Mercury and its form in a dammed reservoir ecosystem during the charging phase

Throughout continents, reservoirs tend to have elevated methylmercury (MeHg) concentration transformed from mercury (Hg/total Hg). This impact may be pronounced in the reservoir with less velocity of water during the charging period resulted in the deposition of sediments. In sediments on favorable conditions, methylation may be enhanced by the decomposition of flood organic material, which can release Hg and enhance microbial activity. However, much less is known about the transfer ratio of Hg and its form MeHg from sediment to biota in the hydrological reservoir during the dam charging phase. The objective of our study was to understand the interrelationship between total Hg and MeHg in two key components sediment and fish in the reservoir ecosystem. This study was performed at the Three Gorges Reservoir (TGR) located on upstream of the Yangtze River in China. At the TGR charging phase, during winter time, the water level was high due to blockade of water by Three Gorges Dam (TGD). Sediment and fish samples were collected in winter season for total Hg, MeHg, and several ancillary parameters. The results showed that total Hg in sediment samples of the winter season were ranged from 6.2 ± 0.001 to 193.3 ± 0.001 × 10⁻³ mg/kg, with an average value of 53.76 ± 51.80 × 10⁻³ mg/kg, and for MeHg was ranged from 12.1 ± 0.04 to 348.7 ± 0.16 × 10⁻² ng/g, with an average value of 98.96 ± 93.07 × 10⁻² ng/g. Total Hg and MeHg in fish samples of the winter season were from 42.48 ± 6.71 to 166 ± 52.56 ng/g, with an average value of 76.22 ± 31.23 ng/g, and from 21.09 ± 2.31 to 61.60 ± 13.30 ng/g, with an average value of 37.89 ± 11.96 ng/g. The relationship of total Hg and MeHg concentrations in fish to those of sediments from corresponding sites showed a negative relationship. This might include a strong association of total Hg with an inorganic component of sediment (e.g., bound to sulfides or coprecipitated with other metal oxides such as manganese and iron). The average concentration of fish MeHg found in this study, at rates greater than 1.72 g/day, was estimated hazardous to human health. This study concludes sediment was acting as sequestrate for total Hg and MeHg in TGR. The bioaccumulation of total Hg and MeHg in fish was not controlled by sediment further investigation about pathological routes and dietary habits of fish needed to be identified for total Hg and MeHg study in TGR.