Oral Presentation Australasian Groundwater Conference 2017

Understanding recharge and mixing on floodplains in the Murray Basin (85)

Ian Cartwright 1 , Adrian Werner 2 , Tariq Laattoe 2 , Juliette Woods 2 , Graham Green 3
  1. Earth, Atmosphere and Environment, Monash University, Clayton, Vic, Australia
  2. School of the Environment, Flinders University, Adelaide, SA, Australia
  3. Department of Environment, Water and Natural Resources , Adelaide, SA, Australia

Floodplains are complex hydrological zones that may receive both discharge from regional groundwater and recharge from river water during high flow events. The floodplains on the Murray River host important ecosystems (particularly the river red gum forests); however, changes to river flows and landuse changes that result in a rise in the regional water table have locally severely affected these regions. This project uses isotope geochemistry (3H and 14C) together with major ions to address the timescales and patterns of floodplain recharge and groundwater mixing on the degraded Pike floodplain (South Australia).

The study sampled groundwater from the shallow low hydraulic conductivity Coonambidgal Formation and the underlying Monoman aquifer. Groundwater 3H activities in the Monoman Formation decrease with depth from ~1 to <0.02 TU while the Coonambidgal groundwater has 3H activities of 0.25 TU. There is no correlation between 3H activities and the proximity to surface water channels on the floodplain. Groundwater 14C activities are between 40 and 95 pMC and are less well correlated with depth. Groundwater Cl concentrations are between 300 and 750 mmol/L and there is a broad inverse correlation with 3H activities.

The combined geochemical data imply that groundwater recharge is dominantly through the floodplain rather than through the channel banks, which in turn requires preferential flow through high hydraulic conductivity zones in the Coonambidgal Formation. The recently recharged waters mix in the aquifers with the regional groundwater on timescales of a few years to decades. Development and preservation of fresh shallow groundwater, which may sustain vegetation, on this floodplain requires that flooding events occur and that regional water table rise is limited.

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