A new study suggests that the degraded breakdown products of oil-spill contaminants in groundwater could be just as important to monitor as the original contamination itself.
At sites where crude oil or petroleum hydrocarbon fuel spills have occurred and contaminants 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.
Results of a recent U.S. Geological Survey study suggest that at oil-spill sites where residual sources are present, the monitoring of metabolites or breakdown products may be an important part of an effective evaluation of the fate and effects of groundwater contaminant plumes.
The study of two crude-oil spill sites in Minnesota focused on the occurrence and fate of the combination of all dissolved organic carbon metabolites in existing contaminant plumes. Contaminants such as benzene, toluene, ethylbenzene, xylene and polycyclic aromatic hydrocarbons are commonly found in groundwater plumes from crude oil. It was the metabolites of those contaminants as well as the less soluble components of the crude oil – the biodegradation products of the original oil contaminants – that were studied.
Scientists discovered concentrations of oil breakdown products at greater concentrations than the typical regulated-compound concentrations. These types of crude-oil metabolites in groundwater plumes at the two sites are not covered by regulatory monitoring and reporting requirements in Minnesota and other states. Yet at study sites where the spilled oil is still present, the total concentration of metabolites can exceed the concentrations of regulated compounds by one to two orders of magnitude. In addition to possible concerns about toxicity, these plumes of metabolites can slow the biodegradation of other compounds in the contaminant plume.
“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,” said USGS co-author Isabelle Cozzarelli.
“This analysis was possible because of USGS scientists’ long-term monitoring of hydrocarbon degradation progression. Long-term collection of hydrologic and geochemical measurements is the foundation of USGS science. This study is a great example of the application of science to inform potential regulatory process updates,” said USGS groundwater specialist and co-author Melinda Erickson.
This new research, “Crude oil metabolites in groundwater at two spill sites,” published in the journal “Groundwater,” is available online.