US Geological Survey Toxic Substances Hydrology Program: Proceedings of the technical meeting, Phoenix, Arizona, September 26-30, 1988

Abstract

In August 1979, the subsurface was contaminated near Bemidji, Minnesota, when a pipeline burst, spilling crude oil on a glacialoutwash aquifer. After cleanup efforts were completed, approximately 400,000 liters of crude oil remained in the aquifer. In 1983, the U.S. Geological Survey began research on the mobilization, transport, and fate of petroleum derivatives at the site. In the 9 years since the spill, petroleum has moved about 30 meters downgradient as a separate fluid phase, constituents dissolved in ground water have moved 200 meters, and vapors through the unsaturated zone have moved 100 meters. Each phase continued to advance during June 1987 through Juty 1988, although the rate of advance of the plume of contaminated water was less than the calculated ground-water velocity. The petroleum source is becoming increasingly viscous, dense, and depleted in volatile compounds. The petroleum derivatives moving with ground water and through the unsaturated zone are being degraded to water, carbon dioxide, and methane. The maximum extent of ground-water contamination eventually may be controlled by the near-equilibrium condition between the rate at which the petroleum is dissolved and transported by ground water and the rate at which it is degraded by microbes. If this hypothesis is confirmed by additional long-term research, it implies that many problems of groundwater contamination by petroleum derivatives might be managed successfully without direct remedial action if the volume of the subsurface affected at the maxium extent of contamination is acceptably small.