GIS-based three-dimensional groundwater flow and microbial transport modeling for an artificial bank filtration site in El Paso, Texas.

摘要

This thesis is composed of four papers. The first paper is a characterization of the bank filtration study site to determine the hydrostartigraphic units and hydrogeologic conditions. Hydrological data from aquifer test and multiwell artificial tracers (bromide and microspheres) test carried out in the site were recorded including drawdown levels, hydraulic conductivity, transmissivity, ground water traveling times, and breakthrough curves. Horizontal hydraulic conductivity of the sand for the model layers ranged from 0.17 to 27.28 meters per day. Vertical hydraulic conductivity ranged from 0.17 to 2.48 meters per day. The site characterization revealed that even in an aquifer composed almost entirely of sand, flow heterogeneity is pronounced. Twelve observation wells and one production well were constructed near the artificial stream to provide geologic and hydrologic information. Lithologic samples were obtained from all boreholes during well construction work using hollow-stem augurs. The soil samples have been analyzed for grain size analysis, porosities, and hydraulic conductivity. The aquifer portion in the study area is composed almost entirely of fine to coarse-grained unconsolidated sediments of sand, deposited in an ancestral channel of the Rio Grande and is in hydraulic connection with the artificial stream.; The second paper aimed at predicting the movement of contaminants in groundwater during bank filtration. The 17.8 day-tracer test provided valuable results that are relevant to the transport of pathogens through the subsurface under riverbank filtration conditions. Our main objectives were to compare the degree of in situ bacteria and protozoa immobilization, apparent dispersion, and retardation to assess the suitability of microspheres as abiotic analogs in future investigations involving the physical aspects of bacteria and protozoa transport behavior and to apply simplified analytical methods to determine dispersion parameters in a three-dimensional dispersion field. Transport velocities were evaluated based on appearance of the first pulse of the tracer plume detected in sampling wells at various distances from the injection points.; In the third paper, a GIS-based three-dimensional finite difference groundwater flow model was developed using MODFLOW under GMS to simulate ground-water flow, quantify hydraulic properties of the riverbed and aquifer material, and to estimate the quantity of river water entering the aquifer. The objective was to better understand ground-water flow processes, contaminant transport, filtration processes, and surface-water/ground-water interactions on the bank filtration site.; In the fourth paper, the ground-water flow model was the base for a particle tracing model using the code MODPATH to analyze the migration processes and to evaluate the transport of infiltrated stream water into the nearby aquifer. The models simulated the flow path (pathlines), travel times, and extension of riverbank infiltration (influence zones of stream water) in addition to identifying the contaminants' transport pathways. These parameters were determined to characterize the interactions between water in the stream and the aquifer. (Abstract shortened by UMI.)