Geochemical and 222Rn constraints on baseflow to the Murray River, Australia, and timescales for the decay of low-salinity groundwater lenses

The distribution of 222Rn activities and major ion concentrations show that during low river flows characteristic of the recent drought conditions, the middle reaches of the Murray River, northern Victoria, were a predominantly gaining system at low river levels (May 2009, May 2010), and were variably gaining and losing at higher river levels (November 2009). The location of gaining and losing reaches are partially governed by the position of the river with respect to the edge of its floodplain; gaining reaches occur where the floodplain narrows or where the river is adjacent to the steep slopes at the floodplain margin. Cumulative inflows along this ∼350km stretch of the river estimated from 222Rn activities were 320-1500m 3/day in November 2009 and 560-4300m 3/day in May 2010. The relatively large uncertainty reflects the heterogeneous and poorly-constrained groundwater 222Rn activities. Major ion variations (notably Cl concentrations and Na/Ca ratios) correlate with 222Rn activities and may be used as indicators of groundwater inflows. Baseflow contribute