. The region is depicted from the perspective of the Bass Strait, looking south.
In Scottsdale, the principal aquifer occurs in Tertiary age sediments deposited in a basin that is underlain and surrounded by Devonian granite and the Mathinna Group. Diffuse recharge occurs throughout the Tertiary basin and in the surrounding hills, where rainfall is higher and the Mathinna Group outcrops and is fractured. The Tertiary sediments form regional and intermediate groundwater flow systems. Groundwater flow occurs towards the coast and follows deep leads, or discharges in watercourses that have been classified as gaining. River leakage also recharges the aquifer across the coastal sedimentary plain during high flow periods.
The major aquifers in the Ringarooma GAA occur in Tertiary basalts and sediments which flank the middle to upper reaches of the Ringarooma River. The basalt and sediments occur as two separate layers in contrast to many of the Tertiary basalt/sediment sequences throughout Tasmania which are inter-layered. The groundwater flow system in the
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uppermost Tertiary basalt aquifer can be considered as being at a local scale from surrounding mountain ridges into the valley, while flow in the Tertiary sediments is at an intermediate scale, extending from recharge areas at the base of the basalt and along the outcropping fractured rocks at the edges of the catchment, through to discharge in gaining streams. The fractured rock aquifers of the Mathinna Group underlie the Tertiary units and exert significant hydraulic influence on the above aquifers, contributing recharge in places, and constitute a significant groundwater resource at various locations (e.g. Ringarooma town water supply). They form local to intermediate flow systems.
Maps of groundwater elevation contours have been produced for the Scottsdale and Ringarooma GAAs (Aquaterra/REM, 2009e; f), which provide a more detailed representation of groundwater flow paths. In most cases, the flow direction is largely controlled by surface topography and inferred to be in the direction of the main surface water drainage features.
Outside of the GAAs, most groundwater extraction occurs in the Pipers catchment in the west of the region. Aquifers associated with Tertiary basalt, the Mathinna Group and Permian sedimentary units are utilised. These are generally fractured rock aquifers and flow systems are likely to be compartmentalised. Quaternary sands also represent local groundwater flow systems and supply groundwater at a number of locations about the coastal fringe of the region.
Figure 40. Conceptual hydrogeological model for the Pipers-Ringarooma region
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6.3 Scenario assessment
6.3.1 Recharge impacts
The WAVES model (Zhang and Dawes, 1998) was used to estimate the change in groundwater recharge across the Pipers-Ringarooma region under a range of different climate scenarios (section 2.3). The historical (1924 to 2007) modelled recharge was assessed to establish any difference between wet and dry periods of recharge. A 23-year period was used, which allows the projection of recharge estimates to 2030 – in other words, to estimate recharge in 2030 assuming future climate is similar to historical climate (Scenario A). Under scenarios Awet, Amid and Adry the recharge does change for a 23-year period compared to the recharge under the entire period of the historical climate (Figure 41). For the recharge that is exceeded in 10 percent of 23-year periods (Scenario Awet), recharge is on average 64 percent greater that the historical mean (that is, a recharge scaling factor (RSF) of 1.64). For the recharge that is exceeded in 50 percent of 23-year periods (Scenario Amid), recharge is on average 15 percent greater than the historical mean (RSF=1.15). For the recharge that is exceeded in 90 percent of 23-year periods (Scenario Adry), recharge is on average 57 percent lower than the historical mean (RSF=0.43) (Table 24).
The recent (1997 to 2006) climate in the Pipers-Ringarooma region has been drier than the historical (1924 to 2007) mean and consequently the calculated recharge decreases 74 percent under Scenario B relative to Scenario A (Table 24).
Figure 41. Spatial distribution of recharge scaling factors in the Pipers-Ringarooma region for the 23-year Scenario A and the 11-year Scenario B relative to the 84-year historical modelled period
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Under Scenario Cwet, recharge increases 8 percent for the region as a whole, but this is not spatially uniform with greater increases in the west of the region (see Figure 42, Table 24). Under Scenario Cmid, recharge increases 2 percent for the region as a whole with the same pattern as under Scenario Cwet. Under Scenario Cdry, recharge decreases overall 8 percent with the greatest decreases in the south of the region.
The difference in diffuse recharge between Scenario C and Scenario D is due to the impact of future forestry upon recharge. The forestry impacts are spread throughout the region where future forestry is assumed to increase (see Figure 4 and Figure 42). The impact is a reduction in recharge under scenarios Dwet, Dmid and Ddry of about 5 percent greater than under scenarios Cwet, Cmid and Cdry respectively (see Table 24).
Figure 42. Spatial distribution of recharge scaling factors in the Pipers-Ringarooma region for the 84-year scenarios C and D relative to the 84-year historical modelled period
To calculate recharge under each scenario, RSFs were derived from WAVES modelling for each GAA (Table 24). For non-modelled areas (i.e. Ringarooma GAA), the RSFs were then multiplied by the historical average recharge rates (Table 23) to calculate scaled recharge under each scenario (Table 25). A slightly different approach was taken for the Scottsdale GAA – the RSFs from WAVES were passed through individual recharge zones of the numerical groundwater model to calculate a scaled recharge rate for the GAA under each scenario.
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Table 24. Aggregated recharge scaling factors for groundwater assessment areas in the Pipers-Ringarooma region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Scottsdale 1.52 1.16 0.53 0.33 1.03 0.99 0.90 1.02 0.98 0.89 Ringarooma 1.65 1.13 0.41 0.22 0.99 0.94 0.84 0.93 0.87 0.78 Pipers-Ringarooma region 1.64 1.15 0.43 0.26 1.08 1.02 0.92 1.02 0.97 0.87
Table 25. Scaled mean annual recharge for groundwater assessment areas in the Pipers-Ringarooma region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Scottsdale 126 99 47 31 102 99 90 100 97 88 Ringarooma 263 180 65 35 158 150 134 148 138 124
In terms of recharge impacts, a significant amount of variability is evident under Scenario A across the different GAAs, with the wettest years from history (Scenario Awet) leading to significantly more recharge than the driest years (Scenario Adry). Recharge under Scenario B is significantly less than the historical 23-year median recharge (Scenario Amid). The reduction is more acute in the Ringarooma catchment, where the historical climate is more variable by comparison to Scottsdale. In Ringarooma, there is a reduction in recharge of 15 to 20 percent under Scenario Cmid. By comparison, the impact of climate change is negligible in Scottsdale. There is less variability evident under historical conditions; however, this is due to the method adopted where scenarios Cwet, Cmid and Cdry are based on Scenario Amid. The forecast expansion in commercial forest cover is thought to be minimal in Scottsdale, with slightly more forest cover forecast for Ringarooma. The recharge reductions associated with forest expansion (which is evident when recharge under scenarios C and D are compared) are 2 GL for Scottsdale and 12 GL for Ringarooma. The combined impact of climate change and forest expansion (Scenario Dmid) represents a recharge reduction of 2 GL for Scottsdale and 42 GL for Ringarooma from the historical median (Scenario Amid).
6.3.2 Modelled impacts to groundwater levels and fluxes in the Scottsdale groundwater assessment area
The existing numerical groundwater flow model for the Scottsdale groundwater assessment area is that developed by Aquaterra/REM (2009e). Figure 43 summarises the conceptual groundwater model for the Scottsdale GAA. The numerical model follows this conceptualisation, consisting of two layers (Quaternary sediments and Mathinna/Devonian Granite outcrop for layer 1, and tertiary sediments and Mathinna/Devonian Granite outcrop for layer 2) with features representing inflows from diffuse recharge, irrigation and river leakage. The outflow components are represented by coastal, evapotranspiration, and extraction well features. The historical recharge time series of the existing model was altered for the current project using the WAVES model record, as detailed in Section 2.4.
Figure 44 shows the location of two monitoring wells that exist in the Scottsdale assessment area which contain historical water level measurements. The map also shows a fictitious reporting well labelled SV1 that has been placed at McKerrows Marsh which is deemed a key reporting / special value site where no real observation wells exist. Ficticious deepest drawdown wells (see next section) DD1, DD2, and DD3 are also shown. Modelled groundwater levels at these location sites where assessed under the four climate and development scenarios. The modelled river gaining and losing regimes have also been assessed and the results are discussed below.
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Figure 43. Conceptual groundwater model for the Scottsdale groundwater assessment area
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Figure 44. Location of the Scottsdale model extent and reporting sites
Under historical climate (Scenario A)
Figure 45a–b shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under scenarios Awet, Amid and Adry for the real monitoring wells. Figure 45c shows the simulated water levels for the DPIPWE model calibration period and under scenarios Awet, Amid and Adry for the SV1 well placed adjacent to McKerrows Marsh which has been identified as a special value site.
Figure 45d–f show modelled hydrograph responses in areas of deepest drawdown due to extraction, for fictitious monitoring wells specified in the model at these points (identified with a ‘DD’ prefix). The results show no significant difference in terms of trend compared to the actual monitoring points, but with additional drawdown that reflects their location within the area of most concentrated extraction. The results indicate that these aquifer systems appear to be quite robust, in that they:
respond rapidly to pumping and establish a new, dynamic hydrological equilibrium show dynamic water level changes in response to climatic and pumping effects (e.g. lower groundwater levels in centres of pumping, increased groundwater levels in response to recharge) show no apparent long term trend of increasing or decreasing levels.
The Waterhouse well shown in Figure 45a is located in the north of the GAA in an area characterised by low-lying topography and bounded by the Great Forester River to the south and the coast to the north. Water levels under scenario Awet show seasonal variability of up 6 m with a final head at 2030 being a couple of metres higher than the simulated water levels at the end of the DPIPWE model calibration period. Water levels under scenario Amid show seasonal variability similar to under Scenario Awet, however, the large peaks in simulated water levels are less frequent. The final water level at 2030 under Scenario Amid is similar to Scenario Awet. Water levels under scenario Adry show maximum variations that are dramatically reduced when compared to those under scenarios Awet and Amid with variations of less than 2 m modelled throughout the prediction period. Water levels under Scenario Adry show a stable time series pattern until 2025, after which a decline of 2 m is simulated between 2025 and 2030.
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Figure 45b shows the observed and simulated water level for the Jetsonville well, located in the centre of the GAA. Water levels under scenarios Awet and Amid show large seasonal fluctuations of up to 7 m. Water levels under both scenarios also display a general rising trend with the final water level at 2030 being ~10 m and ~6 m higher than the water levels at the end of the historical warm-up period under scenarios Awet and Amid respectively. Water levels under Scenario Adry show a general declining trend with the final water level at 2030 being ~3 m lower than the water levels simulated at the end of the DPIPWE model calibration period.
Figure 45c shows the simulated water levels for the SV1 well, located in the north of the GAA adjacent to McKerrows Marsh. Scenario Awet shows season variability of up ~11 m with a final head at 2030 being only a metre higher than the simulated water levels at the end of the DPIPWE model calibration period. Water levels under Scenario Amid show a smaller seasonal variability compared to under Scenario Awet with a maximum range of 4 m. The final water level at 2030 is also similar to that under Scenario Awet. Water levels under Scenario Adry show maximum variations that are dramatically reduced when compared to scenarios Awet and Amid with variations less than 2 m modelled throughout the prediction period. Water levels under Scenario Adry show a stable time series pattern until 2025, after which a decline of 2 m is simulated between 2025 and 2030.
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(a) Waterhouse
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Figure 45. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3
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Figure 45. Groundwater levels for the DPIPWE model calibration period and under Scenario A at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3 (continued)
Figure 46 shows the simulated gaining, losing, and variably gaining/losing river reaches for Scenario Amid between 2007 and 2030. The main river channels of Hurst Creek, Tuckers Creek, Coxes Rivulet and the Great Forester River are always gaining, which is consistent with the conceptualisation (Figure 38) discussed in Section 5.2.2. The model suggests the McKerrows Marsh is gaining, as opposed to the variably gaining/losing classification applied in the conceptualisation. This may reflect a limitation of the model in that the river level does not change with time. For instance, high flow events (when the river may recharge the aquifer in the vicinity of the wetland) are not simulated. The Brid River to the west is mainly always gaining with some sections showing variably gaining/losing conditions. The headwaters and tributaries to the main river channels are either variably gaining/losing or always losing. Such reaches could not be distinguished in the conceptualisation of surface–groundwater interactions due to a lack of data.
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Figure 46. Simulated gaining and losing river reaches under Scenario Amid
Under recent climate (Scenario B)
Figure 47 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and Scenario B. All hydrographs show a similar response to the simulation under Scenario Adry from 2007 to 2030 with the seasonal variation in water levels being quite small. However, unlike Scenario Adry, the simulated water levels post 2025 do not show a steep downward decline and remain relatively steady. The simulated water levels over the prediction period reflect the water levels simulated during the DPIPWE model calibration period during the last 11 years (1997 to 2007) with the final water levels at 2030 showing only a small reduction compared to the 2007 water levels.
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(a) Waterhouse
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Figure 47. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3
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(e) DD2
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Figure 47. Groundwater levels for the DPIPWE model calibration period and under Scenario B at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3 (continued)
Figure 48 shows the simulated gaining, losing, and variably gaining/losing river reaches under Scenario B between 2007 and 2030. There is a slight increase to the simulated losing reaches relative to under Scenario Amid, with a large portion of the variably gaining/losing reaches becoming all losing under Scenario B. The increased pattern of modelled losing reaches is attributed to the modelled water levels reducing in both overall magnitude and seasonal variability due to a decrease in diffuse recharge. This causes modelled baseflow to decrease and river leakage (i.e. river loses) to increase. Further discussion on modelled water balances are discussed in Section 5.3.3.
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Figure 48. Simulated gaining and losing river reaches under Scenario B
Under future climate (Scenario C)
Figure 49 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario C. There are no discernable differences between the simulated water levels under scenarios Cwet, Cmid and Cdry. This is due to the recharge scaling factors under Scenario C showing insignificant change between scenarios Cwet, Cmid and Cdry as shown in Table 24. All modelled hydrographs under Scenario C show very similar responses to those under Scenario Amid. This shows that the impact of climate change on groundwater levels to 2030 is likely to be small compared with the impact of climate variability (as represented by scenarios Awet, Amid and Adry).
