Cairns Water Appendix D Groundwater Modelling Report Contents
Abbreviations vi
Glossary of Terms viii
1. Introduction 1 1.1 Scope of Work 1 1.2 Previous Studies 2 1.3 Regional Conditions 3 1.4 Models Used 3 1.5 Nature and Limitations of this Report 4
2. Background 6 2.1 Study Area 6 2.2 Location and Topography and Drainage 6 2.3 Drainage 7 2.4 Climate 8 2.5 Vegetation and Land Use 8 2.6 Soils 12 2.7 Geology 13 2.8 Surface Water 16
3. Hydrogeology 19 3.1 Field Investigations 19 3.2 Inferred Hydrostratigraphy 21 3.3 Aquifer Tests and Hydraulic Properties 24 3.4 Groundwater Levels 24 3.5 Fresh/Saline Groundwater Interfaces 32 3.6 Groundwater Usage 33 3.7 Groundwater Recharge 36 3.8 Baseflow Analysis 37 3.9 Preliminary Catchment Water Balance 41
4. Model Construction 43 4.1 Code Selection 43 4.2 Model Datasets and Extent 43 4.3 Boundary Conditions 47
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling 4.4 Near Surface Processes and Groundwater Recharge 47 4.5 Groundwater and Surface Water Abstraction 48 4.6 Aquifer Parameters 49
5. Model Calibration 54 5.1 Calibration Quality 55 5.2 Calibration Sensitivity Analysis 58 5.3 Calibrated Model Water Balance 59 5.4 Model Limitations 61
6. Numerically Modelled Impact Assessment 62 6.1 Background and Approach 62 6.2 Bore Field Design 67 6.3 Predicted River Baseflow Impacts 67 6.4 Predicted Surface Water Impacts in Context 70 6.5 Predicted Drawdown Impacts 70 6.6 Predictive Model Sensitivity Analysis 74 6.7 Potential Impacts of Climate Change 75
7. Conclusions 77 7.1 Results 77 7.2 Model Summary 77 7.3 Potential Impacts 78
8. Recommendations 83
9. References 84
Table Index Table 1 Mulgrave River Groundwater Resource Potential 13 Table 2 Investigation Sites 19 Table 3 Aquifer Test Results 24 Table 4 Surface Water Analysis Summary 40 Table 5 Preliminary Water Balance – Mulgrave River Alluvium 41 Table 6 Mulgrave Alluvium Water Balance – Using Calibrated Groundwater Model 60 Table 7 Existing Groundwater Users Potentially Affected by the Proposed Abstraction 71
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Table 8 Existing Groundwater Users Potentially Affected by the Proposed Abstraction Assuming the Lowest Level of River Aquifer Connection 74 Table 9 Predicted Impacts – Dry Season, Average Climatic Year (1997-98) 80 Table 10 Predicted Impacts – After Several Dry Climatic Years (June 2001 to November 2003) 81 Table 11 Predicted River Flow Impacts – Dry Season, Average Climatic Year (1997-98) 81 Table 12 Predicted River Flow Impacts – Aquifer Several Dry Climatic Years (June 2001 to November 2003) 82 Table 13 Soil Parameters for Subsidence Calculations 103 Table 14 Calculated Subsidence (m) 104
Figure Index Figure 1 Location Plan 9 Figure 2 Digital Terrain Model 10 Figure 3 Long Term Average Annual Rainfall 11 Figure 4 Soils 14 Figure 5 Geology 15 Figure 6 Hydrologic Monitoring 18 Figure 7 Geological Surface Depth of Bedrock 22 Figure 8 Geological Surface – Top of Model Layer 2 23 Figure 9 Groundwater Level Hydrographs, 1973 –2004 25 Figure 10 Average June to July Groundwater Levels 26 Figure 11 Average Wet Season Groundwater Levels 27 Figure 12 Average Dry Season Groundwater Levels 28 Figure 13 Average Annual Groundwater Level Fluctuation 29 Figure 14 Average June to July Depth to Water Table 30 Figure 15 Existing Groundwater Users – Primary Bore Purpose 34 Figure 16 Existing Groundwater Users – License Allocation 35 Figure 17 Measured Flow and Salinity Relationships 38 Figure 18 Groundwater Model Grid and Boundaries 45 Figure 19 Aquifer Test Calibration Grid 46 Figure 20 Hydraulic Conductivity Layer 1 50 Figure 21 Hydraulic Conductivity Layer 2 51 Figure 22 Storage Coefficient Layer 1 52
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Figure 23 Storage Coefficient Layer 2 53 Figure 24 Calibrated Steady State Watertable Contours 57 Figure 25 Calibration Sensitivity Analysis 59 Figure 26 Modelled Drawdown Response Scenario 1 63 Figure 27 Modelled Drawdown Response Scenario 2 64 Figure 28 Scenario 1 Sensitivity Analysis: 1500 ML/year Abstractions, Modelled Dry Season Impacts, Minimal River/Aquifer Connection 65 Figure 29 Modelled Drawdown Following Successive Dry Years 66 Figure 30 Abstraction Volume and Baseflow Impacts – Sensitivity Analysis 73 Figure 31 Modelled Drawdown following successive dry years 75
Appendices A Geophysical Logging Inventory B Observed and Modelled Groundwater Level Hydrographs C Observed and Modelled Aquifer Test Drawdowns D Detailed Water Balance Calculations – Mulgrave River Alluvium E PERFECT Model – Assigned Soil Properties F Residual Statistics – Steady State Calibration G Residual Statistics – Transient Calibration H Land Subsidence Analysis
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Abbreviations
ADWG Australian Drinking Water Guidelines
AHD Australian Height Datum
ANZECC Australian and New Zealand Environmental Conservation Council
APSIM Agricultural Production Systems sIMulator
AS/NZS Australian Standard / New Zealand Standard
ASS Acid Sulfate Soils
BFI Baseflow Index
CMB Conductivity Mass Balance
CSIRO Commonwealth Scientific and Industrial Research Organization
DERM Department of Environment and Resource Management
DF Direct Filtration
DNR Department of Natural Resources
DNRMW Department of Natural Resources, Mines and Water
DNRW Department of Natural Resources and Water
DO Dissolved Oxygen
DTM Digital Terrain Model
EC Electrical Conductivity
Eh Redox Potential
EPA Environmental Protection Agency
GHB General Head Boundaries
GPS Global Positioning System
HACCP Hazard and Critical Control Point
HDPE High Density Polyethylene
LIDAR Light Detection and Ranging
LTA Long Term Average
MCPA 2-methyl-4-chlorophenoxyacetic acid
MF Microfiltration
ML Millilitres
NEPM National Environment Protection Measures
NRM Natural Resources and Mines
42/15610/98344 Mulgrave River Aquifer Feasibility Study vi Groundwater Modelling nRMS normalised Root Mean Squared
PAC Powdered Activated Carbon
PACl Poly Aluminum Chloride
PAH Phenoxyacetic Acid Herbicides
PASS Potential Acid Sulfate Soils
QASSIT Queensland Acid Sulfate Soil Investigation Team
QWQG Queensland Water Quality Guidelines
RO Reverse Osmosis
STP Sewage Treatment Plant
SW Surface Water
SWL Standing Water Level
TDS Total Dissolved Solids
USGS United States Geological Survey
WQ Water Quality
WTP Water Treatment Plant
WWTP Wastewater Treatment Plant
42/15610/98344 Mulgrave River Aquifer Feasibility Study vii Groundwater Modelling Glossary of Terms
Abstraction The process of taking water from any source, either temporarily or permanently
Aeration The process by which air is circulated through, mixed with or dissolved
Alluvial aquifer An area of water-bearing sand
Alluvium Soil or sediments deposited by a river or a moving water body. It typically consists of silt, clay, sand and/or gravel.
Aquifer A geological formation that is made up of water-bearing permeable rock or unconsolidated material allowing the storage and transmission of significant volumes of water.
Basalt A common mafic extrusive volcanic rock that is grey to black and fine grained due to rapid cooling of lava. It may contain larger crystals.
Base flow Portion of streamflow that comes from groundwater and not from a runoff
Bore A hole or a passage made by drilling that is used for water sampling
Borefield An area that contains the bores or wells through which the water is extracted
Catchment An area which water is collected / captured, with catchment area referring to an area that is drained by a river
Chlorination The process of adding the element chlorine to water as a method of water purification
Coagulation The process of forming liquid into semisolid lumps
Coastal plain An are of flat, low-lying land adjacent to a coast (sea) and separated from the interior by other features
Conductance The capacity to conduct electricity
Confined aquifer Confined aquifers are permeable rock units that are generally deeper than unconfined aquifers. They are overlain by relative impermeable rock that limits groundwater movement into and out of the aquifer.
Depressurisation Decreasing the pressure
Drawdown Lowering of groundwater or drop in level of water in the ground. It is typically due to pumping of wells / bores.
Filtration A method of water purification where solids are separated from liquids
Floodplain Flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding
Granite A common and widespread intrusive, felsic, igneous rock with a medium to coarse texture. Granites can be pinto to dark grey depending on their mineralogy
Geology The science and study of the solid matter that constitutes Earth
Groundwater flux The rate of flow / discharge of groundwater
Groundwater recharge The process of adding water to an aquifer
42/15610/98344 Mulgrave River Aquifer Feasibility Study viii Groundwater Modelling Hydraulic Device / operation that uses pressure or flow of water
Hydraulic conductivity A property of soil or rock that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and the degree of saturation
Hydrogeological Geologic characteristics that influence the underground flow or movement of water
Hydrograph Graph of water table versus time
Hydrology The study of the movement, distribution and quality of water throughout Earth, addressing the hydrological cycle and water resources
Infiltration The process of water on ground surface enters the soil
Lithology The study / description of rock composition
Metamorphic The term used to describe rocks that have been transformed by extreme heat and pressure
Pilot Something that serves as a model
Pilot trials A precursor / foundation to a full-scale study
Quaternary A Geological Period that began after the Neogene Period, approximately 1.8 million years ago, to the present
Recharge A hydrological process where surface water moves to groundwater, often resulting in water table fluxes
Runoff A term used to describe the movement/flow of water from rainfall or other sources over the land
Sedimentation The term used to describe the deposition by settling of suspended material
Sludge Residual semi-solid material left from a process
Storativity The volume of water an aquifer released
Tertiary A Geological Period that marks the beginning of the Cenozoic Era, extending from approximately 65 million years ago to 1.8 million years ago.
Throughflow Movement of water horizontally beneath land surface
Transmissivity The rate at which water moves / transmitted
Turbidity The cloudiness / haziness of a fluid caused by suspended solids that are too small to settle out.
42/15610/98344 Mulgrave River Aquifer Feasibility Study ix Groundwater Modelling 1. Introduction
As part of the finalisation of its Water Supply Strategy, Cairns Water and Waste is embarking on a program of works to provide necessary information to identify and confirm future water supply source. This Water Supply Strategy includes an assessment of the feasibility of obtaining a sustainable water supply from the Mulgrave River Aquifer system (Mulgrave River Aquifer Scheme). If developed, the groundwater supply will be used to supplement existing sources of water to Cairns. GHD has been engaged to undertake the Feasibility Study into the Mulgrave River Aquifer. The key component of this Feasibility Study is the assessment of the two primary issues considering the aquifer as a supplementary groundwater supply.
Environmental impacts; and Sustainable abstraction volume. In relation to the above a number of potential impacts were identified that required specific assessment during the development of ground water model. The following key impacts of abstraction need to be assessed in conjunction with other investigations. This assessment will be able to determine a sustainable abstraction volume.
Unacceptable drop in groundwater levels for existing groundwater users;
Unacceptable impact on environmental flows in the Mulgrave River and its tributaries; Inducement of saltwater intrusion to the aquifer from coastal areas; Impacts of land settlement; Creation of potential contaminant groundwater migration towards bores extraction points; Inducement of negative environmental impacts through changes in groundwater conditions in areas of acid sulphate soils; and Any other negative impacts to natural resources and the environment. This report details the outcome of the groundwater flow modelling of Mulgrave River Aquifer. It provides input with respect to assessment of the sustainable extraction rate and potential impacts on existing groundwater and surface water users, existing infrastructure and the environment.
1.1 Scope of Work Development and operation of the numerical groundwater model can be broken down into six main tasks: 1. Data review and analysis 2. Conceptual Hydrogeological Model Development 3. Numerical Model Design and Construction 4. Numerical Model Calibration 5. Predictive Modelling – Estimation of Sustainable Yield 6. Model Output and Reporting
42/15610/98344 Mulgrave River Aquifer Feasibility Study 1 Groundwater Modelling The first two main tasks have been completed in earlier studies, and are documented in Mulgrave River Feasibility Study Hydrogeological Report (GHD Draft report 42/14087/01/7410, August 2006). Tasks 3 to 6 are presented in this report.
1.2 Previous Studies There have been a number of earlier studies that have examined the potential of Mulgrave River Aquifer’s groundwater as a water source. These include:
Muller,P.J., 1975: Mulgrave River Groundwater Investigations, Report on Exploratory Drilling. Rec. Geol. Surv. Qld. 1975/17. Leach L.M. and Rose U.E., 1979: Groundwater Storage Behaviour, Mulgrave River Area. Qld Water Res. Comm. Report. Connell Wagner (1992). Russell Mulgrave River Overview Study Report Stages 1-3. (In association with Environment Science and Services). Cairns. Dept. of Natural Resources, Qld Huxley W. and Bjornsson B., 1998a: A review of Groundwater Conditions and Opportunities for Further Development – Mulgrave River Alluvium. Resource Sciences Centre. Dept. of Natural Resources, Qld In 1999 GHD was commissioned by the Department of Natural Resources to review the above reports with respect as to the potential for abstraction from the Mulgrave River Aquifer, or river itself, as a water supply.
GHD (1999) Mulgrave River Aquifer Study. Report on Abstraction to Supply Cairns City. Report for the Department of Natural Resources. GHD Pty Ltd, Cairns. Further to this report with increasing pressures on existing water supplies, Cairns Water engaged GHD in 1994 to review the aquifer option to reflect the most current urban growth and available water supply position.
GHD (2004) Cairns Water Supply Source Options Review. Mulgrave River Aquifer Water Supply Scheme. Report for Cairns Water. GHD Pty Ltd, Cairns. On the basis of previous studies extending back to 1975, the recommendation was made that the aquifer be formally investigated as a supplementary water supply. Previous reports had identified that yields of up to 41 ML/day may be feasible, based on information from hydrogeological surveys of bore productivity (Mullger 1975), modelling of the groundwater characteristics (Leach and Rose, 1979), and preliminary modelling and assessment of bore productivity (DNR, 1998). However, the immediate short to medium term requirement to meet the growing urban demand was estimated to be in the region of 12 to 17 ML/day. The numerical modelling undertaken in this report is therefore based on two scenarios:
A Stage 1 (short to medium term) supply of 15 ML/day to meet the short to medium term demand for a supplementary supply; and A Stage 2 supply of 40 ML/day, to meet medium to longer term projected demand. These limits have been set by previous investigations as being feasible potential requirements of the aquifer groundwater supply.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 2 Groundwater Modelling 1.3 Regional Conditions In a number of instances throughout Australia the development of groundwater supplies has been undertaken in the absence of data to ascertain what the potential impacts of a groundwater development may be. Over 100 years after the first bores were developed in the Great Artesian Basin, there is now a concerted capping program undertaken in Queensland by the Queensland Department of Natural Resources and Water as the cumulative impacts of a century of abstraction are having quantifiable environmental impacts. In more recent times, limited data interpretation and poor understanding of hydrogeological relationships of the abstraction from the groundwater has resulted in salinity and acid sulfate soil conditions being generated in coastal areas. These conditions have impacted the economic and environmental sustainability of these areas (for example, rising saline groundwater in the Bundaberg district). When considering the model, it was is important to differentiate the unique conditions in relation to the Mulgrave River aquifer from that of other coastal groundwater resources currently in use in Queensland.
An annual rainfall which exceeds 9 metres (Mt Bellenden Ker) in the upper catchment of the Mulgrave River, conferring an extraordinary high potential for aquifer recharge unmatched in Australia. This rainfall also ameliorates the risk of saltwater intrusion into the southern section of the aquifer that directly receives surface surface and groundwater recharge from the wettest area in Australia. Topographical position. By comparison with other Queensland coastal aquifers, the Mulgrave River aquifer is relatively sheltered from saline sea water influences. To the north the catchment of the aquifer is defined by a basaltic flow that minimises the marine influences of Trinity Inlet on the aquifer. To the south the aquifer tapers to the narrow estuary at Mutcheroo inlet. East and west uplifted and eroded mountain massifs directly constrain the aquifer to the valley of the Mulgrave River and prevent general coastal seawater incursion into the aquifer. Geological nature of substrate - the alluvium for the majority of the aquifer is extremely deep (over 90 m), with high amounts of sands/silts and highly porous material overlying base bed rock of granite. The high transmissivity of the aquifer confers high recharge potential from the catchment via surface and groundwater water flow.
1.4 Models Used A list of the codes (software) and the datasets used in defining and running the model are listed below. The overall modelling program used is MODFLOW 2000 (Harbaugh et al., 2000). MODFLOW is a finite difference saturated groundwater flow model that has been comprehensively tested, widely utilised and accepted, and is freely available and well documented. Groundwater Vistas was used as the graphical user interface for most of the model construction. Additional supplementary software packages were run for other specific parameters of the overall MODFLOW model. These include:
PERFECT, A computer simulation model of Productivity, Erosion, Runoff Functions to Evaluate Conservation Techniques (Littleboy et al 1989); PEST Model-independent Parameter Estimation (Doherty 2002);
42/15610/98344 Mulgrave River Aquifer Feasibility Study 3 Groundwater Modelling HYSEP: A Computer Program For Streamflow Hydrograph Separation And Analysis. (Slota et al 1996); Rosetta: A computer program for estimating soil hydraulic parameters with hierarchical pedotransfer functions (Schapp et al 2001) In addition to the formal models above, several mathematically specific formulae used for the following:
Calibration of base flow separation methods with streamflow conductivity. Using groundwater levels to estimate recharge The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils.
1.5 Nature and Limitations of this Report This is a technical document for review and consideration by those with experience in the assessment of groundwater modelling. While an attempt has been made to assist the general public in the understanding of this report, much of the terminology is specific to the language of the discipline, and has no direct common interpretation. This report is not intended to be a standalone report, but is to be considered with reference to other investigations being undertaken. An artifice in the model is the separation of the relative impacts of the scenarios modelled between the Mulgrave River and Behana Creek. The quaternary alluvium representing the primary ground water storage area is not separated by any ground water divide between the Behana Creek and Mulgrave River valley floor, i.e., there is only the one aquifer beneath both surface systems. At an early stage in the modelling a request was made to attempt to differentiate the relative impacts on the modelled abstraction scenarios (15ML/day and 40 ML/day) on the surface water characteristics of both water ways. Within the limitations of the available data for surface water conditions at Behana Creek this modelling has assumed conservative values (based on sensitivity analysis of the lacking data) for the estimation of impacts. This report presents the results of a groundwater modelling investigation conducted for the purposes of this commission. It has been prepared specifically for the use of the client who commissioned the works. Reliance by other parties on this report is at their own risk. Data (drill hole or test pit logs, laboratory tests, geophysical tests, etc.) gathered that has been performed and recorded by others is included and used as provided. The responsibility for the accuracy of such data remains with the issuing authority and not with GHD. The advice tendered in this report is based on information obtained from the investigation locations, test points and sample points and is not warranted in respect to the conditions that may be encountered across the study area other than these locations. It is emphasised that the actual characteristics of the subsurface and surface materials may vary significantly between adjacent test points and sample intervals and at locations other than where observations, explorations and investigations have been made. Sub-surface conditions, including groundwater levels and quality can change in a limited time. This should be borne in mind when assessing the data. However, it is our opinion that the test points chosen are representative of conditions for the study area. Should additional data be provided at a later time, GHD reserves the right to amend this report to reflect the new information.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 4 Groundwater Modelling It should be noted that because of the inherent uncertainties in the sub-surface evaluations, changed or unanticipated sub-surface conditions may occur that could affect total project costs and/or execution. GHD does not accept responsibility for the consequences of significant variances in the conditions. An understanding of the site conditions depends on the integration of many pieces of information, regional, site specific, structure-specific and experience based. Hence this report should not be altered, amended or abbreviated, issued in part or issued incomplete in any way without prior checking and approval by GHD. GHD accepts no responsibility for any circumstances, which arise from the issue of the report, which has been modified other than by GHD. The modelling investigation undertaken necessarily contains a large number of simplifications of the observed site conditions and a number of assumptions in the development of the numerical model. Key assumptions of the model are that groundwater flow can be represented by relatively simplistic equations and that material properties are homogenous over broad areas. These assumptions may not accurately reflect the actual site conditions and this may lead to variations between the modelled results and field observations. The purpose of the modelling is to provide a tool to investigate the potential impact of various changes on the behaviour of the groundwater system. The results are intended to be relative rather than absolute. The results of the modelling should be viewed in this context only.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 5 Groundwater Modelling 2. Background
2.1 Study Area The Project Study Area comprises both the catchment of Mulgrave River itself, and that area of the Mulgrave River valley underlain by Quaternary alluvium, referred to as the Mulgrave River aquifer. It is important to differentiate these two areas, as they are the focus of different assessments used in the overall Feasibility Study. The Mulgrave River Catchment per se, includes only a small proportion of the Mulgrave River aquifer, however information related to catchment conditions, particularly as they relate to climate and surface and groundwater features, are critical in developing the numerical groundwater model upon which many of the predictions of impact of the abstraction will be made. By contrast, the impacts of the project will be restricted to the environs of the Mulgrave River aquifer system, which comprises less than ¼ the area of the total river catchment area. For the purposes of this project, the impacted area (the “aquifer area”) is defined as the Trinity Inlet catchment and the lower Mulgrave River valley below 20 m AHD and bounded by points at: 17° 14’ S, 145° 57’ E; 17° 14’ S; 145° 55’ E, 17° 02’ S, 145° 45’ E; and 17° 02’ S, 145° 50’ E. Both the location of the catchment and the aquifer area are described below in further detail. The Study Area is shown in Figure 1. The impact assessment process used in this modelling report has used an all of catchment approach to modelling the potential impacts of the abstraction on the aquifer area.
