Successful subsurface characterization, detection monitoring, and ultimate remediation is predicated on a conceptual understanding of the subsurface environment. Factors that affect the fate and transport of contaminants, determination of possible adverse risks to public health, safety, and welfare, and the degradation of groundwater resources are largely controlled by regional and local subsurface conditions. Successful resolution of issues pertaining to the occurrence and mitigation of subsurface contamination requires adequate geologic and hydrogeologic characterization to gain insight into the subsurface distribution and preferential migration pathways of contaminants, potential receptors, and development of an appropriate remedial strategy.