Modeled Trace Element Concentrations and Partitioning in the San Francisco Estuary, Based on Suspended Solids Concentration

Abstract

Although trace element (Ag, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn) and methylmercury (MeHg) concentrations have been systematically sampled 1-3 times per year throughout the San Francisco Bay estuary for more than two decades, those collections do not capture episodic events that may govern the biogeochemical cycles of these elements in the Bay and adjacent Pacific coastal waters. Analyses of the partitioning of in situ elemental concentrations between particulate and total dissolved (<0.45 microm) phases coupled with optically based measurements of suspended solids concentration (SSC) showed highly significant (p<0.001) associations between all elemental concentrations and SSC in the Bay. Predictive models were developed to estimate the distribution ratio (D), or partition coefficient (Kd), and total concentration of each element in the water column based solely on SSC measurements. Modeled predictions of total element concentrations and distribution ratios were then coupled with measured SSC to predict the concentrations of dissolved trace elements in the water column. These predicted total and dissolved concentrations of trace elements can provide both better diagnostics of biogeochemical cycling within the estuary and better estimates of fluxes to adjacent coastal waters, overcoming the limitations of the long-running but limited direct measurements of trace elements from existing sampling programs.