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Hydrological Controls on Methylmercury Distribution and Flux in a Tidal Marsh
journal contributionposted on 2014-06-17, 00:00 authored by Hua Zhang, Kevan B. Moffett, Lisamarie Windham-Myers, Steven M. Gorelick
The San Francisco Estuary, California, contains mercury (Hg) contamination originating from historical regional gold and Hg mining operations. We measured hydrological and geochemical variables in a tidal marsh of the Palo Alto Baylands Nature Preserve to determine the sources, location, and magnitude of hydrological fluxes of methylmercury (MeHg), a bioavailable Hg species of ecological and health concern. Based on measured concentrations and detailed finite-element simulation of coupled surface water and saturated-unsaturated groundwater flow, we found pore water MeHg was concentrated in unsaturated pockets that persisted over tidal cycles. These pockets, occurring over 16% of the marsh plain area, corresponded to the marsh root zone. Groundwater discharge (e.g., exfiltration) to the tidal channel represented a significant source of MeHg during low tide. We found that nonchannelized flow accounted for up to 20% of the MeHg flux to the estuary. The estimated net flux of filter-passing (0.45 μm) MeHg toward estuary was 10 ± 5 ng m–2 day–1 during a single 12-h tidal cycle, suggesting an annual MeHg load of 1.17 ± 0.58 kg when the estimated flux was applied to present tidal marshes and planned marsh restorations throughout the San Francisco Estuary.
Hg mining operationsMethylmercury DistributionTidal MarshThe San Francisco Estuarynonchannelized flowPalo Alto Baylands Nature Preservegroundwater dischargehealth concernSan Francisco Estuarymarsh restorationsgeochemical variableshydrological fluxespore water MeHgMeHg fluxsurface watermarsh root zonebioavailable Hg speciesHydrological ControlsMeHg load