THE INFLUENCE OF REDOX CONDITIONS, MICROBIAL POPULATIONS, AND HYDROLOGY ON BIODEGRADATION OF SUBSURFACE CRUDE-OIL CONTAMINATION

In 1983, Mary Jo Baedecker and colleagues initiated a study of the fate of crude oil in the subsurface at a spill site near Bemidji, Minnesota. The oil spilled from a burst pipeline in August, 1979, and infiltrated into a surficial glacial outwash aquifer. The ongoing, 21-year study has resulted in almost 250 publications on the fate of the oil and associated groundwater plume. Separate-phase oil is present at residual concentrations in the vadose zone and as an elongated, elliptical 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. In contrast to aerobic systems, the C₁₆ to C₂₅ n-alkanes are degrading before the lighter n-alkanes. Moreover, there is extreme spatial variability in the degradation rates such that most of the n-alkanes are now below detection in the upgradient oil-body limb, but in the downgradient limb n-alkane concentrations are still comparable to those in the original spilled oil. These differences appear to be related to groundwater recharge. Data from two vertical arrays of moisture probes show that in 2002 the more degraded oil limb received over twice the recharge of the less degraded limb.

Baedecker and colleagues showed that the groundwater plume from the oil body undergoes natural biodegradation under strictly anaerobic conditions. Within the anaerobic portion of the plume, methanogenesis and iron-reduction are the dominant processes. By the early 1990’s the degradation reactions had become so effective that the plume advance rate was negligible. However, the fates of the individual BTEX compounds in the plume depend on the redox conditions. Under methanogenic conditions toluene and o-xylene are rapidly degraded, while m- and p-xylene degrade somewhat more slowly. In contrast, benzene and alkylbenzenes persist 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. The legacy of Baedecker’s research approach, which relates the long-term fate of petroleum hydrocarbon compounds to subsurface redox conditions, is still providing a wealth of relevant insights at hydrocarbon-contaminated sites across the nation.