OVERVIEW OF PRELIMINARY FINDINGS OF HYDROCHEMISTRY AND ISOTOPE HYDROLOGY OF LOS ANGELES RIVER BETWEEN RIVER HEADWATERS AND OUTFALL AT PACIFIC OCEAN

This project assesses the water quality, hydrochemical composition, and water sources along the L.A. River, from its headwaters at Arroyo Calabasas to its discharge at the Pacific Ocean. The L.A. River is a highly urbanized, mostly concrete-lined waterway with many inflow sources. Water samples were collected during dry weather over two summers—2022 during a peak drought and 2024 after a historic wet season. Surface water and influent groundwater was tested for various parameters, including nitrate, phosphate, ammonium, sulfate, chloride, TDS, stable water isotopes, and sulfate isotopes, with several samples tested for select trace elements, nitrate isotopes and radioisotopes. This first intensive regional study of the river's isotope hydrology will inform future stream and wetland restoration projects and serve as a baseline to assess anticipated change in the hydrochemistry and flows of the L.A. River. Key findings include: in the upper reach between Arroyo Calabasas and Sepulveda Basin, influent groundwater from high water table affects flows by diluting sulfate and chloride while enriching nitrate and selenium. Selenium levels exceed the aquatic standard of 5 ug/L midway through this reach, sourced from Miocene rocks and erosional sediments in the upper San Fernando Basin. From Sepulveda Basin to Glendale Narrows, treated wastewater dominates, affecting salinity, nutrient content, and isotope signatures. Inflows from Arroyo Seco and rising groundwater alter salinity and nutrient levels slightly from Glendale Narrows to downtown L.A., with this reach mostly unlined due to groundwater flux. From downtown to Long Beach, storm drains and tributaries that issue primarily dry weather urban runoff have minimal impact on overall river hydrochemistry, with slight nitrate and selenium dilution and minimal change in salinity. At the mouth of the L.A. River in Long Beach, tidal fluctuations cause marine water intrusion into the L.A. River, with early data showing the river at half the salinity of seawater at least 1.5 km above the outfall of the L.A. River to the Pacific Ocean.