Petroleum-derived hydrocarbons are an important class of ground water contaminants. Spills of hydrocarbons often produce regions in the subsurface that retain the spilled material trapped as an oily phase. When ground water infiltrates the oily material, the more water-soluble hydrocarbons dissolve and seriously degrade the quality of the water. An established biotechnology for remediating these spills is to perfuse the earth with oxygenated water, amended as necessary with mineral nutrients. This allows microorganisms to degrade the contaminants in situ. Because the rate of treatment is the rate with which oxygen is brought into the contaminated subsurface material, the rate is severely limited by the low oxygen-carrying capacity of water. The rate might be accelerated if air is circulated rather than water, or if organisms in the subsurface are encouraged to emulsify the oil so it can be pumped out for treatment on the surface. (Copyright 1987, Society for Industrial Microbiology.)