PATTERNS OF SUBSURFACE CRUDE-OIL BIODEGRADATION RELATED TO MICROBIAL POPULATIONS AND GROUNDWATER RECHARGE

At a study site near Bemidji, Minnesota, degradation of crude oil varies strongly with location in the subsurface. Separate-phase oil is present in the vadose zone and as an elongated elliptical oil body at the water table 6-8 meters below land surface. Gas concentrations in the vadose zone and Most Probable Number Analyses of microbial populations show that methanogenic conditions prevail throughout the separate-phase oil. Analyses of oil samples collected in 1999-2002 document the change in n-alkanes since the 1979 spill. Unlike aerobic systems where low order n-alkanes are degraded first, under the methanogenic conditions in the subsurface oil body, the high-order n-alkanes are degraded first. In the upgradient limb of the oil body, the n-alkanes are nearly completely degraded, whereas in the downgradient limb, very little degradation has occurred compared to oil that was collected shortly after the spill. These differences appear to be related to groundwater recharge. Recharge above the upgradient and downgradient lobes was estimated during 2002 using two vertical arrays of soil moisture probes. The data show that the upgradient oil lobe receives three times the recharge of the downgradient lobe. The higher recharge location is about two meters lower in elevation and is grass covered, whereas the low recharge area has pine trees at the surface.

Although the relationship to recharge is striking, the underlying mechanism for the variation in degradation is not clear. Possible explanations are enhanced dissolution in the upgradient high recharge area and greater supply of growth nutrients from land surface. Numerical modeling of dissolution and degradation confirms that higher recharge enhances dissolution, accelerating loss of the more soluble hydrocarbons. Total inorganic nitrogen analyses of the aquifer sediments show that nitrogen mass fraction decreases with depth below land surface in the vadose zone. Dissolved carbon-nitrogen ratios in the contaminant plume below the water table indicate nitrogen-limiting growth conditions. Our data show that methanogens and fermenters are higher in number near land surface and in subsurface oil-body sections where recharge is greatest. These results suggest that the higher recharge supplies more nitrogen, thus enhancing the hydrocarbon-degrading populations.