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(a) Waterhouse
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Figure 49. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3
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(e) DD2
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Figure 49. Groundwater levels for the DPIPWE model calibration period and under Scenario C at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3 (continued)
Figure 50 shows the simulated gaining, losing and variably gaining/losing river reaches under Scenario Cmid between 2007 and 2030. There is no significant difference to the losing and gaining regime under Scenario Cmid relative to Scenario Amid.
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Figure 50. Simulated gaining and losing river reaches under Scenario Cmid
Under future development (Scenario D)
Figure 51 shows the measured water levels and modelled hydrograph responses for the simulated DPIPWE model calibration period and under Scenario D. There are no discernable differences between the simulated water levels under scenarios Dwet, Dmid and Ddry. This is due to the recharge scaling factors under Scenario D showing insignificant change between scenarios Cwet, Cmid and Cdry as shown in Table 24. Comparing Figure 51 to Figure 49, the groundwater development under Scenario D has caused long term drawdown of ~1 m, 2 m and 0.6 m at the Waterhouse, Jetsonville and SV1 well locations respectively.
Figure 51 shows no significant difference at the DD bores, compared to under Scenario C, in terms of trend compared to the real monitoring points, but with additional drawdown in the order of 5 m, which reflects their location within the area of most concentrated extraction under Scenario D.
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(a) Waterhouse
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Figure 51. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3
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(e) DD2
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Figure 51. Groundwater levels for the DPIPWE model calibration period and under Scenario D at reporting sites (a) Waterhouse (b) Jetsonville (c) SV1 (d) DD1 (e) DD2 and (f) DD3 (continued)
Figure 52 shows the simulated gaining, losing and variably gaining/losing river reaches under Scenario Dmid between 2007 and 2030. Development in Scenario D has no discernable effects on the rivers gaining/losing regime relative to Scenario Cmid.
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Figure 52. Simulated gaining and losing river reaches under Scenario Dmid
Water balance under scenarios A, B, C and D
Table 26 presents the modelled mean annual water balance under scenarios A, B, C and D for the Scottsdale GAA. The diffuse recharge is the greatest source of inflow for all scenarios, except under Scenario B, which only has a modelled mean annual recharge inflow of 30.7 GL/year. River leakage into the model (river losing conditions) remains relatively consistent between scenarios Awet, Amid, Adry, B, Cwet, Cmid and Cdry with a mean annual range of 31.7 to 39.6 GL/year; which is the largest inflow component for Scenario B (39.6 GL/year). Simulated baseflow to rivers (river gaining conditions) is the highest outflow component for all scenarios with the largest baseflow of 118.3 GL/year under Scenario Awet and the smallest baseflow of 57.8 GL/year under Scenario B.
As explained in Section 2.4, simulating water balance under Scenario D involves:
1. increasing the E/R to 0.25 by increasing the number of extraction wells to enable an increase in the total extraction volume
2. increasing the irrigation area and associated irrigation deep drainage volume to reflect additional surface application from both groundwater and surface water
3. including changes to recharge due to potential future commercial forest plantations as projected by WAVES.
This results in the modelled mean annual irrigation drainage (termed irrigation drainage in Table 26) increasing from 4.1 GL/year to 12.5 GL/year; equivalent to an increase of ~200 percent. The groundwater extraction volume increases from the current modelled rate of 0.0 GL/year to 11.1 GL/year (future extraction is estimated at ~24 GL/year (Table 23) however only 11 GL/year could be sustained by the model (refer to Section 3)).The impact of development under Scenario D on the water balance can be assessed by comparing the river leakage, baseflow, evapotranspiration and storage losses with the Scenario C results and the modelled observations. Results are as follows:
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Model mean annual river leakage (river losing conditions) increases by less than 1 percent. This slight increase to river leakage under Scenario D is caused by the additional future development wells inflicting increased stress on the aquifer system which induces more leakage from the modelled river cells. Model mean annual baseflow (river gaining conditions) decreases by ~ 2 percent. Decrease to modelled baseflow under Scenario D is due to the future development wells capturing more groundwater which leaves less available to report to river as baseflow. Evapotranspiration decreases by ~4 percent. Changes to evapotranspiration is a surrogate for the amount of groundwater available for riparian environments. Model mean annual storage loss increases by ~6 percent (although this represents only a small part of the overall groundwater balance). Groundwater is lost from storage to help support the increase in extraction due to Scenario D’s development wells.
Table 26. Mean annual water balance for the Scottsdale groundwater assessment area under scenarios A, B, C and D
Scottsdale Components Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y River leakage 31.7 33.4 37.6 39.6 33.2 33.5 34.1 33.6 33.8 34.5 Coastal recharge 0.1 0.1 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 Storage loss 27.1 22.8 13.9 9.8 23.3 22.7 21.2 24.7 24.2 22.4 Diffuse recharge 125.7 99.1 46.8 30.7 102.1 98.5 89.8 100.5 97.0 88.4 Irrigation drainage 4.1 4.1 4.1 4.1 4.1 4.1 4.1 12.5 12.5 12.5 Total In 188.8 159.6 102.6 84.3 162.9 159.0 149.3 171.4 167.6 157.9 Extraction 0.0 0.0 0.0 0.0 0.0 0.0 0.0 11.0 11.0 10.9 Baseflow 118.3 100.2 68.2 57.8 102.2 99.8 94.0 99.8 97.6 91.8 Coastal discharge 8.0 7.5 6.7 6.4 7.6 7.5 7.4 7.4 7.4 7.2 Storage out 32.8 27.9 14.0 9.8 28.6 27.8 25.8 30.1 29.2 27.3 Evapotranspiration 29.7 23.9 13.6 10.4 24.5 23.8 22.0 23.7 23.0 21.2 Total Out 188.8 159.6 102.6 84.3 162.9 159.0 149.3 171.9 168.1 158.3 Discrepancy 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -0.5 -0.5 -0.4
6.3.3 Reporting metrics
Extraction relative to recharge
The ratio of extraction relative to recharge (E/R) is shown in Table 27. The ratio is commonly used to assess the potential level of stress within aquifers. Where the ratio is greater than 1.0, the groundwater resources are being extracted at a rate greater than diffuse recharge is able to replenish the groundwater. For the purposes of this report, levels of development are defined as:
low, E/R zero to 0.3 medium, E/R 0.3 to 0.7 high, E/R 0.7 to 1.0 very high, E/R >1.0.
With current groundwater extraction being negligible in Scottsdale and only 1 GL/year in Ringarooma, the level of development remains low under all scenarios.
It is noted that the low E/Rs are achieved under Scenario D partly due to the assumptions surrounding future extraction (no expansion in future extraction was forecast in Ringarooma, and extraction in Scottsdale at 2030 was capped such that E/R remained low). Furthermore, a low E/R does not necessarily mean that extraction rates are sustainable. For instance, concentrated extraction may lead to localised drawdown impacts (such as the dewatering of a significant
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wetland). Such impacts may not be reflected in a regional E/R metric, yet extraction within the catchment could not be considered sustainable.
Table 27. Extraction relative to recharge (E/R) for groundwater assessment areas in the Pipers-Ringarooma region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Scottsdale 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.11 0.11 0.12 Ringarooma 0.00 0.01 0.02 0.03 0.01 0.01 0.01 0.01 0.01 0.01
The future extraction rates used to calculate E/R under Scenario D were derived from Table 23 for Ringarooma, while for Scottsdale the extraction implemented in the model (Table 26) was used to ensure E/R values could be assessed against the modelled hydrographs and water balances. The recharge rates used for all E/R calculations were taken from Table 25, which is consistent with the model for Scottsdale. Extraction relative to baseflow
Table 28 shows the modelled mean annual baseflow volume under scenarios A, B, C and D. When these results are compared to modelled diffuse recharge (see Table 26), baseflow under every scenario except Scenario Awet exceeds diffuse recharge. Simulated baseflows are also less sensitive to changes in climate than recharge under the current model arrangement due to the constant river stage approach invoked in the model (refer to sections 2.4 and 3 for further information on assumptions and limitations). However, given these rivers are conceptualised as predominantly gaining, the actual sensitivity is likely to be far greater than that modelled.
Table 28. Mean annual baseflow volume for Scottsdale groundwater assessment area under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Scottsdale 118.3 100.2 68.2 57.8 102.2 99.8 94.0 99.8 97.6 91.8
Table 29 shows the ratio of extraction relative to modelled baseflow under scenarios A, B, C and D (with values of E derived as per the E/R calculations above). The E/B ratio is considered to be a useful metric for comparing scenarios in deeply-incised geologies such as the Scottsdale GAA because baseflow is a key discharge feature that needs to be managed for environmental purposes. Analogous to the concept of E/R ratios above, an E/B>1 is likely to reflect current or potential future stress, both to groundwater and surface water resources.
The E/B values under all scenarios are well below 0.3 and are classified as low development/stress conditions as defined above for E/R.
Table 29. Mean 24-year extraction relative to baseflow (E/B) for Scottsdale groundwater assessment area under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Scottsdale 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.11 0.11 0.12
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6.4 Impacts of use
6.4.1 Management risks
Whilst the current level of groundwater extraction is thought to be low in much of the region, there is significant uncertainty regarding actual extraction rates and only limited ability to monitor the resource condition. In the case of a continued unregulated groundwater management framework, there is a risk that unsustainable levels of extraction may develop, particularly at the local scale. This would cause a number of adverse impacts (e.g. well drawdown interference, reduced groundwater discharge to streams or groundwater dependent ecosystems). An enhanced monitoring network and the recently introduced Tasmanian permit to drill system will help to mitigate these risks.
6.4.2 Waterlogging and salt accession
A return to wetter conditions or an influx of irrigation water has the potential to cause waterlogging and/or land salinisation, particularly in shallow watertable areas. This can occur through the use of saline water sources and/or by additional recharge associated with irrigation causing saline watertables to rise. This may present a future management issue in shallow watertable areas or where an influx of irrigation occurs. This risk is most pronounced for the northern coastal plains of the region, where salinity has previously been identified. For the remainder of the region, where groundwater salinity is typically low, the risk of salinisation is less significant.
Investigating the modelled depth to watertable results under scenarios C and D reveals the following observations:
Depth to watertable over most of the Scottsdale GAA varies by up to a few metres between scenarios C and D. Depth to watertable is greater in areas that have higher local topography and/or a higher concentration of extraction wells. For an example, Jetsonville well is in an area with topography of ~100 m AHD and thus modelled depth to watertable is ~15 m at the wettest time during the prediction period of Scenario Cmid and Dmid. Depth to watertable in the northern coastal plains of the region can be very shallow during the wetter periods of Scenario C and D. Depth to watertable is generally greater under Scenario D than Scenario C due to additional groundwater extraction. The difference is generally within a few metres in areas identified for development.
The modelled depth to watertable is generally increased under the development scenario as a result of increasing extraction to achieve a maximum achievable modelled E/R value of 0.11. However, if future development was to solely use imported surface water, then the increase in irrigation drainage without additional groundwater extraction may cause watertable levels to rise.
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7 The South Esk region
7.1 Contextual information
7.1.1 Hydrogeology
The location of groundwater assessment areas (GAAs) in the South Esk region is shown in Figure 53. Note some overlap from the Kimberley-Deloraine and Mole Creek GAAs, which are reported in the Mersey-Forth region. Groundwater salinities are shown, which also gives some indication as to where groundwater extraction occurs (predominantly in the Longford GAA and in the south of the region). Surface–groundwater interactions have also been mapped (see Section 7.2.2 for further discussion).
Figure 53. Groundwater assessment areas, salinity of groundwater wells and surface–groundwater interactions in the South Esk region
The oldest rocks of the region are the Ordovician-Devonian metasediments (Mathinna Group), which were intruded by granite during the Devonian (see Figure 2 and maps in Appendix B). The Mathinna Group and Devonian granite outcrop in the north-east corner of the region. This was overlaid by tillite, mudstone, sandstone and limestone during the Permian, and by quartz sandstone, siltstone and mudstone during the Triassic. Extensive sheets of Jurassic dolerite occur, which outcrop over the majority of the region. Thick sequences of sand, gravel, silt and clay were deposited during the Tertiary, particularly in the Longford GAA – a north-west trending, oval-shaped trough to the south of Launceston, which contains the largest area of unconsolidated Tertiary sediments in Tasmania (Matthews, 1983). Extrusion of basalt also occurred during the Tertiary. These basalts are comprised of a number of discrete volcanic flows stacked one on another. In some
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places, sediments have been preserved between the flows. Quaternary deposits are present as dune and coastal sediments, as colluvial talus deposits in elevated areas and as alluvial valley fills along rivers and streams.
The Tertiary sediments of the Longford Tertiary Basin form the major aquifer of the region. Groundwater is stored in the pore spaces of the unconsolidated sediments, which comprise alternating beds of clay, silty clay, sandy clay, sand, gravel and conglomerate. In general, well yields are 1 to 5 L/second. Groundwater salinity is somewhat higher compared to similar aquifers in northern Tasmania (e.g. the Tertiary sediment aquifers of the South Esk region). Recorded salinities range from <500 mg/L to >1500 mg/L.
Tertiary basalts also form locally significant aquifers where they occur in the region. Storage within this aquifer system occurs in the network of fractures and vesicles, and also within the pore spaces of interbedded sediments (a vesicle is a small cavity in an igneous rock formed by the expansion of a bubble of gas during the solidification of the rock). The system is considered to be a dual porosity aquifer with movement of groundwater occurring through interbedded sediments and interconnected vesicles as well as through the joints and fractures. The variability in porosity and fractures makes the aquifer heterogeneous with a broad range of aquifer yields and transmissivities. Well yields are typically <5 L/second, but can be significantly higher (15 L/second) in heavily fractured zones. Groundwater salinities are generally less than 1000 mg/L.
Aquifers associated with Triassic sedimentary units are also used to supply groundwater throughout the region. They are largely regarded as fractured rock aquifers although there is a component of storage in coarser-grained units (Bacon and Latinovic, 2003). Groundwater is sourced from these aquifers to the south of the Longford GAA to augment supplies from Tertiary sediment aquifers that also occur in this part of the region (Taylor, 2000). In general, yields are less than 5 L/second and salinities are around 1000 mg/L.
Elsewhere less utilised aquifers occur (Taylor, 2000). At St. Mary’s, in the east of the region, the town water supply is derived from the underlying Permian aquifer that is overlain by Triassic rocks at the surface. In the north-east corner of the region, the Mathinna Group outcrops. These fractured rock aquifers can supply reliable groundwater resources but have not been highly developed across the South Esk region. In addition, Quaternary alluvium along stream lines may host locally important groundwater flow systems. A significant portion of this region is overlain by Jurassic dolerite, but the dolerite generally possesses moderate aquifer properties with few fractures leading to low yields.
