HYDROGEOLOGIC FACTORS IN SELENIUM LOADING TO SOUTHERN CALIFORNIA WATERSHEDS

We have identified three urban watersheds with elevated selenium in the Los Angeles Basin. These include San Diego Creek Watershed of Orange County, Malibu Creek Watershed of Los Angeles County, and tributaries to the Los Angeles River. Dry weather surface flows in these watersheds contain 20 to 35 ug/L dissolved selenium. Shallow groundwater in these watersheds contains 30 to 300 ug/L dissolved selenium. Concentrations exceed the USEPA chronic criterion for selenium of 5 ug/L for protection of aquatic life. Miocene marine shales in the Monterrey-Modelo-Puente formation appear to be the original sources of selenium in these watersheds. Selenium is leached into groundwater from these low-permeability strata, along with standard inorganic constituents (primarily sulfate) and phosphorous. Elevated selenium concentrations develop in shallow groundwater, and baseflows carry selenium into urban surface streams. Groundwater baseflows account for most of the selenium loading to streams. Positive correlations are observed between nitrate and selenium in both groundwater and surface water in the watersheds we investigated. Previous theoretical calculations showed favorable Gibbs free energies for oxidation of selenium by dissolved nitrate. Empirical batch studies support theoretical calculations for mobilization of selenium by nitrate in marine shales. Positive correlation between nitrate and selenium in our studies appears to be related to nitrate sourced from atmospheric fallout, agriculture, and treated wastewater application. Laboratory leaching experiments carried out on weathered and unweathered Monterrey rocks from the Malibu Creek Watershed showed high selenium concentrations only when nitrate was leached from the rocks in high concentrations. Selenium concentrations were non-detectable or very dilute when nitrate was not leached from rocks. Weathered rock generally had high concentrations of nitrate in leachate, possibly from accumulation from atmospheric fallout. Leachate from non-weathered Miocene rock generally had low nitrate concentrations and low selenium. Miocene shales are exposed in vast areas of Southern California. We are undertaking regional studies to determine if other Southern California Watersheds are affected by elevated nitrate and selenium concentrations.