2.2 Location and Topography and Drainage A Digital Terrain Model of the area, representing land surface topography, is presented in Figure 2. The Mulgrave River valley runs in a roughly north-south line and is bordered to the east and west by ranges reaching as high as 1500 mAHD. The headwaters of the Mulgrave River and its tributaries are located in these high ranges. The valley is narrow 100-1000 m) at the higher elevations, opening to a broad (~5- 8 km wide) relatively flat valley around Gordonvale. Valley floor elevation ranges from around 25 mAHD down to less than 5 mAHD. A minor topographic divide occurs around Gordonvale. Elevation falls from approximately 20 mAHD around Gordonvale to ~5 mAHD in both the northern and southern ends of the valley. This divide directs the Mulgrave River to drain to the valley’s southern outlet. Swamps and mangrove flats occur around both Trinity and Mutchero Inlets (Figure 1). However as noted above the topographic divide near Gordonvale clearly demarcates the influences of Trinity Inlet from the Mulgrave River valley. The Mulgrave River valley is located on a narrow alluvial plain. It forms a flat to undulating floodplain, with river terraces east and south of Gordonvale at 23 m, 9 m and 6 m above the present river level (Muller, 1975).
42/15610/98344 Mulgrave River Aquifer Feasibility Study 6 Groundwater Modelling To the west and east of the floodplain lie the rugged terrain of the Bellenden Ker and Malbon Thompson Ranges respectively. These granitic ranges have been deeply incised to form steep sided ranges falling abruptly to the floodplain. The Bellenden Ker Range to the west of the plain rises to 1592 m at Mount Bellenden Ker Central Peak and 1622 m at Mount Bartle Frere further south. The Malbon Thompson Range rises to 1026 m at Bell Peak North in the east. To the northwest of Gordonvale metamorphic rocks form more subdued ranges rising to 1098 m. Green Hill, approximately 6 km northeast of Gordonvale, is a volcanic feature rising to 131 m elevation. To the south of the Mulgrave River, the Russell River has a similar morphology.
2.3 Drainage The Mulgrave River aquifer is contained within the valley of the Mulgrave River (‘the valley’). The aquifer extends over a length of around 40 km, from the divide just south of Trinity Inlet at Cairns in the north, to Mutchero Inlet in the south, where the Mulgrave River meets the Russell River (Figure 1). Together, both rivers discharge to the ocean via Mutchero Inlet. There is comparably little surface drainage into Trinity Inlet. The Mulgrave River is one of the major coastal rivers in North Queensland. Covering an area of approximately 810 km2 and with a mean annual discharge of 770,000 ML, the Mulgrave River Catchment has one of the highest areas of mean annual runoff of any Australian catchment. The headwaters are in the ranges to the west of the coastal floodplain, which the river enters at Gordonvale, meandering easterly then southerly across the floodplain to Mutchero Inlet. River flows are highly seasonal; mean total flow in the dry season (August to December) is around 161,000 ML (or ~14% of mean total annual discharge), and 849,797 ML (75% of mean total annual discharge) during the wet season (January to May). The remaining 11% of total flow occurs in the period June-July. The deeply incised ranges to the east and west of the floodplain generate the headwaters of a number of streams that form tributaries to the Mulgrave River. Behana Creek is the largest tributary on the western side of the floodplain. A number of smaller streams flow from the ranges to the east, the other more significant streams being Fishery and Figtree Creeks. All the creeks carry significant flows during the wet season, but during the drier months may be reduced to intermittent flows dependent on rainfall events in the upper catchment. River terraces on the floodplain east of Gordonvale form a surface water divide, with the main surface water drainage heading from the divide north to Trinity Inlet, and on the southern side entering into the Mulgrave River and hence into Mutchero Inlet. Mutchero Inlet is at the junction of the Mulgrave River (from the north), and the Russell River (from the south). The clearing of land for sugar cane cultivation has extended to the top of the river banks and has resulted in bank erosion and silting of the river as streams become wider and shallower (Connell Wagner, 1992). Local Landcare groups have undertaken limited revegetation of riverbanks in some areas. Riparian vegetation, however is generally fragmented, impacted by varying historical and ongoing land usages, and discontinuous along the waterways in the study area. Sand and gravel has been extracted from the bed of the Mulgrave River and Behana Creek, resulting in pools several metres deep in areas previously shallow in the dry season.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 7 Groundwater Modelling 2.4 Climate The Mulgrave River catchment is one of the highest rainfall areas of Australia. Long-term average annual (LTA) rainfall on the valley floor ranges from as high as 3800 mm near Mutchero Inlet in the south, to 2000 mm in the area extending from Gordonvale northwards to Cairns and Trinity Inlet (Figure 3). This rainfall is highly seasonal, with distinct wet (January to May) and dry (August to December) seasons. The mean annual evaporation does not vary significantly across the area, with an annual mean of 2251 mm measured at Gordonvale. Cyclones typically develop between January and April and result in heavy rainfall. Four to six tropical cyclones are formed in the Coral Sea each year and an average of two cross the coast in any given year. These systems exert a strong influence on rainfall variability in the region due to their unpredictability. For example, during the 2006 wet season the far north had experienced well above average rainfall records. This was due to continued monsoonal activities through April 2006 with Severe Tropical Cyclone Monica increasing monthly rainfall totals for the study area by greater than 3 times the average. Additionally, Severe Tropical Cyclone Larry crossed the coast near Innisfail on the 20th March 2006. This cyclone was a category 5 and the first of this magnitude to cross the coast since 1918, also at the same geographical location. Conversely, in 2002, Cairns recorded its lowest rainfall since records began in 1882. El Nino and La Nina have a strong impact on the wet tropics system and influence rainfall. Generally the region is getting progressively drier with lower rainfall averages due to the significant increase in El Nino events (Weston and Goosem, 2004). Planning for urban water supplies in dry years has now become a reality for historically deemed wetter areas in Australia. Climate gauging station data are available at Cairns, Gordonvale, and Mt Sophia (Figure 6). Temporally infilled data for the Gordonvale and Mt Sophia gauges obtained from the Queensland Department of Natural Resources, Mines and Water (DNRMW’s) SILO service were used in this modelling study. The LTA rainfall distribution shown in Figure 3 was used to divide the area into two “climate zones” represented by the Mt Sophia and Gordonvale gauge SILO data.
2.4.1 Temperature and Humidity Temperatures are quite uniform throughout the year with typical daytime min/max in mid summer ranging between 23/31oC and 18/26oC mid winter. Occasional cold snaps occur overnight in the winter months but these rarely fall below 14oC in the catchment valley. Relative humidity values are typically high for the region reaching an average of 79% during the months of February to March, and 68-70% during the cooler months, but may reach into the 90’s quite regularly. (BOM website).
2.5 Vegetation and Land Use The hills and mountains bordering the Mulgrave River floodplain are densely vegetated with complex and diverse rainforest communities. In some areas (such as Walsh’s Pyramid), lower rainfall and occasional bushfires have allowed the development of eucalypt woodlands and open forests. The floodplain has been almost completely cleared for sugar cane cultivation and it is only lower-lying areas of poor drainage that may have some remnant vegetation. Sugar cane is also grown on some of the gentle slopes on the margins of the floodplain. Mangroves occur towards the coast at both Trinity Inlet and the lower reaches of the Mulgrave River at Mutchero Inlet
42/15610/98344 Mulgrave River Aquifer Feasibility Study 8 Groundwater Modelling Figure 1 Location Plan
42/15610/98344 Mulgrave River Aquifer Feasibility Study 9 Groundwater Modelling Figure 2 Digital Terrain Model
42/15610/98344 Mulgrave River Aquifer Feasibility Study 10 Groundwater Modelling Figure 3 Long Term Average Annual Rainfall
42/15610/98344 Mulgrave River Aquifer Feasibility Study 11 Groundwater Modelling 2.6 Soils Soils in the Mulgrave River catchment have been surveyed and mapped down to soil series at 1:50,000 scale (Murtha et al., 1996; Figure 4). The mapped soil series have been divided into seven broad groups based on parent material and drainage status. Five of which are the dominant soils in the Mulgrave River catchment - soils of granitic rock origin, metamorphic rock origin, mangrove soils, and well- and poorly- drained soils formed on alluvium (Murtha et al., 1996). Soils of basic rock origin and those formed on beach ridges are also found, although less extensively, in the catchment. Parent material influence is most pronounced on the floodplain margins. This parent material originating from granitic areas soils are coarse textured and dominantly sandy, uniform or gradational textured; while those that develop from metamorphic areas are fine textured and predominantly clayey (Willmott and Stephenson, 1989). On the floodplain, soils range from little-developed uniform textured fine sandy soils on the younger terraces and levees to strongly structured uniform or gradational soils on well drained alluvium and clays on poorly drained areas (Willmott and Stephenson, 1989). In this modelling study, hydraulic properties of each major horizon were calculated using data presented in Murtha et al., 1989. This is described in detail in Section 3.
2.6.1 Acid Sulfate Soils Acid sulfate soils are predominantly soils associated with areas of Quaternary alluvium with high levels of organic matter and sulphidic material present. In the majority of cases (though not all) these areas are typically to be found in swampy and tidally influenced areas (including mangroves). Acid and toxic concentrations of metals can be released into the environment when acid sulfate soils are exposed to air and become oxidised. The Queensland Department of Natural Resources and Water and Queensland Acid Sulfate Soil Investigation Team (QASSIT) have mapped the likely occurrence of potential acid soils (PASS) throughout coastal Queensland and have mapped the lower section of the Mulgrave River as being PASS. These soils are in areas dominated by mangrove and melaleuca wetlands, and in most instances are tidally influenced. Acid sulfate problems exist for some farmers about Mutchero Inlet where vegetation clearing and ground tilling (for sugar cane) has resulted in the generation of actual acid sulfate conditions and subsequent loss in agricultural productivity. The general areas mapped as PASS by DNRW and QASSIT approximate the extent of mangrove intertidal saline soils. There are two distinct conditions associated with acid sulfate soils in the Mulgrave River area. The first condition is that most typically found in Cairns area. That is, acid sulfate soils are generally found below 5 m Australian Height Datum (AHD) and in Holocene sediments (organic-rich sediments and silts). They are usually associated with coastal lowlands and estuarine flood plains and contain pyrites and sulfides. Typically the areas around (and within) any of the mangrove/tidally influenced areas (lower reaches of the Mulgrave River) should be considered to have acid sulfate soils. Under natural conditions acid sulfate soils are usually located below the watertable. When these low-lying areas are exposed through dewatering, excavated or drained, there is potential for the acceleration of soil oxidation (of the pyrites) and subsequent acid leachate generation. The second condition is that of naturally occurring acid soil conditions, where the acidity is generated as a result of organic acids. It should be noted that organic acids (e.g. humic/tannic acids generated in
42/15610/98344 Mulgrave River Aquifer Feasibility Study 12 Groundwater Modelling Melaleuca swamps) are a common feature of tropical coastal ecosystems. These organic acids can produce acid water and sediments with the pH of these usually around 4.5 - 5.5. These sediments do not have the ability to generate additional acid when exposed to air and therefore do not pose the same risk as Potential or Actual Acid Sulfate Soils (PASS and ASS generated from exposure of acid sulfate soils). The lower reaches of the Mulgrave River are in a coastal environment with expansive areas of Melaleuca swamps capable of leaching organic acids with subsequent acid soil conditions. Cane farming in the lower Russell/Mulgrave River reaches, in the area about Mutchero Inlet, has exposed both types of acid sulfate soil conditions. Extensive areas of tea tree swamp has been cleared for farming, and subsequent excavation for drainage works has exposed acid sulfate soils with the consequence of oxidation of PASS and generation of acid runoff which has adversely affected productivity in these areas.
2.7 Geology A detailed description of catchment geology is provided in the Hydrogeological Report, which was prepared by GHD in 2006. The hydrogeological review of the Mulgrave River area identified the Quaternary alluvium as having the highest potential for groundwater development (Table 1). Other aquifers have significantly lower potential yield.
Table 1 Mulgrave River Groundwater Resource Potential
Unit Aquifer Type Groundwater Resource Potential
Quaternary Alluvium Porous media High
Atherton Basalt Fractured rock Low
Tertiary Alluvium Porous media Moderate?
Basement (granite / metamorphics) Fractured rock Low
Quaternary Alluvium is the most widespread unit in the valley and a review of drilling records indicates the generally sandy nature of the area, which provides the best potential for groundwater yield. Groundwater analyses also indicated that water in the Quaternary Alluvium is of generally potable quality.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 13 Groundwater Modelling Figure 4 Soils
42/15610/98344 Mulgrave River Aquifer Feasibility Study 14 Groundwater Modelling Figure 5 Geology
42/15610/98344 Mulgrave River Aquifer Feasibility Study 15 Groundwater Modelling Figure 5 (cont): Geologic Cross Section of the Mulgrave River Valley
2.8 Surface Water The Mulgrave River is one of the major coastal rivers in North Queensland. Covering an area of approximately 810 km2 and with a mean annual discharge of 1,136,165 ML (up-scaled from Peets Bridge gauge). The Mulgrave River Catchment is one of the areas of highest mean annual runoff of any Australian catchment. The headwaters are in the ranges to the west of the coastal floodplain, which the river enters at Gordonvale, meandering easterly then southerly across the floodplain to Mutchero Inlet (refer to for drainage features). River flows are highly seasonal; mean total flow in the dry season is around 161,000 ML (or ~14% of mean total annual discharge), and 849797 ML (75% of mean total annual discharge). The remaining 11% of total flow occurs in the period June to July. These reported flows are up-scaled from Peets Bridge gauge, based upon the total catchment area versus gauged area. The deeply incised ranges to the east and west of the floodplain generate the headwaters of a number of streams that form tributaries to the Mulgrave River. Behana Creek is the largest tributary on the western side of the floodplain. A number of smaller streams flow from the ranges to the east. All the creeks carry significant flows during the wet season. River terraces on the floodplain east of Gordonvale form a surface water divide separating northward and southward drainage across the valley floor. Several surface water flow gauges have been or are actively monitored in the catchment (GHD, 2006). Review and analysis of these data has revealed that they are of variable length and quality. This review is documented in Section 3.8.
2.8.1 Licensed Usage Surface water allocation data for the Mulgrave River catchment were obtained from DNRMW. Licensed surface water abstraction totals around 37,000 ML/year by volume (to 14 licenses), plus an additional 927 ha licensed by irrigated area (to 55 licenses). Assuming an irrigation application of 9 ML/ha/month over a 5-month dry season (Steve Bertocchi, DNRMW South Johnstone, pers. comm.), this equates to a further 42,000 ML/year allocated volume. Therefore, the total surface allocation is 79,000 ML/year.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 16 Groundwater Modelling Around 20% of the total allocation by volume (16,000 ML/year) is licensed to Cairns Water for their Behana Creek off take, and a further 25% (~20,000 ML/year) is allocated to the Mulgrave Mill at Gordonvale, although this is largely non-consumptive (i.e. much of the abstracted water is returned to the river). Furthermore, local knowledge suggests that cane is not irrigated in many years in this area, and is most likely only required, on average, a couple of months of the dry season per year. Therefore the consumptive surface water abstraction in most years is likely to be less than 50% of the total licensed volume.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 17 Groundwater Modelling Figure 6 Hydrologic Monitoring
42/15610/98344 Mulgrave River Aquifer Feasibility Study 18 Groundwater Modelling 3. Hydrogeology
The following section provides a summary of the hydrogeological investigation and conceptualisation detailed in previous report (GHD, 2006). This is to further develop the conceptual hydrogeological model, upon which the numerical model is based.
3.1 Field Investigations Potential sites for detailed field investigation and testing were chosen based on aquifer lithology and groundwater recharge. The potential sites are also chosen based on the following area characteristics.
Sandy vertical profile where seasonal recharge potential from rainfall appears significant; Where runoff from ranges or infiltration from headwaters of smaller streams occurs; With significant aquifer thickness at depth to maximise bore yields; Away from the low permeability Atherton Basalt; and Within the valley where a thick section of aquifer appears to be overlain by semi-confining or confining units that are likely to limit surface water – groundwater interaction. Investigation drilling was completed at four sites, where two sites (Area 2 and Area 3) have pump bores for extended aquifer tests. The extended aquifer tests are conducted to determine the sites’ response to pumping and for the assessment of its parameters.
Table 2 Investigation Sites
Location Depth Drilled (m) Outcome
Area 1 115 Atherton Basalt intersected; no significant sands beneath basalt; bore not constructed
Area 2 94.5 Pump test site; aquifer 31 to 49 m; 3 observation bores; pumped at 40 L/s. Aquifer thickness tested is estimated at ~20 m, with ~10 m observed drawdown at the abstraction bore, and ~2 m in nearby observation bores.
Area 3 84 Pump test site; aquifer 41 to 64 m; 3 observation bores; pumped at 30 L/s. Aquifer thickness tested is estimated at ~20 m, with ~14 m observed drawdown at the abstraction bore, and ~1 to 3 m in nearby observation bores.
