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A light-extinction principle was used to measure the fall speeds of the interfaces that develop during gravity sedimentation of monodisperse, bidisperse, and tridisperse suspensions of noncolloidal particles with small particle Reynolds numbers in the semidilute total particle volume fraction range 0.0003 5 co 5 0.15. For monodisperse suspensions, the hindered settling velocity of the interface at the top of the suspension was found to be well represented by the correlation of Richardson and Zaki, provided that the isolated particle fall speed was chosen by linear extrapolation of the data to c,, = 0, with 90% confidence intervals on the exponent of n = 5.0 k 0.1. For dilute monodisperse, bidisperse, and tridisperse suspensions, the hindered settling velocities of the interfaces showed agreement within experimental uncertainty with the theory of Batchelor, which predicts that the settling velocity decreases linearly with increasing particle concentration and which is based upon pairwise particle interactions.

Wright, 1992). As pore spaces become drier, air pathways become better connected and vapor can more eas-A conceptual model is developed to better understand vadose zone ily migrate under molecular diffusion or pressure driven vapor-phase diffusion within the mesas of the Pajarito Plateau at Los Alamos National Laboratory. We focus on 1,1,1-trichloroethane (TCA) flow. Rapid vapor transport can also be caused by large vapor transport from a liquid-waste disposal site. The conceptual model variations in barometric pressure (Auer et al., 1996) and incorporates several physical processes, including partitioning of TCA pressure gradients resulting from topographic features into the liquid phase, saturation-dependent diffusion, diffusion through (Weeks, 2001). asphalt, and interaction with the atmosphere. Three-dimensional nu-Several factors can act to reduce the ability of vapormerical simulations that use the conceptual model of TCA transport phase VOCs to migrate in the vadose zone. As pore spaces are then calibrated to pore-gas monitoring data. Adjustable paramebecome saturated, vapor-phase tortuosity increases and ters in the numerical simulations are limited to (i) the vapor-phase both the effective vapor-phase diffusion coefficient and diffusion coefficients for the different geologic units, asphalt cover, and the gas-phase permeability are reduced (Jury et al., 1991). land-atmosphere boundary layer and (ii) the fixed concentrations inVolatile organic compounds tend to have low solubility, the two shaft clusters. By including all of the components of the conceptual model in our numerical simulations we were able to achieve but some partitioning into vadose zone water occurs and a reasonable match between the simulated plume and site data for two can slow the migration of VOC vapor (Jury et al., 1990). alternate conceptual models of asphalt, one with asphalt as a diffusivePartitioning is often assumed to be an equilibrium phebarrier and one without asphalt as a diffusive barrier. A goodness-of-fit nomenon; however, Thomson et al. (1997) reported that analysis shows that the best-fit simulations are highly correlated to this may be an oversimplification of field conditions. data points from 21 boreholes. The simulations demonstrate thatAnother process that slows VOC migration is sorption diffusive behavior describes the general characteristics of the current onto mineral surfaces, which may or may not be reverssubsurface vapor plume. Effective vapor-phase diffusion coefficients ible. Slow desorption of VOC in the vadose zone can used in the simulations that best fit the data suggest that barometric lead to long-term sources that are extremely difficult to pumping is not contributing to diffusion in the deep vadose zone; howremediate. Finally, degradation of VOCs by both abiotic ever, it is likely that barometric pumping is occurring in fractures near the mesa edge. We conclude that asphalt is most likely acting as

field observations. To best develop and test conceptual models, supporting data should be derived using a num-The Pajarito Plateau in northern New Mexico, on which the Los ber of observational techniques and include a variety Alamos National Laboratory is situated, is characterized by a thick of data types. vadose zone overlying the regional aquifer of the western Espanola Basin. In this study, conceptual models of vadose zone flow and Los Alamos National Laboratory (the Laboratory or transport processes are presented and then supported through the LANL; Fig. 1) has performed research and development interpretation of field data, including synthesis with numerical models. in nuclear weapons technologies and other national de-The conceptual models differentiate the rate of percolation by their fense activities for more than 60 yr, beginning with the location and surface hydrologic setting, including wet and dry canyons, Manhattan Project in the 1940s. During this time, Laboand wet, dry, and disturbed mesas. Net infiltration beneath wet canratory operations have been accompanied by both disyons is the highest, with rates on the order of a meter per year posal of and intentional or accidental releases of chemi-(100-1000 mm yr Ϫ1 ). Transport to the regional aquifer beneath the cal contaminants into the environment at a variety of wettest canyons is likely on the order of several years to several sites. Contaminants with possible negative impacts to decades, depending on the thicknesses of the various hydrostratigroundwater include high explosives, radionuclides, chemgraphic layers. Perched water is sometimes found beneath wetter canyons and is associated with near-surface alluvial systems and at ical solvents, and metals. Today, the Laboratory is reintermediate depths along low-permeability interfaces such as buried sponsible for ensuring that none of its past contaminant soils or unfractured regions of basalt flows. Percolation through the releases pose a threat to human health now or in the volcanic tuffs is generally considered to be via matrix-dominated flow, future, and to carry out remediation activities to clean whereas fracture flow may play a key role in contaminant transport up contaminated sites. One of the key potential risks is through densely welded tuffs or basalt units beneath wet canyons.groundwater contamination, possibly affecting drinking Infiltration beneath dry canyons and dry mesas is much slower (10 water quality in municipal or private wells. Contaminants mm yr Ϫ1 or less), yielding transport times to the aquifer of hundreds must travel through a thick vadose zone to reach the to several thousands of years. However, long-term surface disturregional aquifer. Therefore, a well-developed conceptual bances at mesa-top locations may alter infiltration rates such that model describing vadose zone flow and transport beneath at a local scale, the infiltration rates temporarily approach those of wetter canyons.

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