Spatial distribution and methylation of mercury in a eutrophic reservoir heavily contaminated by mercury in Southwest China

In Baihua Reservoir (BHR), a Hg-contaminated and eutrophic reservoir in SW China, water and sediment samples were collected in June 2010 for the determination of Hg species, metal, major ion and nutrient concentrations. Using a stable isotope tracer technique, 202HgCl2 and Me198HgCl were spiked into sediment cores to study methylation and demethylation processes. The inorganic Hg concentration range was 600–13,000 ng/g (dry weight, dw) in the top 10 cm of sediment; exceeding the local background concentration (260 ng/g, dw). Concentrations of Hg species in the water column and pore water were similar to non-Hg contaminated reservoirs in the same watershed. Dissolved total Hg(DHg)and dissolved methylmercury (DMeHg) (mean ± SD) in the pore water in BHR were 6.8 ± 3.1 and 0.27 ± 0.20 ng/L, respectively. Dissolved Hg, DMeHg and reactive Hg (RHg) in the water column were 2.3 ± 0.9, 0.23 ± 0.22, and 0.77 ± 0.17 ng/L, respectively. The vertical distributions of Hg species showed inorganic Hg and methylmercury (MeHg) concentrations peaked near the bottom of the water column, implying the impact of thermal stratification and eutrophic conditions on the production and distribution of Hg species in this reservoir. The methylation rate (<0.1%/day) in these sediments was lower and the demethylation rate (17.6%/day) was higher than those reported in other eutrophic reservoir studies.