U.S. Geological Survey Identifies Crude-Oil Metabolites in Subsurface Plumes

At sites where there have been crude-oil or petroleum hydrocarbon fuel spills and contaminants have entered groundwater, metabolites from the crude oil form primarily from biodegradation reactions. The metabolites are more soluble than the parent compounds and are transported from the original source, forming a groundwater plume.

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Contaminants such as benzene, toluene, ethylbenzene, xylene (BTEX), and polycyclic aromatic hydrocarbons (PAHs) are commonly present in groundwater plumes from crude oil. The standard method for quantifying the concentration of a mixture of groundwater contaminants from crude oil involves extraction followed by analysis on a gas chromatograph. Individual concentrations of extracted compounds containing 10 to 28 carbons are added to obtain a total concentration known as diesel range organics (DROs) or total petroleum hydrocarbons in the diesel range (TPHd). Regulatory monitoring and reporting requirements in Minnesota and other states require analyses for concentrations of BTEX and DROs; however, because of limitations in the method, analyses for DROs do not reflect all of the organic compounds entering the groundwater from crude oil. A more comprehensive analysis is nonvolatile dissolved organic carbon (NVDOC), which accounts for all organic carbon remaining after purging the sample of volatile dissolved organic carbon. Metabolites produced during hydrocarbon biodegradation are contained in the NVDOC.

The focus of this study was on the occurrence and fate of NVDOC metabolites in existing contaminant plumes associated with two crude-oil spill sites in Minnesota. The first site is the Toxic Substances Hydrology Program's long-term research site in Bemidji, Minnesota. The second site is located at the South Cass Lake pumping station near the town of Cass Lake, Minnesota. The plume scale NVDOC data for Bemidji are from sampling campaigns in July 1988 (22 wells), 1995 (23 wells), and 2010 (23 wells). Samples for NVDOC were collected from contaminated wells at the Cass Lake site in June 2008 (15 wells) and one background well in August 2009.

Scientists compiled 20 years of monitoring data to understand the occurrence of NVDOC relative to other regulated crude-oil contaminants. Concentrations of DROs and extended range DROs compounds are one-third to one-half of the total concentration of NVDOC present in groundwater at the two crude-oil spill sites the scientists studied. NVDOC was measured at concentrations of 20–50 milligrams per liter in monitoring wells at the sites. These values were 10 times higher than benzene and two to three times higher than DROs in the same wells. Monitoring data from 1988 through 2015 at Bemidji indicate that the plume of NVDOC is expanding more rapidly than the BTEX plume toward a lakeshore located 335 meters (2,000 feet) from the source zone.

Environmental Health Considerations

The NVDOC metabolites in groundwater plumes at the two sites are not covered by regulatory monitoring and reporting requirements in Minnesota and other States; however, at study sites where residual product is present, the total concentration of transformation products can exceed BTEX concentrations by one to two orders of magnitude. In addition to possible concerns about toxicity, these plumes of metabolites consume the electron-accepting capacity of the contaminated aquifer, resulting in the slowing of biodegradation of other compounds in the contaminated plume. Together these results indicate that at oil spill sites where residual sources are present, monitoring of NVDOC may be an important part of an effective evaluation of the fate and effects of groundwater contaminant plumes.

This research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology).