Controls on delta S-34 and delta O-18 of dissolved sulfate in aquifers of the Murray Basin, Australia and their use as indicators of flow processes

The usefulness of stable isotopes of dissolved SO4 (delta S-34 and delta O- 18) to study recharge processes and to identify areas of significant inter- aquifer mixing was evaluated in a large, semi-arid groundwater basin in sou theastern Australia (the Murray Basin). The distinct isotopic signatures in the oxidizing unconfined Murray Group Aquifer and the deeper reducing Renm ark Group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections. delta S-34 values in the unco nfined Murray Group Aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20.8 to 0.3 parts per thousand. The concomitant increasing SO4/Cl ratios, as well as relatively low delta O-18(SO4), values, suggest that Vertical input of biogenically derived SO4 via diffuse recharge is the predominant source of dissolved SO4 to the aqui fer, Further along the hydraulic gradient towards the discharge area near t he River Murray, delta S-34 values in the unconfined Murray Group Aquifer i ncrease, and SO4/Cl ratios decrease, due to upward leakage of waters from t he confined Renmark Group Aquifer which has a distinctly low SO4/Cl and hig h delta S-34 (14.9-56.4 parts per thousand), Relatively positive delta S-34 and delta O-18(SO4) values, and low SO4/Cl in the Renmark Group Aquifer is typical of SO4 removal by bacterial reduction. The S isotope fractionation between SO4 and HS- of similar to 24 parts per thousand estimated for the confined aquifer is similar to the experimentally determined chemical fract ionation factor for the reduction process but much lower than the equilibri um fractionation (similar to 70 parts per thousand) even though the confine d groundwater residence time is > 300 Ka years. Mapping the spatial distrib ution of delta S-34 and SO4/Cl of the unconfined Murray Group Aquifer provi des an indicative tool for identifying the approximate extent of mixing, ho wever the poorly defined end-member isotopic signatures precludes quantitat ive estimates of mixing fractions. (C) 2001 Elsevier Science Ltd. All right s reserved.