Isotopic Investigation of Geologic and Anthropogenic Controls on Nutrient Loading in Malibu Creek Watershed, California

Abstract:

The upper portion of the Malibu Creek Watershed exposes the Monterey-Modelo Formation, a Miocene marine mudstone. This formation has been thought to contribute high concentrations of orthophosphate and nitrate to streams via groundwater leaching and baseflow. However, our recent studies suggest that high concentrations of orthophosphate and nitrate may be dominated by dry weather runoff of imported water (tap and recycled water) from watering of urban landscapes. Our study investigates El Camino Real Creek, a tributary in the Malibu Creek Watershed that traverses Monterey-Modelo Formation strata and is fed predominantly by dry weather runoff. From an initial input at a storm drain where dry weather runoff flows consistently, hydrochemical parameters range from 1.86 to 4.66 mg/L NO₃^-N and 1.06 to 2.28 mg/L PO₄ that decrease to concentrations ranging from 0.15 to 0.59 mg/L NO₃^-N and 0.40 to 0.87 mg/L PO₄ where El Camino Real Creek converges with Las Virgenes Creek. The decrease in nutrient content downstream is due to the transformational processes denitrification, vegetation uptake, and mixing with groundwater baseflow containing lower nutrient content. The average water isotope values for the imported (tap and recycled) endmembers are -9.1‰ δ¹⁸O and -73‰ δD. The average water isotope values for the samples collected at the storm drain range from -6.0‰ to -8.0‰ δ₁₈O and -56‰ to -68‰ δD while isotope values downstream range from -6.0‰ to -6.3‰ δ¹⁸O and -47‰ to -48‰ δD. Stable isotopes of hydrogen and oxygen show mixing of imported water with local groundwater downstream, which demonstrates that nutrients in this creek are not strictly dominated by geologic sources. To further understand the nutrient changes and mixing percentages of imported and local water sources, diurnal studies are being conducted with the integration of nitrate isotopes to help understand the nutrient dynamics in El Camino Real Creek.