Selenium biogeochemistry in the San Francisco Bay estuary: changes in water column behavior

The cycling of dissolved selenium was examined in the North San Francisco Bay estuary using 5 surface water transects from the Pacific Ocean (Golden Gate) to the Sacramento and San Joaquin Rivers, monthly river sampling, and three collections of oil refinery effluents during 1997–2000. By combining these data with earlier results from the mid-1980s, a nearly 16-year record of riverine fluxes, estuarine processes, and anthropogenic inputs was obtained. The Sacramento River concentrations and speciation have remained unchanged over the period, and while the speciation of selenium in the San Joaquin is similar, its dissolved selenium concentrations have decreased by almost one half. More significantly, the concentration of selenium from oil refinery discharges to the mid-estuary has decreased 66% and its speciation changed from one dominated by selenite (66%) to one that is only 14% selenite. This change in refinery effluents occurred while our study was underway, with the result being a pronounced decrease in selenite concentrations (82%), and hence total dissolved selenium, in the mid-estuary. A companion study found that sediment/ water exchange is a minor flux to the estuary, and hence selenium inputs from the Sacramento River, as well as refineries during low flow (summer, fall) periods exert major controls on the dissolved selenium behavior in this estuary. Nevertheless, in situ processes associated with organic matter cycling (photosynthesis and respiration) still modify the distributions and internal transformations of dissolved selenium, notably organic selenide.