Joint work with Chongxuan Liu (Environmental Dynamics and Simulations, Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352; firstname.lastname@example.org)
Where groundwater has been contaminated with synthetic organic chemicals for an extended period of time, there is often a diffusion-driven contamination of adjacent impermeable zones, including aquitards above or below the aquifer region as well as regions within the porous medium (fine-grained layers, aggregated inclusions, or microporous primary grains). Spatial distributions of total contaminant concentration in such impermeable regions will reflect the temporal history of concentration conditions that have existed in the adjacent "mobile" phase (groundwater within the aquifer), as well as the sorption and diffusion properties of the impermeable zone(s). During aquifer remediation (such as by biologically induced transformation or by processes that rely on pore flushing), the reverse diffusion (desorption) from the impermeable regions can be an important controlling factor on overall rate. Diffusive transport under these circumstances can often be modeled as a process of molecular diffusion in pore water, coupled with an assumption of local sorption equilibrium to account for contaminant adsorption and partitioning to the adjacent solid phase. In this context, groundwater concentrations in the "mobile" aquifer act as time-variant boundary conditions for the immobile-zone diffusion.
Unfortunately, the historical conditions of contamination at many hazardous waste sites are rarely well known and the first view of subsurface conditions is often that which is obtained at the time when remediation or risk assessment is initiated. Under such circumstances, measurement of the "initial" spatial distribution of contaminants in immobile regions can provide critically important information, especially when combined with an independent assessment of the diffusion and sorption properties of the immobile zone. In some circumstances, such measurements can be used to solve an inverse problem (or a "forensic" interpretation) that makes inferences about the contamination history in the adjacent mobile phase. In all cases, the "initial" immobile concentration distribution is important to know as an initial condition for direct modeling of future diffusive flux, as relevant to the assessment of risk and the evaluation of alternative management actions.
The above points are illustrated by results obtained at a field-scale investigation of groundwater contamination and remediation at Dover AFB, DE. At this site, high-resolution core sampling (1 to 3 m horizontal spacing, 0.02- to 0.1-m vertical spacing) has allowed us to characterize the initial and post-pumping distributions of perchloroethylene (PCE) and trichloroethylene (TCE) contamination in a groundwater aquifer and its underlying aquitard. An important aspect of the problem is the existence of several clearly distinct geologic strata within the aquitard, with sharply different organic carbon contents and sorption properties. The field data (when used together with our independent characterization of heterogeneity and a layered diffusion model) provide a clear picture of diffusion-controlled contamination, in which the aquitard serves as both a long-term sink and source of contaminant to the aquifer. The presented paper will describe both the characterization efforts and the modeling-based interpretations, including both the "forensic" (inverse) interpretations as well as the direct forward modeling of anticipated fluxes during "clean-up."
Liu, C. and W.P. Ball, "Application of Inverse Methods to Contaminant Source Identification from Aquitard Diffusion Profiles at Dover AFB, DE,² Water Resources Research, 35(7): 1975-1985 (1999).
Liu, C., W.P. Ball, and J.H. Ellis, "An Analytical Solution to the One-Dimensional Solute Advection-Dispersion Equation in Multi-Layer Porous Media," Transport in Porous Media, 30: 25-43 (1998).
Liu, C., and W.P. Ball, "Analytical Modeling of Diffusion-Limited Contamination and Decontamination in a Two-Layer Porous Medium,² Advances in Water Resources, 24(4): 297-313 (1998). Ball, W.P., C. Liu, G. Xia and D. F. Young, "A Diffusion-Based Interpretation of Tetrachloroethene and Trichloroethene Concentration Profiles in a Groundwater Aquitard," Water Resources Research, 33(12): 2741-2758 (1997).
Ball, W.P., G. Xia, D.P. Durfee, R.D. Wilson, M.J. Brown and D.M. Mackay, ³Hot-Methanol Extraction for the Analysis of Volatile Organic Chemicals in Subsurface Core Samples from Dover AFB, DE,² Ground Water Monitoring and Remediation, 17(1): 104-121 (1997).
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