To predict contaminant migration, implement remediation strategies or design a disposal facility, the transport properties must be known for the units beneath the site. The single most important transport property is the hydraulic conductivity which is a non-linear function of the water content. Traditionally, long times are required to attain hydraulic steady-state, but the UFA achieves steady-state in hours using an adjustable, whole-body driving force together with precision flow (Conca, J. L. and J. V. Wright. 1992. Applied Hydrogeology 1:5-24.).
Case Study: Hydrostratigraphic Mapping
Subsurface mapping involves knowing the spatial distribution of particular physical or chemical properties. In the study area, measurements of hydraulic conductivity using the UFA Method allowed detailed spatial mapping of the permeability, a hydrostratigraphic map, beneath the site which replaced a traditional layer-cake geology that did not provide enough detailed permeability information for targeting clean-up activities. The Plutonium Finishing Plant in the 200-West Area at the Hanford Site is the site of a mixed-waste contaminant plume containing carbon tetrachloride (CCl4) with plutonium (Pu) and americium (Am) (Last, G., and V. Rohay.1993. Technical Report PNL-8597, Pacific Northwest Laboratory, Richland, WA). The water table at this site is approximately 73 m (240 ft) below the surface. The overlying vadose zone consists of unconsolidated clastic sediments of poorly sorted glacio-fluvial gravel, sand, and silt (designated as the Hanford formation). Beneath it lie semi-continuous layers of loess, paleosols, and low-permeability paleoplaya lake deposits that have developed extensive caliche (grouped as Early Palouse and Plio-Pleistocene units). These sediments have a variety of water contents and a wide range of field hydrologic properties with respect to water and organic liquids. Previously, traditional layer-cake geology was used to define the hydrostratigraphy (Figure 1), with a single hydraulic behavior for each unit inferred from sediment type and a few saturated hydraulic conductivity measurements. Figure 1 is a projection onto a NE-SW trending vertical plane. Perspective is given by the apparent width of each borehole. Shown are the borehole sample positions and boundaries between different sediment units and subunits. The inset shows the generalized regions of hydraulic behavior roughly correlated with gravel and sands, silty-sand, sandy-silt, and fine silts and clays moving from left to right in the inset and describes the hydraulic behavior of the units of the same shading shown in the cross-section. However, different samples which appear to be similar in appearance, grain-size distribution and other characteristics can have very different transport behaviors. The hydraulic conductivity data provided by the UFA Method allow the subsurface to be mapped with respect to the hydrologic properties of the sediments which is more relevant to predictive modelling, conceptual test plans and actual behavior than sediment type. Figure 2 gives results from one of the boreholes directly beneath Z-18 Crib. Each curve has from eight to twelve independent, unsaturated hydraulic conductivity measurements obtained in three days using the UFA Method.
These hydraulic conductivities can now be assigned the shading of the region from the inset in Figure 1 in which they fall, generating a true hydro-stratigraphic map (Figure 3). This map provides direct information of subsurface transport behavior and shows sufficient detail for developing a defensible restoration strategy and predicting contaminant transport. Compared to Figure 1, Figure 3 illustrates that the Early Palouse unit is not
homogeneous as is indicated in hand specimen and by grain-size distribution, but has two
completely different behaviors.
Perched water was found on top of the right-most subunit which would not have been possible using the inferred hydro-stratigraphy from Figure 1. The Hanford
Fine subunit is also seen to be similar to the Hanford Coarse subunit only with local areas of
different behavior. Hydrostratigraphy with the detail obtainable with the UFA provides answers
to many more questions about the subsurface than can be answered traditionally.
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