Hydrocarbons in Water and Sediment Samples from coal Oil Point Area, Offshore California

Abstract

ABSTRACT Water samples collected within about a l-sq. nautical mile area of the Coal Oil Point submarine oil seeps contain somewhat higher amounts of dissolved hydrocarbons, both heavy (C15+) and light (C3-C8), than do water samples collected 10-15 miles to the ESE in the offshore Pt. Dume area. However, the difference in concentrations is much more pronounced for the dissolved heavy hydrocarbons than for the lighter ones. The heavy hydrocarbon concentrations found in these offshore California water samples ranged from 0.2 to 16 ppb and agreed closely with concentrations found by other workers in Atlantic and Gulf of Mexico waters. Bottom sediments collected close to the submarine seeps contain on the average about 25 times more heavy hydrocarbons than do sediments collected in areas well away from the seeps. The hydrocarbon levels in sediments from the control areas agree well with those found by other workers in Recent sediments off Florida and California. Chromatographic analyses of the hydrocarbons extracted from the sediments clearly distinguish those hydrocarbons introducing into the sediments by the oil seeps from those introduced .by natural biologic and chemical processes. The hydrocarbons in the sediments around Coal Oil Point are mainly derived from the seepage. Water and sediment samples from the Gaviota area 10 miles from the seeps show levels of hydrocarbons and types of hydrocarbons similar to that found in the open ocean or in coastal areas free of any petroleum operations. The hydrocarbons in these sediments are mainly derived from natural material. In conclusion, the active seepage at Coal Oil Point does not appear to be building up hydrocarbon concentrations in the waters and sediments or changing the type of hydrocarbons except very close to the seeps. That is, hydrocarbons from the seeps are not moving over a wide area and being incorporated in the sediments or the water column. They should have little environmental impact. INTRODUCTION The water and sediment samples analyzed in this study were collected by D. E. Brandon on a cruise of the University of Southern California's research vessel, Velero IV, in offshore California waters on September 4 through 8, 1974. The objectives of the study were (1) to determine if light (C3-C8) and heavy (C15+) dissolved hydrocarbon concentrations ln waters near natural oil seeps are significantly higher than in waters remote from the seeps and (2) to determine if hydrocarbon contents and the types of hydrocarbons of the sediments in the vicinity of oil seeps are significantly different from those of sediments remote from the seeps. METHODS Three types of analyses were run on the samples:dissolved light hydrocarbons (C3-Ca) in water samples (determined at Exxon Production Research Company),dissolved heavy hydrocarbons (C15+) in water samples (determined at Exxon Research and Engineering Company, Linden, New Jersey), andheavy hydrocarbons (C15+) in sediment samples (determined at Exxon Production Research Company). The method used for the light hydrocarbons in water is a slightly modified version of that published by McAuliffe. (1)