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Groundwater evolution within a catchment affected by dryland salinity, southeastern Australia

Groundwater evolution within a catchment affected by dryland salinity, southeastern Australia. John Webb and Darren Bennetts. Areas at high risk of dryland salinity in 2000. Study area. Increasing salinisation of the landscape. Gellerts Seep. Boggy Creek Spring. 1952. 1996.

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Groundwater evolution within a catchment affected by dryland salinity, southeastern Australia

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

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

  3. Increasing salinisation of the landscape Gellerts Seep Boggy Creek Spring 1952 1996

  4. Gellerts Swamp today

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

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

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

  8. Groundwater composition dominated by Na and Cl

  9. 36Cl analyses from adjacent area • median of 19 x 10-1536Cl/Cl- • consistent with atmospheric precipitation • in southwest Victoria • contributions from connate water and/or • basalt weathering unlikely - 36Cl/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 (14C) 14C ages may be overestimates, but indicate flow times of several thousand years 4000 years old 7900 years old

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

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

  14. Salinity increases along flow path 1 Mt William Swamp Hopkins River 3 7 9 13 8 8 5 1 Salinity (mS/cm) • 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

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

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

  17. Salinity increases along flow path 2 Cockajemmy Lakes Lake Muirhead Gellerts Swamp 15-30 3 9 22 15 • 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

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