Changes in pore water chemistry of desiccating freshwater sediments with different sulphur contents

Especially in dry summers, such as 2003 in Europe, wetlands may become subject to desiccation and oxidation processes may affect sediment top layers. In this paper, we present the results of a study in which the development of the pore water chemistry (major ions, nitrate, ammonium, phosphate and some metals) was monitored during experimental desiccation of previously anaerobic freshwater sediments. Three sediments with different concentrations of oxidizable sulphur were compared. Sediments appeared to respond very differently to prolonged oxidation due to desiccation. It can be concluded that oxidizable sulphur pools play an important role in freshwater wetlands. Water level fluctuations may have beneficial effects in sediments of which the buffer capacity is large enough to prevent acidification as a result of oxidation of reduced sulphur compounds. Oxidation of such sediments will result in net nitrogen losses and a decrease of the phosphate availability. Desiccation of sediments with high oxidizable sulphur contents, however, might lead to reactions that resemble those observed in acid sulphate soils. Extreme acidification might occur resulting in the mobilisation of high concentrations of potentially toxic metals such as aluminium and zinc. Dissolution of oxidized iron at very low pH will also result in the release of previously adsorbed phosphate. In freshwater systems, high concentrations of reduced sulphur will especially accumulate in reductive and iron-rich sediments which are fed by sulphate-enriched groundwater and which almost never fall dry.