INVESTIGATION OF SELENIUM AND NUTRIENTS INPUTS IN URBAN RIVERINE FLOWS IN MALIBU CREEK WATERSHED, CA

The Monterey-Modelo Formation is a Miocene marine mudstone that has been noted for probable contribution of trace metals and possible contribution of nutrients in groundwater baseflow in the upper Malibu Creek Watershed. Our study collected dry weather runoff, shallow groundwater, surface water, and hyporheic zone water in El Camino Real Creek, a tributary that tranverses across Monterey-Modelo strata in the greater Malibu Creek Watershed. Water samples are analyzed for selenium, nutrients, and isotopes (hydrogen, oxygen, and nitrogen) to further understand inputs and sources. We have found Se concentrations of 9.9 to 32 ug/L from groundwater seeps that flow within an urban storm drain. From the input of a storm drain where dry weather runoff provides perennial flow to El Camino Real Creek, concentrations of Se range from 4.1 to 11 ug/L. Concentrations of Se decrease from 4.1 to 0.74 ug/L downstream, where El Camino Real Creek converges with Las Virgenes Creek. Moderately high concentrations of nitrate-nitrogen are correlated with elevated concentrations of selenium in surface water and groundwater near the creek. Elevated concentrations of nitrate in surface water are associated with reclaimed water used for urban irrigation, and/or legacy nitrate sources from groundwater baseflow. Groundwater samples range from 0.37 to 6.6 mg/L NO₃^-N while surface water samples range from 1.5 to 4.9 mg/L NO₃^-N upstream near the storm drain. Further downstream, after the creek flows move through a riparian area, nitrate concentrations in surface water decrease to 0.12 to 0.59 mg/L NO₃^-N. Seasonal analysis of hydrogen and oxygen isotopes suggest perennial input and mixing of local groundwater throughout the riverine system. Our study overall study results suggest that enriched selenium found in local groundwater is oxidized by elevated concentrations of nitrate found in groundwater and dry weather urban runoff (imported and recycled water) that penetrates the hyporheic zone.