Loading of Nitrate to Miocene Marine Aquifers Oxidizes Selenium and Contaminates Interconnected Streams

Southern California watersheds have not been noted for elevated selenium concentrations until recently.  Three urban watersheds supporting healthy ecological habitats have now been identified with elevated levels of selenium.  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 appear to be the original sources of selenium in these watersheds.  Selenium is leached into groundwater from alluvium and shale-residuum deposits.  Elevated selenium concentrations occur 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 agriculture and treated wastewater application.  Nitrogen and selenium isotopes measured in groundwater samples support our interpretation of selenium-moderated oxidation by nitrate.  Stable isotopes of oxygen and deuterium and other geochemical tracers indicate nitrate was sourced in one stream from application of treated municipal wastewater to urban landscapes built on Miocene rocks.       
      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, and where selenium may threaten habitat.