Historical case analysis of uranium plume attenuation

Groundwater plumes containing dissolved uranium at levels above natural bac kground exist adjacent to uranium ore bodies, at uranium mines, milling loc ations, and at a number of explosive test facilities. Public health concern s require that some assessment of the potential for further plume movement in the future be made. Reaction-transport models, which might conceivably b e used to predict plume movement, require extensive data inputs that are of ten uncertain. Many of the site-specific inputs are physical parameters tha t can vary spatially and with time. Limitations in data availability and ac curacy means that reaction-transport predictions can rarely provide more th an order-of-magnitude bounding estimates of contaminant movement in the sub surface. A more direct means for establishing the limits of contaminant tra nsport is to examine actual plumes to determine if, collectively, they spre ad and attenuate in a reasonably consistent and characteristic fashion. Here a number of U plumes from ore bodies and contaminated sites were criti cally examined to identify characteristics of U plume movement The magnitud e of the original contaminant source, the geologic setting, and the hydrolo gic regime were rarely similar from site to site. Plumes also spanned a vas t range of ages, and no complete set of time-series plume analyses based on the spatial extent of U contamination exist for a particular site. Despite the accumulated uncertainties and variabilities, the plume data set gave a clear and reasonably consistent picture of U plume behavior. Specifically, uranium plumes: . Appear to reach steady-state, that is, they quit spreading, rapidly (with in a few years). . Exceed roughly 2 km in length only in special cases (e.g., where in situ leaching has been carried out). The majority are much smaller. . Exhibit very similar U chemistry between sites. This implies analogous co ntaminant attenuation mechanisms despite their location.