INTRODUCTION Some areas of the underground environment within a few tens of meters of the surface are contaminated with gasoline, particularly in urban centers. Gasoline may leak from underground storage tanks, associated piping, and from distribu­ tion pipelines. The most prevalent sources are automotive service stations. Under­ ground releases may sustain liquid gasoline migration for considerable distances through permeable pathways above or along the water table. Gasoline can reside for decades underground; in some cases, long after the source has been aban­ doned, removed, or converted to other uses. Liquid gasoline is highly volatile, and its vapors are toxic and combustible. A small amount of gasoline introduced into an underground excavation can quickly generate a hazardous atmosphere.