Temporally variable freshwater sources of dissolved chromium to the San Francisco Bay estuary

Chromium is a common contaminant in estuarine environments, originating from chemical manufacturing, steel refining, and leather tanning. However, it is also relatively abundant in the earth's crust, resulting in substantial natural sources from upland watersheds. The objective of this study is to quantify the relative magnitudes of natural and anthropogenic chromium fluxes to San Francisco Bay, an important estuary in the National Estuaries Program. Chromium speciation was determined throughout the estuary during the wet and dry seasons of 1992-1995. Those measurements, hydrographs of freshwater endmembers, and silicate-salinity plots reveal the complexity of chromium sources to the highly modified estuary. Episodic flushing of an agricultural drain transports chromium leached from alluvial sediments to the northern reach of the estuary. In contrast, the principal dissolved chromium source to the southern reach appears to be diagenic remobilization from sediments within the estuary. In both the northern and southern reaches, dissolved chromium fluxes are scavenged back onto particles, showing that sediments serve as both a source and a sink in the geochemical cycle of chromium. Direct anthropogenic chromium inputs are dwarfed by high-flow fluvial inputs but are comparable to low-flow dissolved inputs.