Groundwater Simulation-Optimization Research Under Variable-Density Conditions

The problem of seawater intrusion in coastal aquifers caused by groundwater overexploitation has been existing, and fazing local governments and residents in the southern part of Baldwin County, Alabama, USA since 1980s. With the growing of the population and development of the economy, the demand for ground water has been increasing continuously so that the problem of seawater intrusion has been getting worse day by day and the requirement of reasonable exploitation of ground water has been becoming more and more urgent in this area. This dissertation is focused on the study about the seawater intrusion modeling, the management of groundwater extraction in the study area, and the design and development of an all-purpose parallel groundwater simulation-optimization model under variable-density conditions by using a density-dependent groundwater modeling tool - SEAWAT2000, genetic algorithms and a parallel-computing technique. The main conclusions are as follows:(1) A three-dimensional transient finite-difference groundwater model under variable-density conditions - southern Baldwin seawater intrusion model is developed based on the theory of a transition zone between freshwater and saltwater, to simulate the seawater intrusion in coastal aquifers near the Gulf of Mexico in the southern part of Baldwin County, Alabama, USA. The new model is used to predict the dynamics and trend of seawater intrusion in coastal aquifers in the study area 40 years after 1996. The prediction shows that the situation of seawater intrusion in the future will become worse when all the conditions remain the same as those in 1996, which requires the management of local groundwater pumping wells so as to control the future dynamics and trend of seawater intrusion in the study area.(2) A general form of the objective function suitable for a wide variety of groundwater optimization management problems under variable-density conditions is presented and an all-purpose groundwater optimization management model under variable-density conditions is created.(3) A new all-purpose groundwater simulation-optimization model under variable-density conditions -SWTGA is developed through coupling the improved genetic algorithms (it will skip the evaluation of an individual the same as any individual evaluated before) with the density-dependent groundwater modeling tool based on the theory of a transition zone between freshwater and saltwater -SEAWAT2000.(4) A parallel SWTGA program is further developed by using the Message-Passing Interface - MPI. With this program, the all-purpose groundwater simulation-optimization model - SWTGA can be run in a parallel-computing system, which significantly improves the calculation speed of SWTGA.(5) The parallel SWTGA program is applied to optimize the pumping scheme of ground water in the study area. After the optimization, the optimal pumping scheme and the largest amount of water that can be extracted from coastal aquifers are obtained with the precondition that seawater intrusion is controlled to a certain degree, which can scientifically and technically support the decisions made for the management of groundwater exploitation in the study area.This study does not only construct reasonable and practical models for investigating the dynamics and rule of seawater intrusion in coastal aquifers and determining reasonable strategies of groundwater management in the study area, but also provide a reliable and efficient simulation-optimization tool for solving a wide variety of groundwater optimization management problems under variable-density conditions.