Warren Subbasin Artificial Recharge
The Hi Desert Water District (HDWD) implemented an artificial ground-water recharge program in early 1995 to reverse the decline of ground-water levels in the 19 square mile Warren subbasin of the Morongo ground-water basin (~100 miles east of Los Angeles, California). Ground-water levels declined as much as 300 feet from 1940 to1994, owing to increased pumping. The artificial ground-water recharge program introduced imported surface water to recharge ponds to replenish ground water in the underlying aquifer and, in response, water levels recovered as much as 250 feet in the vicinity of the recharge ponds. However, since the implementation of the artificial-recharge program, ground-water nitrate concentrations have shown a marked increase from 10 milligrams per liter to more than 110 milligrams per liter. Prior to the water-level rise, nitrate concentrations were near background levels, indicating that the nitrates had not reached the water table or that denitrification was occurring in the unsaturated zone. The increase in nitrate concentrations in Warren subbasin ground waters is attributed to the entrainment of septage by rising water levels.
The objectives of this study are to: (1) identify the vertical distribution of nitrogen species in the unsaturated zone within the west hydrogeologic unit (2) determine the potential for denitrification in the unsaturated zone; (3) provide a means of monitoring artificial recharge; and (4) develop water-management strategies that control water levels and nitrate concentrations. This study consists of three principle tasks: (1) characterize the hydraulic, chemical, and microbiological properties of the unsaturated zone; (2) monitor changes in water levels and water quality in response to the artificial-recharge program; and (3) use a ground-water flow and solute-transport model previously developed by the USGS to allow HDWD to manage their water-resource facilities.
Project Chief: Tracy Nishikawa