7.1.2 Surface–groundwater interactions
No previous studies of surface–groundwater interactions have occurred in the South Esk region. The deeply incised nature of many of the streams within the region suggests they may be in close connection with the watertable. The streams will be losing (allow recharge to the aquifer via leakage) when surface water levels are above the watertable, which often occurs during high flow periods. Streams will be gaining (receive groundwater discharge from the aquifer) when the watertable is higher than surface water levels, which often occurs during low flow periods. Observations from other river systems in Northern Tasmania, for example, the Scottsdale GAA (REM/Aquaterra, 2008n), suggest many of the streams will be gaining. Some losing streams may occur in the steep terrain catchment headwaters. Variably gaining/losing streams may occur in parts of the sedimentary plains of the Longford GAA.
7.1.3 Groundwater extraction
Groundwater extraction in Tasmania has not been metered and as such there are no historical records. There are only basic estimates of extraction for the South Esk region.
The National Land and Water Audit (SKM, 2000b) divided the region according to groundwater management units (GMUs). This identified the Longford Tertiary Basin as a distinct hydrogeological province and assigned an extraction volume of 1116 ML/year across the whole of the geological basin.
Outside the Longford Tertiary Basin GMU the Land and Water Audit (SKM, 2000b) assigned the majority of the eastern portion of Tasmania as the Central South East Unincorporated Area (CSE UA). This area covers the remainder of the South Esk region but also includes significant areas outside. Thus while an extraction volume of 10,236 ML/year is
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identified for the CSE UA it is estimated that perhaps only as little as approximately 2,000 ML/year may actually occur within the South Esk region (based on the number of wells that occur within the region).
7.1.4 Groundwater resource protection and management
Regulation of the water well drilling industry has recently been introduced into Tasmania. Drilling contractors are required to hold a Tasmanian Well Drillers Licence. A permit to drill system is in place that requires all landowners to obtain a Well Works Permit prior to the commencement of drilling. Whilst drillers have always been required to return information relating to the construction of wells, there is no regulation or controls on well operation, such as the collection of extraction data and long-term monitoring of groundwater levels or salinity. The state wide groundwater monitoring network in this area is too sparse to adequately monitor groundwater conditions. In lieu of these management and data gaps, DPIPWE is making progress on the development of groundwater management plans and intends to expand plans for expansion of the monitoring network.
Groundwater quality is affected by manmade and natural processes in the landscape. Localised monitored natural attenuation zones exist in the area of point sources of pollution (e.g. sewage lagoons, landfills, cattle feedlots, underground storage tanks and fuel depots). These sites are regulated by the State Government and licensed under the Environmental Management and Pollution Control Act 1994. Diffuse sources of pollution (e.g. broad-acre over application of fertilizers) may result in short term resource degradation of the resource, and may have a larger impact area than point sources of pollution. Dryland salinity is known to occur in the upper catchment of the Macquarie River with salt pans forming during summer and in the low-lying areas of the Longford GAA. The impact on groundwater quality from commercial forest plantations in the east of the region is unknown.
7.1.5 Previous estimates of recharge and discharge
Published estimates of diffuse recharge and extraction for the South Esk region are listed in Table 30.
As part of the National Land and Water Audit 2000, SKM (2000b) conducted an estimate of recharge to determine sustainable yields for groundwater catchments based on a percentage of rainfall infiltration. These catchments were based on aquifer boundaries and covered the extent of the Longford Tertiary Basin and an unincorporated area covering all the South Esk, Derwent-South East and parts of the Mersey-Forth regions. The SKM (2000b) catchments do not coincide with the GAAs defined as part of this project.
Table 30. Previous estimates of groundwater fluxes for the South Esk region
Catchment Recharge Extraction Approximate CSIRO surface water catchment volume volume estimate estimate GL/y Longford (SKM 2000b) 26 1 Meander, Brumbys, South Esk (North Esk) Central South East 730 2 Meander, Brumbys, South Esk, Macquarie (Tamar Estuary, North Esk, Upper Unincorporated Area Derwent, Ouse, Clyde, Jordan, Coal-Pitt Water, Swan-Apsley, Little Swanport, (SKM 2000b) Prosser, Carlton, Lower Derwent, Derwent Estuary, Carlton-Tasman Peninsula, Huon)
7.1.6 Groundwater level and salinity trends
Time-series groundwater level and salinity data within the region are rare. State observation wells that have sufficient data tend to reflect localised conditions more so than regional trends. The monitoring wells are not necessarily completed in the major aquifers and a meaningful analysis of trends between wells is generally not possible on a regional scale. Whilst recognising these limitations, hydrographs for two monitoring wells are presented in Figure 54, which are located in the Longford GAA (see Figure 53). The two major aquifers of the region are represented – the Hagley monitoring well
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(Figure 54a) is completed in Tertiary basalt; the Cressy monitoring well is completed in Tertiary sediments (Figure 54b). Local rainfall data are presented alongside the groundwater level data.
(a) Hagley
0 800
600 2
400 mean (mm)mean 4
Water level (m BGL) Water level (m BGL) 200 Cumulative deviation from mean rainfall (mm) Cumulative deviation from the
6 0 1991 1993 1996 1999 2001 2004
(b) Cressy
12 1000
13 800
14 600 mean (mm)
Water level (m15 BGL) Water level (m BGL) 400
Cumulative deviation from mean rainfall (mm) from deviation Cumulative the
16 200 1991 1993 1996 1999 2001 2004
Figure 54. Hydrographs for the (a) Hagley and (b) Cressy monitoring wells, showing the water level (metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall
No long-term trend in groundwater levels is evident in the Hagley monitoring well. A 1 m decline in groundwater levels occurred in the mid-1990s in the Cressy monitoring well, which may be related to below average rainfall during that period. However, there has been no further decline in groundwater levels despite below average rainfall since 1997.
No long-term trend in groundwater salinity is noted for any of the wells in the region (Ezzy, 2004).
7.2 Groundwater system assessment
7.2.1 Recharge/discharge
Table 31 is a summary of the key components of the groundwater balance for the Longford GAA. These estimates are carried through for the scenario analysis in the following section.
The Longford GAA was not assessed as part of the recent DPIPWE groundwater modelling project. Previous estimates of diffuse recharge were limited to applying arbitrary rainfall factors that were not necessarily linked to local conditions (e.g. SKM, 2000). To ensure that consistency was applied across the current project, recharge for the Longford GAA was estimated using the same methods that were applied in other non-modelled GAAs. Namely, the Chloride Mass Balance method (CMB) and the empirical relationship for estimating ‘excess water’ (Zhang et al., 1999; 2001) were applied to estimate recharge.
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When applying the CMB method, groundwater chloride concentrations were only available for three wells across the GAA, all completed in different geologies. Reported chloride concentrations ranged significantly between the wells (48 to 740 mg/L). Consequently, the calculated recharge rates varied from 4 to 130 mm/year (using a mean annual rainfall of 622 mm/year, and an assumed chloride concentration in rainfall of 5 to 10 mg/L). Hence, there is significant uncertainty regarding the recharge rates calculated using this method and the results were disregarded.
Using the empirical relationship of Zhang et al. (1999; 2001), the mean long-term rainfall value of 622 mm/year together with a land use mix of 94 percent grass/cleared and 6 percent trees (BRS, 2002) were used to estimate the annual excess water as being 102 mm/year for the catchment. In the absence of monitored or modelled streamflow for any of the main rivers in the catchment, it was assumed runoff equals 10 percent of the mean annual rainfall (i.e. 62 mm/year). This runoff value was subtracted from the excess water value to obtain a recharge estimate of 40 mm/year, which equates to 70 GL/year across the GAA.
The likely magnitude of current groundwater extraction was assessed by comparing the current surface water allocations to the likely irrigation requirements of the GAA. The Tasmanian Dairy Industry website
As evident in the above calculations, there is significant uncertainty in the estimates of recharge and current extraction. Hence, a precautionary approach was taken to guide future estimates of extraction, and there was assumed to be no increase in extraction by 2030.
Recharge via stream leakage, groundwater discharge to streams and groundwater losses due to evapotranspiration are significant components of the groundwater balance but could not be quantified for the Longford GAA in this study.
Table 31. Estimated diffuse recharge, discharge to streams and extraction for the Longford groundwater assessment area in the South Esk region
Groundwater Recharge Current Future Discharge to assessment area extraction extraction streams GL/y Longford 70 1 1 na
7.2.2 Surface–groundwater interactions
Interactions between groundwater and surface water account for a significant component of the groundwater balance. The likely nature of these interactions has been mapped for the Longford GAA (see Figure 53). More detailed maps are shown in Appendix B.
There were limited data to inform the surface–groundwater interaction mapping. There was sporadic groundwater level data in parts of the catchment, and an absence of data elsewhere. The groundwater levels had been taken at different times over a 20-year period (1985 to 2005). Some preliminary observations were made based on the data. Groundwater elevations were predominantly higher than the adjacent surface water elevations, suggesting that most of the streams are gaining. However, in parts of the basin where streams pass though alluvial valleys in flatter terrain, groundwater elevations could be above or below the adjacent surface water elevations. Such reaches were classified as variably gaining/losing.
Due to the paucity of available data, there is a low level of confidence in the classifications made in Figure 53. More complex and varied surface–groundwater interactions are likely. For example, streams in the steep, high relief areas of the catchment headwaters may be losing. However, there is no data to determine the location of such reaches.
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7.2.3 Conceptual model
A conceptual model has been developed (Figure 55) for the major groundwater flow systems that occur within the South Esk region. The hydrogeology of the region is depicted from the perspective of a cross-section through the centre of the Longford GAA looking south-east.
In the Longford GAA, the principal aquifer occurs in Tertiary sediments deposited in a basin that is underlain and surrounded by primarily Jurassic dolerite. Tertiary basalts also form locally significant aquifers where they occur, for example, at Campbell Town in the south of the Longford GAA. The Tertiary sediments form regional and intermediate groundwater flow systems, whereas the basalt aquifers form more localised flow systems. Diffuse recharge occurs throughout the catchment. Recharge may also occur from the losing reaches of streams. Groundwater discharge occurs primarily to rivers and is lost to evapotranspiration where the watertable is shallow. Groundwater discharge also occurs via extraction although this appears to be a minor component of the water balance.
Maps of groundwater elevation contours have been produced for the Longford GAA (Matthews, 1983), which provide a more detailed representation of groundwater flow paths. In most cases, the flow direction is largely controlled by surface topography and inferred to be in the direction of the main surface water drainage features.
Outside of the Longford GAA, most groundwater extraction occurs in the south of the region, where groundwater is extracted from Permian and Triassic sedimentary units in addition to the Tertiary aquifers. These are generally viewed as fractured rock aquifers although there is a component of intergranular storage and flow in coarser grained sediments. They are likely to form local flow systems. Quaternary aquifers contain small and generally untapped water resources across the region.
Figure 55. Conceptual hydrogeological model for the South Esk region
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7.3 Scenario assessment
7.3.1 Recharge impacts
The WAVES model (Zhang and Dawes, 1998) was used to estimate the change in groundwater recharge across the South Esk region under a range of different climate scenarios (Section 2.3). The historical (1924 to 2007) modelled recharge was assessed to establish any difference between wet and dry periods of recharge. A 23-year period was used, which allows the projection of recharge estimates to 2030 – in other words, to estimate recharge in 2030 assuming future climate is similar to historical climate (Scenario A). Under scenarios Awet, Amid and Adry the recharge does change for a 23-year period compared with the recharge under the entire period of the historical climate (Figure 56). For the recharge that is exceeded in 10 percent of 23-year periods (Scenario Awet), recharge is on average 61 percent greater than the historical mean (that is, a recharge scaling factor (RSF) of 1.61). For the recharge that is exceeded in 50 percent of 23-year periods (Scenario Amid), recharge is on average 22 percent greater than the historical mean (RSF=1.22). For the recharge that is exceeded in 90 percent of 23-year periods (Scenario Adry), recharge is on average 65 percent lower than the historical mean (RSF=0.35) (see Table 32).
The recent (1997 to 2006) climate in the South Esk region has been drier than the historical (1924 to 2007) average and consequently the calculated recharge decreases 78 percent under Scenario B relative to Scenario A (see Table 32).
Figure 56. Spatial distribution of recharge scaling factors in the South Esk region for the 23-year Scenario A and the 11-year Scenario B relative to the 84-year historical modelled period
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Under Scenario Cwet, recharge increases 21 percent for the region as a whole, but this is not spatially uniform with greater increases in the centre and east of the region (Figure 57, Table 32). Under Scenario Cmid, recharge increases 11 percent for the region as a whole with the greatest decrease in the south-west of the region and increases in recharge through the centre of the region. Under Scenario Cdry, recharge increases overall 3 percent with the greatest decreases in the south-west of the region.
The difference in diffuse recharge between Scenario C and Scenario D is due to the impact of future plantation forests. The forest impacts are spread throughout the region (Figure 57) where forest cover is assumed to increase (see Figure 4).The impact is no reduction in recharge for any of the scenarios (Table 32).
Figure 57. Spatial distribution of recharge scaling factors in the South Esk region for scenarios C and D relative to Scenario A
To calculate recharge under each scenario, RSFs were derived from WAVES modelling for each GAA (Table 32). For the Longford GAA, the RSFs were then multiplied by the historical average recharge rates (Table 31) to calculate scaled recharge under each scenario (see Table 33).
Table 32. Aggregated recharge scaling factors for the Longford groundwater assessment area in the South Esk region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Longford 1.61 1.22 0.35 0.22 1.21 1.11 1.03 1.21 1.11 1.03
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Table 33. Scaled mean annual recharge for the Longford groundwater assessment area in the South Esk region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Longford 113 85 25 15 85 78 72 85 78 72
In terms of recharge impacts, a significant amount of variability is evident under Scenario A, with the wettest years from history (Scenario Awet) leading to significantly more recharge than the driest years (Scenario Adry). Recharge under the prolonged drought (i.e. recent climate) Scenario B, is significantly less than the historical 23-year median recharge (Scenario Amid). There is a reduction in recharge of 7 GL under Scenario Cmid when compared to the historical median. There is less variability evident under the climate change scenarios by comparison to historical conditions; however, this is due to the method adopted where the scenarios Cwet, Cmid and Cdry are based on Scenario Amid. There is little expansion in commercial forest cover forecast in the Longford GAA. Hence, there is no difference in recharge between scenarios C and D.
