Evaluation of the Source and Transport of High Nitrate Concentrations in Ground Water, Warren Subbasin, California

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.