INVESTIGATION OF NITRATE AND SELENIUM SOURCES IN MALIBU CREEK WATERSHED, EMPHASIZING A COMPARATIVE ANALYSIS OF TWO MAJOR CREEKS: LAS VIRGENES & MEDEA CREEK

A comparative study evaluating the sources of nitrate, toxic trace elements, (primarily selenium) and water sources is underway in the urban sections of Malibu Creek Watershed. Our studies show that a major tributary to Malibu Creek, Las Virgenes Creek, has nitrate concentrations of 2 to 8 mg/L NO₃^--N. The precise source(s) of nitrate is not well known. Nitrate may be from use of treated wastewater for irrigation of urban landscapes; from leaching beneath solid sludge disposal plots; and/or from historic agriculture that once was the main activity in this region. Selenium concentrations usually range from 10 to 50 ug/L dissolved Se in Las Virgenes Creek, exceeding concentrations of concern for freshwater aquatic species. Studies of a nearby tributary, Medea Creek, have identified low nitrate concentrations (< 1 mg/L NO₃^--N) in the creek, despite the fact that most of the land use practices and geology are similar to areas near Las Virgenes Creek. Furthermore, selenium concentrations are less than 5 ug/L dissolved Se in Medea Creek. Stable isotopes of oxygen and hydrogen and standard inorganic constituents pinpoint groundwater baseflow as the primary source of water discharging into these creeks during dry weather conditions, with dry weather urban runoff accounting for less that 15% of the observed flows. Sampling of small springs and seeps at Las Virgenes Creek indicates pollutant loading is primarily from groundwater baseflows that are enriched in nitrate and selenium. Sampling of shallow groundwater in springs and seeps that flow into Medea Creek indicates little enrichment of nitrate and selenium. However, we hypothesize that selenium is oxidized by elevated nitrate in shallow groundwater flowing through Monterrey/Modelo marine strata adjacent to Las Virgenes Creek. Shallow groundwater near Medea Creek usually contains little nitrate, and therefore does not have the same oxidizing potential for selenium. Laboratory batch studies are underway to determine the efficacy of nitrate as an oxidizing agent of selenium contained in Monterrey/Modelo strata. Nitrate isotopes are also slated for use in our field studies to help determine the source(s) of nitrate in Las Virgenes Creek.