| Injection well barriers are a common means of mitigating seawater intrusion where freshwater is injected underground to build up local water pressure adjacent to the coastline. This dissertation investigates the seawater intrusion problem at one of three barriers operating in Los Angeles County, the Alamitos Barrier Project. A three-dimensional finite element general-purpose model is adopted to solve the coupled groundwater flow and mass transport equations. A brief validation phase is conducted comparing simulation model output with a published semianalytical solution of the modified Henry's problem. Observation well data from 1992 to 2002 are used to build the model, develop initial and boundary conditions, produce heterogeneous hydraulic conductivity fields for each aquifer, and calibrate the dominant transport parameters. A modified kriging technique called natural-neighbor-kriging is applied to estimate the heterogeneous conductivity field for each aquifer. Next, two management problems are investigated using the calibrated simulation model. In the first management problem the simulation model is linked with a continuous gradient-based optimization solver, MINOS, to minimize the amount of injected water subject to nonlinear constraints on head and concentration. The simulation-optimization model is compared with concurrent operation over the calibration period-of-record and results showed that model performance is superior when the number of decision variables is limited to ten wells or less. In the second management problem the calibrated simulation model is linked with a genetic algorithm (GA) to identify optimal locations for new barrier wells. The GA selects locations for new wells and calls the simulation model to evaluate the performance of the new injection well configuration quantified with one of two fitness functions considered, one hydraulically based and one transport based. Results infer that the transport criteria resulted in superior well placement in terms of preventing future chloride seepage. Finally, the impact of installing a physical barrier parallel to the coast is examined by model modification and further simulation. The physical barrier, a bentonite clay grout curtain, is represented by simply reducing the hydraulic conductivity of elements in the topmost R Aquifer. Results show that the impacts to the state variables are minimal, and the maximum variation is concentrated near the center of the physical barrier with virtually no impact predicted at the hydraulic barrier. |
