A preliminary assessment of the regional dispersivity of selected basalt flows at the Hanford Site, Washington, U.S.A

Dispersivity is one of the hydraulic parameters that controls the distribution in groundwater of chemical constituents migrating from a source region. Therefore, knowledge of the range of dispersivity values along likely flow paths from proposed high-level nuclear waste repositories is important for assessing how well a site will perform in limiting releases of radionuclides to the environment. A primary reason why dispersivity has not been analyzed with regional-scale data is the general lack of suitable environmental tracers that have been in the hydrologic environment for long periods of time. Such tracers could normally result from some natural event, perhaps disruptive, that may have transpired in the geologic past. Such an event may have occurred at the U.S. Department of Energy Hanford Site in the State of Washington, resulting in a chemical enclave of regional proportions. A preliminary interpretation is that the enclave occurs immediately down-gradient from a hydraulic barrier, possibly a fault, which may have placed deeper formations in hydraulic connection with the upper basalts. With this hypothesized source for constituents making up the enclave, the observed concentrations are employed in a preliminary attempt to assess the regional dispersivity. This is the single conceptual model being tested in this paper. The mathematical method employed assumes that the concentration data conform to what would be expected of a perfectly symmetrical enclave, and part of the problem deals with identifying that symmetry. The results obtained are quite reasonable when compared to the range in dispersivities determined in laboratory, tracer, and model-scale studies.