Jonathan Arriaza
Published: 2019
Total Pages: 93
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The Coachella Valley is a large, alluviated and gently sloping valley of about 820 square miles in the Palm Springs-Indio California area. The Coachella Valley Aquifer is beset by long term drawdown in water levels. While recharge of imported Colorado River water has helped maintain aquifer water levels near Palm Springs and La Quinta, other areas of the Coachella Valley Aquifer have experienced long term and unsustainable declines. Understanding natural and artificial sources of groundwater recharge and naturally occurring hydrochemical processes in the aquifer provides information for sustainable water use and planning.Coachella Valley is surrounded by the San Gorgonio Mountains, Little San Bernardino Mountains, Orocopia Mountains, Santa Rosa Mountains, and San Jacinto Mountains. The Coachella Valley Aquifer is composed of four subbasins: San Gorgonio, Mission Creek, Desert Hot Springs, and Indio Subbasins. These subbasins are delineated by bounding faults and groundwater divides. Groundwater flows from the northwest to the southeast in the direction of the Salton Sea. Mountain front recharge components and runoff from the mountains to recharge areas on subbasin floors are considered to be the dominant mechanisms for natural recharge to the Coachella Valley Aquifer.Analysis of the stoichiometric relationships between water chemistry and common rock and sediment dissolution/ion exchange reactions indicates several simple reactions and processes control the water chemistry of the Coachella Valley Aquifer. Major processes include dissolution of carbonate cement, dissolution of gypsum, and dissolution of halite. Cation exchange, favoring exchange of Ca and Mg for bound Na is also an important process in the aquifer system. Pyroxene, amphibole, and feldspar dissolution are less important sources of dissolved ions due to their lower solubility. Clay minerals are important weathering products of these minerals and provide the exchange sites for divalent-monovalent cation exchange. Dissolution of specific minerals is a function of their spatial locations in the basin. Chloride occurs in rainwater, and precipitates in soils as halite when rainwater evaporates. The halite can be dissolved by runoff and carried into groundwater along arroyos and other recharge areas. Evaporation of groundwater by natural discharge and by irrigation recycling further concentrates salts in soils and groundwater. Gypsum and halite are present along the basin floor in the eastern part of the basin, having precipitated in various antecedent surface water systems and in phreatic playas in the Salton Sink. Gypsum also formed in fractures along faults in the Mission Creek and Desert Hills subbasins. Gypsum and halite dissolve when meteoric groundwater comes into contact with these evaporite minerals. Carbonate cements are present in the mountains and precipitate as caliche along mountain fronts and at interior locations in the basin. Carbonate cements are important sources of calcium, magnesium, and bicarbonate in groundwater where meteoric waters come into contact with these relatively soluble rocks.The study also identifies different sources of groundwater recharge in the Coachella Valley Aquifer. Stable water isotopes (18ÎþO and ÎþD) distinguish between recharge by imported Colorado River water and native groundwater sources. Both sources have distinctly different water isotope signature. Artificial recharge by imported Colorado River water dominates in only a few local parts of the Coachella Valley Aquifer. Recharge from intermittent and perennial streams, and recharge of precipitation along mountain fronts and basin floors is a more important source of recharge across the Coachella Valley Aquifer. 18ÎþO and ÎþD proved to be powerful forensic tool for investigating groundwater recharge sources in the Valley. Carbon-14 supplemented the findings for 18ÎþO and ÎþD and helped identify sources of recharge that occurred during pluvial periods of the late Pleistocene and early Holocene Epochs.
