James L. Conca

Diffusion And Flow In Gravel, Soil, And Whole Rock

Transport parameters (diffusion coefficients, D(θ), hydraulic conductivities, K(θ), and retardation factors, Rf were experimentally determined in unsaturated soil, gravel, bentonite, and whole rock over a wide range of water contents, fixed at desired levels using the Unsaturated Flow Apparatus (UFATM). Effective diffusion coefficients in all media were found primarily to be a function of volumeric water cintent (θ) and not material characteristics, except where the characteristics affect or determine water content. At high water contents, D(θ) gradually declines as water content decreases, from 10-5cm2/s at a θ of about 50% to 10-7cm2/s at a θ of about 5%, followed by a sharp decline as surface films become thin and discontinuous, and pendular water elements become very small, from 10-7cm2/s at a θ of about 5% to 10-10cm2/s at a θ of about 0,5%. The several whole rock cores studied behaved similary. In aggregate material such as gravel and soil where the particles themselves have significant porosity, only the surface water content, not the internal water of the particles, contributes to the diffusion coefficient and hydraulic conductivity under unsaturated conditions, although the internal water is very important in retardation and other chemical effects. Experimentally determined K(θ) compares favorably to van Genuchten/Mualem relationships calculated from laboratory-determined water retention versus matric potential data obtained on the same soils. Experimentally determined K(θ) for whole rock appears to validate capillary bundle theory.

Evaluation of Van Genuchten–Mualem Relationships to Estimate Unsaturated Hydraulic Conductivity at Low Water Contents

Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities, K, were experimentally determined as a function of volumetric moisture content, θ, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure K(θ) in the laboratory for 22 soil samples. The van Genuchten model was used to fit mathematical functions to the laboratory-measured moisture retention data. Unsaturated conductivities estimated by the van Genuchten–Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity, Ks, were compared to those obtained by a scaled-predictive method that uses a single K(θ) measurement as a match point near the dry regime. In general, the measured K values and those predicted from van Genuchten–Mualem relationships showed considerable disagreement. This suggests that the use of laboratory-measured Ks results in an inadequate characterization of K(θ) for the desired range of moisture content. Deviations between the measured and predicted K were particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at low θ. However, use of the same moisture retention curve-fitting parameters and a single steady state head control-based K(θ) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse-textured soils considered in this study, results indicate that a K∥θ) measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low θ. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities.

Aqueous Diffusion in Repository and Backfill Environments

Aqueous diffusion coefficients have been experimentally determined in a variety of porous/fractured geologic and engineered media. For performance assessment applications, the purely diffusive flux must be separated from retardation effects. The simple diffusion coefficient, D, does not include any transient chemical effects, e.g., sorption, which lower the diffusion coefficient for some finite time period until equilibrium is reached. D is primarily a function of volumetric water content, {theta}, and not material characteristics. At high water contents, D gradually declines as water content decreases, from 10{sup -5} cm{sup 2}/sec at {theta} {approximately}0.5%. Although surface diffusion has a strong experimental basis in the transport of gases along metal surfaces experimental evidence for aqueous geologic/backfill/engineered systems strongly indicates that surface diffusion is not important, even in bentonite, because of the extremely poor connectivity among electric double-layers and the extremely low diffusivities and high {partial_derivative}C/{partial_derivative}x at small area/point contacts which more than negate the increased flux along intragrain surfaces.

Dealing with Uncertainty in the Chemical Environment in Bentonite Backfill

Analytical and conceptual deficiencies in understanding compositional variability in the smectite clays are expected to generate uncertainty in models used to simulate the chemical environment in bentonite backfill. Equilibrium models accounting for nonstoichiometry in smectite can nevertheless bound ranges in aqueous solution compositions that are an explicit function of the uncertainty in smectite compositions. In one approach, we quantify uncertainty in terms of ranges in concentrations of octahedral and tetrahedral Al, and exchange-site cations and vacancies. Heterogeneous mass transfer in bentonite-water systems is modeled using conventional mass-action relations and standard Gibbs energies for stoichiometric minerals, and the site-occupancy constraints combined with site-mixing relations for smectite. The resultant bounding conditions in groundwater compositions may be large or small depending on which aqueous species are of interest in a given situation, but they are valid irrespective of whether equilibrium in smectite-water reaction is attained or is inhibited by slow intracrystalline reaction rates.

Direct determination of transport parameters in repository materials

A new flow technology has been developed that significantly decreases the time required to obtain transport data on saturated and unsaturated porous/fractured media. This technique is based on open-flow centrifugation and was developed to measure steady-state transport properties in most geologic materials within a matter of hours. Centripetal acceleration does not induce artificial effects in samples i.e., fracturing, collapse of interlayer structures, structural dewatering, compaction, chemical changes, etc., that occur with high-pressure methods. Using this technique, hydraulic conductivities (K) and diffusion coefficients (D) for compacted bentonite and four host rocks have been measured and re-interpreted. Based on these new data, K for compacted bentonite is less than 10{sup -14} m/s, a factor of 1000 lower than previous pressure-gradient measurements, providing further assurance that radionuclide transport through bentonite backfill will be diffusion limited. Measured K for mudstone (1.8 {times} 10{sup -12} m/s) indicates diffusion-limited far-field transport, while advective transport should occur for granite, basalt, and tuff, with expected matrix diffusion coefficients (correlated to measured D values) of 8.3 {times} 10{sup -13} and 2.5 {times} 10{sup -12} m{sup 2}/s for fractured granite and basalt, respectively.