7.3.2 Reporting metrics
The ratio of extraction relative to recharge (E/R) is shown in Table 34. The ratio is commonly used to assess the potential level of stress within aquifers. Where the ratio is greater than 1.0, the groundwater resources are being extracted at a rate greater than diffuse recharge is able to replenish the groundwater. For the purposes of this report, levels of development are defined as:
low, E/R zero to 0.3 medium, E/R 0.3 to 0.7 high, E/R 0.7 to 1.0 very high, E/R>1.0.
With current groundwater extraction estimated to be only 1 GL/year in the Longford GAA, the level of development remains low under all scenarios.
It is noted that the low E/Rs are achieved under Scenario D partly due to the assumptions surrounding future extraction, that is, no expansion in future extraction was forecast. There is also significant uncertainty surrounding the estimates of extraction and recharge. Furthermore, a low E/R does not necessarily mean that extraction rates are sustainable. For instance, concentrated extraction may lead to localised drawdown impacts (such as the dewatering of a significant wetland). Such impacts may not be reflected in a regional E/R metric, yet extraction within the catchment could not be considered as sustainable.
Table 34. Extraction relative to recharge (E/R) for groundwater assessment areas in the South Esk region under scenarios A, B, C and D
Groundwater assessment area Adry Amid Awet B Cdry Cmid Cwet Ddry Dmid Dwet Longford 0.01 0.01 0.04 0.06 0.01 0.01 0.01 0.01 0.01 0.01
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7.4 Impacts of use
7.4.1 Management risks
Whilst the current level of groundwater extraction is thought to be low in the Longford GAA, there is significant uncertainty regarding actual extraction rates and only limited ability to monitor the resource condition. In the case of a continued unregulated groundwater management framework, there is a risk that unsustainable levels of extraction may develop, particularly at the local scale, to cause a number of adverse impacts (e.g. well drawdown interference, reduced groundwater discharge to streams or groundwater dependent ecosystems). An enhanced monitoring network and the recently introduced Tasmanian permit to drill system will help to mitigate these risks.
7.4.2 Waterlogging and salt accession
Groundwater salinity is somewhat higher in the South Esk region than it is in other parts of Tasmania and dryland salinity is known to occur in parts of the region (e.g. at Cressy in the Longford GAA, Tunbridge, Conara-Epping Forest area etc.). The higher salinity may be a reflection of the generally drier climate and longer residence times of groundwater in the aquifer associated with low flow rates across the flat terrain within the basin. A combination of high salinity and low flow rates presents a risk for both waterlogging and salt accession to the aquifer (low flow rates increase the potential for groundwater mounds to develop in irrigated areas because drainage is poor). Therefore, a return to wetter conditions or an influx of irrigation water has the potential to cause waterlogging and/or land salinisation, particularly in shallow watertable areas. This can occur through the use of saline water sources and/or by additional recharge associated with irrigation causing saline watertables to rise. This may present a future management issue in shallow watertable areas or where an influx of irrigation occurs.
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8 The Derwent-South East region
8.1 Contextual information
8.1.1 Hydrogeology
The location of groundwater assessment areas (GAAs) in the Derwent-South East region is shown in Figure 58. Groundwater salinities are shown (where reported), which also gives some indication as to where groundwater extraction occurs. Surface–groundwater interactions have also been mapped (see Section 8.2.2 for further discussion).
Figure 58. Groundwater assessment areas, salinity of groundwater wells, and surface–groundwater interactions in the Derwent-South East region
The Derwent-South East region is underlain by a complex distribution of geology (see Figure 2 and maps in Appendix B). Basement consists of a mix of Precambrian quartzite, lithicwacke, conglomerate, siltstone, mudstone, slate, phyllite, marine dolostones and mudstones. Sandstone, siltstone and conglomerate were deposited in the Late Cambrian, followed by sandstone, mudstone and limestone in the Ordovician. Different geological conditions are recorded during the Devonian in the east and the west of the region. In the east, Devonian granites were intruded and now form islands and peninsulas along the east coast. In the west, sedimentary deposits are preserved with deposits of siltstone, shale, fine grained sandstone and minor limestone. Permian geology reflects significant glaciation and is comprised of tillite, mudstone, sandstone and limestone. The Permian glaciomarine deposits are more widespread in the south and
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south-west of the region. The Triassic sequence is composed dominantly of sandstone, siltstone and mudstone with widespread deposits of quartz sandstone at the base of the sequence and minor deposits of coal throughout. Significant volcanism occurred during the Jurassic, resulting in the most extensive formation of dolerite in the world, which outcrops over the majority of the region. The Tertiary saw the deposition of sediments comprising sand, gravel, silt and clay that is preserved in localised areas in the south of the region. Extrusion of basalt also occurred during the Tertiary. These basalts are comprised of a number of discrete volcanic flows stacked one on another. In some places, sediments have been preserved between the flows. Quaternary deposits are present as dune and coastal sediments, as colluvial talus deposits in elevated areas and as alluvial valley fills along rivers and streams.
The most significant aquifer systems of the region occur within Permian and Triassic sediments. The Permian aquifers are associated with the Lower Parmeener Supergroup, which is composed of mudstone, siltstone and sandstone with minor limestone, conglomerate and tillite of glaciomarine origin. The greatest occurrence of Permian sediments is in the south-west of the region, where they are used to supply groundwater in the Cygnet-Cradoc and Mt Wellington-Huonville GAAs (REM/Aquaterra, 2008p; q). They also represent in a significant local aquifer in the north of the Coal River GAA. Triassic aquifers are associated with the Upper Parmeener Supergroup, which comprises sequences of sandstone, siltstone and mudstone (Leaman, 1977). These are distributed widely across the northern portion of the region and occur throughout the Coal River GAA. Both the Permian and Triassic aquifers are considered to contain local to intermediate groundwater flow systems throughout the region, which are not interconnected at a regional scale. They are largely regarded as fractured rock aquifers although some flow of groundwater occurs through pore spaces in coarser-grained units (Bacon and Latinovic, 2003). Well yields are typically low (~1 L/second), but can be significantly higher in heavily fractured zones. In general, salinities in these aquifers are <1000 mg/L in the Cygnet-Cradoc and Huonville GAAs, but are somewhat higher (>1000 mg/L) in the Coal River GAA.
Tertiary basalts are not widespread in the region but form locally significant aquifers where they occur, such as in the Sorell-Tertiary basalt GAA (REM/Aquaterra, 2008r; Latinovic, 2002), which forms part of the Coal River GAA, and elsewhere near Hamilton and Brighton. The Tertiary basalt aquifers are considered dual porosity aquifers with movement of groundwater via flow through interconnected vesicles as well as through joints and fractures (a vesicle is a small cavity in an igneous rock formed by the expansion of a bubble of gas during the solidification of the rock). This variability in porosity and fracturing makes the aquifer heterogeneous with a broad range of aquifer yields and transmissivities (Bacon and Latinovic, 2003). Well yields, typically 1 – 5 L/second, are somewhat higher in Tertiary basalt compared to other aquifers in the region. Salinities are >1000 mg/L.
Tertiary sediments occur throughout the region commonly as localised deposits to form local to intermediate groundwater flow systems. Tertiary sediments are used to supply groundwater within the Coal River GAA to augment supplies from other aquifers. Well yields are low (~1 L/second) and salinities are >1000 mg/L.
Quaternary sediments may also form locally significant aquifers such as the sand spits of Seven Mile Beach (Cromer, 2006) and Nine Mile Beach (Swansea-Nine Mile Beach GAA) (Cromer, 2003; REM/Aquaterra, 2008s; Aquaterra/REM, 2009a). Reserves of good quality water are stored within the dune systems adjacent to the coast as well as within underlying Quaternary sediments of marine origin. Groundwater is also contained within Quaternary alluvium deposited along watercourses and this can also be locally important although yields are often low.
A significant portion of this region is overlain by Jurassic dolerite, but the dolerite generally possesses moderate aquifer properties with few fractures leading to low yields.
8.1.2 Surface–groundwater interactions
Varied surface–groundwater interactions are likely to occur across the many streams of the Derwent-South East region. Few previous studies of these interactions have occurred and were based on limited information.
In the Coal River GAA, studies conducted in the Sorell area (REM/Aquaterra, 2008r) indicate that streams are connected to the groundwater system and are in close equilibrium. The streams were interpreted to be gaining in their upper and middle reaches, and losing in their lower reaches (although these reaches may be periodically gaining during high flow events).
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Springs are very common throughout the Coal River GAA and often occur at the contact between Jurassic dolerite and Triassic sedimentary rocks, or where topographic depressions intersect the watertable (Leaman, 1971). The widespread occurrence of springs suggests that groundwater discharge to streams is significant.
Studies in the Mt Wellington-Huonville GAA on the Mountain and Huon Rivers (REM/Aquaterra, 2008q) indicated that these rivers are perennial and gaining across their entire length.
The majority of the region has not been previously studied. As in other areas of Tasmania where the watercourses occur in deeply incised valleys, many of these streams are likely to be gaining.
8.1.3 Groundwater extraction
Groundwater extraction in Tasmania has not been metered and as such there are no historical records. A summary of previous estimates of extraction is provided in Table 35.
In the recent DPIPWE groundwater modelling project, groundwater extraction was estimated by comparing the approximate water requirements of irrigated areas to surface water allocations with the difference inferred to be derived from groundwater (REM/Aquaterra, 2008p–s).
The Land and Water Audit (SKM, 2000b) assigned the majority of the eastern portion of Tasmania as the Central South East Unincorporated Area (CSE UA). This area covers all of the Derwent-South East region in addition to a large portion of the South Esk region and some of the Mersey-Forth region. Thus while an extraction volume of 10 GL/year is identified for the CSE UA, it is estimated that perhaps only 6-8 GL/year may actually be extracted within the Derwent-South East region (based on the number of wells that occur within the region).
8.1.4 Groundwater resource protection and management
Regulation of the water well drilling industry has recently been introduced into Tasmania. Drilling contractors are required to hold a Tasmanian Well Drillers Licence. A permit to drill system is in place that requires all landowners to obtain a Well Works Permit prior to the commencement of drilling. Whilst drillers have always been required to return information relating to the construction of wells, there is no regulation or controls on well operation, such as the collection of extraction data and long-term monitoring of groundwater levels or salinity. The state-wide groundwater monitoring network is too sparse to adequately monitor groundwater conditions. In lieu of these management and data gaps, DPIPWE making progress on the development of groundwater management plans and has expanded the monitoring network.
In terms of groundwater quality, contamination plumes are monitored at several large landfills, and at paper and mineral processing sites. Other point sources of pollution include sewage lagoons, underground storage tanks, fuel depots, and recycled water for irrigation from food processing plants. A large hydrocarbon groundwater contamination plume has been identified at the historical site of the Hobart railway depot. Studies have been undertaken at Swansea-Nine Mile Beach on the east coast where a number of septic trenches occur in coastal dunes, presenting a risk of contamination to the underlying aquifer. Salinity studies have been undertaken in the Upper Derwent catchment with localised salinity being identified on properties in the Ouse catchment. Additional salinity studies have also been undertaken in the Coal River GAA and several locations on the east coast.
8.1.5 Previous estimates of recharge and discharge
As part of the DPIPWE groundwater modelling project, REM/Aquaterra (2008p–s) undertook a series of groundwater assessments, which included an estimation of volumes of recharge. These assessments covered the Swansea-Nine Mile Beach, Sorell Tertiary basalt, Mt Wellington-Huonville and Cygnet-Cradoc GAAs. Diffuse recharge rates were estimated using a variety of methods including the steady state Chloride Mass Balance method, Water Table Fluctuation method and the empirical relationship for estimating evapotranspiration derived by Zhang et al. (1999; 2001). A best estimate of recharge was determined from the results of these methods, and is shown in Table 35. Recharge is assumed to occur throughout the catchment.
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A second estimate of recharge was defined during the DPIPWE project for regions where a numerical groundwater flow model was constructed. In the Derwent-South East region, a model was constructed for the Swansea-Nine Mile Beach GAA (Aquaterra/REM, 2009g). Recharge rates were defined across the model domain according to land use and the permeability of different sedimentary units.
In addition, as part of the National Land and Water Audit 2000, SKM (2000b) conducted an estimate of recharge to determine sustainable yields for groundwater catchments based on a percentage of rainfall infiltration. Those catchments were based on aquifer boundaries and used an area covering the South Esk, Derwent-South East and parts of the Mersey-Forth regions. The REM/Aquaterra (2008a–s) catchments coincide with GAAs defined in this project, but not with the SKM (2000b) catchments.
Table 35. Previous estimates of groundwater fluxes in the Derwent-South East region
Catchment Diffuse Extraction Total Approximate CSIRO surface water catchment recharge volume discharge* estimate estimate volume estimate GL/y Swansea-Nine Mile Beach – 1.2 0.05 1.1 Swan-Apsley conceptual model (REM/Aquaterra, 2008s) Swansea-Nine Mile Beach – 0.7 0** 0.7 Swan-Apsley numerical model (Aquaterra/REM, 2009g) Sorell Tertiary basalt – conceptual 0.6 0.1 3.8 Coal-Pitt Water model (REM/Aquaterra, 2008r) Mt Wellington-Huonville – conceptual 8.8 negligible 30 Huon model (REM/Aquaterra, 2008q) Cygnet-Cradoc – conceptual model 4.0 1.5 1.5 Huon (REM/Aquaterra, 2008p) Central South East unincorporated 730 6.8 NA Tamar Estuary, North Esk, Upper Derwent, Area (SKM 2000b) Ouse, Clyde, Jordan, Coal-Pitt Water, Swan- Apsley, Little Swanport, Prosser, Carlton, Lower Derwent, Derwent Estuary, Carlton-Tasman Peninsula, Huon (Meander, Great Lake, Brumbys, South Esk, Macquarie) * Discharge volumes are derived from estimates of groundwater discharge to streams, groundwater extraction and lateral discharge (REM/Aquaterra, 2008p–s). Losses to evapotranspiration are included for modelled areas (Aquaterra/REM, 2009g) but not elsewhere. ** Groundwater extraction was set to zero pending more detailed information regarding pumping rates NA – not available
8.1.6 Groundwater level and salinity trends
The Derwent-South East region covers a diverse range of catchments with a broad range of aquifer geologies. Ten monitoring wells are distributed across the fourteen catchments. Only two of the ten wells are located within the same catchment and they are not completed within the same aquifer. As such, a meaningful analysis of trends between wells is generally not possible on a regional scale. Whilst recognising these limitations, hydrographs for two monitoring wells are presented in Figure 59, which are located in the Coal River GAA (see Figure 58). Two of the major aquifers of the region are represented – the Pawleena Road monitoring well (Figure 59a) is completed in Tertiary basalt; the Tunnack monitoring well is completed in Permian mudstone (Figure 59b). Local rainfall data are presented alongside the groundwater level data.
