The estuarine behaviour of selenium in San Francisco Bay

Abstract

In April and September 1986 concentrations of dissolved selenate, selenite and Se(−II+O), suspended particulate selenium, nutrients, chlorophyll a and total suspended matter, were determined in the San Francisco Bay estuarine system. In addition, dissolved selenium speciation was determined in the Sacramento and San Joaquin Rivers between 1984 and 1987. The April 1986 estuarine sampling occurred during high river discharge, and within the Northern Reach of San Francisco Bay mid-estuarine input of selenite and Se(−II+O) is apparent, while selenate appears to be removed. During September 1986 river discharge rates were approximately four orders of magnitude lower than in April, and the mid-estuarine production of all selenium species is apparent. In contrast, dissolved selenium in the South San Francisco Bay generally shows conservative mixing behaviour during April and September 1986. The source of dissolved selenium in the South Bay appears to be effluents from sewage treatment plants. In the Northern Reach effluents from oil refineries located in the mid-estuary may be major sources of selenium input during low river discharge periods. However, during periods of high river discharge the sources and sinks of dissolved selenium species within the Northern Reach remain unidentified.