Fractionation of sulphur isotopes during biogenic sulphate reduction below a sandy forested recharge area in south-central Canada

Abstract

Detailed groundwater monitoring below a sandy forested recharge area in south-central Canada shows that atmospherically derived SO 2− 4, occurring at concentrations of 6–27 mg l −1 in the shallow aquifer, is completely consumed within a zone of less than 6 m thickness encountered at about 15 m depth. The SO 2− 4 depletion zone is anoxic, which suggests that biogenic sulphate reduction is the principal SO 2− 4-depleting process. The field evidence suggests that, in this environment, the reduction reaction is similar to a first-order decay process with a relatively short half-life of about 3 years. Isotopic fractionation of SO 2− 4-S is observed during the reaction and can be described by a Rayleigh-type distillation process governed by an enrichment factor averaging −15.5‰