Hydrochemical and Isotopic Studies of Los Angeles River Flows Between River Headwaters and Sepulveda Dam

This project assesses the quality and sources of water in the Los Angeles River, from headwaters at confluence of Arroyo Calabasas and Bell Creek to Sepulveda Dam. The L.A. River is a highly engineered, highly urban waterway running through Los Angeles, CA. Water samples collected from various reaches, tributaries, groundwater dewatering infrastructure, and storm drain laterals were tested during dry weather conditions. This study is the first intensive study of the isotope hydrology, source water tracking, and nutrient and trace element loading to the L.A. River in the reach from its headwaters to Sepulveda Dam. 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 above 1500 mg/L. The river transitions to baseflows derived from seepage through mixed sediments eroded out of highlands in the middle part of the watershed, with mixed cation-mixed anion composition and TDS between 800 and 1400 mg/L. While dilution of dissolved solids occurs between the headwaters of the LA River and Sepulveda Dam, the selenium concentration increases from < 4 ug/L total Se to 13 ug/L total Se, due to groundwater loading. Correlation between nitrate and selenium in groundwater implies that nitrate acts as an oxidizing agent for bound selenium in aquifers feeding the LA River. Stable water isotopes suggest that most of the groundwater discharge in the stream reach between headwaters and Sepulveda Dam are derived from recharge of imported water, mostly from Owens Valley.