DIRECT MEASUREMENT OF UNSATURATED TRANSPORT PROPERTIES USING THE UNSATURATED FLOW APPARATUS

Direct measurements of transport properties can be made using the UFA (Unsaturated Flow Apparatus, ASTM D6527) which consists of open-flow centrifugation with precision, non-pulsating flow to fix both flux and driving force within a sample, allowing complete control of Darcy's Law. Centrifugation, like gravity, is a whole-body force and results in similar mechanisms of flow within the sample, unlike the surface forces of a pressure gradient. Use of acceleration allows uncoupling of the matric potential from the Darcy driving force, without adverse effects in the sample, allowing attainment of steady-state in most geologic materials in hours and direct measurment of hydraulic conductivity, diffusion coefficient, and retardation factors. The normal operating range is about 10-4 cm/s (10-1 darcy; 10-9 cm2)] down to about 10-14 cm/s (10-8 darcy; 10-16 cm2), between -20 to 150°C, and flow down to 0.0001 mL/h. Comparisons of unsaturated hydraulic conductivity results among the UFA method, steady-state head control methods, and van Genuchten/Mualem estimation techniques agreed very well. A case study is presented where the UFA was used to obtain data for subsurface hydrostratigraphic mapping and recharge distribution at a mixed waste site having chaotic stratigraphy. The Plutonium Finishing Plant at the Hanford Site in Washington State is a semi-arid shrub steppe with a deep vadose zone made up of various aridisols, paleosols, and sediments. The UFA Method was used to characterize the unsaturated hydraulic conductivity and other transport behavior in 60 split-spoon borehole samples. Each unsatK curve has 8 to12 independent, steady-state measurements. In addition, grain-size distributions, mineralogical analyses and porewater extraction using the UFA were obtained for each sample from each borehole. The total time required to obtain the over 700 steady-state hydraulic conductivity measurements was only 6 months using the UFA. Together with well logs and other field and geophysical information, these data were used to assemble subsurface hydrostratigraphic and recharge maps that showed the spatial variation in subsurface recharge as well as the direction and extent of the plume even after the primary VOC constituents were gone or degraded.