Salinity Controls on Artificial Recharge: Case Study Distinguishing Mixing from Mineral Dissolution in a Saline Aquifer

ABSTRACT Artificial recharge is increasingly important for groundwater management in arid regions, yet its effectiveness depends on how recharge water interacts with existing aquifer salinity. In the southwestern San Fernando Valley Groundwater Basin of Los Angeles, CA, elevated sulfate and total dissolved solids (TDS) have long prevented artificial recharge west of the “bad-water line.” This study evaluated whether modern salinity results mainly from conservative mixing with sulfate-rich groundwater derived from Miocene marine formations or from in situ dissolution of lithic fragments containing gypsum and pyrite in Quaternary basin-fill sediments. Groundwater discharging into the upper Los Angeles River, the only accessible source of modern southwestern basin groundwater, was sampled and analyzed for major ions, stable water isotopes, and tritium, and results were compared with imported water sources and native bedrock groundwater. Stable water isotopes show Los Angeles River groundwater plotting between imported water and bedrock-derived native groundwater, with measurable tritium indicating imported water recharge within roughly 10 to 25 years. Major anion mixing calculations yielded native water fractions of 24 to 29 percent across chloride, sulfate, TDS, and stable water isotopes, demonstrating that observed salinity is reproduced by two end-member conservative mixing. Native groundwater contains far higher dissolved solids than imported water, and mixing trajectories match observed compositions. These results show that salinity in the southwestern basin reflects non-reactive mixing rather than mineral dissolution. Managed recharge using low-TDS water can dilute saline zones and improve groundwater quality when monitoring accounts for bedrock-derived saline inflows.