Selenium speciation,distribution, and transport in a river catchment affected by mercury mining and smelting in Wanshan,China

Selenium (Se) is an important co-existing elemental component of the mineral matrix of mercury (Hg) ore deposits. The hazards associated with Se contamination of the aquatic ecosystems in Hg mining areas; however, are often overlooked by environmental researchers due to a preoccupation with Hg. Selenium may also pose a long-term risk to the local ecosystem, and further complicate the situation as Se may also play an important antagonistic role against Hg. Furthermore, most studies on Se pollution have focused only on total Se, whereas the toxicity, bioavailability, and bioaccumulation of Se in aquatic ecosystems is primarily determined by its site-specific individual species. In this study, the concentrations of total Se, inorganic Se (tetravalent and hexavalent), and organic Se were determined in water samples collected from 41 typical sites selected in rivers, tributaries, and springs in Wanshan, China, where Hg and Se co-occur due to historic Hg mining and retorting activities. Se concentrations were observed to decrease with distance from mine-waste calcines, which indicated that mine-waste calcines may be significant sources of the elevated Se in the rivers, especially in downstream areas within 8 km from the mine-waste calcines. The concentration of total aqueous Se throughout the study area was highly variable (3.8 ± 6.0 μg L^?1) and on average was one order of magnitude greater than that in natural river systems worldwide (0.1–0.3 μg L^?1). The majority of the Se was hexavalent (3.1 ± 4.9 μg L^?1; 65%), followed by tetravalent (0.53 ± 0.86 μg L^?1; 15%) and organic forms (0.85 ± 1.5 μg L^?1; 20%), possibly due to the generally alkaline conditions. Se concentrations in some sampling sites exceeded certain recommended limit of values. However, the existing criteria for Se in aquatic system are mainly based on total Se and the recommended limit of values in different countries or organizations are inconsistent with one another. Therefore, the need to consider Se speciation rather than only total Se is highlighted for future studies.