Damming effects on river sulfur cycle in karst area: A case study of the Wujiang cascade reservoirs

Damming effects on sulfur (S) cycling remains unclear at basin scales due to the lack of basic data prior to dam construction. To explore the influence of damming on the riverine S cycle, we investigated the spatiotemporal variations of water chemistry, sulfate (SO42–) concentration, and δ34S-SO42– in January 2017 and July 2017 in the impounded Wujiang River, and the measured results in this study were compared with the data in 2002 when no dams were built in the middle and lower reaches. Results showed that SO42– was the second dominant anion and accounted for up to 30% of the total anions, and sulfuric acid played a vital role in carbonate weathering. Sulfate reducing process and organic S oxidation were markedly enhanced by seasonal thermal stratification induced by damming, particularly in summer. The seasonal differences of SO42– concentration and δ34S-SO42– in this study were not readily discernable compared to those in 2002, when the average SO42– concentration and δ34S-SO42– in winter were 31.0% and 63.3% greater than that in summer respectively. The δ34S-SO42– from upstream WJD reservoir to downstream YP reservoir in 2002 increased by 17.2% in summer; however, in this study, it drastically increased by 76.7%. The reduced seasonal variations and increased spatial differences of SO42– concentration and δ34S-SO42– reflected the damming effect of cascade dams on S cycling in river waters. Interestingly, SO42– concentration gradually decreased while the δ34S-SO42− value became positive from upstream to downstream reservoir, but both their variations inside reservoirs were homogenized to narrow ranges, which could be derived from the cumulative effects of cascade dams. After damming, the increased hydraulic retention time and water depth caused significant seasonal thermal stratification in reservoirs, which enhanced the S biogeochemical process, and this damming effect was accumulated through bottom released water. Therefore, SO42− concentration together with δ34S-SO42− could be useful indicators in assessing the damming effects of cascade dams in the river-reservoir systems.