High resolution porewater profiling of methylmercury with a novel equilibrium passive sampler

Abstract

Accurate quantification of sediment porewater methylmercury (MeHg) profiles is critical for understanding Hg fate, transport, and informing remediation design, yet direct porewater profiling remains challenging. Equilibrium-based passive samplers (EqPS) offer an alternative, providing fine-scale, time-integrated measurements and convenience. In this study, we deployed an EqPS consisting of activated carbon suspended in agarose gel (ag+AC) in estuarine and freshwater sediment microcosms to obtain porewater MeHg profiles at 0.5 cm vertical resolution. Our objectives were to evaluate whether ag+AC samplers could reproduce porewater MeHg profile shapes and concentrations across redox gradients and contrasting geochemical conditions. Following 4 week deployments across the sediment–water interface ag+AC samplers produced distinct depth profiles that corresponded closely with directly measured porewater profiles and exhibited maxima at redox transition zones where net methylation is most likely. Using a previously derived equilibrium partition coefficient (10²·⁹⁶ L kg⁻¹) ag+AC samplers predicted, on average, directly measured porewater MeHg concentrations to within a factor of about two. Across both sediment types, porewater MeHg alone explained ∼65% of the variability in passive sampler MeHg despite wide ranges in DOC, sulfide, iron, and salinity. These results demonstrate ag+AC EqPS can provide fine-scale porewater MeHg profiles and accurate concentration estimates across diverse sedimentary environments.