Influence of salinity intrusion on the speciation and partitioning of mercury in the Mekong River Delta

The lower Mekong and Saigon River Basins are dominated by distinctive monsoon seasons, dry and rainy seasons. Most of the Mekong River is a freshwater region during the rainy season, whereas during the dry season, salt water intrudes approximately 70 km inland. To understand the role of salinity intrusion controlling Hg behavior in the Mekong and Saigon River Basins, Hg and monomethylmercury (MMHg) in surface water and sediment of the Mekong River and in sediment of the Saigon River were investigated in the dry season. Sediment Hg distribution, ranging from 0.12 to 0.76 nmol g−1, was mainly controlled by organic carbon distribution in the Mekong River; however, the location of point sources was more important in the Saigon River (0.21–0.65 nmol g−1). The MMHg concentrations in Mekong (0.16–6.1 pmol g−1) and Saigon (0.70–8.7 pmol g−1) sediment typically showed significant increases in the estuarine head, with sharp increases of acid volatile sulfide. Unfiltered Hg (4.6–222 pM) and filtered Hg (1.2–14 pM) in the Mekong River increased in the estuarine zone due to enhanced particle loads. Conversely, unfiltered MMHg (0.056–0.39 pM) and filtered MMHg (0.020–0.17 pM) was similar between freshwater and estuarine zones, which was associated with mixing dilution of particulate MMHg by organic- and MMHg-depleted resuspended sediment. Partitioning of Hg between water and suspended particle showed tight correlation with the partitioning of organic carbon across study sites, while that of MMHg implied influences of chloride: enhanced chloride in addition to organic matter depletion decreased particulate MMHg in the estuarine zone. Primary production was an important determinant of inter-annual variation of particulate Hg and sediment MMHg. The bloom year showed relatively low particulate Hg with low C/N ratio, indicating biodilution of Hg. In contrast, the percentage of MMHg in sediment increased significantly in the bloom year, likely due to greater availability of metabolizable fresh organic matter. The overall results emphasize that Hg behavior in the lower Mekong River Basin is strongly connected to the local monsoon climate, via alterations in particle loads, biological productivity, and availability of sulfate, chloride and organic matter.