Constraining hydrochemical pathways in a small internally draining basin (Lake George Basin, NSW) using isotopes of the water molecule

Lake George, located in the Southern Tablelands of New South Wales, is a shallow, intermittent lake with highly variable salinity and water levels. The lake is the focus of drainage for a small internally draining basin containing six sub-catchments. It is currently believed that the lake’s water level is primarily affected by the short- and long-term variations in precipitation, runoff and evaporation. Contributions from the regional groundwater are thought to be low because of the thick (~40 m) clay unit that underlies the modern extent of the lake. As a result, the lake’s hydrograph, which is one of the oldest in the world, has recorded droughts and wet periods of southeastern Australia over the last 200 years. In this project, water chemistry and physical hydrometeorological data are being collected to determine hydrochemical pathways and recharge rates. This will provide much needed information regarding the sustainability of the Bungendore town water supply as well as identify potential risks to water quality in the basin. This project also forms one part of a much larger project to investigate the landscape and human evolution of the Lake George Basin. Four precipitation samplers have been installed at three sites around the catchment – one to the north of the lake at Winderadeen homestead, one to the south of the lake at Bungendore’s post office, and two at a rocky outcrop (‘Rocky Point’) on the eastern shore of Lake George. Three precipitation samplers (one located at each site) are used to collect monthly composite precipitation samples for deuterium and oxygen-18 analysis, and elemental concentrations. The additional sampler located at Rocky Point is used to collect precipitation for tritium analysis. In months when precipitation is too low to collect sufficient sample volume (~1 L), monthly samples are combined. These data are being combined with deuterium, oxygen-18, tritium and elemental compositions, as well as hydrometeorological measurements, of surface water, groundwater and atmospheric moisture from around the basin to help constrain recharge areas and rates, primary hydrochemical pathways and controls on water quality. Deuterium and oxygen-18 compositions indicate that groundwater is primarily recharged by winter rainfall. Despite variable precipitation compositions throughout the year, creek water maintains a winter rainfall/groundwater isotopic year-round, which may indicate that the creeks of the basin are fed by aquifers and through-flow rather than fresh precipitation. This is also supported by low tritium activity. A local evaporation line has also been determined for Lake George during the period July 2013 (after high rainfall) to November (when the lake dried out due high evaporation rates