Fifty years after its discharge, methylation of legacy mercury trapped in the Penobscot Estuary sustains high mercury in biota.

Fifty years ago, the Penobscot Estuary was contaminated by mercury discharged from the chlor-alkali plant located in Orrington, Maine, USA. Almost all of the mercury was discharged from the plant during the late 1960s and early 1970s. Despite the much lower mercury discharges in recent decades, present-day concentrations in surface sediment remain high (averaging 350-1100 ng/g dw) and are still high in blood of marsh birds (up to 10.5 µg/g), black duck muscle (0.8 µg/g), and lobster muscle (0.4 µg/g). Methyl mercury (MeHg) concentrations in marsh birds exceed levels that impair reproduction. There are health advisories for duck hunters and closures of shellfish fisheries. These continuing high mercury concentrations are caused by the trapping of legacy mercury in a mobile pool of sediment that is retained in the upper estuary above a tidally forced salinity front, which travels up and down the estuary each tidal cycle - slowing the transport of particulate mercury to Penobscot Bay. The trapped legacy mercury continues to be available for methylation 50 years after it first entered the estuary. This is demonstrated by the fact that rates of MeHg production are positively related to the inorganic mercury concentration in parts of the estuary with elevated concentrations of legacy mercury. Thus, remediation measures that would lower the THg concentration in surface sediment would lower the MeHg in birds, fish and shellfish. All of this new information leads us to recommend two remediation options. Addition of mercury binding agents may lower mercury concentrations in birds in some wetland areas. System-wide, we also recommend Enhanced Natural Recovery (ENR), a novel approach that involves the partial removal of the contaminated mobile sediment pool followed by replacement with clean-clay particulates to dilute inorganic mercury concentrations, which would lower methylation rates and mercury concentrations in biota.