Abstract

Five oil refineries discharge into a narrow section of the San Francisco Bay, known as the Carquinez Strait. Previous studies have shown that these discharges increase the concentration of Se in both the water and sediments of this area, potentially threatening local bird populations. As a result of a partnership involving Lawrence Berkeley National Laboratory, the Dept. of Energy, and the San Francisco Bay Regional Water Quality Control Board, selenium cycling in wetlands is being investigated. Two sites, each downstream of a major oil refinery, were characterized with respect to selenium concentration and speciation in surface water, suspended sediment, pore water, marsh sediment, plants, and clams.

Sedimentation dynamics were found to be an important factor in Se distribution, because of the high affinity of selenium on suspended sediments and the landward-decreasing energy of the depositional environment. Although more than 99% of sediment selenium is in insoluble forms, concentrations of aqueous selenium in sediment pore water were found to be an order of magnitude higher than in overlying Bay water, suggesting selenium diffusion out of the sediment. Chemical reduction of selenium in both sediments and the water column under experimentally ponded conditions was found to occur on time scales exceeding the duration of tidal flooding of, these environments. The implications of these findings bear strongly on the future approach to selenium monitoring, prompting a focus on the sediment-water interface and further research into selenium association with suspended sediment.

Introduction

Motivated by revelations of selenium toxicity to waterfowl at Kesterson Reservoir, California, selenium (Se) became a focus of public and regulatory attention in the mid 1980's. In response to appeals from public agencies and environmental groups, the San Francisco Bay Regional Water Quality Control Board ("Water Board") adopted in 1986 a Basin Plan aimed at reducing the mass of selenium being deposited in San Francisco Bay. As is now known, oil refineries contribute selenium into the Bay via the effluent of the refining process (Ref. 1). Therefore, the Water Board moved to limit those discharges by issuing permits. In 1991, permits to six refineries (Shell, UNOCAL, Chevron, Exxon, Tosco, and Pacific) were issued which required limiting Se discharge at 1991 levels and set a December 1993 deadline to attain Se concentrations of 50 g/L in refinery effluent. A comprehensive review of the problem and strategy were published by the Water Board in 1992. Toward the end of 1993, it became apparent that goals set forth by the Water Board were not attainable by the set deadline. Out of this realization came a settlement between the Water Board and the refineries, as represented by the Western States Petroleum Association (WSPA). The settlement provided for a $2 million mitigation fund to be used for resolving San Francisco Bay contamination issues. A small part of this find was be used to aid refineries in attaining the 50 g/L selenium standard. Most of it is earmarked for studies of the behavior and transport of Se and other contaminants in the Bay as well as wetlands restoration. As part of this settlement, Bay Area refineries have until July 1st of 1988 to comply with both concentration and loading limits.

Past Research and Unresolved Issues

Se occurs naturally in certain areas of the San Joaquin Valley, generally associated with Cretaceous shales or material eroded from those formations (Ref. 2). The latter was the source of Se which eventually found its way to Kesterson Reservoir. Being that Cretaceous shales are also associated with crude oil deposits, Se is found in relatively high concentrations in San Joaquin Valley (SJV) crude. Therefore, refineries which use SJV crude discharge effluent which is higher in Se than those which use crude from other areas, such as Alaska's North Slope. Se concentrations in effluent from the SJV crude refining process at Shell, UNOCAL, and Exxon refineries fall in the range of 400 to 700 g/L, exceeding the Water Board standard by one order of magnitude. Conversely, the Chevron refinery uses low-selenium crude, resulting in an effluent Se concentration of approximately 25 g/L.

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