Movement and fate of crude-oil in contaminants in the subsurface environment at Bemidji, Minnesota: Chapter C in U.S. Geological Survey program on toxic waste--ground-water contamination: Proceedings of the Third technical meeting, Pensacola, Florida, March 23-27, 1987

Abstract

On August 20, 1979, a pipeline break in a remote area near Bemidji, Minn. (fig. C-l), resulted in the release of 1.5x10 5 L (liters) of crude oil. Although about 1.1x10 5 L were removed from the site as part of the cleanup, some crude oil infiltrated the ground and percolated to the water table. The spill occurred in the recharge area of a local flow system that discharges to a small lake 300 m (meters) downgradient (Hult, 1984). The aquifer is a pitted and dissected outwash plain underlain at a depth of about 20 m by low-permeability till. Crude oil is floating on the water table about 8 m below land surface and has migrated about 20 m as a separate fluid phase. Soluble petroleum derivatives have dissolved in and are moving with ground water. Volatile constituents are migrating through the unsaturated zone by diffusion. The abstracts presented in this chapter provide an overview of ongoing efforts to combine the results of interdependent, interdisciplinary research into the comprehensive understanding of the physical, chemical, and biological processes that will be needed to develop predictive models of contaminant mobilization, transport, and fate. Franzi investigates the relationship between depositional and post-depositional processes and the heterogeneity and anisotropy of the aquifer. Post-depositional subsidence and collapse caused by melting of stagnant ice has created significant geomorphic and hydrogeologic structures. Miller describes a preliminary ground-water flow and chemical-transport model used to assess the effect of these hydrologic discontinuities, estimate aquifer properties, and to guide continuing field work. Abstracts by Siegel, Bennett, and Berndt describe studies of dissolved inorganic constituents and parameters done to characterize the geochemical environments in and around the site. An anoxic zone, probably plume shaped, extends downgradient of the oil pool and into an oxic zone that surrounds the contamination. Carbonate minerals are actively dissolving in the oxic spray zone upgradient of the oil pool where oil is being mineralized, whereas quartz is dissolving underneath the oil by organic-acid complexation. Selective leaching experiments show that fractionation of metals is also occurring where the aquifer matrix is contaminated. Morphological analysis of quartz sand grains clearly show unusual rock-water interactions are occurring in the anaerobic contaminant zone. Preliminary analysis indicates the possibility of an organic/quartz interaction that is significantly increasing the mobility of silica through the system. Predictions of the evolution and ultimate geometry of contaminant plumes resulting from spills require quantitative descriptions of the rate of mass transfer from the organic fluid to ground water. Pfannkuch presents laboratory and field work that describe how the the rate of oil dissolution, and therefore the strength of the contaminant source, is controlled by fluctuations in ground-water velocity and water-table fluctuations.