Area 4 97 Relatively clay rich sequence compared to Area 2 and 3; constructed as observation bore
42/15610/98344 Mulgrave River Aquifer Feasibility Study 19 Groundwater Modelling 42/15610/98344 Mulgrave River Aquifer Feasibility Study 20 Groundwater Modelling 3.2 Inferred Hydrostratigraphy The stratigraphy and variability in lithology across the valley was confirmed based on the results from the geophysical logging of 21 existing government and private bores and the drilling of four bores. An inventory of the geophysical logging undertaken is presented in Appendix A. The results indicated that the valley sediments could be broadly grouped into two layers, separated by a clay or sandy clay section corresponding with the base of Atherton Basalt. This was only clear in the geophysical logs, where a significant change in sedimentation was identified across most bores at depth. The two layers are referred to as Layer 1 and Layer 2 throughout the remainder of this report, where Layer 1 overlies Layer 2. In general, Layer 2 is more clay-rich than Layer 1. In some deeper sections of the valley, Tertiary Alluvium was identified in Layer 2 while Quaternary Alluvium comprises Layer 1. Layer 2 may be thin or absent at the valley margins. The Area 2 pump bore was constructed in Layer 1 while Area 3 was in Layer 2. Each was constructed to abstract from the sandiest portion of the profile at every site. Geological surfaces were constructed for the groundwater model, representing the top of Layer 2 and the top of the underlying bedrock (or base of Layer 2). The depth from ground surface to bedrock is presented in Figure 7 and the top of Layer 2 is presented in Figure 8. It should be noted that data defining the top of Layer 2 was limited largely to the geophysical logging undertaken on four new bores, and on 21 existing government and private bores. This was due to this stratigraphic horizon being perceived only in geophysical logs. It was not clearly discernable in the geological logs. In areas of sparse data, this geological surface was inferred utilising the better-defined bedrock surface, and spatial trends in the top of Layer 2 in areas with available data. The construction of the base of Layer 2 (or basement top) was largely based on earlier mapping by DNRMW (Dept. Natural Resources, QLD 1999). Although this was checked and adjusted in places using the latest available data:
A total of 129 bores were identified in the Queensland bore database with logs identifiying intersection of the bedrock surface (Figure 6); and The four bores drilled during this investigation. The largest area of uncertainty in the construction of this surface is towards Cairns and Trinity Inlet, where the alluvial valley widens significantly and sparse data present intersects the bedrock. The Mulgrave Alluvium is mapped as being 100-200 m thick on average along the deepest line of sediments, thinning to the edges of the alluvium along the east and west valley sides.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 21 Groundwater Modelling Figure 7 Geological Surface Depth of Bedrock
42/15610/98344 Mulgrave River Aquifer Feasibility Study 22 Groundwater Modelling Figure 8 Geological Surface – Top of Model Layer 2
42/15610/98344 Mulgrave River Aquifer Feasibility Study 23 Groundwater Modelling 3.3 Aquifer Tests and Hydraulic Properties Analysis of 100-hour aquifer tests conducted in test Areas 2 and 3 indicated transmissivity and hydraulic conductivity values as shown in Table 3. These were used as initial values in the groundwater model, and the detailed data recorded in observation bores during the tests were used to calibrate the groundwater model (Section 5). The test analysis suggests that the aquifer is a leaky, semi-confined system in both areas. Further details on these tests and analysis can be found in the previous Hydrogeological Report (GHD, 2006). Slug testing was also completed on 18 existing bores as part of this investigation. However, the bores are constructed with gravel pack backfilled almost to land surface. This gives slug test data representative of the gravel pack permeability rather than that of the aquifer. The slug test data were therefore disregarded in the modelling.
Table 3 Aquifer Test Results
Area Transmissivity Hydraulic Conductivity (m/d) Storage Coefficient (m2/day)
2 700 35 2.3 x 10-3
3 400 18 3.3 x 10-4
3.4 Groundwater Levels DNRW maintains and regularly monitors a network of groundwater observation bores in the Mulgrave Aquifer (Figure 6). Groundwater levels in the active monitoring bores are measured on a monthly to quarterly frequency. Various statistics relating to the groundwater monitoring are presented in Figure 9. These were assessed in conjunction with climatic data to determine the best monitoring data to be used. The montiroing data will be used to create representative groundwater level contour maps of the area. These were also assessed to determine the most suitable period against which to calibrate the mode. The resulting groundwater level contour maps are presented in Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 24 Groundwater Modelling Figure 9 Groundwater Level Hydrographs, 1973 –2004
42/15610/98344 Mulgrave River Aquifer Feasibility Study 25 Groundwater Modelling Figure 10 Average June to July Groundwater Levels
42/15610/98344 Mulgrave River Aquifer Feasibility Study 26 Groundwater Modelling Figure 11 Average Wet Season Groundwater Levels
42/15610/98344 Mulgrave River Aquifer Feasibility Study 27 Groundwater Modelling Figure 12 Average Dry Season Groundwater Levels
42/15610/98344 Mulgrave River Aquifer Feasibility Study 28 Groundwater Modelling Figure 13 Average Annual Groundwater Level Fluctuation
42/15610/98344 Mulgrave River Aquifer Feasibility Study 29 Groundwater Modelling Figure 14 Average June to July Depth to Water Table
42/15610/98344 Mulgrave River Aquifer Feasibility Study 30 Groundwater Modelling The contour maps show groundwater flow from the elevated areas of the west, particularly the upper Mulgrave River and west of Edmonton, to the northeast and southeast. A groundwater flow divide is evident in the main irrigation and abstraction area, northeast of Gordonvale. Inspection of average annual groundwater level fluctuations suggests that this is an area of relatively high groundwater recharge (Figure 13). The high groundwater recharge is due to its location in the north and west of Gordonvale, on the flanks of the alluvium. The fluctuations in the main abstraction area are related to increased storage (via seasonal drawdown) and capacity of the aquifer to accept more recharge than areas distal to major abstractions. The fluctuations on the western flanks of the alluvium are due to the coarser, better-drained nature of the soils in this area, and therefore relatively high groundwater recharge. The high heads also observed on the western alluvial flanks are a result of a combination of:
Lower transmissivity, due to thinning of the aquifer along the western alluvial edges, bedrock topography is much steeper beneath the alluvium on the eastern boundaries (Figure 15); and
Higher recharge on the (higher permeability) soils along parts of the western edge of the alluvium, mainly Soils of Metamorphic Rock Origin, and Well Drained Soils Formed on Alluvium (Figure 4). The average groundwater depth below the land surface is presented in Figure 14. It shows that the aquifer is on average near saturation (groundwater generally less than 5 m below ground) across most of its area. The only exceptions to this are typically around the valley margins where the sediments thin out and lap up onto the elevated areas of underlying bedrock. Groundwater levels are, on average, at or above ground surface across large areas of the aquifer in the north and south of the catchment, it is also in the area where Behana Creek crosses onto the alluvium from the elevated bedrock areas. This suggests that Behana Creek may be a potential source of groundwater recharge or that the surrounding area is a location of enhanced recharge. It also suggests that the aquifer in this vicinity is typically recharged to its full capacity. Further discussion on this is presented in Section 3.8. The areas in the north and south of the valley, where groundwater levels are also at or above ground surface, indicate that these are sections of groundwater discharge. The majority of rainfall falling onto these areas is likely to be shed as runoff, because the aquifer is typically saturated to its capacity. The average June-July groundwater level map (Figure 10) will be used to calibrate the steady state groundwater flow model. This period was selected because it straddles the wet - and dry seasons, and it represents the average annual aquifer condition. Hydrographs (Appendix B) show that groundwater levels generally fluctuate seasonally by between 1 m and 7 m. Appendix B presents a selection of bores with hydrographs of sufficient temporal resolutions. These bores provide the most appropriate basis against which to calibrate the groundwater model. Figure 13 depicts the wet season to dry season difference in interpolated groundwater levels, and shows annual fluctuation up to 4 m over much of the aquifer. It should be noted that the wet - and dry season groundwater level maps were constructed using point data available at different times and locations. Therefore, some of the mapped fluctuations may be a source of data insufficiency in one season versus the other in some locations, rather than the true average annual fluctuation. Lower hydrograph fluctuations are observed in bores close to surface water bodies, and greatest fluctuation is observed in bores on the more elevated flanks of the alluvium, particularly in the west.
3.4.1 Inferences on River-Aquifer Connection It is evident in all the constructed maps that Mulgrave River is the preferential path for the majority of groundwater discharge in this catchment. The maps clearly show that the river controls groundwater
42/15610/98344 Mulgrave River Aquifer Feasibility Study 31 Groundwater Modelling levels over large areas, acting as a drain to the Mulgrave Alluvium. Therefore it is considered that the aquifer and river are in direct hydraulic connection in this catchment, and that the river is largely a gaining feature (i.e. is fed by baseflow from the aquifer much of the time, over most of the catchment). Further discussion is presented in Section 3.8. Groundwater levels appear to become increasingly controlled by surface water features (Trinity Inlet and the Mulgrave River) towards the northern and southern catchment outlets. In addition, it is generally higher than surface water stage heights throughout the catchment. Furthermore, the constructed groundwater level maps, particularly for the average wet season, show a strong influence (lowered groundwater levels) of surface water features over large areas. The depth to groundwater map also supports this conclusion (Figure 14). Review of bore hydrograph data at locations close, but up-catchment of river flow gauges provide further evidence that the river is a gaining feature. Comparison of average groundwater levels at bore 11100075 (14.6 mAHD), which is the closest bore with sufficient data located up-catchment of the Gordonvale gauge, shows that groundwater levels are on average 6.7 m. This is higher than the average gauged river height at Gordonvale. This bore is located only 1 km from the gauge, which suggests high hydraulic groundwater gradients (0.007 m/m) to the river from the aquifer upstream of Gordonvale. Assuming an average 20 m thickness of aquifer contributing baseflow to the river, and an average aquifer hydraulic conductivity of 26.5 m/day (from the aquifer tests), the estimated baseflow to the river is 7 ML/day per kilometre of river length (accounting for baseflow from either side of the river). If the same assumptions will be applied to the whole rivers and streams crossing Mulgrave Alluvium, a rough estimate of total baseflow from the main body of the alluvium is 497 ML/day, or 181456 ML/year. This will be used in the conceptual water balance presented in Section 3.9. The proposed abstraction scenarios are for 15 ML/day (3% of the groundwater baseflow in Stage 1) and up to 40 ML/day (8% of the groundwater baseflow in Stage 2).
3.5 Fresh/Saline Groundwater Interfaces The location of the fresh/saline groundwater interfaces expected to exist at Trinity Inlet and Mutchero Inlet are presented in the groundwater level maps (Figure 10, and Figure 12). Groundwater salinity data in DNRMW’s Groundwater Bore Database was used to define these boundaries. Sufficient data were available to delineate the Trinity Inlet boundary. However, there was no real indication of the presence of a fresh/saline interface in the data for Mutchero Inlet from the available monitoring bore data. This suggests that the interface does not extend a significant distance into the Mulgrave Alluvium from the coast. For Mutchero Inlet, a bore approximately 4 km north of Deeral (11100054) shows a single elevated reading of 950 µS/cm electrical conductivity (EC) in 1977, whilst all other readings in this bore are well below 100 µS/cm. EC measurements in other bores further to the south, nearby, and further north show similar low readings. This suggests that the fresh/saline interface (for groundwater, as opposed to surface water) at Mutchero Inlet may not extend far at all up the Mulgrave Alluvium from the inlet, and may exist very close to the coast, near to the mouth of the Mulgrave River with Mutchero Inlet. For the purposes of impact assessment, bore 11100054 will be used conservatively to delineate the location of the saline/fresh groundwater (as opposed to the surface water) nterface at Mutchero Inlet. Delineation of the interface at Trinity Inlet was made much easier with several bores with recorded EC of around 20000-40000 µS/cm. Other bores have readings exceeding 1000 µS/cm, whilst most bores in the
42/15610/98344 Mulgrave River Aquifer Feasibility Study 32 Groundwater Modelling Mulgrave alluvium show recorded EC of around 50-200 µS/cm. The ground interface at Trinity Inlet corresponds closely with the extent of the mangrove flats.
3.6 Groundwater Usage Leach and Rose in 1979 recorded 17 irrigation bores in the area with a total allocation of 5,707 ML per annum. Actual usage in the irrigation season between August and November was thought to be less than 1,710 ML per year, or 30% of allocations. Application rates were thought to comprise three 75 mm applications during the irrigation season. In 1998, DNR reported total groundwater allocation of about 10,500 ML per annum, with the majority in the northern section of the valley. A search of the bores in the declared management area indicated that there are currently approximately 90 bores licensed for groundwater extraction that have a total licensed volume of 22,000 ML/year. This includes the Cairns Water application that has a total volume of 15,000 ML/year. These bores are shown in Figure 15 and Figure 16. The remaining 7,000 ML/year is licensed to private irrigators and smaller town water supplies. The average licence is in the order of 85 ML/year. Despite significant increase in allocated groundwater since 1970s, there has been no observed long-term impact on groundwater levels. This suggests that either actual usage has not increased over the years or that current long-term average abstractions remain less than long term average recharge. Furthermore, seasonal groundwater level fluctuations suggest that there is excess recharge available (i.e. insufficient aquifer storage to accept the recharge) in this catchment. Therefore there is a potential for greater amount of groundwater abstraction. This would also explain the lack of long-term observed abstraction impacts on groundwater levels.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 33 Groundwater Modelling Figure 15 Existing Groundwater Users – Primary Bore Purpose
42/15610/98344 Mulgrave River Aquifer Feasibility Study 34 Groundwater Modelling Figure 16 Existing Groundwater Users – License Allocation
42/15610/98344 Mulgrave River Aquifer Feasibility Study 35 Groundwater Modelling 3.7 Groundwater Recharge Analysis of bore hydrographs indicates that groundwater levels respond rapidly to seasonal rainfall and the annual rainfall is in excess of the aquifer storage capacity (GHD, 2006). Groundwater levels appear to respond more to seasonal (and hence annual) rainfall than the long-term rainfall trend. This suggests that seasonal rainfall exceeds the recharge capacity of the aquifer. After an initial saturation of the soil profile, additional rainfall in any single rainfall event in excess of the saturated infiltration rate cannot be accepted and becomes runoff. Particularly dry years, where rainfall is significantly below annual average, have a much greater impact on groundwater levels than particularly wet years that exceed recharge capacity. This inference is clear in:
The hydrographs in Appendix B, for the years 2002-2003 (around time 2350 on the hydrographs’ x- axes). In these hydrographs, water levels drop only slightly lower than their “base” level, but the recharge peaks are well below the typical peak level. In contrast, there are no extremely wet years (eg. 1999-2001, or time 1000-1700 on the hydrographs’ x-axes), in which the observed peak groundwater levels are considerably higher than the surrounding years. The long-term hydrograph trends remain largely flat over the period of record, with the exception of recharge peaks, and subsequent declines over the dry season, back to their “base” level; and
The observed depth to groundwater statistics presented in Figure 9, in which the minimum depth to groundwater is rarely more than 1 m below ground, and is typically at ground surface. In addition, the average depth to groundwater is rarely more than 5 m below ground surface. Given that the aquifer is generally 50 m to more than 100 m thick (Figure 7), this, in combination with water levels generally remains within 5 m of land surface. This suggests that across the entire aquifer, for both wet and dry seasons, it remains close to saturated. This is also clear in the depth to groundwater map presented in Figure 14. Following a dry period, recovery in groundwater levels occurs quickly when seasonal rainfall returns to average or better conditions. Investigations suggest that recharge commences within hours of rainfall onset and may continue for a week after rainfall events (Leach and Rose, 1979). It also appears, from a comparison of bore hydrographs and rainfall records, that recharge capacity was met by a daily rainfall of 100 mm, with excess rainfall becoming runoff (Leach and Rose, 1979). This is substantiated by the hydrograph and rainfall analysis presented by GHD (2006), and by the modelling reported in Section 5.3. Furthermore, the groundwater level analysis and discussion of aquifer saturation presented in Section 3.4 also supports this inference. An analysis of potential recharge based on the hydrograph fluctuation method, indicates an estimated average total annual gross recharge to the groundwater system of around 200 mm, within a range of 100 and 400 mm. The hydrograph fluctuation method converts the annual variation in groundwater level to a recharge volume based on the storability of the aquifer system and the area of the aquifer. The analysis further indicates that between 10% and 30% of this gross recharge is lost to groundwater discharge from the aquifer, via processes including river baseflow, evapotranspiration, and groundwater abstraction. Analysis identifies that there is sufficient annual recharge potential to allow recovery in groundwater levels following increased groundwater abstraction. However, in successive dry years, increased abstraction may have an observable impact on groundwater levels.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 36 Groundwater Modelling It is considered that runoff recharge onto the alluvium from the bedrock outcrop flanking the valley is likely to be an insignificant component of aquifer recharge. This is because of the greater potential available annual recharge than can be accommodated by the aquifer. However, runoff recharge may be important in occasional heavy rainfall events in dry periods.
3.8 Baseflow Analysis Surface water flow and salinity data for the Mulgrave River and tributaries were collated and analysed in an effort to develop the understanding of groundwater / surface water interactions in the catchment. It is also to provide calibration constraints to the groundwater model. A discussion of the data sources and quality, analysis methods and interpretation of results is included in Sections 3.8.1, 3.8.2, and 3.8.3 below.
3.8.1 Data Sources and Quality Mean daily flow and level data for the Mulgrave River (Fisheries, Peets Bridge and Gordonvale gauges) and Behana Creek at Aloomba were obtained from DNRMW. A gauge at Whites Falls on Behana Creek has only a short record of level only; there are no flow data for this gauge. Daily records of stream conductivity (EC) were also obtained for the Peets Bridge gauge. The only gauges in the catchment with a useable record are Fisheries, Peets Bridge and Gordonvale gauges on the Mulgrave River, and Aloomba gauge on Behana Creek. However, even some of these records are very limited. Discussions with DNRMW hydrographer, Alan Hooper, suggested that the Gordonvale gauge data are questionable This is primarily because during periods of low flows the gauge (which is in the Mulgrave Mill pump house) is bypassed. Low flows are therefore likely to be under-estimated by the Gordonvale gauge. The best quality data are from the Peets Bridge gauge, which also has daily records of stream EC. Although this gauge is located in the upper (thin, narrow) reaches of the Mulgrave River alluvial aquifer, it does provide some insight into groundwater/surface water interactions in the area. This gauge was the focus of the majority of the baseflow analysis. It must be reinforced that the use of the data from a limited area of the aquifer infers a conservative stance to the ground water model. That is, the flow data at Peets Bridge will reflect a lower volume of water than would a gauging station, for example, record in the reach of the Mulgrave River at Aloomba. In order to counter some of the inherent conservatism in the model, an allowance has been made in the model for additional flow into the aquifer below the upper limits. The mechanism of this allowance is described in the following sections.
3.8.2 Methods The Conductivity Mass Balance (CMB) (Stewart et al., 2007) and United States Geological Survey (USGS) local minima (Sloto and Crouse, 1996) approaches were used to estimate the amount of baseflow for each gauged data set. The CMB method utilises the relationship between stream flow and stream electrical conductivity (EC; roughly analogous to salinity) to define the proportion of baseflow within the total stream flow. It requires estimates of stream runoff EC and stream baseflow EC. These data were estimated from the gauged
42/15610/98344 Mulgrave River Aquifer Feasibility Study 37 Groundwater Modelling data at 55 uS/cm and 27 uS/cm, respectively. The USGS methods are less physically based, and are simple digital filters. The CMB method was used to separate out baseflow for the Peets Bridge gauge, and the USGS methods were used on the remaining gauges (where no stream salinity records were available). The CMB analysis on the Peets Bridge gauge was used to calibrate the USGS method, the calibration parameters for which were then applied to the remaining gauges. The use of the CMB method was justified for this catchment given the high degree of correlation between stream flow and stream EC, and reasonable satisfaction of the assumptions inherent in the method (refer to Stewart et al., 2007). It should be noted that river flow data were not naturalised due to lack of information on actual surface water abstraction and discharge. For the river upstream of Peets Bridge gauge, from license allocations, it is estimated that a maximum of 98 ML annual dry season surface water abstraction occurs. Most of which is abstracted between Peets Bridge and the Fisheries. Lack of accounting for these abstractions produces an underestimate of baseflow to the river, whilst lack of accounting for discharges to the river results in an overestimate of baseflow. This should be noted when reviewing the baseflow analysis presented below the analysis presented provides an underestimate of baseflow.
