Forest runoff increase mercury output from subtropical forest catchments: an example from an alpine reservoir in a national nature reserve (southwestern China)

For a typical alpine forest reservoir that does not have other obvious mercury pollution sources, mercury levels in its downstream water can reflect the characteristics of mercury inputs and risks of a remote forest. To confirm this proposal, eight sampling campaigns were carried out in 2012 and 2013 to investigate the distribution patterns of Hg species in the water column and sediment profiles at four sampling stations in an alpine forest reservoir, southwestern China. The concentrations of total Hg (THg) and methylmercury (MeHg) in water samples of DaHong Reservoir varied from 2.8 to 8.2 ng L−1 and 0.06 to 2.9 ng L−1, respectively. The concentrations of these Hg species in the wet seasons (summer and fall) were significantly higher than those in the dry season (winter and spring). Forest field runoff and diffusion of Hg from sediments could be the reasons for elevated concentrations of these Hg species in the wet season. Elevated MeHg concentrations in water samples from the bottom water and water-sediment interface demonstrated an active net Hg methylation in the downstream of DaHong (DH) Reservoir. Dissolved MeHg levels in the pore water of surface sediments and the bottom of downstream were apparently higher than those in the upstream, which indicated that MeHg sources of sediment pore water varied in space. MeHg diffusive fluxes from sediment to overlying water were higher in the wet season than those in the dry season, demonstrating that high temperatures favor Hg methylation processes in sediment.