The crude-oil spill site near Bemidji is one of the better characterized sites of its kind in the world. Results of research conducted on processes affecting the migration and fate of crude oil in the environment have provided fundamental knowledge that has been used to remediate similar sites worldwide. The Bemidji research project was the first to document that the extent of crude-oil contamination at a site can be largely limited by natural attenuation. Scientists studying and documenting natural attenuation at other contaminated sites have used many of the methods and approaches developed at the Bemidji site.
Our long-term study of the evolution of the contaminant plume indicates that biogeochemical processes that degrade the contaminants largely control the transport of hydrocarbons in the plume. Although geochemical processes have changed over time in the aquifer, the plume has not migrated as far as predicted considering the ground-water flow velocities and sorption constants for these compounds (Baedecker et al., 1993). As of 1996, the leading edge of the plume of ground water containing a total BTEX concentration greater than 10 micrograms per liter had moved only about 200 m downgradient, whereas advective flow of ground water since the spill has been about 500 m. The primary reason is that hydrocarbons have biodegraded under oxic and anoxic conditions. Most of the soluble hydrocarbons are degraded in the anoxic and sub-oxic zones, but some compounds, such as benzene, are recalcitrant under anoxic conditions and are transported further downgradient. The fate of the separate BTEX compounds differ in the plume (Eganhouse et al., 1996). Toluene and o-xylene degrade rapidly near the oil body while benzene and ethylbenzene together account for the majority of the mass at the advancing BTEX front.
Our recent research at this site shows that as ferric iron is depleted (Tuccillo et al., 1999), narrow methanogenic zones form in areas of high contaminant flux (Bekins et al., 2001). Cozzarelli et al. (2001) showed that from 1992-1995, although the overall length of the BTEX plume did not grow, expansion of the methanogenic zone resulted in vertical and horizontal growth of the area contaminated with the highest BTEX concentrations. The advancing methanogenic front is marked by nearly constant benzene concentrations upgradient and a steep drop at the location where Fe(III) is still present. As Fe(III) is consumed during hydrocarbon degradation, expansion of the plume under methanogenic conditions may change the outlook for monitored natural attenuation (MNA) at this site as well as other sites contaminated with petroleum products.
The degradation of hydrocarbons in the plume has resulted in the production of microbial metabolites and has altered the chemical composition of the ground water and aquifer solids. Knowledge of the biogeochemical processes occurring in contaminant plumes provides a basis for understanding the hydrogeochemical conditions under which organic compounds persist in the subsurface and is important for making management decisions concerning the remediation of aquifers. Our research at Bemidji has highlighted the importance of long-term monitoring when assessing natural attenuation as a possible remedial option in a contaminated aquifer. Biodegradation of petroleum-derived hydrocarbons in oxic and sub-oxic environments is generally considered a more efficient attenuation mechanism than is biodegradation in anoxic environments. However, research at this site has demonstrated that biodegradation in anoxic environments can remove substantial amounts of hydrocarbons from ground water (Baedecker et al., 1993; Eganhouse et al., 1993; Cozzarelli et al., 1994).
(Modified from Bekins et al., 2001.)
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Click on images to enlarge.
In 1979 a high pressure crude-oil pipeline broke spilling 10,700 barrels of oil. This aerial photograph of the oil spill was taken that year.
After initial cleanup efforts 2,500 barrels of oil remained and percolated to the water table.
Researchers have been studying the site.
Oil infiltrated the ground and is floating on the water table.
Sampling approaches reflect the scale of reaction processes.
Wells are used for plume-scale observations and the freezing drive-shoe coring device can be used for looking at the centimeter scale. The freezing drive shoe sampler, developed at the Bemidji site by Murphy and Herkelrath (1996), enables efficient recovery of saturated, unconsolidated sandy sediments; recovery of such samples was not possible using standard coring methods. The method enabled estimation of oil saturation (the fraction of pore space occupied by oil) in the subsurface.
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