Saltwater intrusion in Southern Eyre Peninsula, December 2009

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2.2 Significance of Seawater Intrusion on EP Several of EP’s most important groundwater sources occur in coastal settings. These include the Uley South and Lincoln Basins, which respectively provide ~70% and ~10% of total reticulated water, as well as Coffin Bay “A” Lens which currently provides 100% of reticulated water to the township of Coffin Bay (i.e. an isolated supply system). These resources lie within the Southern Basins PWA (Figure 2). Given the overall importance of these aquifers, it is important to understand and manage against the risk of seawater intrusion due to either climate change or overextraction. Figure 2. Southern Basins PWA, showing locations of individual groundwater basins and SA Water production bores. Note that water is not extracted from all basins; however three of the key production areas (Uley South, Lincoln and Coffin Bay A) are located adjacent to the coast. 6
Water Level (m AHD) Water Level (m AHD) Water Level (m AHD) Water Level (m AHD) Hydrographs from the four key water supply areas in the Southern Basins are shown (Figure 2.1b) for comparison. Of the four areas shown, the three coastal aquifers (Coffin Bay, Uley South and Lincoln B) currently have relatively steady water levels at around 1m AHD (i.e. 1m above mean sea level). Uley Wanilla is an inland system occurring at a higher elevation, and is not connected to the coast, however it is included here to show the contrast in water level trends. The past two decades of below-average recharge 4 are likely to have contributed to the downwards trend in Uley Wanilla (~4m drop over 20 years, and continuing to fall). Meanwhile, the downward trend is far less pronounced in the three coastal zones, where there is a “boundary condition” of 0 m AHD imposed by the sea. In these areas a reduction in recharge is likely to have led to a reduction in freshwater outflow to the sea. This can theoretically cause inland movement of the saltwater wedge, which serves as an inflow to the aquifer thereby helping to “buffer” further water level decline. The extent of this effect is currently untested, and requires more sophisticated methods than those adopted here. It is clear from other studies of seawater intrusion that stable groundwater levels do not necessarily indicate a stabilised saltwater wedge, because of the difference in lag time between the watertable (e.g. days-months) and saltwater wedge (e.g. months-years) following a change in the hydrological conditions (e.g. recharge, pumping, etc). In coastal aquifers, stable water levels may appear to imply that an equilibrium between recharge and discharge has occurred and that the aquifer provides a sufficient supply of water in terms of quantity, however if seawater is intruding the aquifer from the coast, then water quality may be at risk. 2 Coffin Bay - LKW027 73 Uley Wanila - ULE007 72 1 71 70 0 69 68 67 -1 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 66 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 5 Uley South - ULE102 2 Lincoln B Lens - SLE030 4 3 1 2 0 1 0 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 -1 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Figure 3. Typical groundwater level 5 trends in three coastal aquifers (Coffin Bay, Uley South and Lincoln Basin) and one inland aquifer (Uley Wanilla). Note the “levelling off” of coastal water levels at around 1m AHD, while inland levels have continued to drop. 4 Note that in the Uley Wanilla basin, pumping rates have been greatly reduced over the past two decades in an effort to arrest the decline in water levels. Despite these efforts, water levels have continued to drop sharply relative to the long-term average. As the fall in water levels are not easily attributable to over-extraction, it is thoughts that a reduction in recharge has occurred. 5 Data obtained from “ObsWell”, Primary Industries and Resources SA (PIRSA) https://obswell.pir.sa.gov.au/ 7
Page 1 and 2: Saltwater intrusion in Southern Eyr
Page 3 and 4: Contents Executive Summary v 1. Pro
Page 5 and 6: Executive Summary The Eyre Peninsul
Page 7 and 8: 1. Project Background 1.1 Eyre Peni
Page 9 and 10: Milestones 1-3 Project management o
Page 11: Table 1. Summary of hydrogeologic u
Page 15 and 16: Elevation m AHD Elevation m AHD ULE
Page 17 and 18: Figure 5. Conductivity slice at -53
Page 19 and 20: Figure 7. Interpreted surface for b
Page 21 and 22: 4. Methods Owing to the very high p
Page 23 and 24: On EP, seawater intrusion in unconf
Page 25 and 26: Inland freshwater lens It is worth
Page 27 and 28: Figure 11. Diagram of parameters fo
Page 29 and 30: A Note on Analytical vs Numerical M
Page 31 and 32: Figure 12. Contours of x’ T for d
Page 33 and 34: 1976) but no data exists closer to
Page 35 and 36: 4.3.4 Parameter Summary Table 2. Aq
Page 37 and 38: Figure 13. Contours of x’ T for:
Page 39 and 40: production bores and observation we
Page 41 and 42: Figure 16. Plot of theoretical fres
Page 43 and 44: Figure 17. Contours of x T across a
Page 45 and 46: 5.4 Confidence in Predictions The m
Page 47 and 48: 6. Conclusions The Southern Basins
Page 49 and 50: scheme allows prediction of time-de
Page 51 and 52: References ABS (2008) National Regi
Page 53 and 54: Selby, J. (1972), Lincoln Basin - P
Page 55 and 56: Monitoring site ULE 210 (Tertiary S
Page 57 and 58: ULE 210 ULE 211 ULE 212 ULE 209 ULE
Page 59 and 60: Appendix Figure 3. ULE 210 - Tertia
Page 61 and 62: Appendix Figure 5. ULE 205 Composit

Saltwater intrusion in Southern Eyre Peninsula, December 2009

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