River infiltration to a subtropical alluvial aquifer inferred using multiple environmental tracers

Chloride (Cl^-), stable isotope ratios of water (δ¹⁸O and δ²H), sulfur hexafluoride (SF₆), tritium (³H), carbon-14 (¹⁴C), noble gases (⁴He, Ne, and Ar), and hydrometry were used to characterize groundwater-surface water interactions, in particular infiltration rates, for the Lower Namoi River (New South Wales, Australia). The study period (four sampling campaigns between November 2009 and November 2011) represented the end of a decade-long drought followed by several high-flow events. The hydrometry showed that the river was generally losing to the alluvium, except when storm-derived floodwaves in the river channel generated bank recharge - discharge cycles. Using ³H/¹⁴C-derived estimates of groundwater mean residence time along the transect, infiltration rates ranged from 0.6 to 5 m yr^-1. However, when using the peak transition age (a more realistic estimate of travel time in highly dispersive environments), the range in infiltration rate was larger (4-270 m yr^-1). Both river water (highest δ²H, δ¹⁸O, SF₆, ³H, and ¹⁴C) and an older groundwater source (lowest δ²H, δ¹⁸O, SF₆, ³H, ¹⁴C, and highest ⁴He) were found in the riparian zone. This old groundwater end-member may represent leakage from an underlying confined aquifer (Great Artesian Basin). Environmental tracers may be used to estimate infiltration rates in this riparian environment but the presence of multiple sources of water and a high dispersion induced by frequent variations in the water table complicates their interpretation.