Methylmercury cycling in estuarine sediment pore waters (Penobscot River estuary, Maine, USA)

Abstract

Particulate mercury (Hg) sequestered in coastal marine sediments may be efficiently methylated to highly toxic methylmercury (MeHg), thereby placing exposed organisms at risk of MeHg bioaccumulation. The Penobscot River estuary in Maine, U.S.A., has been subject to Hg contamination from multiple industries including a recently closed chlor‐alkali production facility. Pore‐water depth profiles of total Hg, MeHg, and ancillary chemistry for the midestuary region were collected in August 2006 and June 2007. The profiles are divisible into kinetically discrete intervals with respect to MeHg dynamics. Dominant MeHg production occurs between ~2 and 7 cm in August and ~2 and 8 cm in June, with similar net MeHg production rates between 0.35 x 10‐20 and 4.9 x 10‐20 mol cm‐3 s‐1. A significant decrease in pore‐water MeHg concentration is observed in the vicinity of the sediment‐water interface (SWI). For August, a minimum MeHg consumption rate constant of 1.1 d‐1 may be estimated equal to the diffusive transport rate within a depth interval <0.75 cm. In June, the MeHg consumption zone extends from the SWI to a similar depth as for August, but the consumption rate is slower. MeHg consumption involves both sorption to sediment and demethylation. Intact sediment cores were incubated in the laboratory under various ponding regimes to study the influence of dominant geochemical parameters on in situ Hg methylation. The ponding regime changes the location of the redoxcline, which affects the depth of maximum methylation. Induced shoaling of the redoxcline brought about by the absence of any advective mixing of overlying water results in heightened MeHg efflux from the sediment.