Saltwater intrusion in Southern Eyre Peninsula, December 2009

More documents

Recommendations

Info

7. Future Work 7.1 Ongoing Work 7.1.1 Monitoring of Saltwater Interface in Coastal Wells In Uley South there are currently two long-screened monitoring wells (ULE205 and SLE069), both approximately 500m from the coast. The wells allow the direct measurement of a saline interface by salinity profiling (sonding). At the time of this report, two additional wells were under construction. Previous results of sonding have established what could be considered baseline information for the position of the saline interface. However, to date insufficient temporal information has been collected to evaluate any changes in salinity profiles and other coastal parameters over time (see Section 3.1). A new study is being planned by EPRNM (to commence in 2010) to characterise the dynamic nature of the seawater interface which may potentially vary with water level fluctuations, tidal cycles and even with local atmospheric pressure variations. The study will aim to provide advice regarding the sonding method as an ongoing monitoring technique, that is, to inform its reliability and how regularly monitoring should occur. Any analysis of this nature should be closely coupled to ongoing modeling efforts to allow for a comparison between simulated and measured saltwater wedge changes. Modelling is expected to offer important insights into future monitoring efforts, and vise versa. As a preliminary measure, it is expected that 2 – 4 wells would be sonded every month for 1 year. The program should observe a seasonal water level maximum and minimum, accompanied by transient interface behaviour on the seasonal timescale. Monitoring dates and times can be compared against the known tidal movement in order to assess tidal impacts on the interface position. Once the “normal” variations of the saline interface have been identified, ongoing monitoring will be able to observe any net changes to the system and provide an early warning monitoring system for signaling seawater intrusion that may threaten pumping wells. The newly constructed wells that are sited inland of the wedge and constructed to the aquifer basement will also offer an early-warning measure for any impending seawater intrusion. Due to the presence of vertical gradients, one must be cautious when attempting to directly relate the depth and fluctuation of a saline interface observed in a well via sonding (particularly long screened wells) to the depth of the interface in the aquifer. Despite this, it is anticipated that frequent monitoring will be more useful than sporadic monitoring, and will improve the understanding of relative changes in the interface depth over time. 7.1.2 Scenario Testing Using a Numerical Model Under Milestone 10 of the GAPM project, a numerical model of seawater intrusion in Uley South is expected to be developed through the Research Fellowship. Key benefits of the numerical modelling framework include an ability to consider the aquifer geometry in three dimensions, and the transient nature of the numerical 42
scheme allows prediction of time-dependent impacts on the seawater interface. This will therefore complement the proposed new monitoring regime. The primary purpose of the model is to build a tool that can be used to assess the possible risks associated with future management scenarios, although the predictability of seawater intrusion models at the regional scale is debatable and requires further research. Nonetheless, regional-scale seawater intrusion models are considered the optimal methodology for supporting management decision-making for key coastal aquifers. The current management approach to groundwater allocation is based on estimates of historical and recent recharge volumes (flux-based approach; Werner et al., In prep). “Trigger-level management” presents as an alternative strategy that allows for adaptive management of groundwater abstraction, based on the condition of the aquifer at any given time (Werner et al., In prep.). Trigger-level management involves defining a measurable threshold or series of thresholds, designed to protect important aspects or features of a system. Allocations of groundwater may be reduced or ceased altogether as the defined thresholds are approached or exceeded. The principles of trigger-level management were applied by Alcoe (2009), using a water balance model that considered seawater intrusion (in Uley South) to be an unacceptable risk. One of the major limitations of the work was the application of a steady-state seawater interface to a changing (monthly) water-level step. This limitation can be overcome once the transient numerical model is available, allowing flux-based and trigger-level management approaches to be rigorously compared. 7.2 Suggested Further Work The following ideas have been tentatively discussed between academics at Flinders University, EPNRM, DWLBC and SA Water. At the time of writing this report, these ideas had not necessarily transformed into definite projects and should be read here as a suite of possible future research directions that – if pursued – would feed into an improved understanding of seawater intrusion processes on EP. 7.2.1 Investigation of Heterogeneity An extension to the numerical model (developed under Milestone 10) is currently being discussed, in which 2D “slices” (perpendicular to the coast) are simulated using a high grid resolution, in order to investigate the impacts of small-scale densitydependent processes that may not be captured within the large-cell regional model. By “condensing” the model from 3D to 2D, an extra level of detail can be achieved without increasing computational demands. This enables different scenarios of heterogeneity to be investigated in a fundamental way. Such investigations would help determine whether, for instance, the regional model is likely to be over- or under-predicting the extent of seawater intrusion in Uley South, and whether such effects are significant. 43
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 and 12: Table 1. Summary of hydrogeologic u
Page 13 and 14: Water Level (m AHD) Water Level (m
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: 6. Conclusions The Southern Basins
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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?