Temporal and spatial changes in the concentrations of hydrocarbons and trace metals in the vicinity of an offshore oil-production platform

Abstract

A 4-year multidisciplinary monitoring program was conducted between October 1986 and October 1990 to detect and quantify environmental impacts related to oil and gas drilling in the Arguello Field in southern California. As part of this program, concentrations of trace metals and petroleum hydrocarbons were determined in surface sediments and in trapped sediments before, during, and after drilling had stopped. Barium, a major component of drilling muds, was the only chemical that changed significantly during the period of study. During drilling, barium concentrations in surface sediments increased 30–40% above background. After drilling stopped, barium concentrations decreased but, after 1.5 years, had not approached background concentrations of surface sediments. In trapped sediments, barium concentrations increased 200–300% during drilling but returned to background concentrations within 1 year after drilling had stopped. Hydrocarbon concentrations in sediments from the study area were occasionally elevated, but generally were not consistent with drilling activities. Examination of the petroleum hydrocarbon and PAH distributions indicated that a distinct petroleum signal was occasionally observed in some samples; however, natural seepage rather than drilling discharges is probably responsible for the randomly-elevated hydrocarbon concentrations. The presence of seep-related particles should be recognized in characterizing the ambient hydrocarbon concentrations of sediments in the study area. During peak drilling periods, neither barium nor petroleum hydrocarbons were detected in concentrations that are likely to impact benthic biota in the vicinity of the drilling platform.