408 - Methylmercury biogeochemistry in a tidal Phragmites Marsh: Impact of tides, plant roots, and marsh elevation

We studied the biogeochemistry of methylmercury production and distribution in an oligohaline Phragmites marsh in Berry’s Creek, NJ, toward a conceptual model to aid in potential remediation. Our goals were to better understand the distribution of porewater MeHg with soil depth and marsh elevation, as well as biogeochemical controls and tidal dynamics. We observed typical sequential profiles of electron acceptors with depth, but those profiles were vertically stretched compared to many salt marshes, potentially due to transpiration by deep Phragmites root systems. There was evidence for microbial iron and sulfate–reduction, based on Fe(II) accumulation in porewaters, and sulfate depletion at depth. Deep porewaters (generally below 30 cm) were highly sulfidic. Conservative solutes like chloride were concentrated in porewaters due to plant evapotranspiration, and profiles of Hg and MeHg need to be evaluated with this in mind. MeHg production and accumulation were maximal in the depth zone where sulfate reduction occurred, but above the zone of sulfide accumulation. In low elevation plots, this zone could be as shallow as 10 cm below the soil surface; at higher elevation, it could be 30 cm depth or more, often well away from the zone of exposure to organisms on the marsh surface. The depth of MeHg production and accumulation depended almost exclusively on marsh elevation, rather than tidal stage, despite several feet of tide. Increases in MeHg concentration at low tide in marsh drainage ditches suggested interflow drainage from marsh edge porewaters, although mass flux was not quantified. Flux chambers deployed on the marsh surface showed little or no flux of MeHg from the marsh surface to overlying waters during inundation. A stepwise model for MeHg concentration in marsh pore waters included porewater Fe, sulfide, Hg and DOC aromaticity. Evaluation of potential remediation approaches will need to consider this complexity.

Authors: Cynthia Gilmour, Susan Kane Driscoll, Elizabeth Henry, James Sanders, Upal Ghosh, Grace Schwartz, Steven Brown