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(a) Pawleena Road 0 800 Water level (m BGL) 2 Cumulative deviation from mean rainfall (mm) 600 4
6 400
8 mean (mm)
Water10 level (m BGL) 200
12 from deviation Cumulative the
14 0 1991 1993 1996 1999 2001 2004 (b) Tunnack 0 600
2
4 400
6 mean (mm) 8 200
Water level (m BGL) Water level (m BGL) 10 Cumulative deviation from mean rainfall (mm) Cumulative deviation from deviation Cumulative the
12 0 1991 1993 1996 1999 2001 2004 Figure 59. Hydrographs for the (a) Pawleena Road and (b) Tunnack monitoring wells, showing the water level (in metres below ground level) in the monitoring wells and the cumulative deviation from mean rainfall
Groundwater levels in the Pawleena Road monitoring well declined from 1991 to 2000 when rainfall was below average. The water levels have appeared to stabilise since 2000 despite a continuation of the drought. The Pawleena Road well is also affected by pumping from nearby production wells (Ezzy, 2005). Groundwater levels in the Tunnack monitoring wells remained relatively stable during the monitoring period.
Ezzy (2004) reports that salinity levels in the region have generally remained stable (although fluctuate seasonally) throughout the 1990s and early 2000s, except in a few areas where rising or falling trends appear in response to local conditions.
8.2 Groundwater system assessment
8.2.1 Recharge/discharge
Table 36 presents a summary of the key components of the groundwater balance for GAAs within the Derwent-South East region. These estimates are carried through for scenario analysis in the following section.
The diffuse recharge rates represent an historical annual average. For the Swansea-Nine Mile Beach GAA, the recharge rate was equivalent to that used for the numerical model (steady state only) developed for the DPIPWE groundwater modelling project (Aquaterra/REM, 2009g). In GAAs where no numerical model exists, rates from the ‘preliminary conceptual water balance’ of the DPIPWE groundwater modelling project (REM/Aquaterra, 2008p–r) were used.
The Coal River GAA was not assessed as part of the recent DPIPWE groundwater modelling project (apart from the Sorell-Tertiary basalt GAA, which forms a small portion of the Coal River GAA). To ensure consistency was applied across the current project, recharge for the Coal River GAA was estimated using the same methods that were applied in
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other non-modelled GAAs. Namely, the Chloride Mass Balance method (CMB) and the empirical relationship for estimating ‘excess water’ (Zhang et al., 1999; 2001) were applied to estimate recharge.
When applying the CMB method, groundwater chloride concentrations were only available for two wells across the GAA that were completed in different geologies. Mean chloride concentrations ranged significantly between the wells – 89 mg/L in Tunnack observation well compared to 597 mg/L in the Pawleena Road observation well (Ezzy, 2004). Consequently, the calculated recharge rates varied from 5 to 70 mm/year (using a mean annual rainfall of 623 mm/year, and an assumed chloride concentration in rainfall of 5 to 10 mg/L). Hence, there was significant uncertainty regarding the recharge rates calculated using this method and the results were discarded.
Using the empirical relationship of Zhang et al. (1999; 2001) the mean long-term rainfall value of 623 mm/year together with a land use mix of 90 percent grass/cleared and 10 percent trees (BRS, 2002) were used to estimate the annual excess water as being 99 mm/year for the catchment. In the absence of monitored or modelled streamflow for any of the main rivers in the catchment, it was assumed runoff equals 10 percent of the mean annual rainfall (i.e. 62 mm/year). This runoff value was subtracted from the excess water value to obtain a recharge estimate of 37 mm/year, which equates to 34 GL/year across the GAA.
Current extraction was based on estimates that were derived during the DPIPWE groundwater modelling project (REM/Aquaterra, 2008p–s). For the Coal River GAA, extraction had only been estimated for the Sorell-Tertiary basalt portion of the GAA (REM/Aquaterra, 2008r). This rate was upscaled to the entire Coal River GAA by assuming a constant rate of extraction for each well (i.e. the extraction rate per well was calculated for the Sorell-Tertiary basalt GAA, which was then multiplied by the number of wells in the Coal River GAA).
As evident in the above calculations, there is significant uncertainty in the estimates of recharge and current extraction. Hence, a precautionary approach was taken to guide future estimates of extraction, and there was assumed to be no increase in extraction by 2030.
Recharge via stream leakage, groundwater discharge to streams and groundwater losses due to evapotranspiration are significant components of the groundwater balance but could not be quantified for this study.
Table 36. Estimated diffuse recharge and extraction for the groundwater assessment areas in the Derwent-South East region
Groundwater assessment area Diffuse recharge Current extraction 2007/08 Future extraction 2030 GL/y Coal River 34.0 0.8 0.8 Sorell Tertiary basalt 0.6 0.1 0.1 Mt Wellington-Huonville 8.8 negligible negligible Cygnet-Cradoc 4.0 1.5 1.5 Swansea-Nine Mile Beach 0.7 0.1 0.1 Total 48.1 2.5 2.5
8.2.2 Surface–groundwater interactions
Interactions between groundwater and surface water account for a significant component of the groundwater balance. The likely nature of these interactions has been mapped for the Coal River GAA (see Figure 58). A more detailed map is shown in Appendix B.
There were limited data to inform the surface–groundwater interaction mapping. There was sporadic groundwater level data in parts of the catchment, and an absence of data elsewhere. The groundwater levels had been taken at different times over a 20 year period (1985 to 2005). Some preliminary observations were made based on the data. The upper reaches of the Coal River appear to losing where the river passes relatively flat terrain, associated with dolerite, and groundwater levels are below the adjacent surface water levels. The mid-catchment reaches are predominantly gaining where streams are incised and groundwater levels rise away from the stream channel. Some losing reaches are evident in the lower catchment, where they flow out across alluvial plains and surface water levels are above the groundwater levels.
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Due to the paucity of available data, there is a low level of confidence in the classifications made in Figure 58. More complex and varied surface–groundwater interactions are likely. For example, streams in the steep, high relief areas of the catchment headwaters may be losing. However, there is no data to determine the location of such reaches.
8.2.3 Conceptual model
A conceptual model has been developed (see Figure 60) for the major groundwater flow systems that occur within the Derwent-South East region. The hydrogeology of the region is depicted from the perspective of Pittwater Bay, looking north along a cross-section through the Coal River GAA. Most of the major aquifer systems of the region are represented.
The Permian, Triassic and Tertiary aquifers presented in Figure 60 can be considered as unconfined to semi-confined hydrogeological units regionally. The groundwater flow systems associated with these aquifers are local to intermediate in scale. The Quaternary alluvium aquifer is unconfined and localised. Diffuse recharge occurs at low rates (45 mm/year) throughout the GAA, but is likely to be higher on the ranges. Some additional recharge to the aquifers occurs from losing reaches of streams. Groundwater discharges to the coast and to gaining streams, and is lost to evapotranspiration where the watertable is shallow. Groundwater discharge also occurs via extraction although this appears to be a minor component of the water balance.
In addition to the aquifer types represented in Figure 60, locally significant aquifers can occur in Quaternary sediments in coastal margins of the region (e.g. Swansea-Nine Mile Beach GAA and, Seven Mile Beach etc.). These are local, unconfined flow systems. Due to limited volumetric storage capacities, they tend to recharge rapidly. Discharge occurs to the coast and to surface water features, where present.
Maps of groundwater elevation contours have been produced for the Coal River GAA (Appendix C) and the remaining GAAs (REM/Aquaterra, 2008p–s), which provide a more detailed representation of groundwater flow paths. In most cases, the flow direction is largely controlled by surface topography and inferred to be in the direction of the main surface water drainage features.
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Figure 60. Conceptual hydrogeological model for the Derwent-South East region
8.3 Scenario assessment
8.3.1 Recharge impacts
The WAVES model (Zhang and Dawes, 1998) was used to estimate the change in groundwater recharge across the Derwent-South East region under a range of different climate scenarios (Section 2.3). The historical (1924 to 2007) modelled recharge was assessed to establish any difference between wet and dry periods of recharge. A 23-year period was used, which allows the projection of recharge estimates to 2030 – in other words, to estimate recharge to 2030 assuming future climate is similar to historical climate (Scenario A). Under scenarios Awet, Amid and Adry the recharge changes for a 23-year period compared to the recharge under the entire period of the historical climate (see Figure 61). For the recharge that is exceeded in 10 percent of 23-year periods (Scenario Awet), recharge is on average 67 percent greater that the historical mean (that is, a recharge scaling factor (RSF) of 1.67). For the recharge that is exceeded in 50 percent of 23-year periods (Scenario Amid), recharge is on average 9 percent greater than the historical mean (RSF=1.09). For the recharge that is exceeded in 90 percent of 23-year periods (Scenario Adry), recharge is on average 55 percent lower than the historical mean (RSF=0.45) (see Table 37).
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The recent (1997 to 2006) climate in the Derwent-South East region has been drier than the historical (1924 to 2007) average and consequently the calculated recharge decreases 59 percent under Scenario B relative to Scenario A (see Table 37).
Figure 61. Spatial distribution of recharge scaling factors in the Derwent-South East region for the 23 year Scenario A and the 11-year Scenario B relative to the 84 year historical modelled period
Under Scenario Cwet, recharge increases 19 percent for the region as a whole, but this is not spatially uniform with greater increases in the west and north-east of the region (see Figure 62, Table 37). Under Scenario Cmid, recharge increases 11 percent for the region as a whole. Under Scenario Cdry recharge is overall the same as the historical scenario with increases in recharge in the centre and north-east of the region and decreases in the north-west and south-west.
The difference in diffuse recharge between Scenario C and Scenario D is due to the impact of growth in future commercial forest plantations upon recharge. The forest impacts are minimal throughout the region (see Figure 62, Figure 4). The impact is a small reduction in recharge under Dwet of about 1 percent more than under Scenario Cwet relative to Scenario A (see Table 37). The changes in recharge under Dmid and Ddry compared to Cmid and Cdry are less than 1 percent.
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Figure 62. Spatial distribution of recharge scaling factors in the Derwent-South East region for the 84-year scenarios C and D relative to the 84-year historical modelled period
To calculate recharge under each scenario, RSFs were derived from WAVES modelling for each GAA (see Table 37). The RSFs were then multiplied by the historical average recharge rates (see Table 36) to calculate scaled recharge under each scenario (see Table 38).
Table 37. Aggregated recharge scaling factors for groundwater assessment areas in the Derwent-South East region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Coal River 1.56 1.20 0.44 0.29 1.05 1.11 1.10 1.01 1.07 1.06 Sorell Tertiary basalt 1.64 1.21 0.33 0.20 1.04 1.10 1.09 0.89 0.94 0.94 Mt Wellington-Huonville 1.72 1.16 0.32 0.23 1.03 1.06 1.01 1.01 1.04 0.99 Cygnet-Cradoc 1.64 1.21 0.33 0.23 1.05 1.10 1.05 0.94 0.99 0.94 Swansea-Nine Mile Beach 1.34 1.15 0.74 0.50 1.20 1.19 1.13 1.20 1.19 1.13 Derwent-South East 1.67 1.09 0.45 0.41 1.19 1.11 1.00 1.18 1.11 1.00
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Table 38. Scaled mean annual recharge for groundwater assessment areas in the Derwent-South East region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry GL/y Coal River 53.0 40.8 15.0 9.9 35.7 37.7 37.4 34.3 36.4 36.0 Sorell Tertiary basalt 1.0 0.7 0.2 0.1 0.6 0.7 0.7 0.5 0.6 0.6 Mt Wellington-Huonville 15.1 10.2 2.8 2.0 9.1 9.3 8.9 8.9 9.2 8.7 Cygnet-Cradoc 6.6 4.8 1.3 0.9 4.2 4.4 4.2 3.8 4.0 3.8 Swansea-Nine Mile Beach 0.9 0.8 0.5 0.4 0.8 0.8 0.8 0.8 0.8 0.8
In terms of recharge impacts, a significant amount of variability is evident under Scenario A, with the wettest years from history (Scenario Awet) leading to significantly more recharge than the driest years (Scenario Adry). Recharge under the prolonged drought (Scenario B), is significantly less than the historical median recharge (Scenario Amid). The impact of the drought is particularly acute in the Sorell Tertiary basalt, Mt Wellington-Huonville and Cygnet-Cradoc GAAs; it is less severe in the Swansea-Nine Beach GAA and over the region as a whole. There is a reduction in recharge of about 10 percent under Scenario Cmid relative to the historical median for all GAAs, with the exception of Swansea-Nine Mile Beach where no reduction occurs. There is less variability evident under the climate change scenarios by comparison to historical conditions; however, this is due to the method adopted where scenarios Cwet, Cmid and Cdry are based on Scenario Amid. The forecast expansion in forest cover may lead to a significant reduction in recharge in the Sorell Tertiary basalt and Cygnet-Cradoc GAAs, which when combined with climate change (Scenario Dmid), equates to a reduction of around 20 percent from Scenario Amid for these GAAs.
8.3.2 Reporting metrics
The ratio of extraction relative to recharge (E/R) is shown in Table 39. The ratio is commonly used to assess the potential level of stress within aquifers. Where the ratio is greater than 1.0, the groundwater resources are being extracted at a rate greater than diffuse recharge is able to replenish the groundwater. For the purposes of this report, levels of development are defined as:
low, E/R zero to 0.3 medium, E/R 0.3 to 0.7 high, E/R 0.7 to 1.0 very high, E/R >1.0.
