Impact of Crude Oil Weathering on the Calculated Effective Solubility of Aromatic Compounds: Evaluation of Soils from Ecuadorian Oil Fields

摘要

Effective solubility (Es) is the maximum dissolved concentration of a compound at equilibrium between an aqueous and organic phase. Published evaluations of crude oils suggest that the Es of only the most soluble aromatic compounds exceeds health-based drinking water goals. But because Es is a function of the concentration of a compound in the oil phase, it changes with weathering. The objective of this study was to evaluate weathering's impacts on the Es of aromatic hydrocarbon compounds. The Es of 20 aromatic hydrocarbons was calculated for Ecuadorian crude oil and 734 oiled soil samples collected in Ecuador. For the crude oil, the Es of mono-aromatic compounds exceeded health-based drinking water goals, while there were no exceedances for any polycyclic aromatic hydrocarbon (PAH). For the soil, the Es of benzene exceeded the health-based drinking water goal in nine (1.2 percent) of the 734 samples. There were no other exceedances. To further evaluate the impact of weathering, two concepts are introduced. The weathering index describes the extent of weathering of the bulk oil phase, while the concentrating factor is the relative change in concentration, and hence Es, of individual compounds. The weathering index and concentrating factor were calculated for 107 soil samples. The weathering index evaluation indicated that the oil associated with the soil was highly weathered. This reduced the Es of the mono-aromatic compounds because they were depleted to a greater extent than the bulk oil. Although the concentrating factor of some PAHs was as high as 256, their Es was still well below health-based drinking water goals. These results indicate that because of rapid weathering of the more soluble aromatics and the low effective solubility of larger PAHs, Ecuadorian crude oil impacted soil is unlikely to result in dissolved concentrations that exceed health-based drinking water goals.