Regional geohydrologic considerations in LLW disposal site selection and characterization

In implementation of the Low-level Radioactive Waste Policy Act of 1980, site selection studies for shallow land burial facilities for LLW are needed in geologically very diverse areas of the United States. The basic geohydrologic goals for location of a LLW burial site are (1) that it be in a geologically simple, stable and predictable setting and (2) that subsurface migration of wastes via groundwater be minimized and predictable. The first goal directs the site search toward thick, stable sedimentary formations with simple stratigraphy and homogeneous materials so that site characterization and modeling can be achieved with confidence. The second goal indicates that the burial unit should be substantially above the watertable, or if it is below the watertable, it should have extremely low permeability and good sorptive properties so that migration times are long relative to radioactive decay. In the humid eastern and midwestern sections of the United States, saturated soil conditions (perched or watertable) are common at shallow depths below the surface. Thus units with low permeability, overlying well-defined bedrock conditions, are most promising. In the more arid west and southwest, unsaturated surface formations with great depth to the watertable are more likely to be found. Under such conditions, significantly more permeable units can prove acceptable for burial, and bedrock structure become less critical when the bedrock is separated from the burial unit by thick unsaturated strata