ISOTOPIC AND HYDROCHEMICAL IDENTIFICATION OF SOURCE WATERS AND EFFECTS OF SEASONAL CHANGES ON DRY WEATHER STREAM FLOWS, SANTA MONICA MOUNTAINS, CA

Many streams within Southern California’s Malibu Creek Watershed reportedly flowed intermittently during predevelopment times, and now flow year round. Some creeks in Malibu Creek Watershed contain relatively high total dissolved solids during the lengthy dry season (2000 to 3000 mg/L TDS) while others contain very low total dissolved solids (<500 mg/L TDS). The isotope hydrology and hydrochemistry of Lindero Creek, an urban tributary in the Malibu Creek Watershed, was investigated for a year to test the origins of its relatively high salinity, and the source(s) of its perennial flow.

Analysis of stable oxygen and deuterium isotopes show specific signatures for native water and imported water during dry weather flows (e.g., sampling at least 3 days after precipitation). Water imported from Northern California’s Bay-Delta project is the primary source of water for the communities near Lindero Creek. Isotope data show the source of flow in Lindero Creek is 75% to 90% native, indicating precipitation recharge and base flow from shallow aquifers is the primary source of water to the creek. Isotope data also indicate more mixing of source waters in summer/fall than in winter/spring, suggesting an increased use in imported water during warmer/dryer months. Specific conductance (SC) maintained a range 2,000-4,000 uS/cm with high levels of chloride (100-300 mg/L), sulfate (300-2,500 mg/L) and phosphate (0.2-1.0 mg/L) recorded throughout the year. The data suggest a geologic source is associated with salinity and phosphate values. The Miocene Monterey/Modelo Formation, which underlies much of Lindero Creek, is a mostly fine textured marine shale that appears to release salts (especially sulfate), phosphorus, and heavy metals, which are all found in high concentrations in Lindero Creek surface water. This study suggests perennial flow is due to channel modification augmented by some urban runoff. Poor water quality is a consequence of base flows from naturally saline geologic formations.