Abstract—Groundwater discharge can play a critical role in sustaining dry-weather flow and influencing water quality in heavily urbanized river systems. The Los Angeles River, the largest urban river in Southern California, is now largely sustained by recycled water from municipal wastewater plants; however, projected reductions in wastewater discharge and urban runoff may increase the relative importance of groundwater contributions to baseflow and river chemistry. This study evaluates groundwater discharge to the Los Angeles River and selected tributaries using hydrochemical and isotopic tracers 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 weepholes were mapped through field reconnaissance, and 173 groundwater samples were collected from five reaches and two tributaries, Arroyo Seco and Laguna Channel. Results show spatial variability in groundwater chemistry. TDS ranged from 145 to 5558 mg/L, with elevated values in the Upper Los Angeles River, Arroyo Seco, and Laguna Channel. NO₃-N concentrations reached 25.9 mg/L in upstream reaches, indicating anthropogenic inputs. Selenium concentrations reached 70 μg/L in upstream reaches and were usually below 15 ug/L in other areas studied. Stable water isotopes indicate meteoric signatures with downstream enrichment consistent with mixing between local groundwater and imported water recharge. These findings demonstrate that groundwater discharge remains a persistent and chemically significant component in parts of the Los Angeles River system. As recycled water discharges decline and channel restoration expands, identifying groundwater discharge zones and associated water quality risks is essential for effective river management, ecological restoration, and protection of aquatic habitat.

Keywords—groundwater seepage, groundwatersurface water interactions, channelization, anthropogenic inputs, water quality, urban rivers, Los Angeles River

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Cite: Maria G. Rosas, Tommy J. Rojas, David Molina, Jennifer K. Michel, Caitlin S. Ruelas, and Barry Hibbs, "Mapping Groundwater Discharge Areas to the Los Angeles River, USA Using Hydrochemical and Isotopic Tracers," International Journal of Environmental Science and Development vol. 17, no. 3, pp. 250-264, 2026.

Copyright © 2026 by the authors. This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).