We have control over the chemicals we use, and where and how we use them. Control over the geologic environment, however, is beyond our means. Therefore, we must understand the geologic environment where our urban areas are located and develop methods to minimize or eliminate the potential harmful effects of contaminants upon human health and the environment. A logical œrst step to this end is through an understanding of urban geology, followed by an evaluation of the extent that a given urban area’s geology in¡uences the migration of contaminants. Since water plays a critical role in assessing a region’s vulnerability to contamination, the analyses performed during the evaluation step require an understanding of water occurring at the Earth’s surface and beneath.