Simulated groundwater tracer study of the Alamitos Barrier Project, Los Angeles, California

Abstract

A simulated tracer study is performed on the Alamitos Groundwater Barrier Project (ABP), Los Angeles, California. The ABP consists of 43 injection wells that create a freshwater barrier to prevent intrusion of seawater into the coastal aquifer of southern Los Angeles County. The injection wells currently use a blend of recycled and imported water. Regional water quality regulations dictate the quantity of recycled water that can be injected into the aquifer. The regulations also require that the travel-time between the injection wells and the local production wells be greater than one year. The purpose of this study is to analyze the travel time of injected water into the ABP. A previously calibrated three-dimensional, finite element, coupled groundwater flow and transport model is used to simulate the movement of a conservative tracer in the Alamitos Barrier Project. The results of the simulations show that tracer travel times between the injection wells and the production wells typically exceed 60 years. The model results exhibit a high amount of numerical error when large time steps are used. Time-step sensitivity analysis indicates that a time step of one day or less will minimize model error. Further analysis of local head boundary conditions is recommended in order to provide a wider range of tracer travel times given variations in boundary hydraulic head levels.