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

SummaryThe 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 ∼350 km stretch of the river estimated from 222Rn activities were 320–1500 m3/day in November 2009 and 560–4300 m3/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 contributed <1% of the total water to this part of the river, and consequently has little impact on river water quality. Nevertheless the groundwater inflows remove water from low-salinity groundwater lenses that underlie the river and during the drought conditions these lenses were degrading. These reaches of the River Murray are an example of a system that transitioned from being dominantly losing to being variably gaining due to diminishing surface water flows.

► Long-term losing stretches of the River Murray become variable gaining during drought conditions.
► Radon, major ion concentrations, and geophysical techniques indentify gaining and losing reaches.
► Groundwater inflows are ∼1% of total discharge.
► Low salinity groundwater lenses underlying the river degrade at low river flows.