A diffusion‐based interpretation of tetrachloroethene and trichloroethene concentration profiles in a groundwater aquitard

Abstract

Abstract. Analysis of subsurface soil cores from the site of a field-scale groundwater remediation experiment at Dover Air Force Base, Delaware, has revealed that tetrachloroethene (PCE) and trichloroethene (TCE) contamination extends into an aquitard underlying a groundwater aquifer. The site location is well doWngradient of the locations of contaminant release, and the aquitard contamination is believed to have begun when contaminated groundwater first arrived in the overlying aquifer. Using independent estimates of sorption and diffusion properties in the aquitard layers, mathematical modeling based on diffusion in laminate slabs has been used to make inferences regarding the historical concentration conditions in the overlying aquifer. The results suggest that plume arrival occurred within the last two decades, with some important differences in the inferred TCE and PCE plume histories. The diffusion model was also applied toward predicting future aquitard concentrations and fluxes under scenarios based on the current condition as a starting point and hypothesized conditions of future groundwater cleanup. The results demonstrate how aquitard sampling and diffusion modeling can provide essential information relevant to forensic analysis, risk assessment, and subsurface cleanup. IntroductionClayey subsurface deposits are often of sufficiently low hydraulic conductivity that molecular diffusion is the dominant process of contaminant transport under many typically encountered conditions of hydraulic gradient [Barone el al., 1992; Johnson el al., 1989; Shackelford and Daniel, 1991]. As a consequence, the contamination of clay and silt aquitards underlying polluted aquifers may proceed primarily by this process, and subsequent remediation will be correspondingly slow. Investigations of contaminant diffusion in aquitards can help us to understand the effect of this rate-limiting process on remediation time and can also provide important clues regarding the contamination history of the overlying groundwater.Organic contaminant diffusion i n low-permeability materials has been studied previously iri the context of clay landfill liners or cutoff barriers, using both laboratory experiments [Barone el al., 1992; Molt and Weber, 1991] and field diffusion profiles [Johnson el al., 1989]. Johnson el al. [1989] have reviewed some of the literature related to solute diffusion through clay liners and other low-permeability materials underlying landfill sites and provide compelling evidence that simple Fickian diffusion can be an important mechanism of vertical contaminant transport in many field situations. These authors were among the first to investigate in situ diffusive transport through field material, which they accomplished by means of pore-water analyses from squeezed sections of core taken from directly below a 5-year-old landfill. Results were successfully modeled on the basis of the known exposure time, using the diffusion coefficient as a fitting parameter. Diffusion coefficients of organic Copyright 1997 by the ...

Application of inverse methods to contaminant source identification from aquitard diffusion profiles at Dover AFB, Delaware

“…These authors reported an average tortuosity value 3.67 times higher than that assumed in our prior work, and note that tortuosity values may not be known to better than a factor of 4 without specific evaluation. Because of remaining uncertainties regarding the applicability of repacked column results to our field situation, and more important, for better direct comparison with our prior two-step modeling results [Ball et al, 1997a], we use our prior tortuosity estimates (X = 1.4) in this work. In this regard, we note that the time associated with any particular forensically estimated boundary concentration will scale inversely with effective diffusivity, i.e., as would be associated with constant values of Dt/R (D is the diffusion coefficient, t is time, and R is the retardation factor) for any given length scale of diffusion.…”

“…Our current paper also reports on additional field sampling results that were unavailable at the time of our prior work [Ball et al, 1997a]. More specifically, new coring data have been obtained from several nearby locations [Mackay et al, 1997;Ball et al, 1998], now allowing us to provide comparative estimation results by applying a forensic model to both the prior and new data sets.…”

“…An example of such a situation is our prior analysis of data from Dover Air Force Base (DAFB), Delaware [Ball et al, 1997a], in which the contamination history was assumed to have followed a certain number of step changes in concentration. In such cases, only a limited number of parameters for a known (or assumed) functional form are to be estimated.…”

“…Recently, we conducted an analysis in which we assumed that a timevariant boundary concentration (i.e., source function) for the DAFB contaminated aquitard could be approximated as a step function of time [Ball et al, 1997a]. In that work, we applied error-minimizing parameter estimation techniques together with previously developed analytical solutions ] in order to estimate the step values and their active durations.…”

“…Our analysis uses field-measured concentration profiles in lowpermeability porous media that underlie a contaminated aquifer at DAFB. We have previously analyzed some of the data from this site using a function-fitting approach, in which the source functions (boundary concentrations for the contaminated aquitard) were assumed to be step functions of known shape [Ball et al, 1997a]. In this prior effort, the values and active durations of the step changes at the boundary were estimated by minimizing summed absolute values of the errors between observed and calculated concentrations.…”