Background/Objectives. Geochemical speciation, reaction path, and reactive transportmodels have long been used to address a range of groundwater and surface water qualityproblems, and developed for simulation of early diagenesis, element cycling, and contaminanttransport in aquatic sediments. Geochemical speciation models have also beenadapted for ecotoxicological applications (e.g. biotic ligand model). The scientific appealof these models is their representation of biogeochemical interactions at a molecularlevel, which can provide unique insights into the influence of the underlying reactionmechanisms on the macroscopic system behavior. Geochemical modeling has, in general,seen limited use in investigating contaminant bioavailability, and even less so in the evaluationof sediment remediation strategies targeting bioavailability reduction as theprincipal means of risk reduction. We present an approach using geochemical models toprovide a quantitative framework for understanding mercury (Hg) bioavailability andmethylmercury (MeHg) distribution in contaminated sediments, and demonstrate howthis approach can be used to assess the effectiveness of proposed in situ remediationstrategies involving chemical amendments on Hg methylation potential.Approach/Activities. Sediment and porewater chemical data compiled from several investigationswere used as a test dataset. Geochemical speciation calculations focused ondissolved Hg complexation by sulfide and dissolved organic matter (DOM) and identificationof solubility controls on porewater concentrations. In particular, we focused ondeveloping relationships between total and available Hg based on modeled Hg speciesdistributions and currently accepted Hg methylation mechanisms. Effects of variouschemical amendments on potential Hg methylation were assessed by reaction path simulationsof porewater titration with amendments.Results/Lessons Learned. Porewater Hg concentrations are related to total sediment Hgvia sorption and solubility reactions and porewater chemistry influencing Hg speciation(pH, sulfide and DOM). The concentration of neutral charge Hg sulfide complexes(mainly HgS~0) in porewater represents the fraction of inorganic Hg available for methylationand correlates with maximum porewater MeHg concentrations. Measured MeHgconcentrations represent the balance between methylation and demethylation rates, andcan be lower than predicted from HgS~0 concentrations. Sediment MeHg concentrationsare related to porewater concentrations mainly via sorption to organic matter. The relationshipbetween total sediment Hg and dissolved Hg (and MeHg) in porewater isnonlinear, reaching saturation when total sediment Hg loading exceeds sorption capacityleading to precipitation of metacinnabar (β-HgS). In metacinnabar saturated systems,MeHg concentrations also reach maximum levels, becoming independent of total sedi
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