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Vadose Zone Journal
Vol. 4, No: xx, 2005; Pages: 760–773

Vadose Zone Transport of 1,1,1-Trichloroethane: Conceptual Model Validation through Numerical Simulation

Philip H. Stauffer,* Kay H. Birdsell, Marc S. Witkowski, and John K. Hopkins

Geology, Hydrology, and Geochemistry Group EES-6, Earth and Environmental Sciences Division, LosAlamos National Laboratory, Los Alamos, NM 87545.


Aconceptual model is developed to better understand vadose zonevapor phase diffusion within the mesas of the Pajarito Plateau at LosAlamos National Laboratory. Wefocus on 1,1,1-trichloroethane (TCA)vapor transport from a liquid-waste disposal site. The conceptual modelincorporates several physical processes, including partitioning of TCAinto the liquid phase, saturation-dependent diffusion, diffusion through asphalt, and interaction with the atmosphere. Three-dimensional numerical simulations that use the conceptual model of TCA transportare then calibrated to pore-gas monitoring data. Adjustable parame- ters in the numerical simulations are limited to (i) the vapor-phasediffusion coefficients for the different geologic units, asphalt cover, andland–atmosphere boundary layer and (ii) the fixed concentrations inthe two shaft clusters. By including all of the components of the conceptual model in our numerical simulations we were able to achievea reasonable match between the simulated plume and site data for twoalternate conceptual models of asphalt, one with asphalt as a diffusivebarrier and one without asphalt as a diffusive barrier. A goodness-of-fitanalysis shows that the best-fit simulations are highly correlated to132 data points from 21 boreholes. The simulations demonstrate thatdiffusive behavior describes the general characteristics of the current subsurface vapor plume. Effective vapor-phase diffusion coefficientsused in the simulations that best fit the data suggest that barometricpumping is not contributing to diffusion in the deep vadose zone; however, it is likely that barometric pumping is occurring in fracturesnear the mesa edge. We conclude that asphalt is most likely acting asa barrier to diffusion at this site. The numerical simulations validatethat the conceptual model developed for this study is a useful toolfor analyzing TCA transport within the mesas of the Pajarito Plateau.

Keywords:1,1,1-trichloroethane;deep vadose zone;volatileorganic compounds;Vadose Zone Transport of 1,1,1-Trichloroethane;radionuclide.

Corresponding author:

E-mail: stauffer@lanl.gov


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