FRACTIONATION OF SULFUR ISOTOPES DURING BIOGENIC SULFATE REDUCTION BELOW A SANDY FORESTED RECHARGED AREA IN SOUTH CENTRAL CANADA

Detailed groundwater monitoring below a sandy forested recharge area in south-central Canada shows that atmospherically derived SO42-, 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 SO42- depletion zone is anoxic, which suggests that biogenic sulphate reduction is the principal SO42--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 SO42--S is observed during the reaction and can be described by a Rayleigh-type distillation process governed by an enrichment factor averaging -15.5 parts per thousand.