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Dunedin Groundwater Monitoring and Spatial Observations

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Dunedin Groundwater Monitoring and Spatial Observations Dunedin Groundwater Monitoring and Spatial Observations SC Cox MHJ Ettema SM Mager PJ Glassey S Hornblow S Yeo GNS Science Report 2020/11 June 2020 DISCLAIMER The Institute of Geological and Nuclear Sciences Limited (GNS Science) and its funders give no warranties of any kind concerning the accuracy, completeness, timeliness or fitness for purpose of the contents of this report. GNS Science accepts no responsibility for any actions taken based on, or reliance placed on the contents of this report and GNS Science and its funders exclude to the full extent permitted by law liability for any loss, damage or expense, direct or indirect, and however caused, whether through negligence or otherwise, resulting from any person’s or organisation’s use of, or reliance on, the contents of this report. BIBLIOGRAPHIC REFERENCE Cox SC, Ettema MHJ, Mager SM, Glassey PJ, Hornblow S, Yeo S. 2020. Dunedin groundwater monitoring and spatial observations. Lower Hutt (NZ): GNS Science. 86 p. (GNS Science report; 2020/11). doi:10.21420/AVAJ-EE81. SC Cox, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand MHJ Ettema, Otago Regional Council, Private Bag 1954, Dunedin 9054, New Zealand SM Mager, University of Otago, School of Geography, PO Box 56, Dunedin 9054, New Zealand PJ Glassey, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand S Hornblow, Otago Regional Council, Private Bag 1954, Dunedin 9054, New Zealand S Yeo, University of Otago, School of Geography, PO Box 56, Dunedin 9054, New Zealand © Institute of Geological and Nuclear Sciences Limited, 2020 www.gns.cri.nz ISSN 2350-3424 (online) ISBN 978-1-99-001031-6 (online) http://dx.doi.org/10.21420/AVAJ-EE81 CONTENTS ABSTRACT .......................................................................................................................... V KEYWORDS ........................................................................................................................ VI 1.0 INTRODUCTION ........................................................................................................1 1.1 Sea-Level Rise, Groundwater and Infrastructure ............................................. 1 1.2 Coastal Aquifer Systems and the Water Table................................................. 2 1.3 Dunedin ...........................................................................................................4 2.0 DUNEDIN DATA AND REFERENCE DATUM ............................................................ 7 2.1 Groundwater Monitoring Network .................................................................... 7 2.2 Surveying, Reference Frames, Conversions and Other Data ......................... 13 2.3 Longer-Term Context of 2019 Groundwater Observations ............................. 14 3.0 INTERPOLATED STATISTICAL SURFACES OF GROUNDWATER ...................... 19 3.1 Groundwater Elevation .................................................................................. 20 3.2 Depth to Groundwater ................................................................................... 22 3.3 Effect and Reach of Tides ............................................................................. 24 3.4 Effect and Efficiency of Rainfall ..................................................................... 29 4.0 VERTICAL HETEROGENEITY AND HYDRAULIC PROPERTIES ........................... 31 4.1 Perched Groundwater .................................................................................... 31 4.2 Confined Groundwater ................................................................................... 32 4.3 Vertical Pressure Gradients ........................................................................... 33 4.4 Drawdown and Recovery ............................................................................... 35 4.5 Temperature .................................................................................................. 38 5.0 GROUNDWATER AND WATER NETWORKS ......................................................... 40 5.1 Background ................................................................................................... 40 5.2 Network Position Relative to the Water Table ................................................ 41 6.0 GROUNDWATER AND FLOODING ......................................................................... 44 6.1 Background ................................................................................................... 44 6.2 Heavy (>75 mm) Rainfall Events ................................................................... 46 6.2.1 3–4 June 2015 ................................................................................................. 46 6.2.2 21–22 July 2017 .............................................................................................. 50 6.2.3 1 February 2018 .............................................................................................. 51 6.3 Discussion: Spatial versus Temporal Variability ............................................. 53 6.4 Available Subsurface Storage ........................................................................ 55 7.0 EFFECTS OF SEA-LEVEL RISE.............................................................................. 58 8.0 SUMMARY AND CONCLUSIONS ............................................................................ 64 9.0 ACKNOWLEDGMENTS ........................................................................................... 68 10.0 REFERENCES ......................................................................................................... 69 GNS Science Report 2020/11 i FIGURES Figure 1.1 Changes to the water table associated with sea-level rise, according to different models and local influences on groundwater systems ..................................................................................... 3 Figure 1.2 Number of homes <50 cm elevation above the spring high tide mark, for nine coastal urban areas in New Zealand .................................................................................................................. 6 Figure 1.3 Elevation model of Dunedin from the 2009 LiDAR survey ............................................................ 6 Figure 2.1 Groundwater monitoring sites in Dunedin..................................................................................... 8 Figure 2.2 Definition of ground level and measuring point as used in the Dunedin groundwater monitoring dataset, similar to that used by Environment Canterbury ........................................................... 11 Figure 2.3 Plots of 2019 groundwater elevation against the numerical day of the year (Julian days as 0–365 values) at the four ORC sites with longer-term data ........................................................ 16 Figure 2.4 Plots of 2019 daily rainfall, and groundwater elevation at ORC piezometers compared with the daily rainfall at Musselburgh station averaged over the previous 30 days .................................. 17 Figure 2.5 Plots of 2019 groundwater elevation against percentiles of the observation statistical distribution at the four ORC sites, compared against the longer-term 2010–2018 data records ................... 18 Figure 3.1 Interpolated median groundwater elevation (MedianGWL_2019) .............................................. 21 Figure 3.2 Interpolated model of the range (GWLrange_2019 in m) of groundwater levels in Dunedin. ..... 21 Figure 3.3 Interpolated depth to median groundwater elevation (MedianDTW_2019)................................. 23 Figure 3.4 Electrical conductance of groundwater (μS/cm) as piezometer spot values and an interpolated grid, overlain by modelled %Seawater contours ......................................................................... 25 Figure 3.5 Tidal efficiency at each of the monitoring sites ........................................................................... 28 Figure 3.6 Tidal phase lag at each of the monitoring sites relative to the nearest (ocean or harbour) tidal source, plotted over the tidal efficiency grid................................................................................ 28 Figure 3.7 Graphs of (logarithmic) tidal efficiency (as lnTE) and phase lag (in minutes) against distance from the nearby tidal source ....................................................................................................... 29 Figure 3.8 The RRI ratio as a series of piezometer point observations and an interpolated grid ................. 30 Figure 4.1 Interpreted map-limit of perched groundwater sitting at around 1–1.5 m elevation within dune sands .......................................................................................................................................... 32 Figure 4.2 Plot of the difference in groundwater level for sites where deep and shallow piezometers are situated near each other ............................................................................................................. 34 Figure 4.3 Council St (I44/1124) groundwater level (black in mNZVD2016) and specific conductance during 2019 ................................................................................................................................ 37 Figure 4.4 Examples of temperature data for the period 6 Mar 2019 – 12 Feb 2020 in shallow piezometers ≤6 m deep .................................................................................................................................. 39 Figure 5.1 Position of stormwater network relative
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