Arid zone groundwater recharge and salinisation processes; an example from the Lake Eyre Basin, Australia

Abstract

In arid central Australia, the ∼1.14 M km2 endorheic Lake Eyre Basin (LEB) experiences sporadic floods, and contains saline shallow groundwater. In this basin, where on-ground physical and chemical data are very sparse, new observations of major ion chemistry, stable isotopes (δ2H, δ18O, δ13C), radiogenic isotopes (3H, 14C), and remote sensing (MODIS) data were used to investigate groundwater recharge and salinisation processes in 2006–2007. In January 2007 there was a large and intense local rainfall event, followed by flooding to the normally dry Lake Eyre resulting from heavy rainfall in the northern LEB that took ∼2 months to arrive at Lake Eyre. Groundwater chemistry from this study show no evidence of recharge from the floodwaters or the local rainfall; however this does not preclude the process. Low transit times and low infiltrating volumes may mean that the recharge event has not yet been detected. Over the longer term, the stable isotope chemistry indicates that groundwater is predominantly diffusely recharged during heavy local rainfall events (>100–150 mm/month), and radiogenic isotopes reflect an older shallow groundwater efficiently mixing with recent (decadal) recharge. The high total dissolved solids (TDS) of regional shallow groundwater (up to 52.5 g/L) is intrinsically linked with these recharge processes; where infiltrating diffuse rainfall is evaporated, and heavy rainfall events transport saline pore water (saline due to evaporation in the unsaturated zone) to the groundwater system. The high TDS of shallow groundwater beneath the dry Lake Eyre bed (up to 322 g/L) is also controlled by evaporation. However, although evaporite minerals are prolific in the lake area, the non-conservative behaviour of Br in pore water limits analysis of the role of halite mineral dissolution and precipitation reactions controlling groundwater salinity. Evaporation-driven salinisation of shallow groundwater in arid central Australia is distinct from the transpiration-driven salinisation of groundwater in the Australian Murray Basin (located in a semi-arid climate), and results in relatively high groundwater salinity values compared with many other large arid zone endorheic basins across the world.