1. Groundwater evolution within a catchment affected by dryland salinity, southeastern Australia John Webb and Darren Bennetts

2. Study area Areas at high risk of dryland salinity in 2000

3. Increasing salinisation of the landscape Boggy Creek SpringGellerts Seep 19521996

4. Gellerts Swamp today

5. Topography Hamilton Willaura Grampians Stavely Range Hopkins River Cockajemmy Lakes Boggy Creek Spring Gellerts Seep

6. Surface Geology Greenstone Sandstone/shale Grampians Group Granite Basalt Colluvium Alluvium Stream Alluvium Swamp deposits Cambrian Silurian Devonian Pleistocene Quaternary

7. Hydrogeology – flow paths Flow path 1 Flow path 2 270 260 250 240 250 230 250 220

8. Groundwater composition dominated by Na and Cl

9. 36 Cl analyses from adjacent area median of 19 x 10 -15 36 Cl/Cl - consistent with atmospheric precipitation in southwest Victoria contributions from connate water and/or basalt weathering unlikely - 36 Cl/Cl - ratios from these sources would be zero or ~4 x 10 -15 groundwater Cl - is probably sourced exclusively from cyclic sources (rainfall and/or windblown dust)

10. Hydrogeology – groundwater age (tritium) Only samples in the west contain tritium - recharged after 1950 Waters in centre and east contain no tritium

11. Hydrogeology – groundwater age & rates of movement ( 14 C) 14 C ages may be overestimates, but indicate flow times of several thousand years 4000 years old 7900 years old old

12. Hydrogeology – flow path 1 Flow path 1 270 260 250 240 250 230 250 220

13. Hydrogeology – flow path 1 cross section Mt William Swamp Hopkins River

14. Salinity increases along flow path 1 Mt William Swamp Hopkins River 37 9 8 5 1 Salinity (mS/cm) 13 Progressive salinity increase along flow path due to addition of diffuse recharge from overlying soil zone, where rainfall concentrated by evapotranspiration Note dilution along flowpath due to lateral flow from north 8

15. Hydrogeology – flow path 2 Flow path 2 270 260 250 240 250 230 250 220

16. Hydrogeology – flow path 2 cross section Lake Muirhead Cockajemmy Lakes Gellerts Swamp

17. Salinity increases along flow path 2 Lake Muirhead Cockajemmy Lakes Gellerts Swamp 3 92215 15-30 Increase along flow path again due to addition of saline diffuse recharge from overlying soils, with some addition from salt in bed of Lake Muirhead. very saline brines beneath Cockajemmy Lakes

18. groundwater samples all plot close to local meteoric water line groundwater stable isotope composition becomes heavier downflow probably reflects addition of soil water evaporated under high humidities

19. groundwater becomes more reducing downflow reflects organic content of shallow alluvial aquifer oxidising waters downflow in basalt and basement aquifers

20. Downflow change from kaolinite to smectite stability fields

21. Decrease in Si/Cl ratio with increasing salinity (downflow) probably reflects reaction of groundwater silica with kaolinite to form smectites

22. Marked pH increase downflow probably due to H + removal on clays

23. Conclusions Groundwater chemistry dominated by: rainfall input evapotranspiration Groundwater evolution reflects: progressive addition of saline infiltration from soil zone interactions with clay minerals some oxidation of organic matter in aquifer