Figure 17 Measured Flow and Salinity Relationships
42/15610/98344 Mulgrave River Aquifer Feasibility Study 38 Groundwater Modelling 3.8.3 Results and Discussion Time series records of baseflow for each of the gauging stations were successfully generated. Table 4 summarises the results of the surface water analysis undertaken. The baseflow estimates are derived from the CMB analysis undertaken, and runoff was calculated as the difference between total flow and estimated baseflow. The baseflow index (BFI) was calculated as the proportion of total flow that was estimated to be baseflow. Catchment yield is calculated as the total flow divided by the catchment area upstream of the gauge. The reported average baseflow gain or loss is calculated by subtracting the calculated baseflow of a downstream gauge from that of the next upstream gauge. The main conclusions from this are:
Baseflow into the Mulgrave River on the upper, thin and less extensive reaches of the alluvial aquifer is estimated to be around 50% (~1000 ML/d) of the total flow, as estimated from the Peets Bridge gauge. Data from the Fisheries gauge upstream suggests that as much as 25% of this (~280 ML/d) is derived from the alluvial aquifer. Behana Creek is likely to be a sporadic recharge feature at least upstream of the Aloomba gauge, whereby runoff flowing into this creek during the wet season recharges the alluvial aquifer via leakage from the creek. This is supported by the on-average saturated aquifer condition in this area (refer to Section 3.4). As a result, there is no, or very limited, perennial baseflow in this creek. However, during times of high flow and high groundwater levels, 50% (or ~174 ML/d) of total flow appears to be baseflow. The lack of gauged and questionable quality of data downstream of the Mulgrave Mill at Gordonvale prevents any further inferences being made with regard to surface water / groundwater interaction on the main alluvial deposits of the valley. As suggested by the data, it is possible that the river “loses” water to the aquifer on average, it is unlikely given the high driving groundwater heads up catchment and apparent storage in most years. This is due to the high rainfall and recharge rate. In this situation, the river would be expected to be, on average, a “gaining” stream on the main alluvial deposits. Further analysis of river-groundwater interaction is presented in the numerical modelling undertaken and documented later in this report. This includes model calibration to groundwater level responses in government observation bores located close to rivers and streams in the catchment (refer to Section 5) and detailed model calibration to the aquifer tests undertaken as part of this study. Specifically, on this latter aspect, the Area 3 aquifer test monitored the groundwater response to pumping in a shallow bore located close to the Mulgrave River. As expected, this bore (A3Ob2; Appendix C) responded in a restricted fashion, due the adjacent Mulgrave River acting as a recharge boundary under the stress of aquifer testing. The model was successfully calibrated to the drawdown response in this bore.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 39 Groundwater Modelling Table 4 Surface Water Analysis Summary
Gauge Name Upstream Baseflow Catchment Mean Mean Mean Average Comments (Ordered Area Index Yield Runoff Base Base flow Gain or Upstream to (km2) (BFI) (mm/year) (ML/d) flow (mm/year) Loss Downstream) (ML/d) (ML/d)
111005 Mulgrave River 357 0.5 1560 805 721 737 n/a Suggests reasonable level of A at The Fisheries groundwater discharge from the bedrock aquifers into the river.
111007 Mulgrave River 520 0.5 1528 1174 1003 704 282 Suggests river gains from the Fisheries A at Peets Bridge gauge to Peets Bridge (mainly on bedrock, or thin alluvials), which possibly suggests a small amount groundwater storage and discharge in the thin alluvial deposits high up in the catchment.
111001 Mulgrave River 552 0.4 1323 1210 791 523 -213 Possibly suggests river loses from Peets A at Gordonvale Bridge to Gordonvale, however Gordonvale gauge thought to underestimate low flows, so this is probably an underestimate of baseflow - groundwater/surface water interaction on the main alluvial valley unable to be accurately quantified with the available data
111003 Behana Creek 86 0.4 2054 310 174 738 n/a Sporadic flows (regularly nil flow), C at Aloomba suggesting runoff-dominated water body, which looses much of its flow to the underlying alluvial aquifer. The very low relative catchment yield supports this notion.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 40 Groundwater Modelling 3.9 Preliminary Catchment Water Balance A conceptual water balance has been constructed for the Mulgrave River Alluvium. The purpose of this is to provide an initial (pre-numerical modelling) assessment of the relative proportions of the various water balance components. The numerically modelled water balance can be compared to this conceptual balance. This can provide a framework against which the hydrogeological conceptualisation can be further assessed in light of information gleaned from the numerical modelling. The conceptual water balance is summarised in Table 5 while the inputs and assumptions are detailed in Appendix D.
Table 5 Preliminary Water Balance – Mulgrave River Alluvium
IN (ML/year) OUT (ML/year) OUT (as % of IN)
Recharge 151574 - -
Inflow from up catchment areas 12606 - -
Stream leakage into aquifer 35533 - -
Groundwater Abstractions - 2100 1%
Baseflow - 181456 91%
Groundwater discharge to ocean - 11098 6%
TOTAL 199713 194655 97%
BALANCE DISCREPANCY (IN- 5059 ML/year (3% of estimated recharge) OUT):
The water balance for the alluvium stabilises quite well, despite being based upon simplifying assumptions. The key piece of information obtained from constructing this water balance is the inference that river baseflow is the primary path for groundwater discharge in the catchment. This is common in areas of high rainfall with highly permeable aquifers, such as the Mulgrave River catchment, and suggests that the river and aquifer are in strong connection with one another. The baseflow component of the water balance was initially extrapolated down-catchment from gauged river baseflow yield per unit up-catchment area at Peets Bridge gauge. However, in comparison with the recharge estimates derived from the data analysis, it was found that the baseflow estimated in this manner was too high or that the estimated recharge was too low. It was decided that the baseflow estimate was more likely the source of error. This is due to the extrapolation of baseflow data from up- catchment areas to down-catchment areas on the alluvium, whereas the recharge was estimated from monitoring data on the alluvial aquifer. Baseflow to the river upstream of Peets Bridge is driven by far larger hydraulic gradients than it is on the lower, topographically flatter areas on the main body of the Mulgrave Alluvium. Furthermore, much of the catchment upstream of Peets Bridge is of differing geology and receives much higher rainfall than the lower parts of the catchment (i.e., the alluvium). As a result, the baseflow
42/15610/98344 Mulgrave River Aquifer Feasibility Study 41 Groundwater Modelling estimate was re-visited using a different approach as detailed in Section 3.4.1 and Appendix D, which provided a far more reasonable estimate and smaller water balance error. This conceptual water balance is compared with the water balance derived from the numerical modelling in Section 5.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 42 Groundwater Modelling 4. Model Construction
4.1 Code Selection The selected code with which the numerical modelling was completed is MODFLOW 2000 (Harbaugh et al., 2000). MODFLOW is a finite difference saturated groundwater flow model that has been comprehensively tested, widely utilised and accepted, and is freely available and well documented. Groundwater Vistas was used as the graphical user interface for most of the model construction.
4.2 Model Datasets and Extent The model has been constructed to extend along the entire Mulgrave River valley between Trinity Bay and the outlet of the river at Mutchero Inlet. The extent of the Mulgrave Alluvium mapped in the 1:100000 scale geological map sheets was taken as the extent of the model (Figure 18). The model grid was separated into 150 m square cells and rotated 25q to the west (from north), which parallels the line of the Mulgrave valley and the predominant groundwater flow directions. The model grid does not entirely cover the mapped area of Mulgrave Alluvium, particularly the sediments extending up the Little Mulgrave River. This is due to a lack of data and because the sediments in these areas are often narrower than the model grid cell size. Their thickness and areal extent are severely limited compared to the extent and thickness of the remaining alluvial deposits. Consequently, this slightly limited grid extent will have no significant impacts on the modelled water balance or estimation of sustainable aquifer yield. A second, refined model was extracted from the larger (regional) model for detailed calibration to the aquifer tests undertaken at Areas 2 and 3 outlined in Section 3.1 (Figure 19). This refined model grid was separated into 10 m grid cells in the vicinity of the aquifer tests. Temporally, the regional model is separated into monthly stress periods, each subdivided into ten time steps. The regional model spans the period from June 1996 to January 2007, with 128 stress periods. The refined aquifer test calibration model spans the whole of January 2007. It is separated into variable length stress periods depending on the groundwater abstraction regime, each subdivided into up to ten time steps. Datasets from previous investigations, current study results and long term groundwater monitoring were used in model construction, and include: Digital Terrain Model, sourced from the United States Geological Survey’s Shuttle Radar Topography Mission (Figure 2)
Hydrologic monitoring (Figure 6) Geology () Soils (Figure 4) Depth to basement (Figure 7) Top of Layer 2 (i.e. division of the Mulgrave Alluvium; Figure 8)
Observed groundwater levels (Figure 10, Figure 11, Figure 12 and Figure 14).
42/15610/98344 Mulgrave River Aquifer Feasibility Study 43 Groundwater Modelling The extent of the regional model is shown in Figure 18, indicating the extent of Layers 1 and 2. Two layers were used due to the apparent variability in aquifer parameters with depth as indicated from the geophysical logging and recent investigation drilling. Layer 1 comprises the upper section of the Quaternary Alluvium and the Atherton Basalt north of the river. Layer 2 (Figure 8) consists of the lower section of the Quaternary Alluvium and the Tertiary Alluvium in the deeper sections of the valley. The extent of the aquifer test calibration model is presented in Figure 19. No structural changes (from the regional model) were made to this model, aside from assignation of General Head boundary conditions to the north and south to allow groundwater flux into and out of the model.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 44 Groundwater Modelling Figure 18 Groundwater Model Grid and Boundaries
42/15610/98344 Mulgrave River Aquifer Feasibility Study 45 Groundwater Modelling Figure 19 Aquifer Test Calibration Grid
42/15610/98344 Mulgrave River Aquifer Feasibility Study 46 Groundwater Modelling 4.3 Boundary Conditions At the northern (Trinity Inlet) and southern end (Mutchero Inlet) of the valley MODFLOW General Head Boundaries (GHBs) were applied at mean sea level. GHBs allow groundwater flow into or out of the model dependent on the modelled head in the aquifer, the head assigned to the GHBs, and the assigned boundary conductance. Bedrock areas are poor aquifers, therefore these areas neither contribute nor accept any flow to or from the alluvial aquifer (Layers 1 or 2). The east and west model boundaries (alluvium/bedrock contact) were designated no flow boundaries. The Mulgrave River and Behana Creek are represented using MODFLOW River Boundaries, which simulate interaction between surface water bodies and the underlying aquifer. The modelled rate of gain or loss from/to the river is proportional to the head difference between the river and aquifer and a user specified riverbed conductance. The rate becomes constant and independent of aquifer head when the head drops below the river stage. In such cases, the river loss/gain is dependent only on the assigned riverbed elevation, and the river stage (i.e. it becomes a constant rate). Riverbed conductance is largely controlled by riverbed permeability and given the difficulty of measuring this parameter in the field, it is typically estimated during model calibration. Drain boundaries were assigned to the top of Layer 1. This was designed to allow natural groundwater discharge at times of high groundwater levels (i.e. when groundwater levels rise above the land surface), and to allow the rejection of the assigned recharge (from PERFECT; refer to Section 4.4 and Section 3.9). The Drain boundaries were assigned a drain elevation equal to ground surface elevation, and a sufficiently high conductance so as not to impede groundwater discharge from the model via these boundaries. Use of Drain boundaries provides a means by which to assess the catchment water balance in more detail with the model. Particularly, with the rejection of groundwater recharge at times of high groundwater levels, and the generation of runoff. This is discussed further in Section 5.
4.3.1 Representation of River-Aquifer Interconnection River boundaries were assigned a (temporal) mean stage and riverbed height that was linearly interpolated between surveyed gauge heights and sea level at Mutchero Inlet, taking into account the DTM elevation. Minor tweaks were made to stage height during model calibration in order to provide a better match between modelled and observed groundwater levels in bore located near river boundaries. Due to a lack of sufficient supporting data (i.e., gauged river flow over much of the alluvial aquifer), river stage height was maintained at a constant level through all simulated time periods.
4.4 Near Surface Processes and Groundwater Recharge Several methods were used to quantify the proportion of annual rainfall that results in groundwater recharge (Section 3.7). Potential recharge was modelled for each mapped soil type and climate zone using the PERFECT model (Littleboy et al., 1989). The PERFECT model is based on soil hydraulic properties and can represent simple or complex evapotranspiration and cropping. In this manner, spatially distributed recharge was applied over the entire groundwater model, depending on soil type and climate zone.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 47 Groundwater Modelling Soil hydraulic properties were derived from the particle size distribution data presented in the CSIRO Babinda-Cairns soils mapping report (Murtha et al., 1996). These data were input to the Rosetta pedo transfer model (Schaap et al., 2001) to derive parameters used in the van Genuchten soil-moisture retention model. These data were used to describe the water-holding properties of a soil (van Genuchten, 1980). This model was used to estimate the relevant hydraulic properties (residual moisture content, wilting point, field capacity, saturated water content) for input to PERFECT. The assigned hydraulic properties are presented in Appendix E. The grain size data for each major soil horizon were weighted averaged (based upon the sample thickness relative to the horizon thickness) and adjusted for the presence of gravels. The derived hydraulic parameters are in general agreement with those documented in the earlier NRM modelling. Other soil hydraulic properties required by PERFECT (eg runoff curve number) were taken directly from the default soil files in PERFECT for the relevant Great Soil Group (Appendix E). Crop parameters were largely derived from the APSIM sugar cane module (http://www.apsim.info/apsim/Publish/apsim/sugar/docs/sugar_science.htm). Given the dominance of cane cropping on the Mulgrave Alluvium, it was assumed to cover the entire area and was therefore the only crop modelled in PERFECT. The large areas of mangrove/intertidal communities in the south and north of the catchment are groundwater discharge areas. This simplifying assumption regarding land use has no impact on the groundwater model in these areas. Simple cropping and evapotranspiration was modelled in this case, rather than the detailed crop growth/ratoon model. Rooting depth was set to 1.2 m, and no ratooning or replanting was simulated (i.e. constant evapotranspiration by cane). Crop factors were taken from the earlier DNRMW modelling (DNRMW, 1999). Climate data were obtained from DNRMW’s SILO database. The PERFECT model does not account for shallow groundwater levels and saturated soils, constant free drainage from the soil profile is assumed. These conditions result in overestimated groundwater recharge in PERFECT. Thus, the groundwater model was designed to allow for rejection of recharge by the aquifer in the event that the aquifer became saturated and had no capacity to accept it. This was achieved by assigning MODFLOW Drain cells to Layer 1, which allow groundwater flux out of the model if the modelled head rises above ground surface. In this manner, the groundwater model was used to provide quantification of groundwater recharge and the generation of runoff in response to the aquifer having no capacity to accept further recharge (i.e. towards the end of most wet seasons). To clarify this, PERFECT models runoff generation in response to rainfall magnitude, soil type, and soil moisture deficit. However, PERFECT does not account for saturation of the soil profile from below (i.e., water table raising to surface) - the MODFLOW model was used to model this aspect of runoff generation via Drain cells.
4.5 Groundwater and Surface Water Abstraction Groundwater and surface water abstraction information was sourced from DNRMW to identify the location of licensed users and annual abstraction volumes. This is detailed in Section 2.8.1 and 3.6. Full annual groundwater allocation volumes were assigned to the model at the documented location of each bore. This is a conservative approach to assess the sustainable yield of the resource. For irrigation licenses (i.e. most licenses), the annual allocation was distributed across the wet season according to the soil moisture deficits (and irrigation demand) modelled in PERFECT. This was achieved
42/15610/98344 Mulgrave River Aquifer Feasibility Study 48 Groundwater Modelling using reported average seasonal variation in allowable soil moisture deficits reported in the earlier DNRMW modelling (DNRMW, 1999). For all other types of licenses (aquaculture, industrial, etc), the full annual allocation was assumed, distributed evenly throughout the year. Due to the simple representation of rivers in the catchment and the inadequate data to enable calibration to river baseflow, surface water abstractions were not incorporated into the model.
4.6 Aquifer Parameters Aquifer parameters were derived from the investigation bore pump testing completed as part of this project (Section 3.3). These parameters provided a starting point for the modelling, although were altered during the model calibration process. Final calibrated model parameters are presented in Figure 20, Figure 21, Figure 22 and Figure 23. The hydraulic properties estimated from the aquifer tests are generally higher than the calibrated parameters in the vicinity of the aquifer tests. This is mos likely due to the tested abstraction bores screening and testing discrete coarse sand layers. Whereas the model parameters represent much thicker sequences of alluvium, much of which is likely to be of lower permeability than the discrete coarse layers targeted for aquifer testing. Further discussion of model parameterisation with particular reference to calibration sensitivity is presented in Section 5.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 49 Groundwater Modelling Figure 20 Hydraulic Conductivity Layer 1
42/15610/98344 Mulgrave River Aquifer Feasibility Study 50 Groundwater Modelling Figure 21 Hydraulic Conductivity Layer 2
42/15610/98344 Mulgrave River Aquifer Feasibility Study 51 Groundwater Modelling Figure 22 Storage Coefficient Layer 1
42/15610/98344 Mulgrave River Aquifer Feasibility Study 52 Groundwater Modelling Figure 23 Storage Coefficient Layer 2
42/15610/98344 Mulgrave River Aquifer Feasibility Study 53 Groundwater Modelling 5. Model Calibration
The model calibration process comprised of the following:
The automated calibration code PEST (Doherty, 2002) was used to refine aquifer parameters (vertical and horizontal hydraulic conductivity) in steady state against the monitored average June- July (1998) groundwater level distribution in DNRMW observation bores (35 bores have records over this period). This steady state model was then updated to a transient model, for transient calibration to groundwater level hydrographs. A subset of the model was refined down to a local scale model for detailed transient calibration in the vicinity of the two aquifer tests undertaken at Area 2 and Area 3 (Figure 19). Note that this was the first step in a two-phase transient calibration – the model was also calibrated to transient water levels for the entire Mulgrave Alluvium (refer below). For the aquifer test calibration, General Head Boundaries were used as local model boundaries, all other boundaries and properties were maintained from the calibrated steady state model. Calibration was undertaken against groundwater level drawdown monitored during aquifer tests at Areas 2 and 3 as a subset of the larger valley model. Pump test calibration parameters were input to the larger model. Detailed drawdown data from five monitoring bores constructed during this investigation were used to calibrate this refined model (Appendix C). Aquifer test and steady-state calibration parameters were checked in the main transient model (i.e., covering the entire Mulgrave Alluvium) and refined manually as necessary to provide an adequate transient model calibration for the whole alluvium. This model was calibrated against the monitored groundwater levels from DNRMW observation bores between June 1996 and December 2005 (16 bores were found to have sufficient hydrograph data against which to calibrate over this period). This calibration period was selected due to the following: – The data available from observation bores over this period are of the highest spatial and temporal resolution over the period of all records, and therefore provide a firm base against which to calibrate the model; and – This time period covers particularly wet climatic years and in which the wet season has failed. This provides a robust basis from which to calibrate the model. The calibration was achieved through altering hydraulic conductivity (vertical and horizontal), storage, river stage and riverbed elevation, and riverbed conductance (i.e., the degree to which the river and aquifer are interconnected). For the steady state calibration, complete (unhindered) river-aquifer connection was initially assumed. During transient calibration, river boundary conductance was adjusted in areas where the modelled water levels at monitoring bore locations were sensitive to the modelled interconnection between the river and the aquifer. In these areas, the effective riverbed hydraulic conductivity is 7*10-3 m/day (assuming an average 50 m wide river), a very low hydraulic conductivity for this environmental setting. In all other areas (i.e. where there were no groundwater level observations clearly affected by river-groundwater interaction), complete river-aquifer interconnection was assumed, for lack of information indicating otherwise. Predictive model sensitivity analysis is presented in Section 5, and calibration model sensitivity analysis is discussed in Section 5.2.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 54 Groundwater Modelling Final calibrated parameters are presented in Figure 20, Figure 21, Figure 22 and Figure 23. A comparison between observed and modelled head distribution for the steady state calibration is presented in Figure 24. Transient calibration hydrographs are presented in Appendix B. Aquifer test calibration hydrographs are presented in Appendix C, Appendix F and Appendix G respectively. Bore location are shown in Figure 6.
5.1 Calibration Quality
5.1.1 Steady State Calibration The normalised Root Mean Squared (nRMS) error of the steady state calibration is 6.6%, which is within the nominated 10% limit of acceptance. This error is caused by observation bore 92951, which should be excluded from the calibration. This private bore which displays an unusually high measured groundwater level compared to other nearby bores. Plots depicting calibration various residual statistics are presented in Appendix F for the steady state calibration. The steady state plots show that the measured water level in bore 92951 is likely to be erroneous. The modelled steady state and observed average June-July groundwater head distributions in Figure 24 show generally good agreement. The only major exception to this being towards Mutchero Inlet, where the modelled 1 m head contour does not extend as far up the valley as it does in the mapped head distribution. This could be related to tidal effects on measured groundwater levels in this part of the catchment (i.e. water level observations may have been read at times of high tide, not mean sea level).
