From early 1995 through 2001, nitrate (NO3) concentrations in ground water in the Warren subbasin, California, increased from a background concentration of 10 milligrams per liter (mg/L) to more than the U.S. Environmental Protection Agency (USEPA) water-quality maximum contaminant level of 44 mg/L (10 mg/L as nitrogen). This increase coincided with an artificial ground-water recharge program implemented by the local water district, Hi-Desert Water District (HDWD), to reverse ground-water level declines of as much as 300 ft. Major findings on ground-water quality from this study are the following:

•Septage from septic tanks was the primary source of NO3 to the ground-water system.

•Rising ground-water levels, resulting from the artificial-recharge program, entrained high-NO3 septage stored in the unsaturated zone.

•The potential for ground-water contamination should be evaluated before beginning an artificial- recharge program in an area that uses septic tanks.