Robert Ritzi (Advisor)
Master of Science (MS)
Ground water flow and contaminant transport patterns are largely controlled by the distribution of high- and low-permeability sediments. Therefore, an accurate description of the aquifer architecture is paramount to producing a representative ground water model. Models of contaminant fate and transport in the aquifer near Woburn, Massachusetts, have previously been created by others using a deterministic approach. As a complement to these prior studies, the proportions, geometry, and juxtaposition of the different lithofacies of the aquifer were statistically characterized for developing stochastic models for the aquifer system. The descriptions of lithology from boreholes were separated into eleven categories based primarily on grain size. Hydraulic conductivity values were available for some of the categories and their frequency distributions were analyzed. However, it was not possible to conclusively divide the categories into facies based on permeability because of the overlap in the values. As a result, three classifications (termed A, B, and C) were devised to explore the effect of different classifications. The classifications were designed to represent both the worst- and best-case scenarios with respect to the volumetric proportion of low-permeability facies. In each classification, the study area was divided into three sections: the northern section, the central section, and the southern section. The proportion of low-permeability facies was found to be highest in northern section and lowest in central section. The vertical range of the low-permeability facies was characterized using the transition probability models while the variogram model characterized the lateral range of the low-permeability facies. The results of the stochastic characterization were utilized with a sequential indicator simulator code to produce visualizations under each classification. Using previous results of Ritzi et al., (2000) from transport studies of contamination in simulations of areas similar to this study, an assessment of the likelihood of contamination still residing in the aquifer was made. Ritzi et al. (2000) found that residence time for contamination that encountered low-permeability facies was several orders several orders of magnitude greater than the residence time for contamination that did not encounter low-permeability facies. In addition, they demonstrated that the probability of contamination entering the low-permeability regions increased as the proportion of low-permeability facies increased. Using these results, an estimate of the probability that contamination encountered low-permeability regions was made for each section. The results indicate that the section with the highest proportion of low-permeability facies, the northern section, would be the most likely to still contain contamination while the central section, which had the lowest proportion, would have the least likelihood. Thus, the aquifer may appear to be clean while contamination still persists in small proportions in the low-permeability facies. Over time, contamination could leach out of the northern section and migrate toward the lower regions. Remediation would then have to continue indefinitely if contamination is to be fully removed from the area.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
Copyright 2006, all rights reserved. This open access ETD is published by Wright State University and OhioLINK.