Abstract

Groundwater discharge can play a critical role insustaining dry-weather flow and influencing water quality inheavily urbanized river systems. The Los Angeles River, thelargest urban river in Southern California, is now largelysustained by recycled water from municipal wastewater plants;however, projected reductions in wastewater discharge andurban runoff may increase the relative importance ofgroundwater contributions to baseflow and river chemistry.This study evaluates groundwater discharge to the Los AngelesRiver and selected tributaries using hydrochemical and isotopictracers to characterize spatial variability in total dissolved solids(TDS), nitrate-N (NO3-N), selenium, and stable water isotopes(δ¹⁸O and δ²H). Groundwater seepage, springs, and weepholeswere mapped through field reconnaissance, and 173groundwater samples were collected from five reaches and twotributaries, Arroyo Seco and Laguna Channel. Results showspatial variability in groundwater chemistry. TDS ranged from145 to 5558 mg/L, with elevated values in the Upper Los AngelesRiver, Arroyo Seco, and Laguna Channel. NO3-Nconcentrations reached 25.9 mg/L in upstream reaches,indicating anthropogenic inputs. Selenium concentrationsreached 70 μg/L in upstream reaches and were usually below 15ug/L in other areas studied. Stable water isotopes indicatemeteoric signatures with downstream enrichment consistentwith mixing between local groundwater and imported waterrecharge. These findings demonstrate that groundwaterdischarge remains a persistent and chemically significantcomponent in parts of the Los Angeles River system. As recycledwater discharges decline and channel restoration expands,identifying groundwater discharge zones and associated waterquality risks is essential for effective river management,ecological restoration, and protection of aquatic habitat.