The potential for groundwater contamination depends upon the natural attenuation that may take place between the sources of pollution and the aquifer system. During infiltration through soils and transport in aquifers, many contaminants may be naturally attenuated; however, not all subsurface environments have equally effective purifying capacities. The degree of attenuation generally varies with different aquifer materials and with the distance a pollutant must travel through unsaturated materials to reach groundwater. Several hydrogeologically based susceptibility ranking systems have been developed as aids for the evaluation of the capacity of the subsurface environment to attenuate pollutants; many such systems involve mapping of resultant zones of vulnerability to pollution. These classifications can be used as rapid and cost-saving tools for preliminary screening related to differing land use choices, fertilizers application rates, monitoring needs, and development of groundwater protection strategies.