Biogeochemical controls on methylmercury in soils and sediments: Implications for site management

Abstract Management of Hg-contaminated sites poses particular challenges because methylmercury (MeHg), a potent bio-accumulative neurotoxin, is formed in the environment, and concentrations are not generally predictable based solely on total Hg (THg) concentrations. In this review, we examine the state of knowledge regarding the chemical, biological, and physical controls on MeHg production and identify those most critical for contaminated site assessment and management. We provide a list of parameters to assess Hg-contaminated soils and sediments with regard to their potential to be a source of MeHg to biota and therefore a risk to humans and ecological receptors. Because some measurable geochemical parameters (e.g., DOC) can have opposing effects on Hg methylation, we recommend focusing first on factors that describe the potential for Hg bio-accumulation: site characteristics, Hg and MeHg concentrations, Hg availability, and microbial activity, where practical. At some sites, more detailed assessment of biogeochemistry may be required to develop a conceptual site model for remedial decision making. Integr Environ Assess Manag 2017;13:249–263. © 2016 SETAC Key Points Because of the complex interaction of multiple geochemical, biological, and physical parameters, no single parameter can reliably predict the likelihood of Hg methylation and bioaccumulation or the concentration of methylmercury (MeHg) in the environment. Site managers should focus on a limited number of parameters that have the greatest likelihood of characterizing the sediment as a potential source of MeHg. Sediment total Hg (THg) and MeHg concentrations alone will not provide all the information needed to plan site remediation, but are a valuable starting point.