Characterization of Surface Soils at a Former Uranium Mill

J.A. Johnson; H.R. Meyer; M. Vidyasagar; and C.A. Little (MFG, Inc.)

A site soil characterization survey was conducted at the Dawn Mining Company uranium millsite in Eastern Washington during the summer and fall of 2004 to define the areal extent of soils that require remediation in order to meet the license termination requirements for Washington State. The survey consisted of a gamma scan and surface and subsurface soil sampling. The gamma scan was conducted using shielded NaI detectors mounted on an ATV and a backpack mounted detector, coupled to geographic positioning systems. Gamma exposure rate maps were generated from the data. Composite surface soil samples were obtained from approximately fifty pre-selected 10 x 10 meter grids with gamma exposure rates covering the range of interest. A correlation was established between the average shielded gamma exposure measurement on the grid and the Ra-226 concentration of the composite soil sample using linear regression techniques. Correlation equations were derived for the ATV mounted and backpack mounted systems for Ra-226 concentrations less than 15 pCi/g, the upper bound of the concentration range of interest. The R-square for the linear regression was approximately 0.8 for both systems indicating that 80 percent of the variability in the shielded exposure rate could be accounted for by the Ra-226 concentration. The correlation equations were used to establish a cut-off exposure rate that would indicate surface Ra-226 concentrations less than 5 pCi/g above background.

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