Mercury and Dissolved Organic Matter in Delta Wetlands

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Between 1860 and 1914, hydraulic mining activities sent more than 800,000,000 cubic yards of mercury-laden sediment into the Delta altering the landscape, water flows, and contributing to the leveeing and reclamation of the Delta's marshes. Transport of mercury from historic mining areas continues today. The sedimentary supply of mercury to the Delta and in Delta sediments (cinnabar, metacinnabar, and elemental Hg) typically are insoluble, but will pose an environmental hazard if they are (1) solubilized and (2) methylated in Delta and Estuary wetlands. The relative roles of dissolved organic matter (DOM) and reduced sulfur in facilitating mercury solubilization are elucidated, and the net export of Hg and MeHg off tidal wetlands are quantified. The net export represents the integrated effect of site-specific processes such as solubilization, oxidant supply, physical dynamics, microbial rates, atmospheric and sedimentary loss, as well as other variables.

The specific effects of some of these processes are under active study by others. In close collaboration with these ongoing process, level studies of Hg methylation rates and food web incorporation, we propose to measure net Hg and MeHg export from representative tidal wetlands to help assess the contribution of tidal wetlands to Hg concentrations in the Delta, and compare the Hg concentrations predicted by process-level rate studies with the observed net wetland fluxes. In collaboration with ongoing Delta Mercury studies, two specific elements of Hg cycling in the Delta, not currently addressed by other studies, are addressed. The two specific objectives are to: (1) Quantify the capability of DOM from representative Delta wetlands to solubilize the major sedimentary forms of mercury in the Delta, cinnabar (HgS), metacinnabar, and elemental Hg, and (2) Determine the dissolved and particulate fluxes of Hg and monomethyl-Hg at representative wetland sites in the Delta. The results of this study will provide knowledge and insights helpful for guiding restoration of wetlands in such a way that MeHg production and export are minimized to the extent possible.