With groundwater extraction being low in the Coal River GAA and Swansea-Nine Mile Beach, and negligible in Mt Wellington-Huonville, the level of development remains low under all scenarios for these GAAs. Extraction is more concentrated in the Sorell-Tertiary basalt GAA, where a high level of development is registered under Scenario B, suggesting the groundwater resource may be under stress due to the current drought. Extraction is even more concentrated in the Cygnet-Cradoc GAA. A very high E/R (1.63) is registered for Cygnet-Cradoc under Scenario B, suggesting that groundwater is being extracted at a rate greater than recharge can replenish the resource under the current drought. A continual decline in groundwater levels would result from such conditions, but there are no monitoring wells in this GAA to confirm such a trend. A moderate level of development is registered for Cygnet-Cradoc under the climate change and development scenarios.
It is noted that low-to-moderate E/Rs are achieved in Scenario D partly due to the assumptions surrounding future extraction, that is, that no expansion in future extraction was forecast. There is also significant uncertainty surrounding the estimates of extraction and recharge. Furthermore, a low E/R does not necessarily mean that extraction rates are sustainable. For instance, concentrated extraction may lead to localised drawdown impacts (such as the dewatering of a significant wetland). Such impacts may not be reflected in a regional E/R metric, yet extraction within the catchment could not be considered sustainable.
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Table 39. Extraction relative to recharge (E/R) for groundwater assessment areas in the Derwent-South East region under scenarios A, B, C and D
Groundwater assessment area Awet Amid Adry B Cwet Cmid Cdry Dwet Dmid Ddry Coal River 0.02 0.02 0.05 0.08 0.02 0.02 0.02 0.02 0.02 0.02 Sorell Tertiary basalt 0.10 0.14 0.51 0.83 0.16 0.15 0.15 0.19 0.18 0.18 Mt Wellington-Huonville ------Cygnet-Cradoc 0.23 0.31 1.14 1.63 0.36 0.34 0.36 0.40 0.38 0.40 Swansea-Nine Mile Beach 0.05 0.06 0.10 0.14 0.06 0.06 0.06 0.06 0.06 0.06
8.4 Impacts of use
8.4.1 Management risks
Whilst the current level of groundwater extraction is thought to be low in much of the region, there is significant uncertainty regarding actual extraction rates and only limited ability to monitor the resource condition. In the case of a continued unregulated groundwater management framework, there is a risk that unsustainable levels of extraction may develop, particularly at the local scale, to cause a number of adverse impacts (e.g. well drawdown interference, reduced groundwater discharge to streams or groundwater dependent ecosystems). The recently introduced Tasmanian well permit system and an enhanced monitoring network will help to mitigate these risks. Particular effort should be directed towards capturing more data (including the establishment of monitoring wells) in the Sorell-Tertiary basalt and Cygnet-Cradoc GAAs, where the groundwater resource appears to be stressed under the current drought conditions.
8.4.2 Waterlogging and salt accession
Groundwater salinity is somewhat higher in the Derwent-South East region than in other parts of Tasmania. The higher salinity may be a reflection of the generally drier climate along the east coast and dryland salinity is known to occur in parts of the region (e.g. at Tunbridge and Bothwell). Groundwater salinities exceeding 1000 mg/L are most prevalent in the Sorell-Tertiary basalt and Coal River GAAs (Figure 58), and also occur in the coastal margins of the region, which may be affected by sea-water intrusion.
The generally high groundwater salinity in the region suggests that a return to wetter conditions or an influx of irrigation water has the potential to cause waterlogging and/or land salinisation, particularly in shallow watertable areas. This can occur through the use of saline water sources and/or by additional recharge associated with irrigation causing saline watertables to rise. This may present a future management issue in shallow watertable areas or where an influx of irrigation occurs.
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9 Conclusions
The hydrogeological setting is complex and varied throughout Tasmania, and the available groundwater data are sparse with significant data gaps. The combination of complexity and limited data means that there is significant uncertainty with regards to an analysis of groundwater resources in Tasmania. For example, groundwater extraction is not metered and the existing estimates of extraction are reliant on gross assumptions.
There are currently no management restrictions on groundwater use and monitoring of the resource condition is limited. As a result of these management and data gaps, DPIPWE is currently engaged in developing a well construction permit system, is making progress on the development of groundwater management plans, and has expanded the monitoring network.
Groundwater levels in the few available monitoring wells show a response to rainfall, yet there has been no significant decline associated with the recent drought. Whilst the data from these wells suggests the current extraction rates can be sustained on an aquifer scale, significant well-drawdown has occurred in concentrated areas of extraction, such as Wesley Vale and Mella. This suggests some over extraction is occurring at the local scale, which may be impacting streamflow and groundwater-dependent ecosystems.
In terms of diffuse recharge, the following observations can be made as part of the scenario assessment:
diffuse recharge is significantly lower under Scenario B than under Scenario A over most of the project area there is uncertainty in changes in diffuse recharge under Scenario C with some areas projected to have an increase in recharge from less rainfall. Diffuse recharge is reduced under Scenario D relative to Scenario C due to expansion in plantation forestry, in the Wesley Vale, Mella, Togari and Scottsdale groundwater assessment areas, there appears to be little impact on diffuse recharge due to climate change.
In general, current groundwater extraction is very low compared to diffuse recharge. The ratio of extraction to diffuse recharge remains low for most assessment areas under all climate and development scenarios. Exceptions occur where extraction is currently more concentrated, such as the Wesley Vale area of the Mersey-Forth region and the Mella area of the Arthur-Inglis-Cam region. In these areas, a moderate to high level of development is registered under Scenario B. The low levels of development under Scenario D are a result of the assumptions placed on future extraction (i.e. no development in non-modelled areas and limited development in modelled areas).
Many of the river systems investigated in this study are classified as being connected to the groundwater system and gaining (i.e. receiving groundwater discharge). It is therefore likely that a reduction in recharge due to climate change and/or plantation forestry, or any increased extraction will result in reduced groundwater discharge to streams or cause induced leakage. A reduction in streamflow will result. There is some evidence of this occurring in the scenario modelling undertaken, whereby the length of river reaches that are gaining is reduced under a drier climate and increased extraction. For example, many of the river reaches in the Wesley Vale numerical model changed from gaining to losing reaches under Scenario B.
Modelled groundwater levels do not change appreciably under the different climate and development scenarios. However this does not necessarily indicate that the current rates of extraction are sustainable. For example, there are significant changes to the groundwater balance of the numerical models under Scenario B. Also, diffuse recharge is significantly less under Scenario B than under Scenario Amid, even though groundwater levels do not decline significantly. This induces leakage from streams and decreases groundwater discharge to streams. The subsequent reduction in streamflow could have significant environmental impacts.
Increased irrigation activities have the potential to increase groundwater recharge. This can lead to a local rise in the watertable and cause waterlogging. If the groundwater is saline, such as in parts of the South Esk and Derwent-South East regions, such waterlogging events may lead to land salinisation. The risk is particularly relevant in shallow watertable areas, or where a significant increase in irrigation activities is planned.
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10 References
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A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Water for a Healthy Country Flagship, Australia. Ling FLN, Gupta V, Willis M, Bennett JC, Robinson KA, Paudel K, Post DA and Marvanek S (2009c) River modelling for Tasmania. Volume 3: the Pipers-Ringarooma region. A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Water for a Healthy Country Flagship, Australia. Lyne V and Hollick M (1979) Stochastic time-variable rainfall-runoff modelling. Institute of Engineers Australia National Conference. Publ. 79/10, 89–93.
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Matthews WL (1983) Geology and groundwater resources of the Longford Tertiary basin. Geological Survey Bulletin 59. Tasmania Department of Mines. Rosny Park, Tasmania. Moore WR (1992) Map 2 Hydrogeology of the Scottsdale sedimentary basin. Tasmanian Department of Resources and Energy, Tasmania. Murray-Darling Basin Commission (2001) Groundwater Flow Modelling Guideline. Report prepared by Aquaterra to MDBC. January 2001,
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11 Appendices
Appendix A: Groundwater model benchmarking
A.1 Introduction
This project utilises the Modflow based numerical models that were developed for the Department of Primary Industries, Parks, Water and Environment (DPIPWE). Recharge input to the Modflow models for the DPIPWE project used a simple rainfall-recharge percentage assumption model, whereas this project generates recharge datasets from more complex WAVES modelling (Zhang and Dawes, 1998) by CSIRO. A benchmarking exercise was undertaken by Aquaterra by applying the WAVES recharge to the Wesley Vale Modflow model to confirm this approach achieves: (i) satisfactory calibration to measured water levels with (ii) a water balance consistent with the estimated catchment water balance (Aquaterra/REM 2009c) and the DPIPWE Modflow modelled water balance. Wesley Vale was selected because it is the catchment with a well-distributed monitoring well network for the evaluation of model performance.
A.2 Methods
Three versions of the WAVES recharge time series were generated for the historical period to 2007, and applied to the Modflow model for the benchmarking exercise:
WAVES version 1 represented the recharge time series as a raw output with the standard parameterisation implemented in WAVES. This time series did not take into account any recharge from irrigation. WAVES version 2 represented the recharge time series that was adjusted to include irrigation deep drainage during the summer periods and with no reduction to recharge under forestry. WAVES version 3 was based on WAVES version 2, but with the recharge time series scaled so that the average recharge over the DPIPWE model calibration period matched the average recharge used in the previous DPIPWE project. This time series included recharge from irrigation and included recharge reduction under forestry.
During the benchmarking process, some minor aquifer parameter adjustments were applied to the Modflow model to achieve a satisfactory calibration with the WAVES recharge dataset. Transmissivity (T) applied to the Thirlstane basalt in the north-eastern section of the catchment was increased by a factor of 3.5 (by increasing the horizontal hydraulic conductivity (Kh) from 0.04 m/day to 0.14 m/day locally). This was required to improve model calibration performance to water levels during times of intense groundwater extraction. The higher Kh is still in the range of accepted values defined by DPIPWE. Table 40 below summarises the Wesley Vale aquifer properties set by DPIPWE’s independent reviewer at the commencement of the DPIPWE project.
Table 40. Range of acceptable aquifer parameters for the Wesley Vale groundwater assessment areas
Unit Horizontal hydraulic Vertical hydraulic Confined storage Unconfined conductivity (Kh) conductivity (Kv) (S) storage (Sy) m/d m/d n/a m-1 Moriarty basalt 0.01 – 1 0.001 – 0.1 NA 0.1 Wesley Vale sand 0.01 – 0.5 0.001 – 0.05 5.8 x 10-4 0.1 Thirlstane basalt 0.01 – 0.2 0.001 – 0.02 6 x 10-3 NA
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A.3 Results
The following figures and table are used to assess the benchmarking performances:
Figure 63 shows the location of 20 monitoring wells in the Wesley Vale groundwater assessment area where the simulated water levels were compared to the historical water level measurements. Figure 64a–t displays the simulated and observed water levels during the benchmarking period at these well locations. Modflow model results for all three versions of the WAVES runs are shown for comparison to the observed historical levels. Table 41 compares the estimated water balance (REM/Aquaterra, 2009c) with the DPIPWE model water balance and the three benchmarking models with WAVES recharge input. Figure 65a–d displays the simulated annual average total recharge that was applied to the DPIPWE model and for the three WAVES benchmarking models. Benchmarking Model with WAVES version 1 recharge
The model performance for each of the benchmarking models with WAVES recharge was:
A steady state Modflow model was established for the WAVES version 1 recharge model, and the SRMS (scaled root mean square) percentage was calculated as an objective measure of the calibration performance of the Modflow modelled water levels in relation to the observed levels. The DPIPWE model SRMS was 4.17 percent compared to the benchmarking model SRMS of 6.84 percent, indicating a poorer calibration under the WAVES version 1 recharge input. Referring to Figure 64a–t, more than half of the wells have simulated water levels that are too low when compared to the measured, with the exception of Lloyd’s 3, Lloyd’s 7, MOB1, MIB1, ROB1, ROB4, RIB222, RTIB223 and DOB3. Referring to Figure 64q–t, wells located in the north-eastern section of the catchment (DIB1, DIB2, DOB2 and DOB3) show modelled water levels (for non-pumping times) that initially match the observed water levels quite well. However, these wells are located in an area where intense groundwater extraction commenced in the mid-1980s. The WAVES version 1 modelled recharge (discussed below) is too small to support the extraction in this area and as a consequence, the model cells go dry due to large drawdowns and/or lack of recharge as soon as the Modflow wells are turned on. Table 41 compares the estimated water balance to the simulated annual average water balance components over the benchmarking period for the DPIPWE and benchmarking models. The models for DPIPWE were designed to achieve calibration to known water levels and abstraction data while also maintaining aquifer parameters within the guidelines set by DPIPWE and the independent reviewer, and broadly consistent with the estimated water balance (REM/Aquaterra, 2009c), especially in regard to the estimated recharge volume. When compared to the estimated water balance and DPIPWE model water balance, the WAVES version 1 annual average recharge volume of 3.75 GL/year represents reductions of 77 percent and 86 percent respectively. Figure 65a–b shows the time series of modelled mean annual recharge volumes for the DPIPWE and benchmarking models with WAVES version 1 recharge respectively. The DPIPWE recharge is above 20 GL/year for most years over the calibration period from 1985 to 2007 (the benchmark period for the DPIPWE model), and is never below 16 GL/year. However, the modelled recharge time series for WAVES version 1 ranges between just 0.2 GL/year in 2003 and 17.5 GL/year in 1976. This difference accounts for much of the poor benchmarking model performance in this case.
The relatively low modelled recharge input under the WAVES version 1 algorithm is the reason that the modelled water levels fall substantially at most of the simulated well locations resulting in a model that was not satisfactorily calibrated against observed water levels and previously estimated/modelled water balance components.
