Insight into the mechanisms of denitrification and sulfate reduction coexistence in cascade reservoirs of the Jialing River: Evidence from a multi-isotope approach.

The coexistence of denitrification and bacterial sulfate reduction (BSR) processes is commonly observed in natural water systems. However, its formation mechanism remains unclear at a basin scale due to the difficulty of precise identification of these processes. To address this issue, we investigated the spatial-temporal variations in water chemistry and isotopic compositions (e.g., delta C-13(DIG) delta N-15(NO3), delta O-18(NO3), delta S-34(SO4), and delta O-18(SO4)) in cascade reservoirs (artificial dam lakes) of the Jialing River, SW China in 201 6. The results showed that the denitrification and BSR processes coexisted in the studied reservoirs, which was supported by the positive correlation between delta N-15(NO3) and delta O-18(NO3) and between delta S-34(SO4) and delta O-18(SO4), and by the decreasing concentrations of NO3- and SO42- Moreover, covariation of Delta(CDIC)-C-13, 4 Delta N-15(NO3), and Delta S-34(SO4) indicated the dominance of heterotrophic denitrificalion (HD) in the reservoir waters along with the occurrence of bacterial sulfide oxidation (BSO). In addition to SO42- and NO3-, the coexistence of HD and BSR processes were also controlled by the dissolved organic carbon (DOC) in winter and dissolved oxygen (DO) contents in other seasons. Overall, (he cumulative effect of cascade reservoirs caused delta N-15(NO3) and delta S-34(SO4) to display an upward (rend from upstream to downstream in the Jialing River, while delta C-13(DIC) showed an opposite downward trend, which implying that cascade reservoirs may be in favor of the coexistence of the HD and BSR processes. This study therefore concludes that the multi-isotope approach could be a useful technique to ascertain the coexistence mechanism of HD and BSR processes in reservoir water systems. (C) 2020 Elsevier B.V. All rights reserved.