MEASUREMENT, MONITORING, AND MODELING ANALYSES OF A PROPOSED LOW-LEVEL NUCLEAR WASTE FACILITY IN TEXAS

Groundwater recharge is critical in evaluating performance assessment of nuclear waste sites. Our evaluation of a proposed low-level nuclear waste disposal facility in Eagle Flat, Texas, included assessment of water fluxes in the natural system to predict performance of the proposed facility. A wide range of geomorphic features were examined, including ephemeral stream, interstream, and earth-fissure settings. Soil physics, environmental tracers, and modeling were applied to better understand system performance. Water fluxes were related to geomorphic setting: low upward water fluxes in interstream settings, as shown by upward matric-potential gradients and chloride bulges, and higher downward water fluxes in ephemeral stream settings and fissured sediments, as shown by high matric potentials and low chloride concentrations. However, penetration depth of higher fluxes beneath fissures was restricted to the upper 10 m of the profiles. Modeling analyses indicate that sediments in interstream settings have been drying out since the Pleistocene, approximately 10,000 to 15,000 yr ago. Fissures have limited impact on water fluxes, both laterally (~10–20 m) and vertically (upper 10 m). Therefore, the natural system seems to be extremely suitable for waste disposal. Prototype engineered covers (both resistive and conductive) were installed to evaluate how performance of these covers would compare with that of the natural system. The cover system was much wetter than the natural system because of addition of water and rain during construction. The 4-year monitoring period showed the dynamic relationship between water availability and vegetation productivity and underscores the need for capturing two-way feedback between water availability and vegetation growth in performance models. The studies underscore the importance of evaluating unsaturated flow using physical, chemical, isotopic, and modeling analyses and the critical role of vegetation in controlling the water balance of these systems.