MN OXIDE CONCENTRATION AS EVIDENCE OF A PATHWAY FOR INFILTRATION OF CRUDE OIL INTO A SHALLOW AQUIFER, WEST TEXAS

Abstract

In November 1991, landowners near Abilene, Texas, found crude oil in their water well. Subsequent drilling (four cores and 30 borings) defined a plume of crude oil (∼300 bbl) floating on shallow, perched groundwater. Data suggest that the oil came from a near-surface leak associated with oil-production activities. Crude oil is present in a thin (0.5 ft), silty sand layer 17.7–19 ft below the surface. Because of water level fluctuation, traces of oil also occur along fractures as deep as 35 ft in two cores collected within the crude-oil plume.

The presence of manganese (Mn) oxide coatings along fracture surfaces might prove to be a record of the path of oil as it infiltrated the subsurface. Mn oxide minerals are concentrated along fracture surfaces to depths of 20 ft in two cores located nearest the suspected crude-oil source. Changes in redox conditions and increased microbial activity associated with the crude oil probably caused dissolution, followed by reprecipitation and concentration of Mn oxides.

Other effects of crude-oil degradation include high unsaturated zone methane concentrations in a halo around the oil plume. Methane was measured in boreholes at concentrations mainly between 5–50% but locally as high as 98% at depths of 8–10 ft. The methane is most likely a result of both volatilization and biodegradation of the crude oil. Coincident with the methane plume are zones of high carbon dioxide (as much as 10%) and low oxygen (as little as 1.9%) content.