Geologists today are being called upon not only to find naturally occurring petroleum, but also to help assess and remediate the problem of refined hydrocarbons and other man-made contaminants in the subsurface that may endanger freshwater resources or human health. Petroleum geologists already have many of the skills required and are at ease working with fluid flow in the subsurface. If called for environmental projects, however, they will need to know the language and additional concepts necessary to deal with the hydrogeologic problems.

Most releases of refined hydrocarbons and other man-made contaminants occur in the shallow unconfined groundwater environment. This is divided into three zones: the saturated zone, unsaturated zone, and capillary fringe. All three have unique characteristics, and contamination behaves differently in each.

Gasoline contamination partitions into four phases in this environment; vapor phase, residual phase, free phase, and dissolved phase. Each has a different degree of mobility in the three subsurface zones. Their direction and rate of movement can be estimated using basic concepts, but geologic complexities frequently complicate this issue. This is why we need geologists.