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

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.