Hydrocarbon characterization and weathering of oiled intertidal sediments along the Saudi Arabian Coast two years after the Gulf War oil spill

Abstract

Two years after the 1991 Gulf War oil spill, shoreline areas of Dawhat al Musallamiyah and Dawhat ad Dafi along the Saudi Arabian Coast impacted by the largest oil spill in history were revisited to assess the changes, from weathering, in hydrocarbon composition of residual oil in intertidal sediments. This was done to determine the effect of different geomorphic habitats from exposed to sheltered on the extent of weathering of these oiled sediments. This 1993 study was a continuation of the nearshore geochemistry processes study conducted as part of the scientific expedition on the R/V Mt. Mitchell in the Arabian Sea in 1992. Over 70 surface and subsurface shoreline sediment samples were analyzed for saturated hydrocarbons (SHCs), defined as n-alkanes from C 10 to C 32 and selected isoprenoids, and polycyclic aromatic hydrocarbons (PAHs), defined as individual and alkyl homologues of the two- through six-ringed PAHs including the sulfur heterocyclic compounds. The extent of weathering of hydrocarbons over the two-year period was determined for sediments in each of the different shoreline habitats based on SHC and PAH distribution pattern, and diagnostic ratios, such as ratios of the C 2- and C 3-alkyl homologues of the distribution pattern, and chrysenes/benz(a)anthracenes. A classification depicting four stages in weathering (from 1 [fresh] to 4 [advanced]) was established for both SHCs and PAHs. Some sediments contained relatively fresh oil that showed no change in SHC or PAH distributions over the two years in the environment. Other sediments showed extensive weathering, involving multiple weathering stage changes. For SHCs, approximately 66% of the oiled sediment samples had progressed to the most advanced weathering stage (Stage IV) by the end of the second year. In contrast, only 25% of the oiled samples had PAHs at full Stage IV weathering. For the SHC hydrocarbons, changes in SHC distributions looked to be principally caused by selective microbial degradation of the n-alkanes. Weathering of spilled oil PAHs was a combination of losses of smaller PAH families (naphthalenes) and smaller carbon alkyl groups within a PAH family. The least weathered oiled sediments were predominantly from subsurface sediments in habitats with surface seals or in sheltered muddy areas which had reduced exposure from physical weathering processes involving air, sunlight, tidal flushing, and reworking by waves. Seventy percent of the subsurface samples contained residual oil that was relatively fresh (Stage I or II), whereas 20% of the surface samples had relatively fresh residual oil. The heavily oiled subsurface sediments, even in exposed habitat environments, showed the least weathered oil, particularly where liquid oil remained trapped under hardened near-surface oil residual (crust and pavement). Extremely weathered oiled sediments were found mostly in exposed or moderately exposed near-surface sediment habitats. PAHs in oiled sediments were examined for selective degradation of certain alkyl groups or isomers within an alkyl group by microbial action. PAH distributions of the residual oil indicated that selective microbial degradation of alkyl groups was neglibible and the loss of PAHs by any weathering mechanism (microbial, physical, chemical) looked to follow, at least in this two-year time period after the spill, the weathering pathway predicted by the physical processes (evaporation).