HYDROGEOPHYSICAL INVESTIGATION OF AN ISOLATED RECHARGE EVENT ON A CENTRAL AUSTRALIA EPHEMERAL RIVER

Episodic floods on ephemeral rivers need to be evaluated because they represent an important groundwater recharge source in arid climates. This study characterizes modern groundwater recharge in the western margin of the Great Artesian Basin that covers 22 percent of Australia. This central Australian aquifer has relatively long flow paths (>1,000 kilometers) and residence time (hundreds of thousands of years). This research evaluates changes in aquifer storage and bulk aquifer conductivity following a one-week duration flood in November 2008 on the Finke River. The Finke River last flowed in 2003; thus, this flood provides a temporally-isolated recharge signal in the Great Artesian Basin recharge zone. Ultimately an improved understanding of recharge on the Finke River may be scaled up to estimate recharge across the approximately 100,000-quare kilometer study area. Stream-groundwater interactions are evaluated using temporal gravity surveys and time domain electromagnetic soundings conducted in January 2009 after flow stopped and in August 2009 after groundwater equilibration. Surveys orthogonal to channel are located in upper reaches and where disconnection between river and groundwater occurs. Hydrogeophysical data are compared with a preliminary numerical groundwater model simulating a recharge front. Temporal gravity surveys are expected to show aquifer mass changes below gravimeter resolution. Temporal time domain electromagnetic soundings show recharge along preferential flow paths instead of an idealized, symmetrical recharge front predicted by the flow model. The November 2008 flood was not of sufficient magnitude to recharge the aquifer. Only Finke River floods of exceptional magnitude contribute to modern recharge. This project is part of an Australian government National Water Commission initiative providing science for sustainable groundwater management in the Great Artesian Basin western margin.