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Bekins, Barbara1, Frances Hostettler1, Ean Warren1, William Herkelrath1, Geoff Delin2, Hedeff Essaid1, Isabelle Cozzarelli3
(1) U.S. Geological Survey, Menlo Park, CA
(2) U. S. Geological Survey, Mounds View,
(3) U. S. Geological Survey, Reston, VA

ABSTRACT: The Influence of Microbial Populations and Hydrology on Natural Biodegradation of Subsurface Crude-Oil Contamination

At a 1979 crude-oil-spill site near Bemidji, Minnesota, the fate of the oil and resulting ground-water plume have been studied for the past 20 years. Separate-phase oil is present at residual concentrations in the vadose zone and as an elongated oil body floating on the 6-8-m-deep water table. Gas and water concentrations and microbial data show that methanogenic conditions prevail in the separate-phase oil and the first 50 meters of the dissolved plume. In contrast to aerobic systems, under the methanogenic conditions in the separate-phase oil the C16 to C25 n-alkanes are degraded before the lighter n-alkanes. There is extreme spatial variability such that the n-alkanes are almost gone in the upgradient oil-body limb, but in the downgradient limb n-alkane concentrations are comparable to oil archived from the original spill. These differences appear to be related to groundwater recharge. Data from two vertical arrays of moisture probes show that in 2002 the upgradient oil limb received three times the recharge of the downgradient limb.
Within the anaerobic ground-water plume, where methanogenesis and iron-reduction are the dominant processes, the degradation rates of the individual BTEX compounds differ substantially. Toluene and o-xylene are rapidly degraded while m- and p-xylene degrade more slowly under methanogenic conditions. In contrast, benzene and alkylbenzenes appear refractory in the methanogenic zone but degrade rapidly where iron oxides are still present in the aquifer. Consequently, a front of ~5 mg/L of benzene is expanding in the aquifer at a rate of 2-3 m/yr as iron oxides are depleted.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.