Modeled Trace Element Concentrations and Partitioning in the San Francisco Estuary, Based on Suspended Solids Concentration MICHELLE D. BENOIT, † RAPHAEL M. KUDELA,* ,† AND A. RUSSELL FLEGAL ‡ Department of Ocean Sciences, University of California, Santa Cruz California 95064, and Institute of Marine Science, University of California, Santa Cruz, Santa Cruz California 95064 Received February 2, 2010. Revised manuscript received June 15, 2010. Accepted June 29, 2010. 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 μm) 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 ( K d ), 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. Introduction Total and dissolved trace element concentrations are im- portant to understand when studying complex estuarine ecosystems. Contaminant trace elements can directly affect biological systems in estuaries and are both influenced by and influence anthropogenic usage. Concentrations of several trace elements are high enough to adversely affect biota within San Francisco Bay (1, 2), while high levels of mercury in some fish (3) exceed criteria for human consumption (4). Phillips et al. (5) determined that concentrations of silver (Ag), cadmium (Cd), copper (Cu), chromium (Cr), nickel (Ni), lead (Pb), and/or zinc (Zn) contributed to the toxicity to bivalve larvae in the northern reach of the Bay. Others (6, 7) have associated elevated Ag concentrations with sublethal toxicities in marine invertebrates throughout the estuary, and Se toxicity has been documented in waterfowl within the Bay’s drainage basin (8). In response to these issues, trace element concentrations have been extensively monitored throughout the Bay, including both site specific studies and systematic sampling of near total (aqua regia digest) particulate and total dissolved (<0.45 μm) phases of a suite of trace elements (Ag, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, Zn) and associated parameters (e.g., nutrients, salinity, chlorophyll, suspended particulates). This sampling was initiated over two decades ago (9) and has been institutionalized as the San Francisco Estuary Regional Monitoring Program, RMP (10). It is the longest running, and perhaps most extensive, monitoring program for trace elements in any estuarine system in the U.S. and elsewhere (11). However, even the RMP trace metal data are insufficient, because limited sample collections (1-3 times per year) usually miss extreme or unusual events (e.g., major floods, high wind episodes, algal blooms, atypical tides, or storms) that strongly influence the temporal and spatial distribution of trace elements within the estuary and adjacent oceanic waters (12). Optically derived suspended sediment concentration (SSC) has previously been used to assess water quality for estuarine and coastal waters (13-15). We investigated the potential for utilizing the relationship between optically derived SSC and trace element concentrations to augment in situ measurements of these elements. The United States Geological Survey (USGS) maintains 13 continuous SSC monitoring stations in San Francisco Bay (16), and satellite remote sensing provides synoptic coverage of SSC in the Bay at reduced spatial and temporal scales (e.g., ref 13). Theo- retically, remote sensing has the ability to sample any location in the Bay and adjacent coastal waters at daily temporal resolution. Although this is difficult in reality due to various constraints, time-averaged spatial maps of SSC and associated trace element concentrations could be developed for the San Francisco Bay system to augment existing programs such as the RMP. That potential was tested with a 10-year triannually collected RMP data set (1993-2003), and a 3-year RMP data set (2004-2006), when samples were collected biannually. A model of the relationship between the distribution ratio and total concentration of a suite of trace elements (Ag, As, Cd, Co, Cr, Cu, Fe, Hg, MeHg, Mn, Ni, Pb, Se, and Zn) with SSC was derived from the 10-year data set. Accuracy of the model was evaluated using the independent 3-year data set, and model outcomes were applied to the 3-year data set to estimate the dissolved concentrations of those trace elements. These estimates were then compared with measured dis- solved trace element concentrations to determine the ac- curacy and bias of the optically based model. These models cannot replace actual measurements but can give snapshot or overview estimations, and can be used to estimate concentrations (with known model error) when in situ data are not available. Study Area. San Francisco Bay is a turbid, shallow estuary that covers an area of 1240 km 2 (Figure 1), and can be divided into three hydrographic regions. The Sacramento and San Joaquin river system contributes 90% of the Bay’s fresh water (17) and 83-86% of its fluvial sediment (18) into the northern reach of the estuary, which extends from the deltaic con- fluence of those two rivers (Delta) to San Pablo Bay (19). * Corresponding author phone: 831.459.3290; e-mail: kudela@ ucsc.edu. † Department of Ocean Sciences. ‡ Institute of Marine Science. Environ. Sci. Technol. 2010, 44, 5956–5963 5956 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 15, 2010 10.1021/es1001874  2010 American Chemical Society Published on Web 07/12/2010