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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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slide1

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

John Webb and Darren Bennetts

slide2

Areas at high risk of

dryland salinity in 2000

Study area

slide3

Increasing salinisation of the landscape

Gellerts Seep

Boggy Creek Spring

1952

1996

slide5

Topography

Grampians

Boggy Creek

Spring

Willaura

Hopkins River

Hamilton

Gellerts Seep

Cockajemmy Lakes

Stavely Range

slide6

Surface Geology

Quaternary

Swamp deposits

Stream Alluvium

Alluvium

Colluvium

Pleistocene

Basalt

Devonian

Granite

Silurian

Grampians Group

Cambrian

Sandstone/shale

Greenstone

slide7

Hydrogeology

– flow paths

250

270

240

Flow path 1

260

230

250

220

Flow path 2

250

slide9

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)
slide10

Hydrogeology – groundwater age (tritium)

Only samples in the west

contain tritium - recharged

after 1950

Waters in centre

and east contain

no tritium

slide11

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

slide12

Hydrogeology

– flow path 1

250

270

240

Flow path 1

260

230

250

220

250

slide13

Hydrogeology – flow path 1 cross section

Mt William Swamp

Hopkins River

slide14

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
slide15

Hydrogeology

– flow path 2

250

270

240

Flow path 2

260

230

250

220

250

slide16

Hydrogeology – flow path 2 cross section

Cockajemmy Lakes

Lake Muirhead

Gellerts Swamp

slide17

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
slide18

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
slide19

groundwater becomes more reducing downflow

  • reflects organic content of shallow alluvial aquifer
  • oxidising waters downflow in basalt and basement aquifers
slide21

Decrease in Si/Cl ratio with increasing salinity (downflow) probably

reflects reaction of groundwater silica with kaolinite to form smectites

slide23

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