5.1.2 Transient Calibration The nRMS error of the transient model calibration, incorporating all data from all bores, is 9.8% within the 10% target acceptable level of error adopted in this study. Much of the error is due to discrepancy between the DTM elevations and the surveyed elevations of monitoring bores. The calibration hydrographs presented in Appendix B and Appendix C show a reasonable agreement between modelled and observed temporal and spatial variation in groundwater head and drawdown in the aquifer tests. Seasonal variation in the major aquifer stresses – groundwater recharge and abstraction for irrigation – appear to be adequately represented in the model. The calibration to observed drawdown in the aquifer tests conducted at Area 2 and Area 3 shows a good match. This is with the exception of bore A2Ob2, in which the modelled drawdown is lower than that observed. There is no clear reason for this, but the calibration could not be improved for this bore without adversely affecting that of the others. This suggests either a conceptual model error (e.g. model not of sufficient lithologic detail) or systematic error in the testing and data gathering process (e.g. in instrumentation). The former explanation is the most likely. Plots depicting calibration various residual statistics are presented in Appendix G for the transient calibration. For the transient calibration, the plots show a greater scatter in the residual error at higher observed groundwater levels. Further discussion on this is presented below. In the identified area of greatest groundwater recharge (Section 3.4), many of the model hydrograph peaks do not match those of the corresponding observation bores in particularly wet years (1998-1999 to 2000-2001, 2003-2004; days 3000 through 3900 on the x-axes of each chart). Upon detailed model
42/15610/98344 Mulgrave River Aquifer Feasibility Study 55 Groundwater Modelling inspection, it was found that this is due to DTM used to represent the land surface for each 150 m by 150 m model grid cell, the elevation values of which differ from the surveyed elevation of many of the bores. The DTM is typically up to 3 m higher than the surveyed bore elevation. This is an artefact of all DTM’s, which average the land surface elevation over an area and have an inherent degree of error with respect to the true land surface elevation. The bore elevations on the other hand represent one point within the land surface area represented by a model grid cell. This will differ from the averaged land surface elevation for the broader surrounding area, as represented by the DTM. This means that modelled groundwater levels may rise higher than those observed in some of the observation bores. The modelled groundwater levels may rise up to the elevation of the DTM. This is not considered to compromise model predictions in any way, because at the scale of the model, the DTM provides a good representation of land surface topography. Despite this issue, the overall seasonal response to recharge, and lack of response in the particularly dry years, are adequately represented. The calibration hydrographs for bores located in close proximity to the Mulgrave River and Behana Creek (e.g. bores 11100098, 11100043, 11100042, 11100049) show poorer agreement between modelled and observed groundwater head. The modelled base level heads are too low and are controlled by the river boundary in the model. At times of low flow it is evident that the lack of detailed data pertaining to stage heights and riverbed levels is inadequate to provide a detailed picture of the river stage variations through the catchment. The calibration could potentially be improved in these areas through riverbed and stage surveys, and potentially the acquisition of a more detailed Digital Terrain Model for the floodplain, using data such as LIDAR. However, it should be noted that these issues are unlikely to significantly affect model predictions. These predictions provide relative rather than absolute impact (i.e. drawdown) assessment. The benefit of undertaking detailed surveys and acquiring more detailed topographic data was considered to be low for this assessment given the associated high costs.
5.1.3 Summary The described calibration quality indicates that the model is sufficiently calibrated given the quality and spatial and temporal resolution of the available data. Model residual error is within the prescribed limits. If there is a poor match between modelled and observed data, logical reasoning can explain the sources of error. The model is therefore considered sufficiently calibrated for the purposes of the investigation, however uncertainty and sensitivity analysis will be used to indicate the ambiguity in model predictions.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 56 Groundwater Modelling Figure 24 Calibrated Steady State Watertable Contours
42/15610/98344 Mulgrave River Aquifer Feasibility Study 57 Groundwater Modelling 5.2 Calibration Sensitivity Analysis The results of a sensitivity analysis show that the horizontal hydraulic conductivity in particular areas are the parameters to which the model calibration is most sensitive (Figure 20 and Figure 21). This is generally observed around the main abstraction and cane irrigation area (north of Gordonvale in model Layer 1. In Figure 20 and Figure 21 the points represented by the largest circles are the parameters to which the model is most sensitive. Conversely those with the smallest circles represent the parameters to which the model is least sensitive. The points to which the calibrations was least sensitive were adjusted by PEST to their specified maximum These values were later adjusted down to the average of the other pilot points (to which the calibration was sensitive) in order to maintain a physically realistic parameterisation that is not skewed by parameter points to which the calibration is insensitive. The calibration was far less sensitive to vertical than horizontal hydraulic conductivity. Sensitivity analysis of the calibration to other key parameters – riverbed conductance and recharge, was also undertaken (Figure 25). It shows that (reduced) riverbed conductance is another significant parameter to the calibration. However increasing the conductance much higher than the calibrated values changes th model calibration very little. The This suggests that the river is almost entirely connected to the aquifer in the calibrated model. This theory is supported by the Hydrogeological conceptualisation and data analysis. Reduction in the degree of river-aquifer connection results in a poorer match between the modelled and observed results (Figure 26) Consequently the modelled degree of connection in the calibrated model is considered provide an adequate reflection of the true connection. As a result of this high degree of river-aquifer interconnection, model predictions are likely to be largely controlled by the river boundaries in the model. This is because river boundaries provide the primary avenue of groundwater discharge from the alluvium (i.e., they largely control the groundwater head distribution). Thus, any stress on the aquifer is likely to manifest itself as a change to inflows or outflows across the river boundaries. Sensitivity analysis of model predictions to riverbed conductance will however be conducted to assess the potential range in predicted extents of drawdown in the aquifer. This is in response to increased groundwater abstraction. It should be noted that the lowest value for conductance tested in the sensitivity analysis is the equivalent to a riverbed hydraulic conductivity of 7*10-5 m/day. This amount of riverbed hydraulic conductivity is considered to be extremely low. This value is at the lowest end of the physically possible levels of hydraulic conductivity and is not supported by the conceptualisation or model calibration. However, this was the level to which river-aquifer connection had to be dropped in order to achieve a noticeable change in the modelled responses to groundwater abstraction (i.e., drawdown and impact on river flows). The sensitivity analysis of the model to recharge suggests that the model is more sensitive to reductions in applied groundwater recharge than the degree of river-aquifer interconnection. The model is less sensitive to increase in recharge. This supports the conclusion reached earlier in this report (Section 3.9 and earlier reporting (GHD, 2006)) that aquifer storage is completely filled in most wet seasons and the excess recharge is rejected by the aquifer as surface runoff. Lower rates of recharge rapidly reduce the quality of the calibration, suggesting that the modelled rates of recharge are a reasonable reflection of the true rates.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 58 Groundwater Modelling Further on the topic of river/groundwater interaction, during the transient model calibration, it was noted that the calibration hydrograph for several bores located particularly close to the river were highly sensitive to river bed conductance. Initially, the conductance was set extremely high under the assumption that the river is in complete (unhindered) connection to the aquifer. Under this assumption, the bores located close to the river showed flat hydrographs (i.e. they were unresponsive to recharge events) with groundwater levels completely hinged to the model river stage, whilst the observed data suggested otherwise. Typical annual river fluctuations (2-3 m) do not account for most of the observed bore hydrograph fluctuation in these areas (6-8 m). This suggests that it is more likely that river/aquifer connectivity is the controlling parameter on bore hydrograph fluctuations, rather than river stage dynamics. The river conductance was thus lowered iteratively until the modelled response to recharge events matched those in the observed data. This provides useful information on what the true river/aquifer connectivity may be in those areas where bores are adjacent to the river. This also provides confidence in the model predictions in these areas, which are discussed below.
Figure 25 Calibration Sensitivity Analysis
5.3 Calibrated Model Water Balance The purpose of this Section is to use the output from the calibrated model to further refine the water balance for the Mulgrave Alluvium that was presented in Section 3.9. The model provides a more detailed (numerical) assessment of the groundwater components of that water balance. The model is a 3-dimensional, spatially distributed model, and directly links the recharge modelling with the groundwater
42/15610/98344 Mulgrave River Aquifer Feasibility Study 59 Groundwater Modelling model, whereas the preliminary water balance was constructed at a more conceptual level. The modelled water balance is summarised in Table 6.
Table 6 Mulgrave Alluvium Water Balance – Using Calibrated Groundwater Model
IN (ML/year) OUT (ML/year) OUT (as % of IN)
Recharge 179678 - -
Surface Water Leakage into 29063 - - aquifer
Inflow into alluvium from up 14020 - - catchment alluvia
GWABS - 4034 2%
Baseflow - 127947 57%
Rejected recharge and - 65326 29% discharged to surface
Through flow to ocean - 25516 11%
TOTAL 222761 222823 100%
BALANCE DISCREPANCY: -62 ML/year (0% of total annual recharge to the alluvial aquifer) The recharge component of the water balance was estimated at 460 mm using the hydrograph fluctuation method. This equates to ~151574 ML/year for the Mulgrave Alluvium, which compares well with modelled recharge rate of 179678 ML/year. Other key differences between this calibrated water balance and the initial conceptual water balance are:
Modelled baseflow is lower than the initial estimate, with the model indicating 127947 ML/year of the overall balance, whilst the estimated baseflow was 181456 ML/year; The groundwater discharge to ocean via Trinity and Mutchero Inlets was modelled as being higher than the initial estimate (25516 ML/year versus 11098 ML/year); The modelled leakage from surface water into the alluvium is lower than the conceptual water balance estimate (29063 ML/year versus 35533 ML/year); and The modelled groundwater abstractions are higher than the initial estimate purely through conservatism – the full license allocation of each bore was applied to the modelling, whereas the best estimate of actual usage was used in the initial water balance. The largest modelled versus conceptual water balance relative discrepancies are baseflow and throughflow to the ocean. These discrepancies are due to better (distributed, 3-dimensional) representation of groundwater head gradients, aquifer parameters, groundwater recharge, and aquifer boundary conditions in the numerical model. With respect to the baseflow discrepancy, this indicates inaccuracy in the broad-brush approach that was used in the conceptual water balance, in the absence of gauged river flow data on the main body of the alluvial aquifer.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 60 Groundwater Modelling 5.4 Model Limitations The key issue is the lack of flow data in the middle reaches of the Mulgrave River, and limited data for Behana Creek. Consequently the model assumed that there is no surface or subsurface recharge of these systems below Peets Bridge (which is not the practical case) but as the Peets Bridge flow data was the longest most reliable flow data, the model was calibrated against a far lower recharge than certainly actually exists to avoid extrapolation beyond the sensitivity analysis conducted (as mentioned below). The lack of flow data also skews interpretation of the modelled potential impacts on the same conservative basis, i.e., by assuming no surface or subsurface recharge (particularly for Behana), the modelled impacts are disproportionate for the different scenarios between Behana Creek and the Mulgrave River. With further data these modelled impacts could be greatly refined and would (most likely) show a far lesser proportional impact than is presented in the current model. Following is a summary of the identified limitations of the numerical model developed in this investigation, and associated consequences. These limitations should not be uniformly regarded as negative attributes, but are the primary elements in the conservative outputs and assumptions derived for this model.
Lack of calibration to river baseflow due to insufficient gauged river data against which to calibrate. This results in uncertainty in the degree of river – aquifer interaction in the model, and has the flow- on effect of uncertainty in model predictions of drawdown and induced river leakage into the aquifer. This results in modelled impacts greater than actually exists. As described in earlier and later sections of this report, some of this uncertainty is minimised via sensitivity analysis, and confidence in model predictions is not affected significantly by the remaining uncertainty. Lack of data on river and creek stage heights in the lower parts of the catchment. This would cause a minor impact on the quality of the model calibration in the lower catchment. It does not significantly affect model predictions and again the impacts modelled err on the conservative side, adopting lower conditions than would be normally expected in such a high rainfall area. Lack of recorded groundwater usage data. In this investigation, the entire allocated volume has been modelled (as opposed to the actual usage which is a fraction of the actual legal licensed allocations). This means that existing groundwater abstraction is overestimated in the model, but as groundwater usage is a small component of the water balance, thus results are insensitive; Groundwater observation data are concentrated around the main groundwater abstraction / irrigation area north of Gordonvale. This results in a model that is better-constrained in this area than in others, particularly the far north and south of the Mulgrave Alluvium; The existing groundwater observation data are not of sufficient temporal resolution to identify the timing and magnitude of seasonal peaks in groundwater recharge. For average climatic years this is not an issue, but does mean that for wetter than average climatic years groundwater recharge is underestimated and does compromises the quality of the calibration, ie: assumes less recharge than actually happens. The model does not incorporate runoff-recharge events from the bedrock outcrop bordering the east and west valley sides. The water balance assessments undertaken indicate that the major components of the water balance have been accounted for. Lack of representation of this process does not compromise the modelling outcomes, but does increase the conservatives of the model through assuming less run off than is likely to occur.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 61 Groundwater Modelling 6. Numerically Modelled Impact Assessment
6.1 Background and Approach Assessment of the impacts of groundwater abstraction from the Mulgrave Alluvium has been undertaken through the simulation of two scenarios. Both of these scenarios have been derived from the recommendations of previous studies into the Mulgrave Aquifer. 1. Abstraction of 15000 ML/year (40 ML/day) from two bore fields located in the two areas found to be most promising in terms of aquifer yield (“Area 2” – near Gordonvale, and “Area 3”- south of Behana Creek). 2. Abstraction of 5475 ML/year (15 ML/day) from only the “Area 2” bore field. No abstraction was modelled in “Area 3”. Both scenarios was assumed that pumping would be from Layer 1 in Area 2, and from Layer 2 in Area 3 based upon the geology encountered during the drilling investigation, the conceptual hydrogeological model, and the test bores’ construction. Scenario 2 (reduced abstraction localised to the “Area 2” bore field) was modelled subsequent to review of the predictions obtained from Scenario 1, which indicate potential impacts on surface water flows (refer to Section 6.3). In reviewing previous reports and recommendations and in discussion with Cairns Water it was proposed that a reduced abstraction scenario be the first phase implementation of the groundwater supply. This will allow for environmental monitoring of impacts in response to the abstraction. Should no significant impacts be identified during this first phase groundwater supply, the supply could be expanded to greater supply volumes with the collected data during the first stage. This will provide technical support to such expansion. Should no significant impacts be identified during this first phase groundwater supply, the supply could be expanded to greater supply volumes, with the collected data during the first stage providing technical support to such an expansion. Monitoring data collected during this first stage could be used to refine the groundwater model to provide stronger support for prediction of the impacts under expansion of the groundwater supply. The calibrated transient model was used as the basis for the predictive modelling. Therefore the predictive models were run for the same period as the calibrated model, with the same climatic inputs. The modelled drawdown in response to the abstraction for each model layer is presented in Figure 26 and Figure 27 respectively. The predictive model was also run with reduced river boundary conductance for sensitivity analysis of model predictions to the degree of river-aquifer connection. The results of this predictive model sensitivity analysis are presented in Figure 28 and Figure 29 for Scenarios 1 and 2, respectively. The effects of existing abstractions and water level variations on modelled drawdown have been removed so that only the impact in response to the proposed abstraction is presented. Particle tracking analyses were also conducted in the predictive modelling using MODPATH in steady state mode. Such analyses provide an indication of all potential sources of water being abstracted from each bore. The particle tracking results can be used as a guide to the potential groundwater source areas within the valley, and potential water quality issues.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 62 Groundwater Modelling Figure 26 Modelled Drawdown Response Scenario 1
42/15610/98344 Mulgrave River Aquifer Feasibility Study 63 Groundwater Modelling Figure 27 Modelled Drawdown Response Scenario 2
42/15610/98344 Mulgrave River Aquifer Feasibility Study 64 Groundwater Modelling Figure 28 Scenario 1 Sensitivity Analysis: 1500 ML/year Abstractions, Modelled Dry Season Impacts, Minimal River/Aquifer Connection
42/15610/98344 Mulgrave River Aquifer Feasibility Study 65 Groundwater Modelling Figure 29 Modelled Drawdown Following Successive Dry Years
42/15610/98344 Mulgrave River Aquifer Feasibility Study 66 Groundwater Modelling 6.2 Bore Field Design The initial modelled arrangement of the bore fields at “Area 2” and “Area 3” was iteratively derived via spreadsheet modelling using Theis (1935) analysis. The Theis (1935) analysis suggested a minimum bore spacing of 400 m. The arrangement of the bore field was determined through:
Utilising the ideal minimum bore spacing; The likely maximum total groundwater abstraction required (40 ML/day); Likely achievable bore yields; Local physical constraints such as land ownership (road reserve versus private land); and Existing nearby groundwater users at each area. Three iterations of the modelling were conducted to determine a final bore field arrangement that minimised the drawdown impacts of abstraction. The resulting bore field arrangement is shown in Figure 26 and Figure 27. An ideal bore field arrangement is not constrained by land availability. Ideally, each bore should be evenly spaced from one another in a symmetrical spatial distribution in order to minimise drawdown interference between bores. This is also to minimise the area of land required for the bore field. However, the model-predicted drawdown in response to abstraction (Section 6.5) suggests that the (physically-constrained) bore field arrangement does not cause any undue drawdown and bore interference effects on any of the other bores in the bore field. The modelled drawdown distribution is largely controlled by transmissivity (thickness) variations in the alluvial aquifer (Figure 26 and Figure 27), particularly at Area 2. This where the drawdown cone is offset to the west of bore field centre towards the edge of the alluvial aquifer The modelled drawdown within the Area 2 and Area 3 bore fields indicates that the proposed average bore depths (50 m at Area 2 and 65 m at Area 3) are sufficient to provide the required available drawdown under the maximum proposed abstraction scenario (40 ML/day) This can be done with a maximum modelled drawdown of 5 m at Area 2 and 2 m at Area 3. The modelled bore field drawdown, particularly at Area 2, is offset to the west of bore field centres due to thinning of aquifer to the west (Figure 8). The aquifer thinning results in reduced aquifer transmissivity in this direction. Therefore, the bore fields have been located as far to the east as practicable.
