ABSTRACTNatural concentrations of nitrate are quite low in most ground waters in the eastern part of the San Joaquin Valley. High nitrate contents are related to sewage effluent percolation ponds, septic tank disposal systems, industrial waste‐waters, and agricultural fertilizers. Hydrologic factors are closely related to the occurrence of nitrate. Transmissibility of the aquifer, hardpan development in the soil, canal recharge, and cobble zones in the subsurface are the primary factors of importance. Nitrate is stratified in the aquifer beneath unsewered metropolitan areas and highest contents occur in the upper 50 or 60 feet. Water quality hydrographs were used to show long‐ and short‐term trends in nitrate. Chloride and nitrate hydrographs, trilinear diagrams, the distribution of other constituents, and hydrologic data were used to effectively delineate sources of nitrate in areas where numerous potential sources were present. Conclusions from ground‐water data agree well with previous studies in other areas beneath unsewered tracts and near sewage treatment plants in which attention was focused primarily on the soil or the unsaturat