Geomorphic controls on groundwater evolution in the arid Cooper Creek system, SW Queensland, Australia: inferences from element and stable isotope hydrogeochemistry

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International Association of Hydrogeologists
Quaternary climatic changes have had a remarkable impact on the biological and geomorphological evolution of the Australian continent, and in turn can exhibit considerable control on the current hydrological cycle. In the absence of glaciation, changes in precipitation and wind strength have resulted in alternating fluvial, aeolian and lacustrine deposits over much of inland Australia. In the currently arid anabranching floodplain-channel system of Cooper Creek (SW-Queensland), this is manifest as extensive late Pleistocene fluvial and aeolian sand bodies overlain by floodplain and channel mud deposits. The alluvial muds are the result of the much reduced Holocene transport capacity of the Cooper Creek system, and are punctuated at the surface by remnant aeolian sand dunes which are stratigraphically connected to the underlying sand bodies. These Quaternary sand bodies (Chookoo dune-floodplain complex) have in turn become the main aquifers for the region, where the water table is ~10-12m below the floodplain surface. The presence of shallow groundwater is especially crucial for ecosystems in arid environments because evaporation quickly removes any available surface waters. Considering the importance of this resource, and the fragility of the hydrological cycle in arid zones, the shallow groundwaters in this region have received surprisingly little attention. This study aims to determine the basic recharge/evaporation processes of the Chookoo dune-floodplain-channel system using major and minor element chemistry together with water stable isotopes (δ18O, δ2H) and dissolved sulfate isotopes (sulfate-δ34S and δ18O). We hypothesise that groundwater recharge predominately occurs as diffuse rainfall infiltration via the dunes while chemical variations also occur through the dunes, modifying the original chemistry of the recharged water. Presented chemical data suggests that the main channel of the creek has little or no hydraulic connectivity with the shallow aquifers except during large flood events when the mud seal over their base is scoured and fresh water temporarily recharged. Major-element chemistry: All waters are Na-Cl-rich with appreciable amounts of Ca and SO4. All major elements increase along a transect from the sand dunes to the floodplain within the same aquifer. In general, major element ratios show a marine derived signal for groundwater, while a few surface water samples deviate from marine ratios (Fig. 1A). This difference is interpreted as an event based signature, with most solutes incorporated from dissolution of surfaces salts. Evaporation models also provide evidence of major element evolution (Fig. 1B). Simple evaporation of surface waters cannot reproduce the concentrations found in the groundwater. Only when a mixture of surface and groundwater and/or dissolution of previously precipitated salts along the recharge path are considered, do the evaporation models match with the observed concentrations.
Geomorphology, Ground water, Queensland, Australia, Streams, Stable isotopes, Geochemistry, Climatic change, Aquifers, Rain
Cendón D. I., Larsen J. R., Jones B. G., Rickleman D. & Nanson, G. C. (2007). Geomorphic controls on groundwater evolution in the arid Cooper Creek, SW Queensland, Australia: inferences from elemental and stable isotope hydrogeochemistry. Paper presented to the XXXV IAH Congress of the International Association of Hydrogeologists, "Groundwater and Ecosystems", Lisbon, 17-21 September 2007, (pp. 553-554).