Characterizing Water Inputs to Catchments in the Santa Monica Mountains with δ18O and δD

Water inputs to Santa Monica Mountains National Recreation Area (SMMNRA) watersheds include local precipitation, runoff of imported water and treated wastewater derived from imported water. Imported water from northern California is isotopically distinct from southern California precipitation. This contrast allows source flow differentiation. To determine the relative contributions of water sources to creeks and streams in the SMMNRA, stable water isotopes [δ18O, δD] are used. Sampling of rainwater, stream water, tap water and treated wastewater was done from January through December 2008. Analysis includes a focused study of McCoy Creek. Results show tap water and treated wastewater to be fairly reliable end members and local precipitation to be unreliable as an end member. The isotopic signatures of McCoy Creek samples appear to be affected either by evaporation or changes in imported waters' isotopic signatures.