Nitrate‐Driven Selenium Release From the Monterey Formation in Coastal California Watersheds—A Recycled Water Conundrum

ABSTRACT Recycled municipal wastewater is increasingly used for non‐potable irrigation in California, but its moderate nitrate content may mobilize redox‐sensitive trace elements in geologic units such as the Miocene Monterey Formation, enriched in selenium, nickel, and uranium. This study evaluates nitrate‐driven selenium release in the Las Virgenes Creek Watershed, where selenium regularly exceeds aquatic life criteria. Monitoring incorporated hydrochemical, isotopic, and redox characterization of groundwater, surface water, recycled water, and municipal supplies. Five water groups exhibited distinct signatures, with Monterey and Calabasas groundwaters most saline (TDS ≈ 2400 to 2900 mg L −1 ) and selenium‐rich (averages of 46 and 23 μg L −1 ), whereas recycled water showed elevated nitrate‐nitrogen (5 to 10 mg L −1 ). Selenium occurred primarily as selenate (Se‐VI), consistent with oxidizing conditions. Selenium and nitrate were correlated in Monterey groundwater ( R 2 = 0.76), suggesting nitrate‐enhanced oxidation of mineral‐bound selenium. Groundwater baseflow was the primary selenium source to the creek, explaining downstream co‐enrichment of nitrate and selenium. Nitrate isotope values indicated dominant manure or septic contributions, with secondary recycled water influence. Findings show anthropogenic nitrate sustains oxidizing conditions that increase selenium mobility in Monterey‐derived strata.