Sources of subsurface water contamination can generally be assigned to one of three basic categories: The direct introduction of pollutants deep within the earth by injection through wells; Percolation of pollutants from surface and near surface sources such as septic tanks, leaching ponds, sanitary landfills, and pesticides and fertilizers used in agricultural practices; and Intrusion of salt water into fresh-water aquifers as a result of reductions in fresh-water flow in coastal areas or the breaching of impervious strata in inland areas. These pollution problems are reviewed concerning the increasing demands for water of acceptable quality for the various types of water needs in the United States.