Vadose water flow around a backfilled drift located in tuff

Through the technique of computer simulation, the authors investigate the flow characteristics of groundwater from surface influx into a host medium of unsaturated tuff. The study is designed to assess the potential influence of backfilled drifts on the groundwater flow past vertically emplaced waste canisters in a prospective nuclear waste repository. Numerical modeling with the code SAGUARO is used to determine the magnitude and direction of flow in the vicinity of a waste package below a drift backfilled with various materials. Sand and clay represent potential backfill materials which are significantly different in hydrologic properties. Results indicate that clay in a drift reduces the flow immediately adjacent to the waste package to 91 to 96% of the natural flow. Sand in the same drift acts as an extremely effective barrier to flow; however, below the drift at the waste package level, the groundwater flow is 81% to 92% of the natural volumetric flow. Therefore, backfilling a drift does not provide a significant reduction of flow in the vicinity of a vertically emplaced waste package. The effect of the drift on flow extends only slightly over a drift length below and a drift width to the side of the backfilled region. This study is part of the Nevada Nuclear Waste Storage Investigations