University of California Water Resources Center

Trace metal inputs to estuarine ecosystems can have substantial impacts on their environmental quality. However, it is imperative to understand background processes which affect trace metal cycling in order to quantify the impacts of human activities. Chromium is an important industrial metal, with known sources surrounding San Francisco Bay. It also has substantial natural sources, being the tenth most abundant element on the planet. Therefore, this research is directed at quantifying the relative magnitude of natural and anthropogenic fluxes of Cr to the San Francisco Bay estuary.

Episodic flushing of a wetlands area, the Yolo Bypass, appears to be the predominant source (up to 980 kg d-l) of dissolved chromium to the San Francisco Bay estuary. Depletion of [Cr]/[Al] in suspended particulate matter within the entrapment zone of the estuary suggests that that chromium is solubilized by weathering of aluminosilicates. Conversely, other processes (e.g., internal inputs, in-situ reduction) appear to govern the net flux of chromium within the estuary when the bypass is not being flushed. All of these processes can be demonstrated by examination of ancillary measurements (e.g., hydrographs of freshwater endmembers, nutrient data, other trace metal measurements), which reveal the complexity of the chromium biogeochemical cycle within the hydrologically bifiucated and highly modified estuary.

Sediments located at Green Sands Beach, adjacent to the Mare Island Naval shipyard are known to be impacted by disposal of metal-enriched tailings from sandblasting operations. These 4 sediments were an order of magnitude enriched compared to the baywide average [Cr]/[Al] ratio. This enrichment showed a gradient leading away from the wetlands above the beach.