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

Fluxes of total mercury (Hg_T) 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 Hg_T 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 Hg_T and those of NH ₄ ⁺ and SiO₂, and between dissolved groundwater concentrations of MMHg and those of Hg_T and NH₄ ⁺. However, such relationships did not account for the majority of the variability in concentration data for eithermercuryspecies in groundwater. Fluxes of Hg_T 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 Hg_T fluxes are substantially greater than net atmospheric inputs of Hg_T 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 Hg_T and MMHg to coastal waters along the central California coast.