UNSATURATED-ZONE CASE STUDY AT THE IDAHO NATIONAL ENGINEERING AND ENVIRONMENTAL LABORATORY: CAN DARCIAN HYDRAULIC PROPERTIES PREDICT CONTAMINANT MIGRATION?

Performance assessment of contaminated sites often requires the predictive extension in space and time of a limited set of contaminant or tracer detections. The quantitative models used for such predictions utilize hydrogeologic properties whose values are measured, estimated from experience, or calculated by an inverse model. Several decades of investigations of the 200-m thick unsaturated zone at the Idaho National Engineering and Environmental Laboratory (INEEL), composed of fractured basalt with interbedded sedimentary layers, provide a basis for testing the overall predictive process. A notable example is a 1999 field study in which a chemical tracer applied to INEEL infiltration basins during a period of high-volume input was detected over distances of at least 1.3 km within a few months. Horizontally extensive layers of rubble or intensely fractured basalt apparently constitute an effective conduit for this large-scale transport. Layers of interbedded fine sediment or dense basalt may establish the anisotropy needed to inhibit tracer loss from the unsaturated zone by downward flow. The dip of sedimentary interbeds with respect to the eight unsaturated-zone sampling wells where tracer was detected and the three where it was not suggests the driving force to be the component of gravity parallel to the interbeds, possibly augmented by pressure gradients associated with an effectively continuous hydraulic connection between standing water in the infiltration basins and the points of detection. The driving force of these indicates thresholds of hydraulic conductivity (K) and anisotropy that must be exceeded for the observed transport to take place according to Darcy’s law. We have compiled a large data set of measured unsaturated hydraulic properties of the sedimentary interbeds and have developed a statistical model to estimate these properties over a substantial portion of the INEEL. Assuming perfect lateral homogeneity, these values, with available estimates of basalt properties, barely meet the required K and anisotropy thresholds calculated from the tracer experiments. This comparison suggests that there must be remarkable uniformity in hydraulic properties over km-scale distances, or that mechanisms other than gravity-driven Darcian transport cause long-range contaminant migration.