RECENT LOCALIZED SULFATE REDUCTION AND PYRITE FORMATION IN A FISSURED CHALK AQUIFER

Sulphate reduction and pyrite formation are well-known diagenetic processes in marine sediments during the early stages of burial. Sulphate reduction has also been detected in aquifers, however, the identification of pyrite which was precipitated during the circulation of meteoric water is uncommon. Here we describe petrographic and sulphur isotope data which show that pyrite was formed in fissures while the aquifer contained meteoric water. The occurrence of active sulphate reduction is demonstrated by the recovery of sulphate-reducing bacteria (SRB's) and detection of hydrogen sulphide. Hence organic matter may be metabolised and diagenetic minerals formed during late, meteoric-water diagenesis of sediments. This process affects groundwater quality by removing sulphate, Fe and trace metals, and by altering the redox state of the system.