Enhanced migration of gasohol fuels in clay soils and sediments
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Clay soils have low hydraulic conductivities in the presence of high polarity pore fluids, such as water. Low polarity fluids, including hydrocarbon fuels and halogenated organic solvents, typically cannot migrate into clay pores because they cannot displace water. Oxygenated additives in gasoline, such as alcohols and methyl-tert-butyl ether, are increasingly being used to control air pollution emissions. These relatively polar and water-soluble compounds may facilitate displacement of pore water and enhance migration of fuels and solvents through clay-rich soil strata. In the reported research, the migration of gasoline-alcohol fuel mixtures (gasohol) through consolidated clay was examined. Prepared kaolinite clay samples were consolidated from slurry, and various combinations of gasoline, alcohol, and water were applied to the clays under 152 Pa pressure in a fixed-wall permeameter. Movement of the fluids into the clay samples was monitored by measuring displaced pore fluid and by magnetic resonance imaging of the samples. Changes in clay aggregate structure were examined using environmental-scanning electron microscopy. Results of the research suggest that alcohol added to hydrocarbon fuels can enhance their migration through clays. Gasoline did not migrate into water-saturated clay, even after 14 days under pressure. The gasohol mixture migrated readily into the clay in only 20 minutes. Increased hydraulic conductivity of the clay in the presence of gasohol is hypothesized to be due to the collapse of the clay's pores thereby creating larger pores. Increasing pore diameter decreases the capillary pressure needed for the gasohol to replace water and allows gasohol to migrate through the clay.