Two groundwater plumes in north central Minnesota with residual crude oil sources have 20 to 50 mg/L of nonvolatile dissolved organic carbon (NVDOC). These values are over 10 times higher than benzene and two to three times higher than Diesel Range Organics in the same wells. On the basis of previous work, most of the NVDOC consists of partial transformation products from the crude oil. Monitoring data from 1988 to 2015 at one of the sites located near Bemidji, MN show that the plume of metabolites is expanding toward a lakeshore located 335 m from the source zone. Other mass balance studies of the site have demonstrated that the plume expansion is driven by the combined effect of continued presence of the residual crude oil source and depletion of the electron accepting capacity of solid phase iron oxide and hydroxides on the aquifer sediments. These plumes of metabolites are not covered by regulatory monitoring and reporting requirements in Minnesota and other states. Yet, a review of toxicology studies indicates that polar metabolites of crude oil may pose a risk to aquatic and mammalian species. Together the results suggest that at sites where residual sources are present, monitoring of NVDOC may be warranted to evaluate the fates of plumes of hydrocarbon transformation products.

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Factors that determine water quality at a site include climate, geology, biology, and land use (anthropogenic practices). Water quality varies naturally as the result of differing natural environments, but anthropogenic practices can override these natural variations. During the last several decades, research and regulatory efforts in the United States have largely focused on understanding and limiting water quality effects from specific or localized anthropogenic sources, which are termed point sources. Point sources include farms; industrial facilities where chemicals are manufactured, used, or stored; landfills; sewage treatment plants; hazardous waste sites; mines; and construction sites.