Assessing the impact of water reuse on water quality in the Los Angeles River

Abstract

Water reuse through water reclamation plants (WRPs) is an approach used by Los Angeles to reduce dependence on imported water. Three WRPs in the Los Angeles River (LAR) watershed, located in southern California, contribute up to 70% of the river discharge during dry weather periods. Increasing water reuse will lead to flow reductions in-stream, which could affect water quality. The purpose of this research is to identify the impact of water reuse on loads and concentrations of copper, zinc, lead, total suspended solids (TSS), and total dissolved solids (TDS) in the LAR. First, an empirical water quality data analysis was accomplished by collecting concentration data from monitoring programs and evaluating the relationships between pollutants and discharge. A water quality module was then added to a calibrated EPA-SWMM hydrologic model for water years 2011 to 2017 on a daily time-step, which was developed in prior work. Land use characteristics, event mean concentrations, urban baseflow concentrations, and WRP effluent data were added to the model. The model was then calibrated at four locations along the river mainstem. Water reuse scenarios were applied to the calibrated model and the simulated daily loads and concentrations were assessed using water quality regulatory compliance. Results show that TSS and metals are strongly correlated with each other and that discharge is a moderate predictor of pollutant concentration. Median daily loads for all pollutants generally decrease under scenarios of varying WRP flow reduction, but median daily concentrations either increase or decrease depending on the major sources for each pollutant. Copper, zinc, and lead regulatory compliance are not greatly impacted by WRP flow reduction, but TDS compliance is degraded. The results from this study will help inform regional water managers of potential impacts of flow reductions on water quality in the LAR and help aid decisions on water recycling policies.