Decadal trends of silver and lead contamination in San Francisco Bay surface waters

Over the past decade, San Francisco Bay surface waters have remained enriched with dissolved (<0.45 μm) silver and lead concentrations (decadal means of 5.7 ng kg^-1 Ag_[filtered] and 31 ng kg-¹ Pb_[filtered]) compared with those (0.26 ng kg-1 Ag_[filtered] and 2.7 ng kg^-1 Pb_[filtered]) of adjacent oceanic surface waters of the northeast Pacific, despite efforts to reduce pollutant loadings to the Bay during that period. While time series models show that there has been a 40% decline in total lead concentrations in the southern reach of the estuarine system between 1989 and 1999, the filtered lead fraction has not changed significantly during that time. That persistence is attributed to (i) the ongoing input from previous atmospheric depositions and industrial lead to its drainage basin, which are slowly being advected into the estuary and (ii) the internal recycling of lead between the surface sediments and the water column within the Bay. In contrast, both filtered and total silver concentrations in the southern reach have declined by 70% and 40%, respectively, within the past decade. These temporal declines are attributed to a 2-fold decrease in silver loadings from publicly owned treatment works and a comparable decline in the silver concentration of surficial sediments within that region during the past decade. In the northern reach, silver and lead concentrations have remained essentially constant between 1989 and 1999, reflecting invariable anthropogenic input of these elements to this embayment over that decade.