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Characterization Results <br />Gamma Survey Results <br />Gamma surveys are highly effective for delineating the distributions and relative magnitudes of Ra -226 <br />concentrations in soils, mine wastes, or other solid materials residing at or near the ground surface. At <br />many sites, including the Schwartzwalder Mine, Ra -226 concentrations are strongly correlated with <br />uranium concentrations. A gamma survey of the surface of the alluvial fill pad was conducted in 2007 <br />(Tetra Tech, 2007) and additional surface gamma surveys were conducted by Cotter in the fall of 2010 to <br />verify earlier results and assess any significant changes due to site activities since 2007. <br />Based on the gamma survey data, a kriging program in ArcGIS® (ESRI, 2008) was used to develop <br />continuous estimates of gamma exposure rates across the alluvial fill pad. Kriging is a geostatistical <br />interpolation procedure that fits a mathematical function to a specified number of nearest points within a <br />defined radius to determine an output value for each location. A given "location" is represented by a cell <br />of specified areal dimensions that may or may not include any measured data points. Values closer to the <br />cell are given more weight than values further away, and distances, directions, and overall variability in <br />the data set are all considered in the predictive semivariogram model. Kriging is a useful interpretive tool <br />that can help improve understanding of overall survey results. A kriged rendering of gamma survey data <br />across the alluvial fill pad is shown in Figure 6. <br />One finding of this study is that subsurface source term materials across the alluvial fill pad are not well <br />correlated with gamma readings at the surface because a 1 -2 foot layer of clean fill from the adjacent <br />Black Forest Mine had once been used as cover, presumably to shield gamma radiation and help to reduce <br />the potential for erosion and migration of radiologically elevated materials. Exceptions are apparent near <br />the current concrete ponds and in areas associated with the former lined overflow pond and ore <br />sorter /crusher which were previously excavated to a soil Ra -226 cleanup criterion of 5- pCi /g. <br />8 <br />