How much oil is actually removed by evaporation, photo-oxidation and microbial weathering?

Crude oil is a complex mixture of thousands of organic compounds including alkanes, aromatics, asphaltenes, resins and waxes. A number of physical, chemical and biological weathering processes, as well as, vertical sinking followed by burial in sediments act on oil once it is released into the marine waters. The weathering processes cause oil's initial concentration/composition to change into oil residues, and allow natural mechanisms for oxidation and conversion of reduced organic carbons in oil back to CO₂ and biomass. Once the lighter compounds are gone due to weathering, and some new oxidative compounds are formed, the heavier constituents left behind as residues ultimately sink to the bottom and/or strand at the coasts. The oil and oil residues can persist much longer in soil and sediments (20–40 years) than in the water column (<6 months), and can have long-term environmental impacts. Thus, it is important to know the amount and fates of the residues produced and transported to the seafloor and/or to the coastal marshes after early oil weathering in marine water column. The understanding of likely fates and behavior of oil allows us to choose and optimize a most appropriate response option. Recent studies have considered hopane as degradation resistant, used hopane normalization to determine the loss of select oil constituents via weathering, and have concluded that a significant proportion of spilled oil quickly is removed. Such analysis, however, are based on percentage removal of GC-amenable alkanes and PAHs, and may not represent the actual amount of loss of spilled oil via weathering processes as a large fractions of the crude oils (higher alkanes, PAHs, oxidative products, asphaltenes, resins and wax) are not GC-amenable which are not accounted in GC-based analysis. In the present study, we conducted a series of laboratory weathering experiments for direct estimation of the actual amount of loss of crude oil via evaporation and biodegradation. A known amount (mass) of BP surrogate oil was mixed into the natural seawater and allowed for weathering for 30 days, simulating natural physical conditions and periodically flushing the water to prevent accumulation of biomass. At the end of the experiment, oil residues were carefully collected, solvent extracted followed by evaporation of solvent and weighing the residues left behind. The comparison of pre-post mass, and mass balance including flushed water, provides direct estimation of loss of oil via weathering. This is a work-in-progress and results will be presented during the conference.