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.