Cl/Br ratios and environmental isotopes as indicators of recharge variability and groundwater flow: An example from the southeast Murray Basin, Australia

Systematic variations in Cl/Br ratios together with R36Cl values, 14C activities, and δ18O values reflect differences in groundwater recharge and define groundwater flow paths in the Riverine Province of the southeast Murray Basin. In groundwater from the shallowest Shepparton Formation, homogenisation of Cl/Br and δ18O values and a decline in 14C activities with depth imply that vertical flow dominates. The 14C activities define variations in pre-land clearing recharge. In the palaeovalleys of present day rivers (“deep leads”), which contain sandier sediments, higher recharge (0.5 to 1.4 mm/year) produced relatively fresh groundwater (TDS < 3000 mg/L). By contrast, away from the deep leads, lower recharge (0.1 to 0.4 mm/year) resulted in higher degrees of evapotranspiration producing more saline groundwater (TDS up to 60,000 mg/L). Cl/Br ratios correlate with these differences in recharge and groundwater salinity. Groundwater from the deep leads has average molar Cl/Br ratios of 530 to 660, while that from adjacent areas has average molar Cl/Br ratios of 980 to 1090. The variation in Cl/Br ratios is interpreted as due to small differences in the volume of windblown halite that the groundwater has dissolved during recharge, as confirmed by an inverse correlation between Cl/Br ratios and R36Cl values in groundwater from the Goulburn subcatchment. Zones of groundwater with variable Cl/Br ratios in the deeper Calivil–Renmark aquifer also allow the detection of rapid leakage from the surface that has the potential to compromise groundwater resources.