6.3 Predicted River Baseflow Impacts
6.3.1 Introduction Potential river baseflow impacts have been predicted based upon the outcomes of the groundwater model. The groundwater model is inherently limited in so far as that it must assume:
Pumping and drawdown is constant, that is, water is being continually abstracted from the aquifer. Abstraction intensity is uniform, i.e, the same volume of water is being withdrawn constantly. In the above for Stage 1, the model assumes that 5475 ML/year would be withdrawn at the rate of 15 ML/day over the entire year, and similarly for Stage 2, that the maximum licensed allocation of 15000 ML/year would be withdrawn uniformly at the rate of 40ML/day over the entire year.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 67 Groundwater Modelling Neither of these assumptions reflect the operational parameters under which the bore field would operate. Both 15ML/day (Stage 1) and 40 ML/day (Stage 2) represent the uppermost daily limit of abstraction, in accordance with the existing groundwater licence allocation of a maximum of 15000 ML/year. However the timing of the abstraction would not be constant, nor the rate of abstraction. There may be prolonged periods of the year when water is not being abstracted from the aquifer and as the daily limit would not be exceeded the actual volume of abstracted water may be significantly less that the total modelled abstraction volume. The proposed abstraction is a supplementary supply only, not a major development in its own right. During the months January to April, for example, there may be no abstraction from the bore, therefore the estimation of impacts as modelled may exceed the actual operational impacts of the bore field by 30%. In this regard the predicted modelled impacts have been very conservative in assuming uniform pumping and constant drawdown when in actuality the bore field may not be used for up to four months of the year. The daily rates of pumping will not be exceeded and the net result would be an overall decrease in the total volume and therefore total modelled impacts of the proposal. In considering the modelled impacts, the model has been run over the entire duration for which climatic data is available, ie: 60 years of quantifiable, reliable climatic data. When assessing withdrawal from the aquifer to the relationships with surface flows consideration must be given to the fact that these impacts may take place over 60 years. The model water balance was based on the assumptions of constant demand on the aquifer and assumed no further baseflow or runoff into surface waters below Peets Bridge. Subsequently the drawdown for each bore field (Figure 26 and Figure 27) show that the drawdown for each bore field is largely controlled by the nearest river boundary, suggesting that the abstraction is inducing movement of water from the river into the aquifer Further investigation into the model water balance suggests that the maximum abstraction regime of 15000 ML/year (or 40 ML/d) induces 7 ML/day of leakage from the river into the aquifer, and a reduction in groundwater discharge into the river by 34 ML/day. This is not an instantaneous relationship (modelled over 60 years), and as strongly indicated, does not reflect actual operation of the borefield. On average (over the duration of the modelled abstraction) all of the abstracted groundwater will impact surface flows via a combination of reduction of supply to the baseflow of surface waters, and induced leakage into the aquifer from the surface waters. This relationship is not uniform, and will depend on seasonal factors (wet seasons, prolonged dry seasons) and on bore field operational practices. Over the long term (the 60 years of the model) the ratio of the volume of abstracted groundwater to the total volume of river flow is predicted to be 1:1. To put this into context, this has assumed uniform pumping, and no surface water recharge or base flow below Peets Bridge, both extremely unlikely scenarios. On a more seasonal, yearly basis the degree of river impact will vary, but in no instance would there be a situation where the impact on the surface water would exceed that of the volume of abstracted groundwater. In catchments with a highly connected groundwater and surface water system, the phenomenon of a 1:1 abstraction to surface water impact is not unusual. In this catchment it is due to the Mulgrave River providing the only major means of groundwater discharge out of the alluvial aquifer. The east and west sides of the the valley are bordered by the elevated and relatively impermeable bedrock. Also there are no other discharge avenues for groundwater other than Mutchero Inlet and Trinity Inlet. Trinity Inlet is however hydraulically separated from the Mulgrave River by the groundwater level divide north of Gordonvale (Figure 11). This hydraulic separation of the groundwater between Trinity Inlet and the
42/15610/98344 Mulgrave River Aquifer Feasibility Study 68 Groundwater Modelling Mulgrave aquifer is the key parameter restricting the potential impacts of salinity intrusion into the Mulgrave aquifer as a result of any lowering of the groundwater table. This leaves only Mutchero Inlet for groundwater discharge in the southern part of the catchment. This part of the catchment is of very limited capacity in terms of discharge due to its narrow nature, distance down catchment, and vast volumes of recharge entering the alluvial aquifer up-catchment each year. Therefore, groundwater levels in the alluvium rise towards ground surface under this restricted aquifer discharge capacity, and groundwater is forced to discharge into surface water features as baseflow. As a result, any significant volume of water removed from the aquifer will result in a direct reduction in the net groundwater discharge to surface water. These surface flow impacts have been derived over the duration of the model, i.e., over a period of 60 years assuming constant withdrawal from the aquifer, so these impacts are neither immediate, nor irreversible. Operational requirements of the bore field indicate that the bore field (as a supplementary water supply) may not be utilised for number of months in each year (the length being seasonally dependent) so the degree of impact will be temporal and as previously iterated the maximum case only has been modelled. The surface water flow impacts are predicted to affect both the Mulgrave River and Behana Creek. Figure 30 graphically presents the relative impacts upon each of these surface water features for abstraction rates varying from 15 ML/day (5450 ML/year) up to 40 ML/day (15000 ML/year). Under the higher abstraction scenarios, most (~two thirds) of the impact is on the Mulgrave River, whilst at the lowest modelled abstraction rate the impacts on Behana Creek and the Mulgrave River are roughly equal. Figure 30 also shows that in the average dry season, most of the impact on surface water is the result of reduced baseflow in response to drawdown in the aquifer under the modelled abstraction, particularly for the Mulgrave River. For the Mulgrave River, under very dry conditions such as those in November 2003, the impacts tend towards to be more a result of induced leakage from the river rather than reductions in baseflow. This is because groundwater levels fall below the river stage under such dry conditions, hydraulically encouraging water to flow from the river into the aquifer. For Behana Creek under such conditions, the relative impact derived from baseflow reductions and induced river leakage remains roughly the same as in average climatic conditions. This is due to the rapid rise of the land surface up Behana Creek towards the edge of the alluvium. Consequently, Behana Creek’s stage height rises above groundwater elevations up-catchment. This suggests that Behana Creek under both average and dry conditions, much of its upper reaches (that section still within the Mulgrave alluvium) naturally “lose” water to the underlying aquifer. This is because the creek stage height in this area is generally higher than the surrounding groundwater levels. Subsequently as Behana Creek further rises the basement of the creek becomes the parent granite material of the Bellenden Ker range that intersects the alluvium of the Mulgrave River west of the Bruce Highway. .As the creek basement changes to granite, and rises with elevation, the potential for withdrawal from the Mulgrave alluvium to affect surface or base flows of Behana Creek on the granite basement is nil. Abstraction from the Mulgrave alluvial aquifer cannot induce a surface or subsurface response from that elevated section of Behana Creek which has a granite basement. That area of granite bedrock is within the World Heritage Area, and surface flows (there is no subsurface flow through the granite) of Behana Creek within the World Heritage Areas cannot be affected.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 69 Groundwater Modelling 6.4 Predicted Surface Water Impacts in Context In an average dry season (1998), mean total river flow is in the order 866 ML/day at the Peets Bridge gauge. Conservatively assuming that the river gains no further baseflow or runoff downstream of this location (unlikely, and certainly not the case in the calibrated model), it could be estimated that the proposed abstraction of 15000 ML/year (40ML/day) may reduce low flows by up to 5%. In particularly dry periods1, the reduction in river flow may be as high as 34% of total river flow as a result of the modelled abstraction. Note that this is predicted to be a 1 in 60 year event. Prior to such a level of abstraction (Stage 2) data would be obtained from gauging stations to be established on the mid-reaches of the Mulgrave River (within the likely drawdown area) and on Behana Creek (also in the drawdown area). This data would be used to refine the model, and provide further detailed information for the necessary detailed assessment that would be required prior to Stage 2 being implemented. For implementation of Stage 1 (5450 ML/year), the volume is small by comparison with the recharge potential of the aquifer and surface/subsurface flows of the Mulgrave and Behana systems. The conservatism of the model assumes a constant withdrawal rate modelled over 60 years( that would not reflect actual operation which may be 30% less in duration than modelled) and assumes no net baseflow or recharge below Peets Bridge (which does not happen, but as the gauging data from Peets Bridge was the most reliable data then this conservative point was adopted). The model has identified that even with these conservative approaches incorporated that impacts on river baseflow in an average seasonal year, over 60 years, would be less than 1%.
6.5 Predicted Drawdown Impacts The predicted extent of drawdown is limited to the north and east of Aloomba by the Mulgrave River as shown in Figure 26 and Figure 27. These model predictions have been made using the calibrated model. To the west drawdown extends to the margin of the valley and into the lower section of the Behana Creek valley. To the south, drawdown extends to the Meerawa area. The 0.5 m drawdown contour extends to a distance of ~10 km from Mutchero Inlet. Consequently, there is minimal potential for extraction to alter the current groundwater – seawater interface, which is thought to occur at a maximum of 4-5 km north of Deeral, based on extremely limited data (refer Section 3.5). The predicted drawdown is highly unlikely to induce saline intrusion from the south of the model (around Mutchero Inlet), and certainly not from the north (Trinity Inlet). Because the drawdown resulting from abstraction is predicted to induce leakage to the aquifer from the upper-most tidal section of the Mulgrave River (around the junction with Behana Creek; Section 6.3), it is possible that this will result in deterioration of groundwater quality within the aquifer in these areas. This is likely to be minimal however, because the average salinity of surface water is likely to decline with distance up-catchment. In addition, the volume of water derived from the Mulgrave River relative to the total abstracted volume is on average / minimal (Figure 30). Thus, the water quality in the Mulgrave River is not predicted to have any significant impact on the quality of abstracted groundwater. The drawdown does not extend to the sensitive wetland vegetation communities and acid sulfate soils around Mutchero Inlet, not northwards towards those around Trinity Inlet (shown in Figure 26 and Figure
1 The end of the historical dry climatic years of June 2001 to November 2003 (a 1 in 60 year climatic event) was used as the reference (i.e., November 2003)
42/15610/98344 Mulgrave River Aquifer Feasibility Study 70 Groundwater Modelling 27). The draw down is localised to the bore field and locality, with the extent of drawdown confirmed for an extended period of dry years in the sensitivity analysis presented in Section 6.6. In order to assess the existing users that may be affected by drawdown in the aquifer from the abstraction a threshold of 0.5m modelled drawdown was specified (this being the minimum modelled limit of the drawdown. Figure 26 identifies that four irrigation/domestic supply bores (78268, 92900, 109373 and 109846) may be affected (>0.5 m drawdown), with a combined total annual allocation of 258 ML. Details of these bores are presented in Table 7.
Table 7 Existing Groundwater Users Potentially Affected by the Proposed Abstraction
BoreID License License Bore Use Bore Depth Modelled % Reduction Volume (m) Drawdown (m) in Available (ML/year) Drawdown
Irrigation, Domestic 109373 173970 1.5 Supply 18 1.2 11%
92900 92900K 2 Irrigation 24 1.1 8%
109846 182129 6 Irrigation 39 1.7 6%
78268 78268K 248 Irrigation 18 (estimated) 0.5 5%
NOTES: a. To estimate the depth of bores without recorded depths, it was assumed that bores were drilled to 10 m below the average mapped groundwater elevation at the bore location. b. The modelled drawdown presented in this table is derived from a particularly dry (1 in 60 year) dry season (November 2003 was used as the reference) In terms of induced land subsidence, in the worst case, the modelling predicts up to 0.26 m in the immediate vicinity of abstraction points, and up to 0.09 m over the broader area (Appendix H). Any subsidence will be gradual over several years and is anticipated to be relatively uniform over the area. It is not expected to affect infrastructure or drainage. Settlement within the sands is expected to occur soon after groundwater drawdown and aquifer depressurisation (“immediate settlement”), whilst the settlement within clays will take a long period of time to complete. On average, approximately one third of total settlement will occur immediately in response to groundwater drawdown, while the residual settlement will occur over a long period of time. The results indicate that there will be minimal subsidence impacts in the area. The particle tracking results for Scenario 1 in Figure 26 and Figure 27 (40 ML/day) identifies that there is little potential for abstraction of contaminated groundwater. This could happen if it is assumed that the nearest potential contaminant point source is Gordonvale (waste water treatment plant and Mulgrave mill) on the northern side of the Mulgrave River, or south of Aloomba (capped and sealed landfill), or near Babinda (Babinda waste water treatment plant). All of these areas are well away from the modelled source zone of both abstraction points. However, the simulated abstraction is predicted to induce leakage from the river into the aquifer. Thus, contaminated surface water poses a potential risk to groundwater quality. The most likely source of surface water contamination is the Gordonvale sewage treatment plant (STP). Gordonvale STP discharges a waste stream into the river upstream of the proposed abstraction area (and the identified area of drawdown/river flow impact). However, as already described, and shown in Figure 29, the relative proportion of abstracted groundwater modelled as being derived from the Mulgrave River is, on
42/15610/98344 Mulgrave River Aquifer Feasibility Study 71 Groundwater Modelling average / minimal. Therefore, mixing of surface water with the large volume of water within the aquifer will reduce any potential contaminant concentrations. The particle tracking analyses undertaken in this investigation suggest the shortest groundwater travel times between the river and the bore field are around 1 year in the calibrated model for the Area 3 bore field, and 18 years for the Area 2 bore field. These predicted travel times suggest that the aquifer will have significant time and capacity to naturally attenuate any contaminants that may enter the aquifer from the river under induced river leakage into the aquifer.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 72 Groundwater Modelling Figure 30 Abstraction Volume and Baseflow Impacts – Sensitivity Analysis
42/15610/98344 Mulgrave River Aquifer Feasibility Study 73 Groundwater Modelling 6.6 Predictive Model Sensitivity Analysis The calibrated model was altered to investigate the effects on model predictions of the minimum level of river-aquifer connection. The results of this sensitivity analysis with respect to river boundary conductance are presented in Figure 28. The river boundary conductance in this case was lowered to an equivalent riverbed hydraulic conductivity of 7*10-3 m/day (assuming an average river width of 50 m). Thereby reducing the degree of river-aquifer interconnection to the lowest, but still realistically parameterised level. This provides a conservative indication of the maximum likely drawdown extent in response to the proposed abstraction. In contrast, the equivalent riverbed hydraulic conductivity of the calibrated model is 0.07 m/day. As expected, the predicted maximum extent of drawdown in this case covers a larger area than the higher river conductance model. The 0.5 km drawdown contour extends a further 2 km southwards, beyond Meerawa, to the south of the abstraction area. Whereas, to the north of the abstraction area, the 0.5 m drawdown contour extends a further 3 km to the northeast, beneath the Mulgrave River, towards the edge of the thinning alluvium. The model was also used to assess the impacts of a period of successive dry years, with the lowest likely level of river - aquifer interaction. June 2001 to November 2003 was a period of two particularly dry consecutive years, equivalent to a 1 in 60 year dry climatic event. The modelled drawdown in November 2003 is presented in Figure 29. This indicates that the drawdown induced by continuous abstraction of 15000 ML/year under such conditions would extend from northeast of Gordonvale by ~15 km to the south, to ~4 km southeast of Meerawa. The drawdown in this case extends towards the main abstraction area north of Gordonvale. It impacts an additional nine existing groundwater users (Table 8), but does not extend sufficiently far south to affect the saline/fresh groundwater interface north of Deeral.
Table 8 Existing Groundwater Users Potentially Affected by the Proposed Abstraction Assuming the Lowest Level of River Aquifer Connection
BoreID License License Bore Use Bore Depth Modelled % Reduction Volume (m) Drawdown (m) in Available (ML/year) Drawdown
Irrigation, Domestic 109373 173970 1.5 Supply 18 6.1 59%
78268 78268K 248 Irrigation 18 (estimated) 4.3 43%
92900 92900K 2 Irrigation 24 3.2 23%
109846 182129 6 Irrigation 39 4.2 15%
45030 45030K 203 Irrigation 16 (estimated) 0.6 6%
45975 45975K 10 Irrigation 33.5 1.1 6%
45845 45845K 12 Irrigation, Stock 29.5 1.0 5%
45312 45030K 203 Irrigation 21.9 0.6 4%
45145 45145K 160 Irrigation 40.3 1.1 4%
45029 45029K 170 Irrigation 27.4 0.6 3%
109675 45030K 203 Irrigation 29.7 0.6 3%
42/15610/98344 Mulgrave River Aquifer Feasibility Study 74 Groundwater Modelling 45126 45126K 380 Irrigation 33.8 0.6 2%
109325 178518 40 Irrigation 42 0.5 2%
NOTES: 1. To estimate the depth of bores without recorded depths, it was assumed that bores were drilled to 10 m below the average mapped groundwater elevation at the bore location. 2. The modelled drawdown presented in this table is derived from a particularly dry (1 in 60 year) dry season (November 2003 was used as the reference) This sensitivity analysis of river conductance shows that river baseflow impacts resulting from 15000 ML/year of groundwater abstraction are reduced with decreasing river-aquifer connection (Figure 31). The sensitivity analysis also supports the above conclusions of Section 6.5. Predicted drawdown does not extend to the southern maximum extent of the saline/fresh groundwater interface, nor to the protected ecological communities or acid sulphate soils around either Mutchero Inlet or Trinity Inlet. Despite increased drawdown extent, there are unlikely to be significant impacts in terms of land subsidence, saline intrusion into the aquifer, or induced contamination of the groundwater supply from Gordonvale, or any other urban areas.
Figure 31 Modelled Drawdown following successive dry years
6.7 Potential Impacts of Climate Change Given the large proportion of the annual water balance shed as rejected recharge (i.e. runoff, Section 3.9 and Section 4.4), it is considered unnecessary to explicitly model the recharge and groundwater flow
42/15610/98344 Mulgrave River Aquifer Feasibility Study 75 Groundwater Modelling model impacts of climate change scenarios. Instead, a water balance impact approach has been utilised to show that the net excess recharge available in this catchment each year is larger than the potential impacts of climate change. This assessment is based upon information presented by the CSIRO report “Climate Change in Queensland under Enhanced Greenhouse Conditions” (Walsh et al., 2002). The assessment is as follows:
The CSIRO report that net annual impact on soil moisture is estimated at a maximum of –40 mm per degree of warming in this region of Queensland (Walsh et al., 2002); The predicted maximum dry season temperature increase over this area of Queensland is between ~1 and ~4qC by 2050 (mean annual is between ~0.5 and ~3qC) (Walsh et al., 2002); In the worst case (~4qC warming by 2050), a worst-case negative impact of 160 mm on soil moisture can be assumed. This is highly conservative because the calculation assumes the worst-case warming across the entire year (i.e. does not consider seasonal variation in warming); In the study area, assuming that this is a consistent change across the entire Mulgrave Alluvium, this equates to an extra ~6327 ML/year increase in available aquifer or soil moisture storage, or around 8% of the model-predicted potential recharge that is, under the current climate, rejected by a seasonally fully-saturated aquifer. This assumes a soil porosity of 0.12 after Leach and Rose, 1979. In summary, when very conservatively considering climate change in the worst case, there is predicted to remain an excess potential groundwater recharge in this catchment. If it were assumed that the increase in soil moisture deficit under climate change occurs wholly in the dry season, this would result in a slight delay in the initial groundwater recharge response than would otherwise have occurred. The recharge modelling undertaken in this study indicates that the soil moisture deficit is largely replenished within 30 days of the wet season onset. The groundwater model operates on a monthly time step, and therefore the impacts of climate change will not be evident in the model. This also indicates that any changes in groundwater levels in response to climate change at the onset of the wet season are likely to be unnoticeable. Although this is a simple water balance approach to the problem of climate change, it can be reasonably concluded that the worst-case predictions of climate change will result in minimal net losses to the aquifer water balance. This is due to the current net lack of aquifer storage with respect to the potential groundwater recharge in this catchment.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 76 Groundwater Modelling 7. Conclusions
7.1 Results The hydrogeological data analysis, conceptualisation presented and other supporting investigations in this report were used to construct and calibrate a numerical groundwater flow model of the Mulgrave River Alluvium. The model is considered sufficiently calibrated for the purposes of the investigation, however uncertainty and sensitivity analyses have been carried out to produce a range of possible outcomes. The calibrated transient model was used as the basis for simulating the following two scenarios: 1. 15,000 ML (40 ML/day) constant annual abstraction from the two areas found to be most promising in terms of aquifer yield (“Area 2” – near Gordonvale, and “Area 3”- south of Behana Creek); and 2. 5450 ML/year (15 ML/day) constant annual abstraction from Area 2 only. The latter was modelled to provide the basis for a staged implementation of the proposed abstraction. It has a view to expand the groundwater abstraction up from 15 ML/day in the future, dependent on the outcomes of environmental impact monitoring during the first stage. Several bore field arrangements were modelled to provide the least impact, yet practicable spatial bore field arrangement. In terms of aquifer drawdown impacts, summary results for an average climatic year (1997-98) and following a particularly dry period (June 2001 to November 2003) are presented in Table 9 and Table 10, respectively. In terms of river flow impacts, results for the same climatic periods are presented in Table 11 and Table 12. All reported impacts refer to dry season drawdown and river flow impacts, for conservatism.
7.2 Model Summary A range of model predictive runs has been undertaken using the calibrated model and assuming ‘high river-aquifer connection’ and ‘low river-aquifer connection’ interaction between the aquifer and the Mulgrave River. The calibrated model provides the ‘best estimate’ basis for predicting impacts of groundwater abstraction, given the currently available data. The ‘high river-aquifer connection’ scenario assumes direct connection between the aquifer and the river effectively maximising the volume of abstracted water derived from the river. Conversely, the ‘low river-aquifer connection’ case assumes a very low riverbed conductivity of 7*10-3 m/day (assuming an average river width of 50 m) thereby minimising abstraction-induced leakage from the river to the aquifer. This represents the lowest, physically realistic level of river-aquifer connection. The modelled arrangement of the bore fields at Area 2 and Area 3 was iteratively derived via spreadsheet modelling using Theis (1935) analysis, which suggested a minimum bore spacing of 400 m. Three iteration of modelling were conducted to determine a final bore field arrangement that minimised the drawdown impacts of abstraction. The resulting bore field arrangement is shown in Figure 26 and Figure 27. The model-predicted drawdown in response to abstraction (Section 6.5) suggests that the proposed bore field arrangement has minimum drawdown and bore interference effects on other bores in the area with
42/15610/98344 Mulgrave River Aquifer Feasibility Study 77 Groundwater Modelling potentially only 4 bores impacted at 40ML/day Stage 2 abstraction and only 1, marginally 2 at most, being affected at a Stage 1 abstraction of 15 ML/day. In all instances the potentially affected bores are shallow bores and are not withdrawing from the main quaternary alluvial field, but from shallower lenses in the upper reaches of the aquifer. Compensatory measures may be as simple as extending the depth of the existing bores. The modelled drawdown within the Area 2 and Area 3 bore fields suggests that the proposed average bore depths (50 m at Area 2 and 65 m at Area 3) are sufficient to provide the required available drawdown under the maximum proposed abstraction scenario (40ML/day).
