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 ...

First-order mass transfer models are commonly used as a means of interpreting sorption-related mass transfer in laboratory columns, often with the intent of approximating diffusion-based processes. We have fitted first-order model parameters to computer-simulated breakthrough curves from hypothetical column experiments in which Fickian diffusion into spherical particles limited the rate of sorption and desorption. Using both step and pulse inputs, we show that the fitted first-order coefficient is a function not only of the intrinsic diffusion rate, but also of the column length, the step experiment's duration, the input pulse width, the fluid velocity, and the solute retardation factor. For a range of typical column run conditions and a given diffusion rate, we show that the fitted first-order coefficient varies over three orders of magnitude in a manner roughly predictable through proper definition of a dimensionless timescale. In general, step inputs (as opposed to pulse inputs) provide a more consistent and predictable relationship between fitted coefficients and underlying diffusion rates. For either type of input, we recommend cautious use of the first-order model, since many observed variations in fitted rate constants are not the result of mechanistic phenomena. ß YOUNG

Diffusion coefficients in an aquitard material were measured by conducting miscible solute transport experiments through a specially constructed macropore column. Stainless steel HPLC columns were prepared in a manner that created an annular region of repacked aquitard material and a central core of medium-grained quartz sand. The column transport approach minimizes volatilization and sorption losses that can be problematic when measuring hydrophobic organic chemical diffusion with diffusion-cell methods or column-sectioning techniques. In the transport experiments, solutes (tritiated water, 1,2,4trichlorobenzene, and tetrachloroethene) were transported through the central core by convection and hydrodynamic dispersion and through the low-permeability annulus by radial diffusion. All transport parameters were independently measured except for the effective diffusion coefficient in the aquitard material, which was obtained by model fitting. Batch-determined retardation factors agreed very closely with moment-derived retardation factors determined from the column experiments, and no evidence of pore exclusion was found. A model with retarded diffusion was found to apply, and the effective tortuosity factor of the aquitard material was estimated at an average value of 5.1 (based on estimates that ranged from 3.7 to 7.7).

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