Over half a million sites in the US have been contaminated by leaking underground fuel tanks. A quick, inexpensive method would be helpful to delineate the resulting subsurface hydrocarbon contamination and assist in proposing cleanup strategies. Our study site is located near Bemidji, Minnesota, where a crude-oil pipeline ruptured in 1979, contaminating the surficial sand-and-gravel aquifer with about 1700 m3 of light crude oil. About 75% of the oil was removed and the remaining oil infiltrated into the unsaturated zone forming three oil bodies at the water table. Previous studies show that methane produced from biodegradation of the oil moves towards the land surface along the concentration gradient. Methane mixes with oxygen in the unsaturated zone and is oxidized to carbon dioxide by methanotrophic bacteria. This reaction is exothermic, and reaction rates are sufficiently high to measurably increase unsaturated zone temperatures by up to 2.7 °C. When surface temperatures are low, the temperature gradients near the surface above the oil body should be greater than those at background sites. Temperature loggers were placed at 0.5 and 1.0 meters below land surface in a grid pattern over the oil body and allowed to come to equilibrium over 3 hours. Temperature gradients ranged from a maximum of 3.8 °C m-1 over the oil to 1.6-2.6 °C m-1 in uncontaminated areas. (Positive values denote temperatures at 1.0 m are higher than those at 0.5 m.) In general, higher temperature gradients were found over the known location of the oil body but one location outside oil body also exhibited a high gradient. Subsequent coring showed that oil is present in this location, which was previously thought to be uncontaminated. The results of this study indicate temperature gradients may be used to find subsurface fuel and oil, decreasing time and cost of source zone delineation.

Subsurface oil body delineation at a spill site using near-surface temperature gradients

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.