Improper disposal of hazardous wastes is a threat to the nation's ground water supply. Methods which prevent contamination are probably the most effective techniques to protect ground water. Once contamination problems occur, there are a number of in situ techniques that can be used to cleanse the ground water and at least partially restore the aquifer. Chemical and physical treatment techniques include processes such as neutralization, chemical reaction, extraction and immobilization. Biological techniques for in situ treatment generally involve enhancing the degradative capacity of the indigenous microflora or the addition of organisms acclimated to degrade the contaminants. (Copyright (c) 1987 Elsevier Science Publishers B.V.)