Reduced sulfur accumulation in salinised sediments

Environmental context The accumulation of reduced sulfur compounds following salinisation is an emerging risk to inland waterways. Disturbance of these materials can lead to acidification, mobilisation of heavy metals and oxygen depletion. Knowledge of the rates of reduced sulfur accumulation in salinised waterways and the speciation of the reduced sulfur products is crucial for effective management of aquatic ecosystems. Abstract The accumulation of reduced sulfur species in the sediments of salinised inland waterways poses a serious environmental risk to many historically freshwater environments. Here the effects of salinity (and associated sulfate concentration), organic carbon load and temperature on reduced sulfur accumulation and speciation in closed microcosms containing sediments from a wetland that had not previously been salinised are examined. At conductivities of up to 10 000 µS cm–1, extant sediment carbon was sufficient to allow reduction of the entire sulfate load. Sulfate reduction was carbon limited at higher salinities. The rate of sulfate reduction approximately tripled with an increase in temperature from 20 to 30°C. Speciation studies showed that elemental sulfur and an unidentified sulfur species – probably reduced organic sulfur – were the dominant reduced sulfur species present during the early stages of sulfate reduction. By the end of the incubation period (226 days), reactive forms of S (elemental sulfur and acid-volatile sulfide) dominated. In the low conductivity treatments (0 and 1000 µS cm–1) reduced sulfur was approximately equally distributed between the two forms; acid volatile sulfide comprised ~75% of the reduced sulfur at higher salinities. Formation of less reactive di-sulfide minerals was inconsequential over the timescale of this experiment.