Author: Peter Wawn Supervisor: Faisal Anwar

1. Author: Peter Wawn Supervisor: Faisal Anwar Saline and Freshwater Interaction:Future Implications for Freshwater Management at Rottnest Island Lake Negri Freshwater Seep

2. Background and Objectives of Research Background Freshwater sources for fauna are extremely limited on Rottnest A small amount of freshwater seeps remain Several species are tolerating record salinity Rainfall in Western Australia is decreasing The relationship between Rottnest’s groundwater, surface water and rainfall remains poorly understood Objective Investigate and identify how the availability of fresh water sources may change in the future Investigate and identify how salinity in the Rottnest Aquifer may change in the future

3. Wadjemup Aquifer and Discharge Points Pink Lake Freshwater Seep

4. Critical Cross Section Lake and Seep Monitoring Locations Cross Section to be Modelled

5. Collection of Site Data

6. Methodology Software Utilised: Model: SUTRA Version 2.2 (USGS) Graphical Pre and Post-Processor: SUTRA GUI (USGS) Commercial Interface: Argus ONE (GIS and Quad Mesh Modules) Model: Two dimensional radial mesh assumed Elevations of the critical cross section 9625 nodes in total

7. Setting up the Model Graphical Representation of the Aquifer

8. Aquifer Parameters

9. Boundary and Initial Conditions Boundary Conditions Static Seawater Pressure at Nodes Below Sea Level Rainfall Recharge at Nodes Above Sea Level Specified Concentration at Nodes Along the Salt Lakes Specified Concentration at Nodes Along the Right Boundary Initial Conditions Salt Water Concentration at All Nodes 600mm Recharge for 200 Years Scenarios Velocity and concentration at all nodes over 250 – 900mm annual rainfall (50mm steps) over 10 and 20 years

10. Results: Salt and Freshwater Mixing 200mm Over 20 Years 900mm Over 20 Years

11. Results: 0.5% Concentration Lines

12. Accuracy of 0.5% Concentration Lines Approximate Thickness Reported by Davidson (1991)

13. Results: Discharge Concentration

14. Accuracy of Discharge Concentration Measured Concentration Lakeside Piezometers (Sept 2011)

15. Results: Discharge Velocity

16. Accuracy and Limitations Aquifer parameters poorly understood Assumed as 2D radial model Local topography will influence results Local evapotranspiration will influence results Preferential flow paths and impermeable layers unknown Discharge occurs at multiple points Lake Baghdad Freshwater Seep

17. Conclusions Ecological balance is threatened by continued rainfall decline. Freshwater sources may disappear completely. Direct association between rainfall reductions and: - Salinity Increases at seeps - Less water discharging at seeps - Reductions in the thickness of freshwater lens Average rainfall of 400mm may cause: - 50% increase in salinity at seeps - 16% less water discharging at seeps - 28% decrease in lens thickness

18. Recommendations The Rottnest Island Authority is encouraged to: - Continue the Current Lake and Seep Monitoring - Investigate contingencies should freshwater sources become unavailable - Note the implications for the bore field based on the predicted reduction in freshwater lens thickness Contingencies may include - Artificial aquifer recharge - Installation of artificial drinking holes

19. Issues with Implementing Contingencies Rough cost of implementing artificial sources or aquifer recharge: - Assuming 2000m of 50mm copper pipe from the water tanks near Geordie Bay ($86/m) - Assuming $100,000 for Installation of artificial sources or recharge - Assuming $80,000 for design - Assuming $40,000 for environmental management Rough Cost = $400,000 +/- 50% Ongoing Cost = Desalinated Water $1.5-2 / kilolitre Class A Reserve (Most Protected Type of Crown Land) - May have issues with permits and approval for any contingency construction work

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