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Our results show that subsurface crude-oil degradation rates at a long-term research site were strongly influenced by small-scale variations in hydrologic conditions. The site is a shallow glacial outwash aquifer located near Bemidji in northern Minnesota that became contaminated when oil spilled from a broken pipeline in August 1979. In the study area, separate-phase oil forms a subsurface oil body extending from land surface to about 1 m (3.3 ft) below the 6–8-m (20–26 ft)-deep water table. Oil saturation in the sediments ranges from 10–20% in the vadose zone to 30–70% near the water table. At depths below 2 m (6.6 ft), degradation of the separate-phase crude oil occurs under methanogenic conditions. The sequence of methanogenic alkane degradation depletes the longer chain n-alkanes before the shorter chain n-alkanes, which is opposite to the better known aerobic sequence. The rates of degradation vary significantly with location in the subsurface. Oil-coated soils within 1.5 m (5 ft) of land surface have experienced little degradation where soil water saturation is less than 20%. Oil located 2–8 m (6.6–26 ft) below land surface in areas of higher recharge has been substantially degraded. The best explanation for the association between recharge and enhanced degradation seems to be increased downward transport of microbial growth nutrients to the oil body. This is supported by observations of greater microbial numbers at higher elevations in the oil body and significant decreases with depth in nutrient concentrations, especially phosphorus. Our results suggest that environmental effects may cause widely diverging degradation rates in the same spill, calling into question dating methods based on degradation state.
Barbara Bekins is a research hydrologist with the U.S. Geological Survey, specializing in transport and biotransformation of organic contaminants in groundwater. She obtained a B.S. degree in mathematics from University of California, Los Angeles, an M.S. degree in mathematics from San Jose State University, and a Ph.D. in geology from University of California, Santa Cruz.Fran Hostettler is a research chemist with the U.S. Geological Survey whose main interests are in environmental chemistry, especially in fuel oil spills and natural oil seeps. She obtained a B.S. degree in chemistry from Simmons College in Boston and an M.S. degree in organic chemistry from the University of Wisconsin.
Bill Herkelrath is a physicist in the U.S. Geological Survey's Water Resources Division National Research Program in Menlo Park, California. His recent research interests include multiphase flow and solute transport modeling in shallow groundwater systems where oil or brine is present. He received his Ph.D. in physics from the University of Wisconsin at Madison.
Geoff Delin is a hydrologist with 27 years of experience with the U.S. Geological Survey. He is the U.S. Geological Survey groundwater specialist in Minnesota and site coordinator for the U.S. Geological Survey crude-oil spill research site near Bemidji, Minnesota. He obtained a bachelor's degree in geology from the University of Minnesota.
Ean Warren works for the U.S. Geological Survey, investigating how microbial populations affect the fate of anthropogenic contaminants in the environment. He graduated from the University of California, Santa Cruz with a B.S. degree in biochemistry and molecular biology and from Stanford with an M.S. degree in environmental engineering and science.
Hedeff Essaid is a research hydrologist with the U.S. Geological Survey specializing in modeling the fate and transport of organic contaminants in groundwater. She obtained a B.S. degree in geophysics from the University of Baghdad and an M.S. degree and a Ph.D. in hydrogeology from Stanford University.