Sediment and porewater profiles and fluxes of mercury and methylmercury in a small seepage lake in northern minnesota

Mercury (HQT) and methylmercury (MeHg) were measured at 1-2 cm resolution in sediment porewater and sediment cores from Spring Lake in the Marcell Experimental Forest of northern Minnesota. Recent sediment accumulation of Hg_T was 21.4 μ m^-2yr^-1 (1990-2000), 2 orders of magnitude greater than the accumulation of MeHg (0.20 μg m^-2 yr^-1). The highest solid phase concentrations of MeHg were observed persistently at the sediment surface and declined sharply with depth. Porewater profiles showed a small diffusive flux of MeHg from sediment to water (5 ng m^-2 month^-1). Springtime porewater concentrations of MeHg were relatively low (∼0.5 ng L^-1) and increased by late summer to early fall (1.5-2.2 ng L^-1), showing distinct peaks roughly correlated with maxima in sulfate reducing activity at 5 and 15 cm. Advective transport carrying MeHg deeper into the sediment was evident in summer and fall. The percent of Hg_T present as MeHg was highest in the water column above the sediment (10%) and decreased with sediment depth in both the solid and porewater phases. Sediments at this study site are a net sink for MeHg, although diagenetic processes of demethylation and methylation are evident within the lake - sediment environment.