Mixing vs Capping: Thin-Layer Placement of Activated Carbon Reduces Porewater Methylmercury While Limiting Sediment Accumulation

Remediation of mercury (Hg)-contaminated sediments seeks to reduce Hg and methylmercury (MeHg) transport and exposure. Sorbent amendments like activated carbon (AC) mixed into sediments or applied as part of a thin cap can reduce porewater concentrations, benthic exposure, and flux to surface water. However, mixing AC into bulk sediments has sometimes increased the sediment MeHg levels. This study tested whether applying AC as a thin layer could prevent the increase in sediment MeHg associated with the mixing of AC into the active methylation zone. We compared four amendment strategies in Hg-contaminated sediment microcosms: a thin AC layer (5 wt % of sediment) beneath a 2 cm sand cap, sand cap alone, AC layer alone, and AC fully mixed into the sediment. Sediment and aqueous profiles were measured directly and with equilibrium-based passive samplers to evaluate the amendment effects. All amendments decreased MeHg porewater concentrations and potential fluxes to surface water, with the greatest effectiveness for thin-layer AC under a sand cap. As expected, mixing AC into sediments significantly increased the level of sediment MeHg. In contrast, application of the same AC dose as a thin-layer AC cap did not increase sediment methyl mercury. Additionally, surface water MeHg was lowest in microcosms with AC caps, providing a promising strategy for the reduction of MeHg flux from sediments. The use of passive samplers enabled high-resolution, integrative depth profiling at 0.5 cm intervals, providing new insights into MeHg dynamics within treatment layers.