CONTROLS ON RATES OF OIL AND METHANE DEGRADATION AT A CRUDE OIL SPILL SITE

A study of the fate of crude oil in the subsurface at a 1979-spill site near Bemidji, Minnesota, shows the effect of the availability of nutrients and oxygen on oil and methane gas degradation. Separate-phase oil is present in the surficial glacial outwash sediments 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 area with separate-phase oil.

Within the separate-phase oil, substantial degradation of the n-alkane fraction of the oil has occurred under methanogenic conditions. The n-alkanes are mostly degraded in the upgradient oil-body limb, but in the downgradient limb n-alkane concentrations are comparable to oil archived from the original spill. Most Probable Number (MPN) microbial data show that numbers of methanogens are ~10 times greater in the more degraded limb. Data from two vertical arrays of moisture probes show that in 2002 the more degraded oil limb received over twice the recharge as the less degraded limb. Typically, samples located near the top of the 1-m-thick floating oil body, are more degraded than those located 10-30 cm lower. Consistent with the degradation state, numbers of methanogens and fermenters are generally greater near the top of the oil body compared to the lower locations. The lateral and vertical variation in degradation state and microbial data together suggest that a nutrient supplied from land surface enhances microbial growth and degradation rates.

Between land surface and the oil body floating on the water table, a narrow methanotrophic horizon exists in the vadose zone where both oxygen from the land surface and ~20 % (by volume) methane from methanogenic oil degradation are present. MPN data for methanotrophs show similar patterns to those for aerobes suggesting that these are the same population. The largest methanotrophic activity (as indicated by aerobe MPN data) occurs just above a silt layer that restricts upward migration of methane and downward migration of oxygen. Changes in physical properties between this silt layer and an overlying unit of coarse sand result in greater than a 1000-fold increase in aerobe numbers over a vertical distance of less than 10 cm.