RADON DISTRIBUTION IN THE VADOSE ZONE AND IN GROUNDWATER: TOOLS FOR HYDROGEOLOGIC CHARACTERIZATION IN DIVERSE GEOLOGIC SETTINGS

Radon occurrence and distribution at the 10⁰ to 10³ meter scale is generally poorly understood. Characterizing radon degassing in structurally controlled terranes like fractured bedrock, represents an opportunity for developing new and improved tools for hydrogeologic characterization. Our research discusses ongoing work at three test sites in different regions of the United States with diverse geologic settings. At Devens, MA, in a region underlain by igneous and metamorphic bedrock, detailed geologic mapping, surface and borehole geophysical surveys, synoptic water-level measurements, and chemical analyses of groundwater were done to identify large-scale fractures and other geologic features potentially relevant to groundwater flow. Numerous boreholes were drilled into the bedrock to confirm fractured-controlled flow pathways. In subsequent phases of ongoing work, arrays of radon detectors were installed in the vadose zone and groundwater samples were collected from several monitoring wells for radon analysis using liquid scintillation methods. These data are being compiled to provide additional validation of the location of fractures previously identified and to verify whether radon samples collected from the vadose and groundwater zones can be used as a potentially much less expensive means of identifying hydrogeologic pathways in suitable geologic settings. We are presently collecting similar data at a former uranium processing mill located on the Colorado Plateau in Arizona (sedimentary rocks), and a former uranium mining site located in Okanagan Highlands region (Columbia Plateau) of Washington state (igneous and metamorphic rocks). We will discuss preliminary findings, ongoing work, and future research.