Geochemical Drivers of Elevated Sulfate in California Coastal Basins: Evolution From Marine Sediments to Watersheds and Aquifers

ABSTRACT Sulfate concentrations of 500–2700 mg/L are widespread in Los Angeles Basin groundwater and surface waters, often exceeding chloride by factors of three to eight. This sulfate‐dominant pattern contrasts with the chloride‐driven salinity typical of coastal aquifers influenced by seawater intrusion or marine aerosols. Data from the western San Fernando Valley, Upper Los Angeles River, and Santa Monica Mountains indicate a geologic origin tied to Miocene marine sedimentary rocks of the Monterey, Modelo, and Topanga Formations. Oxidation of diagenetic pyrite and dissolution of secondary gypsum release large sulfate loads to groundwater and streams, confirmed by sulfate isotope signatures consistent with geologic rather than anthropogenic sources. Groundwater discharge contributes substantial sulfate to the Los Angeles River, while springs in the Santa Monica Mountains show similar isotopic values, underscoring the regional extent of this process. This study develops an integrated geochemical model linking marine deposition, microbial reduction, pyrite formation, tectonic uplift, oxidation, gypsum precipitation, and dissolution. The model highlights how geologic processes continue to shape modern groundwater chemistry, provides a framework for interpreting sulfate enrichment in Southern California coastal basins, and emphasizes the need to incorporate geologic sources into water quality management strategies.