Browsing by Subject "Groundwater flow -- Mathematical models"
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Item A finite difference model for flow of a pollutant through unsaturated porus [i.e. porous] media(Texas Tech University, 1987-05) Acharya, Binayak PrasadThe purpose of this study was to develop a two dimensional finite difference model to estimate the solute concentration at different depths in various types of soils. The results obtained with the model will be used to determine the potential of possible groundwater contamination due to the flow of pollutants through unsaturated porous media. Three different soils were studied. Inputs into the model were soil physical properties, grid sizes of the domain, and a specified time increment. The soil physical properties include the relationships between moisture content, pressure, hydraulic conductivity, soil porosity, and soil field capacity. Solute concentrations were estimated at different depths within a domain of 10 feet by 10 feet using a grid size of one foot in both the horizontal (X) and vertical (Z) directions. During the testing of this model for Amarillo silt clay soil and Poudar river sand, the solute was shown to reach a depth of two feet from the soil surface within a time period of 45 days after a one day application period. The computation showed movement of moisture as well as solute through a depth of soil and in X direction for continuous application condition. For the fine sand soil the flow was much faster compared to the other two test soils. Solute moved to a depth of seven feet and horizontally for a distance of three feet along the X direction within a time period of 60 minutes. The estimates which were developed with the model showed that the flow of solute through unsaturated soil were dependent on the initial soil moisture content. A variation in solute concentrations at different depths of the soil with the initial moisture content was established. The computed results for the fine sand soil showed a little deviation from the type of results expected. Hence necessary modifications are suggested for better accuracy and flexibility of the model. It is also suggested to test the model for more different types of soil. Then the model can be used to estimate the potential threat to groundwater Zpollution for different areas.Item Development and validation of a system identification methodology for the characterization of contaminated sites(Texas Tech University, 1998-12) Jayakody, Kankanamalage Geethani KumariThe quality of groundwater has become a major concern in the United States and many other industrialized countries since the discovery of numerous sites with contamination from hazardous wastes and leaked fuels. Investigation and monitoring have begun at many of these sites, but execution of remedial plans has often been delayed due to regulatory and financial constraints as well as limited understanding of the processes that control the distribution of contaminants in the subsurface. One of the primary difficulties encountered in the site remediation process is the inability to determine the site heterogeneity in an adequate manner. In this research, a mathematical technique known as System Identification Methodology (SID) is used in conjunction with the flow and contaminant transport equations to address the above problem. In this mathematical procedure, a finite volume formulation with an upwind velocity scheme was used to discretize the flow and transport equations. In order to determine site heterogeneity, independent contaminant transport parameters were assigned to each rectangular element in the flow domain. The finite volume formulations of groundwater flow and contaminant transport equations were then reorganized into the standard state-space form that is commonly used in system identification procedures. The determination of the distribution of unknown parameters at the site was then accomplished by minimizing the error between both measured contaminant concentration and hydraulic head and calculated contaminant concentration and hydraulic head by the above finite volume models for groundwater flow and contaminant transport. The Levenberg-Marquardt algorithm was used as the optimization scheme. Once the mathematical model was developed, test cases were run to verify the mathematical accuracy of the model. Sensitivity analyses were performed to determine the relative significance of the heterogeneities in dispersion, retardation, and decay terms on contaminant flow. The model was validated by applying it to actual observations from four selected case studies. As the first step in the validation, the SID methodology was applied to contaminant concentration data obtained from large-scale sand tank tests. Subsequently field validation was accomplished by applying the SID methodology to site in the Southern High Plains of Texas, Jordan aquifer in Iowa and landfill site at Borden.Item Verification of the validity of Darcy's law for unsaturated media using finite difference method(Texas Tech University, 1987-12) Nagaich, SharadNot available