Predicted water-level and water-quality effects of artificial recharge in the Upper Coachella Valley, California, using a finite-element digital model

s From 1936 to 1974, water levels declined more than 100 feet in the Palm Springs area and 60 feet in the Palm Desert area of the upper Coachella Valley, Calif. Water from the Colorado River Aqueduct is presently being recharged in the Whitewater River subbasin to help retard this trend of water-level decline. Water quality in the basin varies from a sodium sulfate type in the northern three subbasins (Desert Hot Springs, Mission Creek, and Garnet Hill) to a calcium bicarbonate type in the southernmost Whitewater River subbasin. Dissolved-solids concentration ranges from a minimum of 200 milligrams per liter in some parts of the Whitewater River subbasin to a maximum of more than 1,000 milligrams per liter in the Desel I I lot Springs sillohitsin. The dissolved-solids concentration in the artificial recharge water ranges from 600 to 750 milligrams per liter. Using projected pumpage, the finite-element digital-model simulation indicates that without artificial recharge the 1974 water levels near Palm Springs would decline 40 feet by 1981, 120 feet by 1991, and 200 feet by 2000. It is planned to recharge the aquifer at a rate of about 7,500 acre-feet per year in 1973, increasing to 61,200 acre-feet per year in 1990 and thereafter. If the artificial recharge is applied at this increasing rate, the 1974 water levels near Palm Springs would decline only 20 feet below the 1974 level by 1991 and recover to the 1974 level by 2000. The solute-transport (water-quality) finite-element digital-model simulation, using dissolved-solids concentration as the model parameter, indicates that the artificial-recharge plume (hounded by the 300-milligram-per-liter concentration line) would move 1.1 miles downgradient from the recharge ponds by 1981, 3.1 miles by 1991, and 4.5 miles by 2000; the plume boundary would cross Indian Avenue between 1991 and 2000. Water-quality model simulations using dissolved-solids 'concentrations of 650 and 750 milligrams per liter for the recharge water resulted in similar plume sizes and in locations -.that were within 650 feet of each other. A simulation to determine the effects of recharging with water from the State Water Project from 1991 to 2000 showed that the plume would be atrophied at the upstream end and would decrease in concentration at the downstream end. INTRODUCTION Ground-water levels declined more than 100 ft from 1936 to 1974 in some areas of the upper Coachella Valley. In an effort to counteract these declining water levels, the Desert Water Agency and the Coachella Valley County Water District contracted with the State of California to purchase water imported from northern California through the California Aqueduct. Because no branch aqueduct presently exists from the California Aqueduct to the Coachella Valley, the water agencies agreed to exchange their annual entitlements to California Aqueduct water with the Metropolitan Water District of Southern California for water from the Colorado River Aqueduct. Since 1973 this imported water has been recharged to the ground-water basin through the Whitewater River channel into the spreading basin where the permeable channel is diked to form ponds (fig. 1). Because this recharge water has a higher dissolved-solids concentration than the native ground water, there is a need to (1) delineate the areal extent of present-day water-quality conditions and (2) develop methods to evaluate and predict changes in water-level and water-quality conditions that are brought about by this recharge. PURPOSE AND SCOPE This study was begun in mid-1973 by the U.S. Geological Survey in cooperation with the Desert Water Agency (DWA) and the Coachella Valley County Water District (CVCWD). The purpose of the study was to (1) analyze the present waterquality conditions of the ground-water basin, (2)