ARE CRUDE OIL METABOLITES EMERGING CONTAMINANTS? (Invited Presentation)

Emerging contaminants include compounds that have not been historically considered contaminants. Risk assessments at oil and fuel spill sites are based mainly on concentrations of benzene and Diesel Range Organics (DRO), but data from a 30-year study of a crude oil spill site show that concentrations of metabolites of the crude oil are 10-20 times those of benzene and three times higher than DRO. Thus, the DRO and benzene concentration data required by regulators do not reflect many of the organic compounds migrating in groundwater from residual petroleum hydrocarbon source zones. The study site, located near Bemidji, Minnesota, was contaminated in 1979 when a pipeline rupture spilled ~1.7 million liters of light crude oil of which ~25% infiltrated the subsurface. Although the 0.05 mg/L contour of benzene was stable between 1995 and 2010 at about 130 m downgradient, the plume of metabolites, or nonvolatile dissolved organic carbon (NVDOC), extends well beyond the benzene plume and is expanding. Modeling indicates that concentrations of NVDOC decrease under anaerobic conditions at a first-order rate of 0.13%/day, but the position of the 5 mg C/L contour has expanded from 125 m downgradient in 1988 to over 200 m in 2010. The NVDOC is a mixture of potentially thousands of partially oxidized hydrocarbons. They are more polar and soluble than their parent hydrocarbons with an overall average molecular formula of C₁₉H₂₄O₆. Efforts to identify individual metabolites at other hydrocarbon-contaminated sites have identified hundreds of compounds but the identified compounds represent a small proportion of those present. Few studies have addressed the toxicity of mixtures of polar metabolites, but published results indicate they pose a risk to both aquatic and mammalian species. Specific toxic compound classes that have been identified at other sites include naphthenic acids (monocyclic and polycyclic alkanes containing a carboxylic functional group) in oil sands process affected waters and phthalate esters at fuel release sites. Thus, hydrocarbon metabolite mixtures can be considered emerging contaminants because they: (1) have not historically been considered contaminants; (2) are probably present in groundwater wherever residual oil or fuels are present, and (3) are known to have aquatic and human toxicity.