7.3 Potential Impacts
7.3.1 Interaction with Surface Flows Over the period of the modelling (based on 60 years of data) in the longer term the maximum abstraction regime of 15000 ML/year (or 40 ML/d) induces 7 ML/day of leakage from the river into the aquifer. It also suggests that a reduction in groundwater discharge into the river is by 34 ML/day. This leakage and reduction is not immediate, and the output figures are averaged over the 60 life of the model. These figures are the maximum scenario, based on the assumptions of continual abstraction with no allowance for operational aspects, and the model assumed no surface water recharge or base flow entering the surface water system downstream of Peets Bridge. This was assumed owing to the paucity of gauge derived flow data within the immediate alluvial area, and consequently imparts a high level of conservatism. Surface discharges into the Mulgrave River downstream of Peets Bridge include surface and subsurface run off from numerous features but as the data is not present for these features, their input into the model was limited. On average, all of the abstracted groundwater will impact the Mulgrave River and Behana Creek by a combination of combination of reduction in baseflow to the river and creek, and by induced leakage from the river and creek back into the aquifer. The degree of impact will vary seasonally and upon the operational requirements of the bore field, however in the long term the ratio of the volume of abstracted groundwater to the total volume of river flow is predicted to be a one to one ratio. The surface water flow impacts will affect both the Mulgrave River and Behana Creek. Most (~two thirds) of the impact of abstracting 40 ML/day is on the Mulgrave River, whilst at an abstraction rate of 15 ML/day the impacts on Behana Creek and the Mulgrave River are roughly equal. Care must be exercised in adopting these figures as the final impact regime. As has been strongly reinforced, no accounting has been made of surface recharge and subsurface flow in the model downstream of Peets Bridge (a conservative approach adopted owing to the limited data for the middle reaches of the Mulgrave River and Behana Creek) and of the regime of the bore field operation which may not pump for up to 4 months of the year (recognising the bore field is a supplementary supply). Therefore the likely practical (as opposed to modelled) impacts on Behana Creek (and Mulgrave River) will be significantly less than the model prediction. The installation of gauging stations in the middle reaches of the Mulgrave River (within the affected drawdown area) and on Behana Creek will supply essential data to refine the model for a more detailed assessment prior to the implementation of Stage 2. For Stage 1 (15 ML/day) the abstraction, as modelled, will result in a less than 1% impact on surface flows. In consideration of the conservative limits
42/15610/98344 Mulgrave River Aquifer Feasibility Study 78 Groundwater Modelling as outlined previously, the practical impacts of the abstraction on the surface flows of Mulgrave and Behana will be less than that modelled. The model does identify that when comparing the predicted river flow impacts to gauged flow at Peets Bridge, the 40 ML/day of river flow impact represents a relatively small percentage of the estimated long- term average and peak river flows. However, under the simulated abstraction in the climatic case of the particularly dry 2003 dry season, model predictions suggest flows may be reduced by up to 34% (c.f. daily gauged mean total flow at Peets Bridge of 122 ML/day on November 23rd, 2003). Note that this is an unusually dry (1-in-60 year) climatic event. In the average dry season (November 1998), this impact is predicted to be in the order of 5% of the gauged mean total flow.
7.3.2 Drawdown Impacts The predicted drawdown may extend up to 10 km from north to south in dry periods, centred on the proposed abstraction area. The predicted maximum aquifer drawdown in the immediate vicinity of the abstraction points is 5 m. However, this is predicted to be 2 m over the broader area, within 2 to 3 km of either the Area 2 or Area 3 bore field. The modelled drawdown has the potential to affect up to four existing groundwater users, with a combined total annual allocation of more than 258 ML. The maximum predicted reduction in any of the bores’ available drawdown is less than 11%. From the information supplied by NRW on the groundwater bores, these groundwater users can be compensated by the extension of the depths of their bores. The model has not identified that drawdown will significantly affect the saline groundwater interface, and there is therefore a low risk of inducing saline intrusion into the aquifer. The predicted drawdown does not extend to areas of acid sulphate soils or the significant ecological communities at either Trinity Inlet or Mutchero Inlet. In terms of induced land subsidence, in the worst case, the modelling predicts up to 0.26 m in the immediate vicinity of abstraction points, and up to 0.09 m over the broader area. Any subsidence will be gradual over several years and is anticipated to be relatively uniform over the area and is not expected to affect infrastructure or drainage. The results indicate that there will be minimal subsidence impacts in the area.
7.3.3 Groundwater Contamination There is little potential for abstraction of contaminated groundwater if it is assumed that the nearest potential contaminant point source is Gordonvale (waste water treatment plant and Mulgrave mill) on the northern side of the Mulgrave River, or south of Aloomba (capped and sealed landfill), or near Babinda (Babinda waste water treatment plant). All of these areas are well away from the modelled source zone of both abstraction points. However, the simulated abstraction is predicted to induce leakage from the river into the aquifer, and therefore contaminated surface water poses a potential risk to groundwater quality. The most likely source of surface water contamination is the Gordonvale sewage treatment plant (STP). The Gordonvale STP discharges a waste stream into the river upstream of the proposed abstraction area (and the identified area of drawdown/river flow impact). However, the relative proportion of abstracted groundwater modelled as being derived from the Mulgrave River is, on average / minimal. Therefore, the mixing of surface water with groundwater will reduce any potential contaminant concentrations. The particle tracking analyses undertaken in this investigation suggest the shortest groundwater travel times
42/15610/98344 Mulgrave River Aquifer Feasibility Study 79 Groundwater Modelling between the river and the bore field are around 1 year in the calibrated model for the Area 3 bore field. Whilst Area 2 bore field has 18 years of groundwater travel time. These predicted travel times suggest that the aquifer will have significant time and capacity to naturally attenuate any contaminants that may enter the aquifer from the river under induced river leakage into the aquifer.
7.3.4 Summary of Model Predicted Impacts The following figures represent the raw outcomes of models constructed for this assessment. These figures are not absolutes. The figures are the result of modelling which has assumed a number of conservative approaches owing to limited data available for some attributes. These approaches have included assumptions of constant pumping and drawdown of the aquifer (which may be up to 30% in excess of potential operational requirements), and no surface recharge or subsurface flow downstream of the most reliable gauging data (Peets Bridge, upstream of the aquifer). Therefore the modelled has undervalued the contribution of the inputs from other surface features as data was not available. In particular, this has had a very strong bearing on the modelled figures of potential river flow impact for Behana Creek. Behana Creek has limited flow data, and in accordance with the model parameters, assumed no recharge at all of Behana by additional subsurface and surface flows. This has resulted in comparative impacts between the Mulgrave River and Behana Creek being highly skewed, with Behana Creek showing a disproportionate river flow impact by comparison with the Mulgrave River. As data from the recommended gauging station on Behana Creekbecomes available and is able to be included into the model, the model then will be able to assume subsurface and surface water inputs into Behana Creek from its catchment. It is then highly likely that the propotional impacts between the figures in the tables below will significantly alter, with Behana Creek surface flowing absorbing a much small proportion of the river flow impact than is illustrated below.
Table 9 Model Predicted Impacts – Dry Season, Average Climatic Year (1997-98)
Impact Under High River- Impact Under Low River- Best Estimate Impact Aquifer Connection Aquifer Connection
Maximum Drawdown (m) 9 10 5
Drawdown Extent (to 7 10 8 0.5 m) (km)
Number of Existing 2 5 4 Users Affected
Total Licensed Volume 8 268 258 of Affected Users (ML/year)
River Flow Impacts 41 (27 Mulgrave River, 14 21 (16 Mulgrave River, 6 41 (26 Mulgrave River, 16 (ML/day) Behana Creek) Behana Creek) Behana Creek)
Subsidence (m) 0.12 (0.04 over broader area) 0.13 (0.04 over broader area) 0.12 (0.04 over broader area)
Drawdown at Saline 0 0 0 Interface (m)
42/15610/98344 Mulgrave River Aquifer Feasibility Study 80 Groundwater Modelling Table 10 Model Predicted Impacts – After Several Dry Climatic Years (June 2001 to November 2003)
Impact Under High River- Impact Under Low River- Best Estimate Impact Aquifer Connection Aquifer Connection
Maximum Drawdown (m) 5 10 5
Drawdown Extent (to 10 16 10 0.5 m) (km)
Number of Existing 4 11 (13 bores) 4 Users Affected
Total Licensed Volume 258 1233 258 of Affected Users (ML/year)
River Flow Impacts 41 (27 Mulgrave River, 14 24 (20 Mulgrave River, 4 41 (25 Mulgrave River, 16 (ML/day) Behana Creek) Behana Creek) Behana Creek)
Subsidence in 0.12 (0.04 over broader area) 0.26 (0.09 over broader area) 0.12 (0.04 over broader area) immediate vicinity of bores (m)
Drawdown at Saline 0 0.1 0 Interface (m)
Table 11 Model Predicted River Flow Impacts – Dry Season, Average Climatic Year (1997-98)
Impact Under High River- Impact Under Low River- Best Estimate Impact Aquifer Connection Aquifer Connection
Total Mulgrave Behana Total Mulgrave Behana Total Mulgrave Behana River Creek River Creek River Creek
Induced Leakage from 15 11 4 0 0 0 7 2 5 River (ML/day)
Reduced Groundwater 25 16 10 22 16 6 34 23 10 Discharge to River (ML/day)
Total Predicted River 41 27 14 22 16 6 41 26 15 Flow Impact (ML/day)
NOTES: 1. The best estimate reduction in groundwater discharge to the river is higher than that in the high river-aquifer connection case because the reduced river-aquifer connection in the best estimate case causes a greater pumping-induced drawdown extent in the aquifer. This therefore leads to a larger area over which groundwater discharge to the river is reduced. 2. These numbers have been rounded to the nearest integer. Therefore minor discrepancies between the totals and the constituents may occur.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 81 Groundwater Modelling Table 12 Model Predicted River Flow Impacts – Aquifer Several Dry Climatic Years (June 2001 to November 2003)
Impact Under High River- Impact Under Low River- Best Estimate Impact Aquifer Connection Aquifer Connection
Total Mulgrave Behana Total Mulgrave Behana Tota Mulgrave Behana River Creek River Creek l River Creek
Induced Leakage from 28 21 7 16 13 3 23 15 8 River (ML/day)
Reduced Groundwater 13 6 7 8 7 1 18 10 8 Discharge to River (ML/day)
Total Predicted River 41 27 14 24 20 4 41 25 16 Flow Impact (ML/day)
NOTES: 1. The best estimate reduction in groundwater discharge to the river is higher than in the high river-aquifer connection case because the reduced river-aquifer connection in the best estimate case causes a greater pumping-induced drawdown extent in the aquifer. This therefore leads to a larger area over which groundwater discharge to the river is reduced. 2. These numbers have been rounded to the nearest integer. Therefore minor discrepancies between the totals and the constituents may occur.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 82 Groundwater Modelling 8. Recommendations
The following recommendations should be undertaken, should the proposed groundwater supply from the Mulgrave Alluvium will commenced:
The groundwater supply is initiated via a staged approach, with an initial stage of abstraction at 15 ML/day. The supply could then be gradually expanded to greater volumes. This will depend on the identification of any impacts associated with the operating groundwater supply.
The bore field layout presented in this report should be used to plan any drilling works. This layout could be further improved if private land were sought to place the bore field, rather than being restricted to public land. Detailed monitoring of river flows and drawdown in the aquifer during the first stage of supply operation. These data can be used to provide support to future expansion of the groundwater supply. They may also be used to provide a detailed data set against which the groundwater model may be re-calibrated, and used to provide better-supported model predictions of potential impacts of the future expansion of the supply. Confidence in the numerical model and the resulting impact predictions presented above could be improved through:
The installation of permanent river flow gauges in the lower catchment and regular spot gauging of dry season flows along the length of the river and key tributaries, such as Behana Creek. Specifically: – Gauge installation and daily monitoring of Mulgrave River flows at the tidal limit (high priority), and at the catchment outlet if possible (Mutchero Inlet) (lowest priority). – Rehabilitation / replacement of the Gordonvale gauge so that it measures total flow at all times (high priority). – Regular spot flow gauging at times of low flows, for the purposes of flow accretion profiling. This should be completed at: o Several (say more than 5) locations along the Mulgrave River between Peets Bridge and the Catchment outlet (high priority); o Several locations along the alluvial deposits of Behana Creek (medium priority); and o Along any other major tributaries to the Mulgrave River (lowest priority). Detailed assessment of river flow data once sufficient data are collected in order to better understand river-aquifer interaction in the area. It is recommended that drilling investigations be undertaken around Mutchero Inlet to confirm the location of the saline/fresh groundwater interface.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 83 Groundwater Modelling 9. References
Cook P.G. and Herczeg A.L. (1998) Groundwater Chemical Methods for Recharge Studies. Part 2 of L. Zhang and G. Walker (ed.) The Basics of Recharge and Discharge. CSIRO Publishing. Cook, P.G, Healy R.W., 2002. Using Groundwater Levels to Estimate Recharge. Hydrogeology Journal 10: pp. 91-109. Dept. Natural Resources, QLD 1999: Report on Mulgrave Groundwater Model Doherty, J. E., 2002. PEST Model-independent Parameter Estimation, Users Guide. Watermark Numerical Computing. GHD, 2006. Mulgrave River Feasibility Study Hydrogeological Report (GHD Draft report 42/14087/01/7410, August 2006) Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000. MODFLOW-2000, the U.S. Geological Survey modular ground-water model – user guide to modularization concepts and the ground- water flow process, U.S. Geological Survey Open-File Report 00-92. Leach L.M. and Rose U.E., 1979: Groundwater Storage Behaviour, Mulgrave River Area. Qld Water Res. Comm. Report. Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R. and Hammer, G.L. 1989. PERFECT, A computer simulation model of Productivity, Erosion, Runoff Functions to Evaluate Conservation Techniques. Queensland Department of Primary Industries, Bulletin QB89005, 119 pp. Muller,P.J., 1975: Mulgrave River Groundwater Investigations, Report on Exploratory Drilling. Rec. Geol. Surv. Qld. 1975/17. Murtha G., Cannon M. and Smith C. 1996: Soils of the Babinda - Cairns Area, North Queensland. CSIRO Aust. Div. Soils, Divl Rpt. No. 123. Schapp, M.G., Leij, F.J., and van Genuchten, M.Th. (2001). "Rosetta: A computer program for estimating soil hydraulic parameters with hierarchical pedotransfer functions". Journal of Hydrology 251 (3): 163– 176 Sloto, R.A., Crouse, M.Y., 1996. HYSEP: A Computer Program For Streamflow Hydrograph Separation And Analysis. U.S. Geological Survey Water-Resources Investigations Report 96-4040 Stewart, M., Cimino, J., and Ross, M. 2007. Calibration of Base Flow Separation Methods with Streamflow Conductivity. Ground Water Vol. 45, No. 1, January–February 2007, pp 17–27. Theis, C.V., 1935. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage. Trans. Amer. Geophys. Union 2, pp. 519-524. van Genuchten M.Th. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 1980;44:892-898. Walsh, K., Cai, W., Hennessy, K., Jones, R., McInnes, K., Nguyen, K., Page C., and Whetton, P., 2002. Climate Change in Queensland under Enhanced Greenhouse Conditions Final Report, 2002. Report on research undertaken for Queensland
42/15610/98344 Mulgrave River Aquifer Feasibility Study 84 Groundwater Modelling Departments of State Development, Main Roads, Health, Transport, Mines and Energy, Treasury, Public Works, Primary Industries, and Natural Resources. CSIRO 2002. Western, A., McKenzie, N., 2006. Soil hydrologic Properties of Australia. Rev. 1.01. Cooperative Research Centre for Catchment Hydrology, University of Melbourne, Victoria, Australia. Willmott, W.F. and Stephenson, P.J. 1989. Rocks and landscapes of the Cairns district. Queensland Dept. of Mines.
42/15610/98344 Mulgrave River Aquifer Feasibility Study 85 Groundwater Modelling Appendix A Geophysical Logging Inventory
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Bore Status
11100002 Not monitored for many years, not located, suspect bore is abandoned
11100003 Not monitored for many years, not located, suspect bore is abandoned
11100004 Not monitored for many years, not located, suspect bore is abandoned
11100005 Not monitored for many years, not located, suspect bore is abandoned
11100006 Not monitored for many years, not located, suspect bore is abandoned
11100007 Not monitored for many years, not located, suspect bore is abandoned
11100008 Not monitored for many years, not located, suspect bore is abandoned
11100009 Okay, gamma and induction
11100010 Obs bore for 0009
11100011 Obs bore for 0009
11100012 Not monitored for many years, not located, suspect bore is abandoned
11100013 Not monitored for many years, not located, suspect bore is abandoned
11100014 No records
11100015 No records
11100016 Okay, gamma and induction
11100017 Okay, gamma and induction
11100018 Equipped with auto recorder, no log possible
11100019 Okay, gamma and induction
11100020 No records
11100021 Not monitored for many years, not located, suspect bore is abandoned
11100022 Not monitored for many years, not located, suspect bore is abandoned
11100023 No records
11100024 Not monitored for many years, not located, suspect bore is abandoned
11100025 Okay, gamma and induction
11100026 Obs bore for 0025
11100027 Obs bore for 0025
11100028 Not monitored for many years, not located, suspect bore is abandoned
11100029 Not monitored for many years, not located, suspect bore is abandoned
11100030 Not monitored for many years, not located, suspect bore is abandoned
11100031 No records
11100032 Not monitored for many years, not located, suspect bore is abandoned
11100033 Okay, gamma and induction
11100034 Obs bore for 0033
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Bore Status
11100035 Relined, new pipe too small to fit logging probes, no logging possible
11100036 No records
11100037 No records
11100038 Okay, gamma and induction
11100039 Relined, new pipe too small to fit logging probes, no logging possible
11100040 Relined, new pipe too small to fit logging probes, no logging possible
11100041 No records
11100042 Not monitored for many years, not located, suspect bore is abandoned
11100043 Not monitored for many years, not located, suspect bore is abandoned
11100044 Okay, gamma and induction
11100045 Okay, gamma and induction
11100046 Obs bore for 0045
11100047 Obs bore for 0045
11100048 Relined, new pipe too small to fit logging probes, no logging possible
11100049 Relined, new pipe too small to fit logging probes, no logging possible
11100050 Not monitored for many years, not located, suspect bore is abandoned
11100051 No records
11100052 No records
11100053 Not monitored for many years, not located, suspect bore is abandoned
11100054 Relined, new pipe too small to fit logging probes, no logging possible
11100055 Not monitored for many years, not located, suspect bore is abandoned
11100056 Okay, gamma and induction
11100057 Okay, gamma and induction
11100058 Okay, gamma and induction
11100059 Okay, gamma and induction
11100060 No records
11100061 No records
11100062 No records
11100063 No records
11100064 No records
11100065 No records
11100066 No records
11100067 No records
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Bore Status
11100068 No records
11100069 No records
11100070 Okay, gamma and induction
11100071 Not monitored for many years, not located, suspect bore is abandoned
11100072 Okay, gamma and induction
11100073 Okay, gamma and induction
11100074 Okay, gamma and induction
11100075 Okay, gamma and induction
11100076 Okay, gamma and induction
11100077 Okay, gamma and induction
11100078 Blocked/kinked at 5m, no logging done
11100079 Okay, gamma and induction
11100080 No records
11100081 Okay, gamma and induction
11100082 No records
11100083 No records
11100084 No records
11100085 No records
11100086 No records
11100087 No records
11100088 No records
11100089 No records
11100090 No records
11100091 No records
11100092 No records
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Appendix B Observed and Modelled Groundwater Level Hydrographs
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling www.ghd.com.au GHD Tel. (03) 8687 8000 Fax. (03) 8687 8111 180 Lonsdale Street Melbourne Vic 3000
Enter Client/Project Name Here Enter Spreadsheet Name Here
Appendix C 28/05/2007 5:53 PM G:\42\14087\MODFLOW\02\02e\output\Appendix D\Appendix C.xls Page 1 of 2 www.ghd.com.au GHD Tel. (03) 8687 8000 Fax. (03) 8687 8111 180 Lonsdale Street Melbourne Vic 3000
River stage height and the degree of river- aquifer interconnection is poorly represented in this (down-river) area of the model
The hydrographs on this page are from bores located in the south of the catchment. It is evident that river stage dynamics and the degree of river-aquifer interconnection play an important role in controlling groundwater levels and fluctuations. The model is unable to represent these dynamics in its current state (uncalibrated to river baseflow due to a lack of data), and therefore the calibration quality deteriorates down catchment towards Mutchero Inlet.