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Benchmarking Model with WAVES version 2 recharge
Referring to Figure 64a–t, the simulated water levels under the WAVES version 2 recharge algorithm show a marginal increase in water levels when compared to WAVES version 1. Referring to Figure 64q–t, wells located in the north-eastern section of the catchment (DIB1, DIB2, DOB2 and DOB3), no longer go dry due to pumping, which is an improvement due partly to the increased recharge volume, and also to the aquifer parameter changes applied to the benchmarking Modflow model. Referring to Table 41, the WAVES version 2 modelled recharge volume has increased from 3.75 GL/year to 6.49 GL/year when compared to WAVES version 1; an increase of 73 percent. However, WAVES version 2 recharge is still 60 percent and 73 percent smaller than the estimated water balance and the DPIPWE Modflow model water balance respectively. Figure 65c shows the time series of modelled mean annual recharge volumes for the WAVES version 2 model, which ranges between 0.6 GL/year in 2003 and 24.1 GL/year in 1976. The WAVEs version 2 recharge volumes are still significantly smaller for most years when compared to the DPIPWE recharge time series in Figure 65a. Benchmarking Model with WAVES version 3 recharge
Referring to Figure 64a–t, the simulated water levels for the benchmarking Modflow models with the WAVES version 3 recharge algorithm show a closer calibration match at most wells to the observed water levels than the previous WAVES versions. This is most noticeable in Lloyd’s wells 1, 5, 6, 8, 9 and 10. Referring to Table 41, the WAVES version 3 modelled recharge volume has increased, such that it now averages 23.8 GL/year, matching the average recharge simulated in the DPIPWE model (by definition). Accordingly, other water balance components also match more closely to the DPIPWE model including river leakage, extraction, baseflow and evapotranspiration. Figure 65d shows the time series of modelled annual recharge volumes for the WAVES version 3 model. Even though the recharge remains highly variable from one year to the next, the average recharge over the DPIPWE model calibration period is equal to the average recharge simulated in the DPIPWE model of 23.8 GL/year (shown as a red line in Figure 65a and Figure 65d). A comparison of WAVES version 3 with the original DPIPWE model is shown in Figure 66.
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A.4 Conclusion
Recharge input for the Wesley Vale Modflow model from the WAVES version 3 algorithm produces the best calibration in terms of: (i) simulated versus observed groundwater level matches and (ii) modelled water balance agreement with the estimated water balance and the previous DPIPWE model water balance. From this benchmarking exercise, WAVES version 3 algorithm has been selected as the most suitable WAVES method to estimate the change in groundwater recharge under a range of different climate scenarios for application to the three numerical groundwater models (Wesley Vale, Mella, Togari and Scottsdale) to assess the impacts on groundwater levels and water balance components under a range of climate and irrigation development scenarios for this project.
Figure 63. Location of modelled wells used in the benchmarking exercise (Wesley Vale groundwater assessment area)
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(a) Lloyd’s 1
45 observed WAVES_v1 WAVES v2 WAVES v3 40
35 Waterlevel (m AHD)
30 1965 1970 1975 1980 1985 1990 1995 2000 2005
(b) Lloyd’s 3
95
90
85
80 observed WAVES_v1 Waterlevel (m AHD) WAVES_v2 WAVES_v3 75 1965 1970 1975 1980 1985 1990 1995 2000 2005
(c) Lloyd’s 4
90 observed 85 WAVES_v1 80 WAVES_v2 WAVES_v3 75
70
Water level (m AHD) 65
60 1965 1970 1975 1980 1985 1990 1995 2000 2005
(d) Lloyd’s 5
175 observed 150 WAVES_v1 125 WAVES_v2 WAVES_v3 100
75 Water level (m AHD)
50 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 64. Simulated results with WAVES recharge, and observed groundwater levels for the benchmarking period at the 20 monitoring wells (see Figure 63 for locations)
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(e) Lloyd’s 6
210 observed 185 WAVES_v1 160 WAVES_v2 WAVES_v3 135
110
Water level(m AHD) 85
60 1965 1970 1975 1980 1985 1990 1995 2000 2005
(f) Lloyd’s 7
75 observed WAVES_v1 70 WAVES_v2 WAVES_v3
65 Waterl level(m AHD)
60 1965 1970 1975 1980 1985 1990 1995 2000 2005
(g) Lloyd’s 8
175 observed WAVES_v2 170 WAVES_v1 WAVES_v3
165
160
155 Water level(m AHD)
150 1965 1970 1975 1980 1985 1990 1995 2000 2005
(h) Lloyd’s 9
160 observed WAVES_v1 155 WAVES_v2 WAVES_v3
150
145
140 Water level (m AHD)
135 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 64. Simulated results with WAVES recharge, and observed groundwater levels for the benchmarking period at the 20 monitoring wells (see Figure 63 for locations) (continued)
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(i) Lloyd’s 10
55 observed WAVES_v1 50 WAVES_v2 WAVES_S3
45
40
35 Water level (m AHD)
30 1965 1970 1975 1980 1985 1990 1995 2000 2005
(j) Lloyd’s 12
30 observed WAVES_v1 WAVES_v2 WAVES_v3 25
20 Waterlevel (m AHD)
15 1965 1970 1975 1980 1985 1990 1995 2000 2005
(k) MOB1
75
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55
45 observed 35 WAVES_v1 WAVES_v2 Water level (m AHD) 25 WAVES_v3 15 1965 1970 1975 1980 1985 1990 1995 2000 2005
(l) MIB1
70
60
50
40 observed WAVES_v2 Waterlevel (m AHD) WAVES_v1 WAVES_v3 30 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 64. Simulated results with WAVES recharge, and observed groundwater levels for the benchmarking period at the 20 monitoring wells (see Figure 63 for locations) (continued)
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(m) ROB1
85 observed WAVES_v1 75 WAVES_v2 WAVES_v3
65
55 Water level (m AHD)
45 1965 1970 1975 1980 1985 1990 1995 2000 2005
(n) ROB4
65
60
observed 55 WAVES_v1 WAVES_v2 Water level (m AHD) WAVES_v3 50 1965 1970 1975 1980 1985 1990 1995 2000 2005
(o) RIB222
90
70
50 observed WAVES_v1 30 WAVES_v2 Waterlevel (m AHD) WAVES_v3 10 1965 1970 1975 1980 1985 1990 1995 2000 2005
(p) RIB223
80
65
50 observed WAVES_v1 35 WAVES_v2 Water level (m AHD) WAVES_v3 20 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 64. Simulated results with WAVES recharge, and observed groundwater levels for the benchmarking period at the 20 monitoring wells (see Figure 63 for locations) (continued)
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(q) DIB1
80
60
40 observed 20 WAVES_v1
Water level (m AHD) WAVES_v2 WAVES_v3 0 1965 1970 1975 1980 1985 1990 1995 2000 2005
(r) DIB2
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40 observed WAVES_v1 20
Water level (m AHD) WAVES_v2 WAVES_v3 0 1965 1970 1975 1980 1985 1990 1995 2000 2005
(s) DOB2
70
50
30 observed WAVES_v1 10 WAVES_v2 Water level (m AHD) WAVES_v3 -10 1965 1970 1975 1980 1985 1990 1995 2000 2005
(t) DOB3
80 70 60 50 40 observed 30 WAVES_v1 WAVES_v2 Waterlevel (m AHD) 20 WAVES_v3 10 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 64. Simulated results with WAVES recharge, and observed groundwater levels for the benchmarking period at the 20 monitoring wells (see Figure 63 for locations) (continued)
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Table 41. Simulated mean annual water balance volumes during the benchmarking period for the DPIPWE model and the benchmarking models with WAVES recharge
Components Estimated DPIPWE WAVES v1 WAVES v2 WAVES v3 GL/y River leakage 12.9 18.4 17.6 14.0 Coastal inflow 0.1 0.2 0.2 0.2 Storage loss 1.3 1.4 1.9 3.5 Total recharge 16.4 23.8 3.75 6.49 23.8 Total In 16.4 38.1 23.8 26.3 41.4 Extraction 4.8 3.1 2.6 2.9 3.1 Baseflow 5.5 23.5 14.0 15.3 26.2 Coastal discharge 4.8 3.2 3.3 4.2 Storage increase 1.4 1.3 1.8 2.9 Evapotranspiration 5.4 2.9 3.2 5.2 Total Out 10.3 38.2 23.9 26.5 41.6 Discrepancy 6.1 -0.1 -0.1 -0.2 -0.2
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(a) DPIPWE Modflow model
120 Recharge 100 Recharge average 80
60
40 Recharge (GL/y) 20
0 1967 1972 1977 1982 1987 1992 1997 2002 2007
(b) WAVES version 1
120
100 Recharge
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0 1967 1972 1977 1982 1987 1992 1997 2002 2007
(c) WAVES version 2
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100 Recharge
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0 1967 1972 1977 1982 1987 1992 1997 2002 2007
(d) WAVES version 3
120 Recharge 100 Recharge average 80
60
40 Recharge (GL/y) 20
0 1967 1972 1977 1982 1987 1992 1997 2002 2007
Figure 65. Simulated total recharge for (a) DPIPWE Modflow model (b) WAVES version 1 (c) WAVES version 2 and (d) WAVES version 3
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(a) Lloyd’s 1
45 observed WAVES_v3 40 DPIPWE
35 Water level (m AHD) 30 1965 1970 1975 1980 1985 1990 1995 2000 2005
(b) Lloyd’s 3
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observed 80 WAVES_S3 Waterlevel (m AHD) DPIPWE 75 1965 1970 1975 1980 1985 1990 1995 2000 2005
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90 observed 85 WAVES_S3 80 DPIPWE 75
70
Water level (m AHD) 65
60 1965 1970 1975 1980 1985 1990 1995 2000 2005 l
(d) Lloyd’s 5
175 observed 150 WAVES_v3 125 DPIPWE
100
75 Water level (m AHD)
50 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 66. Simulated model results for DPIPWE model and benchmarking model with WAVES version 3 recharge, and observed groundwater levels for the DPIPWE model calibration period at the 20 monitoring wells
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(e) Lloyd’s 6
210 observed 185 WAVES_v3 160 DPIPWE 135
110
Water level(m AHD) 85
60 1965 1970 1975 1980 1985 1990 1995 2000 2005
(f) Lloyd’s 7
75 observed WAVES_S3 70 DPIPWE
65 Water level (m AHD)
60 1965 1970 1975 1980 1985 1990 1995 2000 2005
(g) Lloyd’s 8
175 observed 170 WAVES_v3 DPIW 165
160
155 Water level(m AHD)
150 1965 1970 1975 1980 1985 1990 1995 2000 2005
(h) Lloyd’s 9
160 observed 155 WAVES_v3 DPIPWE 150
145
140 Water level (m AHD)
135 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 66. Simulated model results for DPIPWE model and benchmarking model with WAVES version 3 recharge, and observed groundwater levels for the DPIPWE model calibration period at the 20 monitoring wells (continued)
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(i) Lloyd’s 10
55 observed 50 WAVES_S3 DPIPWE 45
40
35 Water level (m AHD)
30 1965 1970 1975 1980 1985 1990 1995 2000 2005
(j) Lloyd’s 12
30 observed WAVES_v3 25 DPIPWE
20 Waterlevel (m AHD)
15 1965 1970 1975 1980 1985 1990 1995 2000 2005
(k) MOB1
75
65
55
45
35 observed WAVES_S3
Water level (m AHD) 25 DPIPWE 15 1965 1970 1975 1980 1985 1990 1995 2000 2005
(l) MIB1
70
60
50
observed 40 WAVES_v3 Waterlevel (m AHD) DPIPWE 30 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 66. Simulated model results for DPIPWE model and benchmarking model with WAVES version 3 recharge, and observed groundwater levels for the DPIPWE model calibration period at the 20 monitoring wells (continued)
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(m) ROB1
85 observed 75 WAVES_v3 DPIPWE 65
55 Water level (m AHD)
45 1965 1970 1975 1980 1985 1990 1995 2000 2005
(n) ROB4
65
60
55 observed WAVES_S3 Water level (m AHD) DPIPWE 50 1965 1970 1975 1980 1985 1990 1995 2000 2005
(o) RIB222
90
70
50
observed 30 WAVES_S3 Waterlevel (m AHD) DPIPWE 10 1965 1970 1975 1980 1985 1990 1995 2000 2005
(p) RIB223
80
65
50
observed 35 WAVES_v3 Water level (m AHD) DPIPWE 20 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 66. Simulated model results for DPIPWE model and benchmarking model with WAVES version 3 recharge, and observed groundwater levels for the DPIPWE model calibration period at the 20 monitoring wells (continued)
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(q) DIB1
80
60
40 observed 20 WAVES_v3 Water level (m AHD) DPIPWE 0 1965 1970 1975 1980 1985 1990 1995 2000 2005
(r) DIB2
80
60
40 observed 20 WAVES_S3 Water level(m AHD) DPIPWE 0 1965 1970 1975 1980 1985 1990 1995 2000 2005
(s) DOB2
70
50
30 observed 10 WAVES_v3
Waterlevel (m AHD) DPIPWE
-10 1965 1970 1975 1980 1985 1990 1995 2000 2005
(t) DOB3
80 70 60 50 40 30 observed WAVES_S3 Waterlevel (m AHD) 20 DPIPWE 10 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 66. Simulated model results for DPIPWE model and benchmarking model with WAVES version 3 recharge, and observed groundwater levels for the DPIPWE model calibration period at the 20 monitoring wells (continued)
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Appendix B: Surface–groundwater interaction maps for selected catchments
Figure 67. Surface–groundwater interactions map for the Duck catchment showing surface geology and location of available groundwater level data
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Figure 68. Surface–groundwater interactions map for the Montagu catchment showing surface geology and location of available groundwater level data
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Figure 69. Surface–groundwater interactions map for the Inglis-Flowerdale catchment showing surface geology and location of available groundwater level data
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Figure 70. Surface–groundwater interactions map for the Cam, Emu and Blythe catchments showing surface geology and location of available groundwater level data
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Figure 71. Surface–groundwater interactions map for Flinders Island showing surface geology and location of available groundwater level data
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Figure 72. Surface–groundwater interactions map for the Leven and Forth-Wilmot catchments showing surface geology and location of available groundwater level data
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Figure 73. Surface–groundwater interactions map for the Rubicon catchment showing surface geology and location of available groundwater level data
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Figure 74. Surface–groundwater interactions map for the Great Forester-Brid catchment showing surface geology and location of available groundwater level data
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Figure 75. Surface–groundwater interactions map for the Ringarooma catchment showing surface geology and location of available groundwater level data
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Figure 76. Surface–groundwater interactions map for the Longford groundwater assessment area showing surface geology and location of available groundwater level data
