Surface-water and ground-water quality in the Yucaipa area, San Bernardino and Riverside Counties, California, 1996-98

The quality of surface water and ground water in the Yucaipa area was evaluated to determine general chemical characteristics and to identify areas of recent ground-water recharge. Water samples, collected from 8 sites on 3 creeks and from 25 wells, were analyzed for general chemistry, nutrients, tritium, and stable isotopes of hydrogen and oxygen. At one production well (1S/2W-25R4), water samples were collected at discrete depths during pumping and a continuous profile of the vertical flow rate inside the well casing was recorded. In addition to general-chemistry samples, tritium and carbon-14 samples were collected at this well to interpret the age of water at different depths.Results indicate that most water in the Yucaipa area is a calcium-bicarbonate type. The general chemical composition of surface water resembles that of ground water, although the concentration of most constituents is higher in ground water. The chemical composition of most ground-water samples is similar. Elevated concentrations of nitrate in some ground-water samples may indicate recharge from agricultural areas.In surface water that recharges ground water tritium activity ranged from 7 to 18 picocuries per liter. The range of tritium activity found in ground water indicates different times since recharge and possible mixing along ground-water flow paths. The oldest ground-water sample had a tritium activity less than 0.3 picocuries per liter, indicating more than 50 years since recharge. Water samples that had tritium activity greater than 0.3 picocuries per liter indicate that some of the water was recharged since 1952. The youngest ground water (greater than 7 picocuries per liter) was found near the hills and mountains surrounding the Yucaipa area; the oldest ground water (less than 0.3 picocuries per liter) was found in the Western Heights subbasin. Testing of the vertical contribution of ground water to well 1S/2W-25R4 showed that more than one-half of the water flowed into the well between depths of about 450 and 600 feet below land surface; the rest of the water flowed into the well between 600 and about 850 feet below land surface. At a discharge rate of 750 gallons per minute, virtually no water was contributed to the well below a depth of about 850 feet. The water samples collected at this well ranged in age from less than 50 to less than 400 years before present.