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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Source Mass Balance Calculated from Changes in Composition of Spilled Crude Oil in the Subsurface near Bemidji, Minnesota

by

Matthew K. Landon (U.S. Geological Survey, Mounds View, Minn.) and Marc F. Hult (U.S. Geological Survey, Mounds View, Minn.)

Abstract

The transport and fate of crude oil that spilled from a break in a buried pipeline near Bemidji, Minnesota, on August 20, 1979, has been monitored by the U.S. Geological Survey since 1983. The oil percolated to the water table in a glacial outwash aquifer. The oil has moved about 30 meters as a separate fluid phase in the aquifer and is preferentially losing soluble and volatile compounds through dissolution into ground water and vaporization into the unsaturated zone. As part of the research effort at the site, the evolution of the composition and physical properties of the oil in the subsurface was studied. The primary objectives were to (1) quantify changes in the composition and physical properties of the oil at the site, (2) compare field results with oil alteration under controlled laboratory conditions that simulated the field environment, and (3) estimate mass balances for the total oil and for individual compounds mobilized during natural alteration of the oil source.

Analysis of oil collected from monitoring wells screened in the oil bodies during 1988 and 1989 indicated that areal differences in the physical properties of the oil have developed because of spatially and temporally variable rates of alteration of the originally uniform oil. The composition of the oil fraction lighter (with a lower carbon number) than dodecane (a normal alkane with 12 carbon atoms) of field and laboratory aged samples of oil was determined using gas chromatography/mass spectroscopy (GC/MS).

The oil has selectively lost aromatic compounds and aliphatic compounds (normal, branched, and cyclic alkanes) more volatile than nonane (normal alkane with nine carbons). About 50 detectable compounds have been depleted in the fraction of the oil that was analyzed. Maximum depletions of the total mass of C3 (compounds with three carbons), C4, C5, C6, C7, C8, C9, and C10 compounds were 100, 100, 95, 81, 66, 48, 49, and 25 percent, respectively. Depletion of the total mass of aromatic compounds ranged from 30 to 84 percent and exceeded depletion of the total mass of aliphatic compounds, which ranged from 0 to 73 percent. The greater depletion of aromatic compounds may indicate that dissolution of aromatic compounds into water initially occurs at greater rates than losses of aliphatic compounds by volatilization. Because of the much greater abundance of aliphatic compounds in the oil, however, more than 70 percent of the mass loss from the oil likely occurs as a result of volatilization. Losses of C6 and C7 aliphatic compounds account for a majority of the total loss of mass from the oil. Total mass loss from the oil calculated from changes in oil composition agrees within 0 to 3.8 percent (median of 1.6 percent) to mass loss calculated from changes in specific gravity, refractive index, and kinematic viscosity, indicating that any of these properties can be used for mass-balance calculations. Minimum estimates of total mass loss from the oil based on changes in composition and physical properties range from 0 to 11 percent (average is 3.8 percent) since the spill occurred in 1979. The results of mass loss calculations provide an independent estimate of the spatially variable source term for transport models of volatile and soluble hydrocarbons emanating from the oil source in the subsurface.

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