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Talk Abstract:
Diffusion-Limited Contamination and Decontamination in a Layered
Aquitard: Forensic and Predictive Analysis of Field Data
William
P. Ball
Department of Geography and Environmental Engineering
Johns Hopkins University
bball@jhu.edu
http://www.jhu.edu/~dogee/ball.html
Joint work with Chongxuan Liu
(Environmental Dynamics and Simulations, Environmental Molecular
Science Laboratory, Pacific Northwest National Laboratory, Richland,
WA 99352; chongxuan.liu@pnl.gov)
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."
Related Publications
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).
Back to Workshop Schedule
Confinement and Remediation of Environmental Hazards
1999-2000
Reactive Flow and Transport Phenomena
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