Quantitative characterization of aquifer heterogeneity and simulation of contaminant transport in a solvent-contaminated aquifer

摘要

Quantitative characterization of hydraulic conductivities of aquifers is of fundamental importance to the study of groundwater flow and contaminant transport in aquifers. A conditional approach is used to represent the spatial variability of hydraulic conductivity. Briefly, it involves using qualitative and quantitative geologic borehole-log data to generate a three-dimensional (3D) hydraulic conductivity distribution, which is then adjusted through calibration of a 3D groundwater flow model using pumping-test data and historic hydraulic data. The approach consists of several steps. First, classify the lithological information obtained from geologic borehole-logs into representative texture categories; second, establish a quantitative correlation between laboratory measured corescale hydraulic conductivities and texture; third, generate a 3D hydraulic conductivity distribution using a genralized kernel-estimator method; fourth, upscale the core-scale hydraulic conductivity values such that the vertically averaged value at each location matches the field-scale value estimated from pumping tests; and fifth, use hydraulic data to calibrate the 3D field-scale distribution to account for regional-scale characteristics. The approach is applied to a trichloroethene (TCE) contaminated large-scale Superfund site. Based on the good agreement between simula.tions and observations, the results are considered reasonable and realistic. A number of nonideal processes and factors may contribute to the decreasing contaminant removal rate observed at the site. Most of the quantitative analyses of nonideal transport behavior have been conducted using data collected from column or small-scale field experiments. Studies extending such analyses to regional-scale contaminant transport are rare. In this study, a fully 3D transport model is developed to evaluate the effects of various processes/factors on the regional-scale nonideal TCE transport. Based on the analyses, it is found that while large-scale heterogeneity of hydraulic conductivity and ratelimited desorption have significant impacts on TCE transport and cause some nonideal behavior, their impact is not sufficient to account for the extensive tailing exhibited by the observed concentrations in the groundwater entering the treatment plant. Rate-limited dissolution of immiscible liquid appears to be the most likely primary cause of the extensive nonideal transport behavior observed at the site. The impact of the nonlinear sorption and the local-scale heterogeneity on TCE removal appears to be insignificant.