Weathering and the Fallout Plume of Heavy Oil from Strong Petroleum Seeps Near Coal Oil Point, CA CHRISTOPHER FARWELL, † CHRISTOPHER M. REDDY, ‡ EMILY PEACOCK, ‡ ROBERT K. NELSON, ‡ LIBE WASHBURN, § AND DAVID L. VALENTINE* ,† Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, California 93106, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, and Department of Geography and Marine Science Institute, University of California, Santa Barbara, California 93106 Received September 11, 2008. Revised manuscript received December 28, 2008. Accepted January 6, 2009.  w This paper contains enhanced objects available on the Internet at http://pubs.acs.org/est. The Coal Oil Point (COP) seeps offshore Goleta, CA, are estimated to release 20-25 tons of oil daily, providing an ideal natural laboratory to investigate the fate of oil in the coastal ocean. To address the long-term fate of COP oil, we collected 15 sediment samples down current from the seeps and quantified petroleum content and individual biomarkers using traditional and comprehensive two-dimensional gas chro- matography. Similarities in the distributions of hopane biomarkers link the oil in the sediments to fresh seep oil ( n ) 5) and underlying reservoirs ( n ) 3), although sediment oil is heavily weathered. The spatial distribution of oil forms a plume along the continental shelf that we suggest represents a chronic fallout pattern for heavy oil from the persistent surface slicks; average surface currents appear to modulate the distribution of the fallout over a period of 0.4 -5 days. The extent of hydrocarbon loss is consistent for all sediments, indicating a common limit to oil weathering with contributions from evaporation, biodegradation, and dissolution. Considering the amount of oil and quantity of sediment impacted, we estimate a sediment oil burden of 0.3 × 10 12 to 3 × 10 12 g in the study area, equivalent to 8-80 spills of the Exxon Valdez accident of 1989. Introduction A decreasing global supply of light crude oil is leading to increases in production, transportation, and use of heavier oils (1). One concern stemming from increased offshore oil activity is a greater probability of oil spills in the coastal zone. The transport, transformation, and fate of oil spilled in the sea remains the subject of ongoing study, with a compre- hensive update compiled recently by the National Research Council (2). An important emerging issue is the fate of heavy oils introduced into the sea, where heavy oil is defined here as having API gravity of <17°. Heavier oil often has a substantially greater proportion of the high molecular weight resin and asphaltene fractions. To address the fate of heavy oil in the coastal ocean, we turned to the natural seeps at Coal Oil Point (COP), offshore Goleta, CA. These seeps provide a unique setting to investigate the transport, transformation, and fate of heavy oil in the sea, as they provide a consistent input of oil to the coastal ocean and are readily accessible (3). Active oil seeping from this location can be viewed in the video accompanying this work. The oil emanating from the seeps is derived from the Monterey Formation and comprised of ∼30% hydrocarbons and ∼70% resins plus asphaltenes (4, 5). These seeps have been active for hundreds to thousands of years (6, 7) and have served as a study site for numerous previous investiga- tions (recent works by Wardlaw et al. (8), Del Sontro et al. (9), and Mau et al. (10), and references therein). Previous studies of COP seep oil have also addressed the subsurface biodegradation of oil (8), initial physical weather- ing of surface slicks (8), wind-driven evolution of surface slicks over short time scales (<2 h) (11), and tar accumulation and transport on local beaches (9). However, we are aware of no published studies in which the long-term fate of COP oil is quantified in sediments. Additionally, marine sediments are known to retain oil for longer than contaminated beaches or open ocean environments (12). Here, we assess the oil burden in an area west of the COP seep field to determine the magnitude of oil deposited from the COP seep field into the benthic environment. Furthermore, we assess the patterns and extent of hydrocarbon weathering in seep oils deposited to the sediments. Sediment grab samples were collected down current from the seep field, analyzed for hydrocarbon content using gas chromatography with flame ionization detection (GC-FID) and comprehensive two-dimensional gas chro- matography with time-of-flight mass spectrometry (GC × GC-ToF-MS), and the resulting spatial variations were related to regional current patterns as determined with a high- frequency radar array and current meters. Experimental Sediment Sampling and Bulk Analyses. Sediment samples were collected at 15 locations in a 90 km 2 grid starting 4 km west of the COP seep field on July seventh, 2007 from the R/V Atlantis. Sample stations were arranged in five longitudinal transects with three water depths (40, 60, and 80 m) for each transect, with one additional comparison sample obtained from within the seep field (labeled BC-16). The locations of each sample are provided in Table S1 and Figure S1. The samples were collected using a small grab-core sampling device with a collection volume of 3000 cm 3 (10 cm × 12 cm × 25 cm). Subsamples were taken from the top 3-5 cm of sediment using a solvent-cleaned stainless steel spoon, sealed in glass jars with PTFE lined caps, and immediately placed in a -20 °C freezer on board the R/V Atlantis. Samples were returned to the shore-based laboratory and analyzed for carbon and nitrogen abundance (CN), carbonate content, bulk organic carbon isotopic composition (δ 13 C-TOC), and bulk nitrogen isotopic composition (δ 15 N-TN), as described previously (13, 14). Bulk sediment density was measured for dried sediment (100 °C for 24 h) with a Pycnometer (AccuPyc 1330). * Corresponding author phone: 805-893-2973; fax: 805-893-2314; e-mail: valentine@geol.ucsb.edu. † Department of Earth Science and Marine Science Institute, University of California, Santa Barbara. ‡ Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution. § Department of Geography and Marine Science Institute, Uni- versity of California, Santa Barbara. Environ. Sci. Technol. 2009, 43, 3542–3548 3542 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 10, 2009 10.1021/es802586g CCC: $40.75  2009 American Chemical Society Published on Web 03/05/2009