The San Luis Project, a major new unit in the Central Valley Project, is currently under construction. It will transport Feather River water to the west side of the San Joaquin Valley. The State of California Water Plan envisions a program of cyclic use of ground water during periods of deficiency, and storage of excess surface water when supplies are plentiful. Recharge of underground storage basins will involve the percolation of water through substrata, and the quality of the percolating water will be influenced by salts present in the substrata.

A series of papers in this issue present a method of predicting the quality of percolating waters in substrata (Part III), utilizing field data obtained from substrata profiles in the west side of the San Joaquin Valley (Part I). Theoretical considerations and approximations (Parts II and III) are formulated in the computer programs, for calculating simultaneously the major physicochemical reactions that occur during recharge.