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Demand for ground water in the 96-square mile Anza-Terwilliger area has increased in recent years because of population growth and agricultural development. During 1950-73, the water table declined and chemical analyses showed elevated concentrations of nitrate in water from five wells. The U.S. Bureau of Indian Affairs is concerned that continued growth will cause greater water-table declines and further degrade ground-water quality at the Cahuilla Indian Reservation. Water-level measurements indicate that, as in previous years, ground water normally moves toward streams in Anza and Terwilliger Valleys. However, during the summer, when pumping is at a maximum, ground water moves toward two areas of ground-water withdrawals in Anza Valley. One area where ground-water levels were lowered extends to the northern boundary of Cahuilla Indian Reservation. Ground-water use during 1986 is estimated at 10,000 acre-feet, 6,000 acre-feet more than in 1973. The water table, however, generally has risen since 1973. The rise is due to the wet climatic conditions that generally have prevailed since 1976, resulting in above average recharge. Dissolved-solids concentrations in water from the wells sampled ranged from 184 to 1,320 milligrams per liter. During 1986, water from 14 wells of the 38 locations sampled had dissolved-solids concentrations greater than 500 milligrams per liter, which is the U.S. Environmental Protection Agency recommended limit for public water supplies. Two of these 14 wells are on Cahuilla Indian Reservation. Nitrate (as nitrogen) concentrations ranged from 0.0 to 170 milligrams per liter. Water from 8 wells of the 38 locations sampled had nitrate concentrations higher than the U.S. Environmental Protection Agency recommended limit of 10 milligrams per liter. None of these eight wells are on Cahuilla Indian Reservation. The observation-well network was found to be generally adequate for detecting changes in water levels and water quality, but the addition of one water-level observation well and two water-quality observation wells would further enhance its effectiveness.

Ground-water conditions in the Anza-Terwilliger area, with emphasis on the Cahuilla Indian Reservation, Riverside County, California, 1973-86