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Figure 77. Surface–groundwater interactions map for the Coal River groundwater assessment area showing surface geology and location of available groundwater level data
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Table 42. Surface geology code index for Figure 67 to Figure 77
Symbol Description Quaternary Q Undifferentiated Quaternary sediments Qh Sand gravel and mud of alluvial, lacustrine and littoral origin Qp Glacial, periglacial and fluvioglacial sediments including till and interglacial deposits Qpg Pleistocene glacial and glacigene deposits Qpl Limestone Qps Coastal sand and gravel Qpt Talus, vegetated and active Tertiary TQ Undifferentiated Cainozoic sediments Tb Basalt (tholeiitic to alkalic) and related pyroclastic rocks Tc Conglomerate, gravel and grit Tf Ferricrete, silcrete, laterite and derived lag deposits Tm Marine limestone Ts Dominantly non-marine sequences of gravel, sand, silt, clay and regolith Cretaceous Ka Appinitic lava and intrusives, associated with lamprophyre dykes (Cape Portland area) Jurassic Jd Dolerite (tholeiitic) with locally developed granophyre Triassic R Undifferentiated Triassic fluviolacustrine sequences of sandstone, siltstone and mudstone Rq Dominantly quartz sandstone Rv Dominantly lithic sandstone with felsic volcaniclastics Rvc Lithic sandstone, siltstone and mudstone with some coal and basal quartz sandstone Rvv Dominantly siltstone, lithic sandstone and mudstone Permian-Late Carboniferous P Undifferentiated Late Carboniferous-Permian glacial, glaciomarine and non-marine sedimentary rocks PR Undifferentiated Parmeener Supergroup rocks Pc Freshwater sandstone with coal measures Pf Freshwater and paralic sandstone and mudstone with some coal measures Pl Lower glaciomarine sequences of mudstone, pebbly mudstone, pebbly sandstone, minor limestone and Tasmanite oil shale Pt Basal tillite Pu Upper glaciomarine sequences of pebbly mudstone, pebbly sandstone and limestone Early Carboniferous-Early Devonian Dd Dolerite dykes Dga Undifferentiated alkali-feldspar granite/granite/adamellite (I-type) Dgaa Dominantly adamellite/granite Dgaas Dominantly adamellite/ granite Dgaf Dominantly alkali-feldspar granite (I-type) Dgafs Dominantly alkali-feldspar granite Dgas Undifferentiated alkali-feldspar granite/ granite/ adamellite Dgn Dominantly granodiorite / adamellite Dgr Dominantly granodiorite Early Devonian-Silurian OD Turbidite sequences, undifferentiated (Mathinna Group) ODq Micaceous quartzwacke turbidite sequences (Mathinna Group) SD Shallow marine quartz sandstone, siltstone and shale (Eldon Group correlates) SDb Siltstone, shale and fine-grained sandstone (Bell Shale, McLeod Formation and correlates) SDf Shallow marine quartz sandstone (Florence Quartzite, Currawong Quartzite and correlates)
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Table 42. Surface geology code index for Figure 67 to Figure 77 (continued)
Symbol Description Ordovician Ol Shallow marine limestone sequence with minor siltstone and sandstone (Gordon Group) Os Shallow marine sandstone- mudstone +/- conglomerate +/- limestone sequences, typically grey, trace fossils and tubicular burrows in places. Ordovician fossils in places. Includes Moina Sandstone, Pioneer Beds, Butler Island Formation Cds Dominantly sedimentary sequences; with minor volcanic and volcaniclastic units Cdt Upper sequence of felsic to intermediate volcaniclastic, volcanic and sedimentary rocks, with late Middle Cambrian (Boomerangian) fossils in places. Tyndall Group and correlates, including Huskisson Group, part of lower Dundas Group, Radfords Creek Group CO Undifferentiated shallow marine – non-marine siliciclastic conglomerate – sandstone sequence – Owen Group and correlates COb Basalt, typically hematite-altered, fine-grained, purple-weathering, within Owen Group COc Mainly siliciclastic conglomerate sequences with sandstone interbeds, shallow marine to non-marine, pebble to boulder grade, typically thick-bedded to massive. Includes Middle Owen Conglomerate, Mt Zeehan Conglomerate, Roland Conglomerate COcd Pebble-cobble siliciclastic conglomerate and coarse-grained sandstone with abundant quartzite and chert clasts (Duncan Conglomerate) COcg Interbedded purple mudstone and fine sandstone with minor chert bearing siliciclastic conglomerate (Gnomon Mudstone) Cod Tholeiitic dolerite (Black Bluff Range) COms Marine sandstone- siltstone- conglomerate sequences, typically turbiditic, siliciclastic to polymict, Late Cambrian fossils in places. Includes correlates of upper Dundas Group, Rosebery Group, Newton Creek Sandstone Cos Shallow marine sandstone, typically pink, cross-bedded to thin-bedded, with interbedded siltstone and minor conglomerate. Includes Upper Owen Sandstone Early-Mid Cambrian Ccw Mafic volcaniclastic sandstone- siltstone- mudstone- chert- minor carbonate sequences with intercalated tholeiitic basalt flows. Considered allochthonous. Cleveland-Waratah Association and correlates Ccwc Chert, pale to dark grey, faintly banded to massive or brecciated. Includes Barrington Chert and correlates. Part of Cleveland-Waratah Association Cda Dominantly andesitic lavas, breccias, volcaniclastic rocks and possible intrusives. Typically calc-alkaline, commonly feldspar-pyroxene-phyric Cdai Major intrusive bodies related to andesites. Includes Beulah Cdd Doleritic intrusives within Mt Read Volcanics belt Cdq Felsic, quartz-feldspar-phyric volcanic, volcaniclastic and intrusive rocks, typically with granitic and porphyry bodies. Eastern Quartz-Phyric Sequence and correlates Ccwb Areas of tholeiitic basalt lava within Cleveland- Waratah Association Cdq Quartz-feldspar-phyric volcanic, volcaniclastic and intrusive rocks of Eastern Quartz-Phyric Sequence and correlates Cds Dominantly sedimentary sequences; with minor volcanic and volcaniclastic units Cdsq Dominantly siliciclastic sandstone-siltstone-conglomerate units of metamorphic derivation. Includes Sticht Range Beds, Miners Ridge Sandstone, Animal Creek Greywacke Cdsv Marine volcano-sedimentary and sedimentary sequences of sandstone, siltstone, mudstone, conglomerate and breccia with some volcanic rocks, felsic to andesitic. Middle Cambrian fossils in places Cdsvl Felsic lava within Western Volcano-Sedimentary Sequence and correaltes Cdt Felsic to intermediate volcaniclastic, volcanic and sedimentary rocks. Late Middle Cambrian fossils in places. Tyndall Group and correlates, including part of lower Dundas Group, Huskisson Group Cdtl Felsic lava, usually quartz-feldspar-phyric, within Tyndall Group Cm Polymict conglomerate, lithicwacke, siltstone and mudstone with rare marine fossils (Scopus Formation) Cqfp Quartz and feldspar +- biotite porphyry Cs Undifferentiated ultramafic rocks
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Table 42. Surface geology code index for Figure 67 to Figure 77 (continued)
Symbol Description Neoproterozoic Laa Amphibolite (Arthur Metamorphic Complex) Lac Chloritic schist with minor phyllite, dolomite and magnesite (Arthur Metamorphic Complex) Lap Phyllite with minor pelitic schist, foliated quartzite and dolomite, and rare conglomerate Ldv Turbiditic volcaniclastic – mafic volcanic rocks (Crimson Creek Formation and correlates) Ldd Dolerite dykes (Port Sorell area) Ldp Strongly faulted sequence of pyritic, carbonaceous and cherty siltstone, chert, greywacke, laminated siltstone, dolomite and basalt (Port Sorell Formation, possible correlate of Success Creek Group) Lo Unmetamorphosed quartzwacke turbidite sequences ( Burnie and Oonah Formations and correlates) Lob Alkali basalt and dolerite (within Burnie and Oonah Formations; includes Cooee Dolerite) Lod Dolomitic mudstone, siltstone and sandstone Lrc Dominantly dark, laminated, commonly pyritic siltstone and mudstone (Cowrie Siltstone and similar sequences) Lrd Well-bedded, cross-bedded, orthoquartzite and subordinate siltstone (Detention Subgroup) Lri Laminated grey siltstone, mudstone and dolomite (Irby Siltstone) Lrj Well-bedded, cross-bedded, orthoquartzite, platy quartzite and siltstone (Jacobs Quartzite) Lm Tholeiitic dolerite dykes (Rocky Cape Dyke swarm) Lsb Tholeiitic basalt (Spinks Creek Volcanics, Bernafai Volcanics and correlates) Lsc Basal siliceous conglomerate and sandstone (Forest Conglomerate and Quartzite, Donaldson Formation and correlates) Lsd Shallow marine dolomite and minor limestone (Smithton dolomite and correlates) Lsr Pale-weathering, thin bedded, laminated quartz siltstone with subordinate interbedded fissile shale. Commonly silicified Lss Shallow marine dolomite, chert, shale and diamictite (Black River dolomite, Savage dolomite and correlates) Lsv Turbiditic mudstone, siltsone, lithicwacke and diamictite with dominantly mafic detritus Lt Undifferentiated pelitic rocks and quartzite sequences, with greenschist facies metamorphism Ltb Dominantly schistose conglomerate (Goat Island Conglomerate and correlates) (Ltb) Ltp Dominantly phyllite Ltpg Fine- to coarse-grained, often thinly banded, pelitic, garnetiferous quartz-mica and mica-quartz schist, commonly containing phengite, biotite, almandine, albite and chlorite. Relatively high metamorphic grade Lts Dominantly quartzite Lw Undifferentiated Weld River Group rocks and correlates Mesoproterozoic Lr Undifferentiated the Rocky Cape Group rocks Lrc Interbedded black, grey or green, locally pyritic, laminated siltstone and mudstone, with rare sandstone and mud- pellet conglomerate (Cowrie Siltstone and correlates)
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Appendix C: Groundwater map of the Coal River groundwater assessment area
Figure 78. Groundwater elevation contours for the Coal River groundwater assessment area
164 ▪ Groundwater assessment and modelling for Tasmania © CSIRO 2009
Appendix D: Surface water catchments of the CSIRO Tasmania Sustainable Yields Project area
Figure 79. Surface water catchments of the CSIRO Tasmania Sustainable Yields Project area
© CSIRO 2009 Groundwater assessment and modelling for Tasmania ▪ 165
Tasmania Sustainable Yields Project reports
Region reports CSIRO (2009) Water availability for Tasmania. Report one of seven to Ling FLN, Gupta V, Willis M, Bennett JC, Robinson KA, Paudel K, the Australian Government from the CSIRO Tasmania Post DA and Marvanek S (2009) River modelling for Sustainable Yields Project, CSIRO Water for a Healthy Tasmania. Volume 1: the Arthur-Inglis-Cam region. A report to Country Flagship, Australia. the Australian Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Water for a Healthy CSIRO (2009) Climate change projections and impacts on runoff for Country Flagship, Australia. Tasmania. Report two of seven to the Australian Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Ling FLN, Gupta V, Willis M, Bennett JC, Robinson KA, Paudel K, Water for a Healthy Country Flagship, Australia. Post DA and Marvanek S (2009) River modelling for Tasmania. Volume 2: the Mersey-Forth region. A report to the CSIRO (2009) Water availability for the Arthur-Inglis-Cam region. Australian Government from the CSIRO Tasmania Sustainable Report three of seven to the Australian Government from the Yields Project, CSIRO Water for a Healthy Country Flagship, CSIRO Tasmania Sustainable Yields Project, CSIRO Water for Australia. a Healthy Country Flagship, Australia. Ling FLN, Gupta V, Willis M, Bennett JC, Robinson KA, Paudel K, CSIRO (2009) Water availability for the Mersey-Forth region. Report Post DA and Marvanek S (2009) River modelling for four of seven to the Australian Government from the CSIRO Tasmania. Volume 3: the Pipers-Ringarooma region. A report Tasmania Sustainable Yields Project, CSIRO Water for a to the Australian Government from the CSIRO Tasmania Healthy Country Flagship, Australia. Sustainable Yields Project, CSIRO Water for a Healthy CSIRO (2009) Water availability for the Pipers-Ringarooma region. Country Flagship, Australia. Report five of seven to the Australian Government from the Ling FLN, Gupta V, Willis M, Bennett JC, Robinson KA, Paudel K, CSIRO Tasmania Sustainable Yields Project, CSIRO Water for Post DA and Marvanek S (2009) River modelling for a Healthy Country Flagship, Australia. Tasmania. Volume 4: the South Esk region. A report to the CSIRO (2009) Water availability for the South Esk region. Report six of Australian Government from the CSIRO Tasmania Sustainable seven to the Australian Government from the CSIRO Yields Project, CSIRO Water for a Healthy Country Flagship, Tasmania Sustainable Yields Project, CSIRO Water for a Australia. Healthy Country Flagship, Australia. Ling FLN, Gupta V, Willis M, Bennett JC, Robinson KA, Paudel K, CSIRO (2009) Water availability for the Derwent-South East region. Post DA and Marvanek S (2009) River modelling for Report seven of seven to the Australian Government from the Tasmania. Volume 5: the Derwent-South East region. A report CSIRO Tasmania Sustainable Yields Project, CSIRO Water for to the Australian Government from the CSIRO Tasmania a Healthy Country Flagship, Australia. Sustainable Yields Project, CSIRO Water for a Healthy Country Flagship, Australia. Technical reports Post DA, Chiew FHS, Teng J, Vaze J, Yang A, Mpelasoka F, Smith I, Graham B, Hardie S, Gooderham J, Gurung S, Hardie D, Marvanek S, Katzfey J, Marston F, Marvanek S, Kirono D, Nguyen K, Bobbi C, Krasnicki T and Post DA (2009) Ecological impacts of Kent D, Donohue R, Li L and McVicar T (2009) Production of water availability for Tasmania. A report to the Australian climate scenarios for Tasmania. A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Water for a Healthy Country Flagship, Project, CSIRO Water for a Healthy Country Flagship, Australia. Australia. Harrington GA, Crosbie R, Marvanek S, McCallum J, Currie D, Viney NR, Post DA, Yang A, Willis M, Robinson KA, Bennett JC, Richardson S, Waclawik V, Anders L, Georgiou J, Middlemis H Ling FLN and Marvanek S (2009) Rainfall-runoff modelling for and Bond K (2009) Groundwater assessment and modelling Tasmania. A report to the Australian Government from the for Tasmania. A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Project, CSIRO Water for CSIRO Tasmania Sustainable Yields Project, CSIRO Water for a Healthy Country Flagship, Australia. a Healthy Country Flagship, Australia.
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