HYDROCHEMICAL STUDIES OF LOS ANGELES RIVER FLOWS FROM HEADWATERS TO SEPULVEDA DAM - A COMPARISON OF DRY AND WET YEAR SAMPLING EVENTS

This project evaluates the quality, composition, and sources of water within the upper reaches of the Los Angeles River, spanning from its headwaters at the confluence of Arroyo Calabasas and Bell Creek to Sepulveda Dam. The LA River is a highly engineered urban waterway that traverses Los Angeles, CA. Water samples were collected during dry weather conditions from various reaches, tributaries, groundwater dewatering infrastructure, and storm drain laterals. These samples were analyzed for field index parameters, as well as nitrate, phosphate, ammonium, sulfate, chloride, and total dissolved solids. Selected samples were also tested for stable water isotopes, sulfate isotopes, dissolved selenium, and dissolved nickel. The primary study period occurred during the summer dry season (May to August 2022), with a smaller resampling event in May 2024. This timeline captured a transition from a 10-year drought ending in late 2022 to a two-year wet period spanning 2023 to 2024.

A significant finding of the study is the substantial groundwater loading observed between the headwaters and Tampa Avenue, which comprises the first 50% of the study reach. This groundwater baseflow input contributes notable amounts of nitrate (2 to 14 mg/L NO₃-N) and selenium (8 to 35 µg/L dissolved Se) to the river. In contrast, downstream from Tampa Avenue to the upper Sepulveda Basin, no groundwater loading was detected, and hydrochemical changes were driven solely by dry-weather urban runoff. From the Sepulveda Basin to Sepulveda Dam, the river’s hydrochemical profile is dominated by large volumes of treated wastewater discharged into the river.

In the groundwater baseflow-dominated section above Tampa Avenue, groundwater inputs increase river concentrations of nitrate by 5 to 6 mg/L NO₃-N and selenium by 10 to 12 µg/L, while diluting chloride and sulfate concentrations. During the wet period, selenium levels in groundwater seepage points and springs feeding the river increased by approximately 50% at resampled stations between 2022 and 2024. This change is attributed to pulses of selenium-oxidizing recharge water reaching the saturated zone near the river’s baseflow sections.