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
  
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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