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Submarine groundwater discharge of total mercury and monomethylmercury to central California coastal waters

Abstract:

Fluxes of total mercury (HgT) and monomethylmercury (MMHg) associated with submarine groundwater discharge (SGD) at two sites on the central California coast were estimated by combining measurements of Hg T and MMHg in groundwater with the use of short-lived, naturally occurring radium isotopes as tracers of groundwater inputs. Concentrations of HgT were relatively low, ranging from 1.2 to 28.3 pM in filtered groundwater, 0.8 to 11.6 pM in filtered surface waters, and 2.5 to 12.9 pM in unfiltered surface waters. Concentrations of MMHg ranged from < 0.04 to 3.1 pM in filtered groundwater, < 0.04 to 0.53 pM in filtered surface waters, and 0.07 to 1.2 pM in unfiltered surface waters. Multiple linear regression analysis identified significant (p < 0.05) positive correlations between dissolved groundwater concentrations of HgT and those of NH 4+ and SiO2, and between dissolved groundwater concentrations of MMHg and those of HgT and NH4+. However, such relationships did not account for the majority of the variability in concentration data for eithermercuryspecies in groundwater. Fluxes of HgT via SGD were estimated to be 250 ± 160 nmol day-1 m-1 of shoreline at Stinson Beach and 3.0 ± 2.0 nmol m-2 day-1 at Elkhorn Slough. These HgT fluxes are substantially greater than net atmospheric inputs of HgT reported for waters in nearby San Francisco Bay. Calculated fluxes of MMHg to coastal waters viaSGDwere 10 ± 12 nmol day-1m-1 of shoreline at Stinson Beach and 0.24 ± 0.21 nmol m-2 day -1 at Elkhorn Slough. These MMHg fluxes are similar to benthic fluxes of MMHg out of surface sediments commonly reported for estuarine and coastal environments. Consequently, this work demonstrates that SGD is an important source of both HgT and MMHg to coastal waters along the central California coast. © 2009 American Chemical Society.