ISOTOPIC AND HYDROCHEMICAL IDENTIFICATION OF SOURCE WATERS AND POLLUTANTS IN MALIBU CREEK WATERSHED, CA

The Malibu Creek Watershed is located within the Santa Monica Mountains in Los Angeles and Ventura County, California. Streams within the watershed contain pollutants that have not met water quality standards set by the US Environmental Protection Agency. The Monterey-Modelo Formation, a Miocene marine mudstone deposit is exposed in the upper portion of the watershed and has been thought to contribute high concentrations of phosphorus and nitrate to streams via groundwater leaching and baseflow. Newer studies suggest that high concentrations of phosphorus and nitrate may be more a result of dry weather runoff from urban landscapes, especially in areas where treated wastewater is applied. As follow up to these conflicting studies, our new study investigates El Camino Real Creek, a tributary in the Malibu Creek Watershed that traverses Monterey-Modelo Formation strata. Due to local development, an increase of imported and recycled water is contributed to the creek. This study investigates sources of excess nutrients to further understand the geologic and anthropogenic controls on nutrient loading to El Camino Real Creek. The study results to date show a decrease in nitrate and phosphate concentrations downstream. From an initial input at a storm drain where dry weather flows consistently contain more than 1.5 mg/L NO3-N and 1.5 mg/L PO4 decrease to concentrations less than 0.5 mg/L NO3-N and 0.6 mg/L PO4 where El Camino Real Creek converges with Las Virgenes Creek. The decrease in nutrient content downstream is due to transformational processes such as denitrification, vegetation uptake, and mixing with nutrient dilute groundwater baseflow. The study results also show an increase in salinity downstream as a result of evapotranspiration processes and saline groundwater baseflow. Stable isotopes of hydrogen and oxygen show mixing of recycled water with local groundwater downstream, which demonstrates that nutrients in this creek are not strictly dominated by geologic sources. Special studies will be conducted to understand effects of groundwater base flow mixing and the changes of source flow from the storm drain within diurnal periods. Being able to identify source flows and hydrochemical changes within the Malibu Creek Watershed is needed to support future regulatory decision making.