Optimal mitigation model for inland saltwater plume in coastal basins with seawater intrusion potential - case study: West Basin in Los Angeles County, California

Optimized hydraulic gradient control can be applied to cost-effectively remove contaminant plume from groundwater supply basins. When production of groundwater from coastal basins exceeds its natural and/or artificial replenishment, it results in landward movement of salt water. Seawater intrusion not only threatens to contaminate existing basin water but also prevents the use of extremely valuable underground storage in future years. The proposed study takes a seawater intrusion mitigation case where a line of recharge wells (barrier wells) were used to prevent seawater intrusion by providing sufficient hydraulic gradient to reverse the direction of seawater flow. The barrier wells, however, were aligned in such a way that an already intruded saltwater was “entrapped” and is moving landward with a combined gradient produced by the seawater barrier line and inland production wells.; The methodology developed identifies efficient strategies for the clean-up of the saltwater plume without causing further seawater intrusion, while meeting total water demand, minimizing the net increase in imported water demand and providing maximum drought and emergency supply capabilities. Optimal control of the system is obtained by applying multi-objective decision making theory where by the multi-objective problem is reduced to a single objective function made up of one of the listed objectives constrained by the rest. The cost of operation and maintenance is set to be the key objective.; Hydrogeologic parameters and protective heads at the seawater barrier wells, as well as preselected well location alternatives, beginning plume boundaries, and starting heads, are input to the model with the listed constraining conditions. System constraints include physical bounds, economic, operational and institutional requirements. A two-stage optimization is proposed to linearize the management problem. Linear programming is employed to select wells and determine the pumping schedules that most effectively stop migration the plume and ultimately remove it. Responses generated by groundwater simulation are assumed to be linear so the principle of superposition can be used. Drawdown is assumed small compared to the saturated thickness of the unconfined aquifer to maintain linearity. For the confined case, the transimissivity is presumed to be independent of pumping rates and therefore, the heads are linear functions of pumping rates.; The model will identify the optimal hydraulic gradient control and plume clean-up device under specified legislative, social and budgetary constraints. Outputs are recharge and extraction schedules, reclaimed and imported water amounts for a given management period. The model is tested using existing data for the West Coast area in the West Basin of Los Angeles County, California.