Appendix C 28/05/2007 5:53 PM G:\42\14087\MODFLOW\02\02e\output\Appendix D\Appendix C.xls Page 2 of 2 Appendix C Observed and Modelled Aquifer Test Drawdowns
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling www.ghd.com.au GHD Tel. (03) 8687 8000 Fax. (03) 8687 8111 180 Lonsdale Street Melbourne Vic 3000
Enter Client/Project Name Here Enter Spreadsheet Name Here
Appendix D 28/05/2007 5:51 PM G:\42\14087\MODFLOW\03\03b\OUTPUT\Appendix_D_03b_Drawdowns.xls Page 1 of 1 Appendix D Detailed Water Balance Calculations – Mulgrave River Alluvium
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling www.ghd.com.au GHD Tel. (03) 8687 8000 Fax. (03) 8687 8111 180 Lonsdale Street Melbourne Vic 3000
Cairns Water / Mulgrave River Aquifer Groundwater Model Mulgrave River Alluvium Water Balance - Detailed Calculations
WATER BALANCE INPUTS Units BALANCE COMMENTS
RECHARGE Area of Mulgrave Alluvium 329508824 m2 Recharge 460.0 mm From hydrogrpah fluctuation method Total Recharge 151574 ML IN
Stream leakage into aquifer Assume 2% of Behana creek annual flow (176660ML) recharges aquifer. This is the Leakage 35533 ML IN component used to balance the water balance
Groundwater Inflows From Upstream Alluvials Assuming average hydraulic conductivity of 26.5m/d (from aquifer tests), gradients estimated from water level mapping (10m/800m), cross sectional areas from constructed geological Behana Creek Upstream Alluvials 994 ML surfaces (depth to bedrock; 10m thickness, 300m width) Assuming average hydraulic conductivity of 26.5m/d (from aquifer tests), gradients estimated from water level mapping, cross sectional areas from constructed geological surfaces (depth to Mulgrave River Upstream Alluvials 11612 ML bedrock) Total Inflows 12606 ML IN
Groundwater Abstractions Total Allocations 22000 ML Unused (Cairns Water) 15000 ML
Used Allocation 2100 ML OUT Assume 30% of allocation is used (Leach and Rose, 1979) and most are irrigation licenses
BASEFLOW Estimated average gradient in vicinity of average stage height at Gordonvale gauge, and Hydraulic gradient to river 0.007 average groundwater level in bore 11100075, located 1km upstream of this gauge Aquifer thickness 20m Estimated thickness of aquifer contributing to baseflow 67000 m Estimated total major river/stream length draining the alluvium Hydraulic conductivity 26.5 m/d Average hydraulic conductivity from aquifer tests Baseflow 181456 ML OUT Detailed discussion of this calculation in Section 3.4.1 of report
GROUNDWATER THROUGHFLOW Trinity Inlet cross-sectional area 402268 m2 From modelled geological surfaces Trinity Inlet hydraulic gradient 0.002 m/m From mapped groundwater levels (average June-July) Trinity Inlet hydraulic conductivity 26.5 m/d From aquifer tests Throughflow via Trinity Inlet 8979 ML Mutchero Inlet cross-sectional area 85455 m2 From modelled geological surfaces Mutchero Inlet hydraulic gradient 0.003 m/m From mapped groundwater levels (average June-July) Mutchero Inlet hydraulic conductivity 26.5 m/d From aquifer tests Throughflow via Mutchero Inlet 2119 ML TOTAL Throughflow 11098 ML OUT
Balance - alluv only (2) 11/07/2007 1:30 PM G:\42\14087\Tech\Water balance\water balance.xls Page 1 of 1 Appendix E PERFECT Model – Assigned Soil Properties
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Soil Horizon Sa Si Cl Depth Adjusted Adjusted Adjusted Adjusted Adjusted Bulk CONA U Great (%) (%) (%) To ThetaR ThetaS Ksat Wilting Field Density Group (m) (cm/d) Point Capacity
Bg_Bulgun A 31.8 22.3 45.9 0.3 0.09 0.45 9.60 0.17 0.37 1.1 4 9.5 Euchrozem
Bg_Bulgun B 27.6 25.3 47.1 1.2 0.09 0.46 11.33 0.17 0.38 1.1 4 9 Euchrozem
Cn_Canoe A 65.6 14.3 20.1 0.3 0.06 0.38 21.09 0.10 0.27 1.2 3.75 8.5 Earth
Cn_Canoe B 74.3 9.9 15.8 1.2 0.06 0.38 34.18 0.08 0.24 1.2 3.5 6.75 Earth
Cn_Canoe C 81.0 8.0 11.0 1.5 0.05 0.38 77.08 0.06 0.20 1.2 3.5 6.75 Earth
Ct_Clifton A 70.8 17.0 12.2 0.4 0.05 0.37 41.32 0.07 0.23 1.2 3.5 6.75 Earth
Ct_Clifton B 61.3 20.6 18.1 1.1 0.04 0.30 17.18 0.07 0.21 1.3 3.75 8.5 Earth
Ct_Clifton C 61.8 22.0 16.2 1.7 0.05 0.37 24.65 0.08 0.26 1.2 3.75 8.5 Earth
Et_Edmonton A 23.5 44.3 32.3 0.1 0.08 0.45 12.11 0.12 0.38 1.1 4 9 Euchrozem
Et_Edmonton B 30.5 41.1 28.4 1.1 0.08 0.43 11.34 0.11 0.36 1.1 4 9 Euchrozem
Il_Inlet A 43.0 20.8 36.2 0.2 0.08 0.42 6.34 0.14 0.33 1.1 4 9.5 Euchrozem
Il_Inlet B 36.8 24.5 38.7 1.6 0.09 0.44 5.34 0.15 0.35 1.1 4 9.5 Euchrozem
Il_Inlet D 16.1 32.8 51.2 2.8 0.10 0.49 17.44 0.18 0.41 1.1 4 9 Euchrozem
In_Innisfail A 32.5 25.4 42.1 0.4 0.09 0.44 6.70 0.16 0.36 1.1 4 9.5 Euchrozem
In_Innisfail B 44.1 25.8 30.2 1.5 0.08 0.41 6.64 0.12 0.32 1.1 4 9 Euchrozem
In_Innisfail C 67.5 15.8 16.8 2.1 0.05 0.38 27.49 0.09 0.26 1.2 3.75 8.5 Euchrozem
Km_Kirrama A 64.2 5.5 30.4 0.5 0.06 0.35 12.62 0.12 0.27 1.2 4 9 Earth
Km_Kirrama B 52.9 2.3 44.7 1.8 0.07 0.33 15.05 0.15 0.27 1.3 4 9.5 Earth
Li_Liverpool A 64.8 13.9 21.4 0.2 0.06 0.38 19.28 0.10 0.27 1.2 3.75 8.5 Earth
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Soil Horizon Sa Si Cl Depth Adjusted Adjusted Adjusted Adjusted Adjusted Bulk CONA U Great (%) (%) (%) To ThetaR ThetaS Ksat Wilting Field Density Group (m) (cm/d) Point Capacity
Li_Liverpool B 69.7 12.6 17.7 1.2 0.06 0.38 26.51 0.09 0.26 1.2 3.75 8.5 Earth
Li_Liverpool D 76.0 11.0 13.0 1.5 0.05 0.38 48.66 0.07 0.23 1.2 3.5 6.75 Earth
Mb_Malbon A 54.0 20.0 26.0 0.2 0.06 0.36 10.40 0.11 0.27 1.2 4 9 Earth
Mb_Malbon B 64.0 13.2 22.8 1.0 0.05 0.29 12.75 0.08 0.22 1.3 3.75 8.5 Earth
Mb_Malbon D 20.1 36.4 43.5 1.9 0.09 0.47 12.41 0.16 0.39 1.1 4 9.5 Earth
Ms_Mission A 68.0 12.0 20.0 0.1 0.09 0.44 11.69 0.15 0.37 1.1 3.75 8.5 Earth
Ms_Mission B 70.9 10.8 18.3 0.8 0.06 0.33 8.63 0.11 0.27 1.2 3.75 8.5 Earth
Ms_Mission C 79.6 11.1 9.3 1.6 0.05 0.27 7.22 0.09 0.23 1.3 3.5 6.75 Earth
Mn_Mangrove A 50.5 24.8 24.8 0.1 0.09 0.47 12.44 0.16 0.39 1.0 3.75 8.5 Euchrozem
Mn_Mangrove B 40.0 25.5 34.5 0.3 0.09 0.47 12.44 0.16 0.39 1.1 4 9 Euchrozem
Mn_Mangrove D 37.0 27.0 36.0 0.6 0.09 0.47 12.44 0.16 0.39 1.1 4 9.5 Euchrozem
Pg_Pin_Gin A 12.0 35.5 52.5 0.3 0.09 0.47 12.32 0.15 0.39 1.1 4 9 Euchrozem
Pg_Pin_Gin B 9.5 36.9 53.6 1.7 0.09 0.47 12.30 0.15 0.39 1.1 4 9 Euchrozem
Th_Thorpe A 69.9 11.0 19.1 0.4 0.06 0.31 8.15 0.10 0.26 1.3 3.75 8.5 Earth
Th_Thorpe B 71.6 8.8 19.6 1.0 0.06 0.29 7.57 0.09 0.24 1.3 3.5 6.75 Earth
Th_Thorpe C 82.8 7.9 9.4 1.6 0.04 0.22 5.91 0.07 0.19 1.4 3.5 6.75 Earth
Th_Thorpe D 72.5 15.8 11.8 1.8 0.09 0.47 12.32 0.15 0.39 1.0 3.5 6.75 Earth
Ti_Timara A 8.5 61.3 30.3 0.2 0.09 0.47 12.44 0.16 0.39 1.1 4 9 Euchrozem
Ti_Timara B 6.0 58.3 35.7 0.7 0.09 0.47 12.44 0.16 0.39 1.1 4 9 Euchrozem
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Soil Horizon Sa Si Cl Depth Adjusted Adjusted Adjusted Adjusted Adjusted Bulk CONA U Great (%) (%) (%) To ThetaR ThetaS Ksat Wilting Field Density Group (m) (cm/d) Point Capacity
Ti_Timara 2.0 4.6 50.2 45.2 1.0 0.09 0.47 12.44 0.16 0.39 1.1 4 9.5 Euchrozem
Ti_Timara D 29.4 48.6 22.0 1.4 0.09 0.47 12.44 0.16 0.39 1.0 3.75 8.5 Euchrozem
Vi_Virgil A 61.2 15.1 23.6 0.3 0.09 0.47 12.32 0.15 0.39 1.0 3.75 8.5 Earth
Vi_Virgil B 59.3 19.9 20.8 2.2 0.09 0.47 12.29 0.15 0.39 1.0 3.75 8.5 Earth
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Appendix F Residual Statistics – Steady State Calibration
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling
Appendix G Residual Statistics – Transient Calibration
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling
Appendix H Land Subsidence Analysis
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Introduction Aquifer depressurisation induced land subsidence is the gradual settling of ground surface due to reduction in water pressure and a corresponding increase in effective stresses in the ground. Subsidence is commonly caused by the compression of soils and rock in and around areas of l arge scale groundwater pumping. Land subsidence due to groundwater withdrawal has been recorded in Asia, Europe, South America, North America and Australia. At Mulgrave River, aquifer depressurisation during groundwater extraction operation is not expected to result in significant land subsidence.
Land Settlement Mechanisms The physical process that links fluid withdrawal to settlment is fundamental and essentially means that for a confined aquifer system the two physical processes are chained together. Given the areal extent of the aquifers at Mulgrave River means that vertical compression of the aquifer systems at depth will result directly in settlement in land surface without significant three-dimensional (i.e. differential) effects. Initially the weight of overburden (soil and water) above an aquifer is in equilibrium being carried by support forces consisting of water pressure and sediment grain-to-grain stress. As water is removed from the aquifer, the fluid pressure decreases. Because the weight above the aquifer does not change with time this weight must continue to be carried by the aquifer system. The portion of overburden weight that was initially supported by the water decreases and an increasing portion is carried by the soil structure. The skeletal structure of the soil becomes more densely packed to achieve a new equilibrium resistance to the overburden load. The result is a decrease in porosity within the aquifer system and corresponding settlement of the land surface. In addition, the slow draining, low permeability clay members of an aquifer system are often found to be more compressible than sands. This results in a time lapse between changes in water pressures and cumulative compression of the entire system. Although settlement of sand units is relatively fast and occurs quickly, volume changes within the clay soils are delayed and occur over a long period of time. The settlement behaviour of clay soils is usually dependant on its stress history and can be expressed in engineering terms as being in either one of two states: 1. Normally consolidated: Where the in-situ stress state of the soil has seen little or no increase in overburden pressure in the past. This is usually expressed as the preconsolidation pressure (Pc) being equal to the current stress state (Po). (i.e. Pc=Po). 2. Overconsolidated: Where the in-situ stress state of the soil is presently less than in the past, due to removal of overburden pressure. This is usually expressed as the preconsolidation pressure (Pc) being greater than the current stress state (Po). (i.e. Pc>Po). The compressibility behaviour of clay soils between normally and overconsolidated states is usually one to two orders of magnitude apart. Soils that are defined as normally consolidated are considered to be more susceptible to settlement than overconsolidated soils and have a correspondingly higher compressibility index value. The degree of over consolidation greatly reduces the compressibility index of the soil and is directly reflected by the how much the preconsolidation pressure (Pc) exceeds the current overburden pressure (Po). Subsequently, the effective stress range within sediment sequence in relation to its
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling preconsolidation pressure (Pc) will have a major bearing on the magnitude of land subsidence that will occur in response to a given reduction in groundwater levels.
Settlement Model In this study settlement is assumed to occur in vertical direction only in response to changes in aquifer pressure due to the operation of the production bores. The process of consolidation settlement has a log-linear relationship. The differential equation that describes the process of consolidation of horizontal clay beds presented below is widely used in the numerical simulation of subsidence.
ª § ' pp iio )()( ·º «CRS H log¨ ¸» when Po + 'P > Pc ¦ 3 i ¨ p ¸ ¬« © io )( ¹¼»
ª Pc § ' pp iio )()( ·º «CRS H log CR H log¨ ¸» when Po + 'P < Pc ¦ 2 i p 3 i ¨ p ¸ ¬« io )( © io )( ¹¼»
Where: S = the sum of primary consolidation settlement Cc CR2 = Compression Ratio 1 eo Cs CR3 = Compression Ratio 1 eo
Cc = compression Index Cs = Swell Index Hi = Initial thickness of sub-layer i
Po(i) initial average effective overburden pressure for sub-layer i
'P = increase in effective stress in each sub-layer of soil analysed Pc = maximum preconsolidation pressure
In applying the settlement model to the Mulgrave River site the adopted stratigraphic soil profile was simplified at two selected locations, as follows: Area 2 Clay 0 to 9 m depth;
Sand 9 to 14 m depth; Clay 14 to 20 m depth;
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Sand 20 to 65 m depth; Weathered Granite 65m to 92 m depth Granite Fresh >92 m depth
Area 3 Clay 0 to 5 m depth;
Sand 5 to 10 m depth;
Clay 10 to 18 m depth; Sand 18 to 29 m depth; Clay 29 to 38 m depth;
Sand 38 to 60 m depth; Weathered Granite 65m to 90 m depth Granite fresh >90 m depth
The settlement discussed considers Quaternary and Tertiary units only. Settlement within the underlying basement rock units is anticipated to be negligible in the context of the magnitudes assessed (ie, settlements within rock units are likely to be insignificant compared with those in Quaternary and Tertiary). Groundwater levels are 9 m and 6 m below ground surface in areas 2 and 3 respectively at the start of groundwater extraction from the aquifers. All soil units were divided into 1 m thick sub-layers for subsidence modelling purposes. Secondary consolidation settlements (ie, creep) have not been included in this assessment of subsidence and their contribution will depend on the degree of overconsolidation present if any.
Overconsolidation Ratio
General At this time we have no information at our disposal to indicate if the soil strata at the Mulgrave River site is normally consolidated or over consolidated.
Sea Level Changes Major sea level changes occurred during the Late Pliocene (128ka to 10ka BP) to Holocene (10ka to 0ka BP), which have influenced the geological process within the Mulgrave River site including depositional and erosional cycles. Sea level changes during this time were caused by the most recent global ice age, which resulted in widespread glaciation of the northern and southern hemispheres. Worldwide studies of sea level changes in response to glacio-eustatic events during the late Quaternary and Holocene periods have been well studied. The worldwide fluctuations in mean sea levels since the last interglacial period are generally based on studies completed in this area by Chappel and Shackelton (1986), Pirazzoli (1991) and others. These studies indicate that around 128ka before present (BP) the sea levels were close to today’s levels and over the period from 117ka BP to 17ka BP gradually fell by approximately 120 m coinciding with glacial maximum period of 17ka to 24ka BP. After this event sea levels gradually
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling rose returning to present day levels at around 7ka BP. It is understood that approximately 20 such cycles may have occurred during the Quaternary period which begun 1 800ka BP.
Adopted Approach Based on published papers on similar sites around the world a range of possible overconsolidation conditions have been adopted to gauge the likely range of subsidence that may be triggered by withdrawal of groundwater. The largest magnitudes of land settlement will occur when the soil strata being affected is normally consolidated. At the other end of the scale the minimum subsidence will occur when this strata is heavily overconsolidated. The most likely outcome is expected to be somewhere in the middle.
Land Settlement Estimate Sections below detail subsidence calculations including the total long term settlements as well as indication of short-term settlement component.
Soil Parameters Physical soil parameters presented in Table 13 have been adopted for analyses of aquifer depressurisation induced settlement at Mulgrave River.
Table 13 Soil Parameters for Land Settlement Calculations
Geological Unit CR2 (kPa) CR3 (kPa) Excess Predominant Material Preconsolidation Type Po to Pc >Pc Pressure (kPa)*
Upper Clay 0.03 0.240 100 Clay
Sand Units 0.013 0.100 100 Sand
Middle and Lower Clay 0.03 0.240 100 Clay Units
Weathered Granite 0.006 0.050 1000 Clay
Note: * - The following upper bound and lower bound estimates for Excess Preconsolidation pressure of 0 kPa and 300 kPa respectively have also been adopted.
Groundwater Levels Hydrogeological modelling of extraction from the Mulgrave River aquifer indicates development of a drawdown cone centred on the Aloomba – Charinga area. For settlement estimates we have adopted the 30-year drawdown contours. Average drawdowns across the area resulting from groundwater extraction have been estimated from the numerical modelling. These have been interpreted at 25 m and 35 m below ground surface for Areas 2 and 3 respectively.
Estimated Total Settlement Table 14 summarises the best estimate and possible ranges of total settlement at the two locations.
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Table 14 Calculated Settlement (m)
Initial Drawdown Settlement (m) Water (m) Level (bgl) Best Range Estimate Minimum Maximum
Area 2 9 3 0.03 0.03 0.26
Area 3 6 3 0.05 0.05 0.39
Note: Minimum and maximum estimates are based on over consolidation of 0 kPa and 300 kPa respectively. The results indicate that under the most likely scenario there will be minimal subsidence impacts in the area. Any subsidence will be gradual over several years and is anticipated to be relatively uniform over the area. It is not expected to effect infrastructure or drainage. Settlement at the maximum calculated could affect hydraulic gradients in the Mulgrave River and may influence local flooding, for example. The settlement within the sands is expected to occur soon after groundwater drawdown and aquifer depressurisation (“immediate settlement”), whilst the settlement within clays will take a long period of time to complete. On average approximately one third of total settlement will occur immediately in response to groundwater drawdown, while the residual settlement will occur over a long period of time.
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling Appendix I Groundwater Quality Analysis
42/15610/98344 Mulgrave River Aquifer Feasibility Study Groundwater Modelling