Challenge. Develop a simulation tool to evaluate the effectiveness and optimize the performance of the Alamitos Barrier injection well system for protecting a groundwater supply from seawater intrusion.

Solution. Pumping in the Central Basin, one of Los Angeles County’s largest groundwater basins, has caused inland water levels to drop, creating a landward hydraulic gradient. The Alamitos Barrier is a system of injection wells that produces a hydraulic ridge to prevent seawater from moving inland. The injection water consists of water from a municipal supply mixed with a percentage of recycled water (tertiary-treated wastewater). INTERA developed groundwater flow and transport models to help assess the flow of recycled water away from the barrier towards inland production wells, model the transport of chloride near the barrier, and evaluate the efficacy of the barrier and optimize various barrier operations. Before developing the numerical models, we developed a conceptual hydrogeologic model of the region and a conceptual water balance. The groundwater flow model was developed and calibrated to average conditions over the past decade. A transient flow model was then developed and calibrated to hydraulic head measurements over this same time period. A solute transport model was developed to predict the potential movement of injected recycled water as well as movement of seawater-tainted groundwater past the injection barrier. INTERA has also provided annual model updates to meet the permit requirements associated with injecting treated wastewater.