Mercury Dynamics in Sulfide-Rich Sediments: Geochemical Influence on Contaminant Mobilization and Methylation within the Penobscot River Estuary, Maine, USA

Research concerning the fate and biogeochemical cycling of mercury (Hg) within coastal ecosystems has suggested that microbially-mediated diagenetic processes control Hg mobilization and that ligands with strong affinity for Hg control Hg partitioning between the dissolved and particulate phases. Of specific further concern is the likelihood that Hg sequestered in coastal marine sediments may be efficiently methylated to highly toxic methylmercury (MeHg), thereby placing exposed organisms at significant risk of MeHg bioaccumulation. We have studied total dissolved Hg (HgT) and methylmercury (MeHg) cycling in the sediments of the Penobscot River estuary in Maine, USA using a combination of equilibrium porewater samplers, kinetic modeling, and Hg-specific reactive gels. The Penobscot estuary has been subject to Hg contamination from multiple industries including a recently closed chlor-alkali production facility. Pore water depth profiles for the Penobscot estuary are divisible into kinetically discrete intervals with respect to both Hgr and MeHg dynamics. Modeling results suggest that (1) while estuary sediments act as a net sink for particulate Hg inputs they may simultaneously function as a source of dissolved Hg to the overlying water and (2) dominant MeHg production occurs at shallow sediment depths, with the sharp decrease in porewater Mel lg concentration observed near the sediment water interface (SWI) likely explained by active demethylation. Intact sediment cores were incubated in the laboratory under various hydrodynamic regimes to assess the extent to which MeHg profiles (such as those just described) are sensitive to variation in geochemical and/or hydrodynamic conditions. Results demonstrate that ponding regimes change the location of the redoxcline and affect the sediment depth at which maximum net methylation occurs. Moreover, induced shoaling of the redoxcline demonstrates the potential for heightened MeHg efflux from the sediment. This flux may represent a distinct aqueous phase exposure pathway for coastal biota. The lability of porewater and sediment Hg was further examined by deploying mercapto-substituted siloxane gels within estuary sediments. The resultant observations of low general Hg reactivity support the hypothesis that porewater Hg may be defined as a function of porewater ligand production, highlighting the importance of microbially-mediated diagenesis in controlling Hg cycling within estuary sediments