After an oil spill, the degraded breakdown products of contaminants in groundwater could be just as important to monitor as the original contamination, according to a study by the U.S. Geological Survey (USGS).

At two crude oil spill sites in Minnesota, researchers found that groundwater concentrations of residual crude oil metabolites exceeded concentrations of diesel range organics, yet these are are largely unmonitored.

Where crude oil or petroleum hydrocarbon fuel spills have occurred and contaminants such as benzene, toluene, ethylbenzene, xylene and polycyclic aromatic hydrocarbons have entered groundwater, naturally occurring microbes in the soil can digest or break down the original crude oil, producing byproducts known as metabolites. The metabolites are more soluble in groundwater than the parent compounds and are transported from the original source, forming a groundwater plume.

The study, published in the journal Groundwater, suggests that at sites where residual sources are present, regulatory monitoring of metabolites or breakdown products may be an important part of an effective evaluation of the fate and effects of groundwater contaminant plumes.

“We compiled 20 years of monitoring data to understand the occurrence of the metabolites relative to other regulated crude-oil contaminants,” said USGS hydrologist and lead author of the study, Barbara Bekins. “Concentrations of the regulated crude oil compounds are one-third to one-half of the total concentration of the metabolites found in groundwater at our study sites. The values of the metabolites are 10 times higher than benzene and two to three times higher than the standard regulatory measure of diesel-like compounds in the same wells. Monitoring data show that the plume of metabolites is expanding more rapidly than the benzene plume.”

“Metabolites of hydrocarbon degradation, such as those found at these crude oil spill sites, are also common at sites contaminated by other fuels such as gasoline leaks and spills,” added USGS co-author Isabelle Cozzarelli.