T IThe first detailed comprehensive simulation study to evaluate fate and transport of low-level, mixed, and transuranic wastes buried in the Subsurface Disposal Area (SDA) at the Idaho National Engineering and Environmental Laboratory (INEEL) has recently been conducted. The study took advantage of pertinent information relating to describing aqueous-and vapor-phase movement of contaminants in the primarily fractured basalt subsurface. The study included spatially and temporally variable infiltration, barometric pressure changes, positive down-hole air pressure during well drilling, vapor-vacuum extraction, and regional hydraulic gradients. Use of the TETRAD simulation code allowed all the pertinent information to be included into a single comprehensive model of the SDA subsurface. An overview of the model implementation and comparisons of calibrated model results to the observed vadose zone water distribution, volatile organic vapor concentrations, and aqueous concentrations of volatile organics and nitrate are presented. Additionally, comparisons between simulated and observed concentrations for other contaminants which were not used for model calibration are made.As part of this modeling exercise, inadequacies in the available data relating to characterization of nonsorbing aqueous-phase transport have been identified. Even with the identified data inadequacies, the comparisons between simulated and observed contaminants along with the calibration results give confidence that the model is a conservative representation of flow and transport in the subsurface at the SDA. The results from this modeling study are being used to guide additional data collection activities at the SDA for purposes of increasing confidence in the appropriateness of model predictions.

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A large-scale test of infiltration into a thick sequence of fractured Snake River Plain basalts was performed during the summer of 1994 on the Idaho National Engineering Laboratory. Monitoring of moisture and tracer movement during this test provided a set of quantitative measurements from which to obtain a field-scale hydrologic description of the fractured basalts. An inverse modeling study using these quantitative measurements was performed to obtain the representative hydrologic description. This report describes the results of the inverse modeling study and includes the background and motivation for conducting the infiltration test; a brief overview of the infiltration test; descriptions of the calibration targets chosen for the simulation study, the simulation model, and the model implementation; and the simulation results with comparisons to hydrologic and tracer breakthrough data obtained from the infiltration test.

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