ISOTOPE HYDROLOGY AND HYDROCHEMISTRY OF THE UPPER LOS ANGELES RIVER, CA

This project assesses the quality, composition, and sources of water within the upper to middle reaches of the Los Angeles River, from headwaters at the confluence of Arroyo Calabasas and Bell Creek to the confluence with Arroyo Seco. The LA River is a highly engineered, highly urban waterway running through Los Angeles, CA. Much of its length is concrete-lined, and there are many inflow sources throughout the city. The motivation behind this project is to provide baseline data for future work on and around the LA River and to inform potential hydrologic policy decisions. Water samples collected from various reaches, tributaries, groundwater dewatering infrastructure, and storm drain laterals were tested during dry weather conditions. All samples were tested for field index parameters, nitrate, phosphate, ammonium, sulfate, chloride, TDS, stable water isotopes, and sulfate isotopes, and select samples were tested for dissolved selenium and nitrate isotopes. This data will help identify the fingerprint of the water in order to better understand the river’s composition.

Salinity and nutrient gradients in the Los Angeles River and its tributaries have been identified thus far. Observational results show gradational trends in the river associated with baseflows sourced from sedimentary marine strata and derivative sediments in the western Los Angeles River watershed, with TDS between 1500 and 2500 mg/L and dry weather river flows containing mainly a mixed-cation/sulfate hydrochemical signature. The river transitions to baseflows derived from seepage through low-solubility sediments eroded out of igneous highlands in the middle part of the watershed, with mixed-cation/mixed-anion composition and TDS between 500 and 1300 mg/L. Where wastewater inputs to the river appear in the Sepulveda Basin, the geologic imprint is masked and nitrate increases from about 0.5 mg/L NO3-N to 5 mg/L NO3-N. Masking does not occur in many tributaries to the main stem where wastewater is not added to tributary flows. Geologic impressions on river chemistry, where baseflows are present, are preserved.