A COMBINED STABLE AND RADIOISOTOPE STUDY OF THE SOURCES OF RECHARGE IN THE COACHELLA VALLEY AQUIFER SYSTEM, RIVERSIDE COUNTY, CALIFORNIA

For the stakeholders in the Coachella Valley, the Coachella Valley Aquifer system is the main source of water, and has been under drawdown conditions since 1936. Presently, Coachella Valley water authorities import up to ~225,000 Af/y of Colorado River water to combat continued draw-down conditions. In this study, stable and radioisotopes of water and carbon in approximately 80 samples of combined surface, spring and well waters from the study area are used to contrast and compare the key findings of Tyleys’ 1974, USGS “Analog Model Study of the Ground-Water Basin of the Upper Coachella Valley, California”.

Water isotope data demonstrate that springs located in the San Jacinto and San Bernardino mountains along with selected wells and precipitation plot along a Local Meteoric Water Line. San Gorgonio subbasin underflow and runoff along with Whitewater River and Mission Creek watershed runoff and underflow mix and recharge the Indio subbasin; with very little mixing of groundwater from the Desert Hot Springs subbasin, or imported Colorado River water. Nested and single wells located near the Windy Point recharge facility show shallow wells have water isotope values similar to imported Colorado River water, with deep wells having water isotopes values similar to wells and low elevation springs located in the watersheds of the San Gorgonio subbasin and Whitewater River watershed. Stable water isotopes in two wells proximal but downgradient from the recharge basins have a ~ 40% and 60% mixture of Colorado river water with native groundwater, respectively. By comparison, Piper diagram analysis of mid-depth wells show an ~50% mixture of Colorado River water and native groundwater and validates the stable water isotope data. Stable and radioisotopes of water, C14 and Δ18O in sample waters indicate recharge to the Coachella Valley Aquifer system occurs mostly as winter precipitation as snow in the watersheds of the flanking San Jacinto and San Bernardino mountains; with limited recharge occurring on the Coachella Valley floor.