Sediment mercury and methylmercury dynamics during reservoir operation.

Mercury (Hg) and methylmercury (MeHg) are highly neurotoxic substances, and reservoirs play a critical role in Hg storage while providing geochemical environments conducive to MeHg production. However, the mechanisms of Hg transport and methylation during reservoir regulation remain poorly understood. This study investigates the spatial distribution of sediment Hg and MeHg in both horizontal and vertical profiles within the Nuozhadu Reservoir, located in the upper Mekong River. Using the diffusive gradients in thin films (DGT) technique, we observed that sediment Hg and MeHg concentrations initially increased and then slightly decreased from the tail to the dam front of the reservoir. This trend is primarily attributed to the transport of fine sediment particles influenced by reservoir interception. MeHg production was observed during sediment transport, but hydraulic conditions driving sediment accumulation emerged as the dominant source of sediment MeHg. Furthermore, vertical fluctuations in DGT-Hg and DGT-MeHg values were noted, likely caused by periodic reservoir operations. Our findings indicate that sediments release Hg into the overlying water, establishing sediments as a significant source of Hg pollution in reservoirs. This study enhances our understanding of Hg transport and methylation processes under the influence of reservoir regulation.