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Whetstone, <br />Associates <br />Technical Memorandum <br />The top surface elevation of soil with radium -226 values at the 7 pCi/g threshold is considered to be <br />at the ground surface for 30 TR and DP series sample locations (Table 1). Concentrations at five <br />sample locations did not reach the threshold value (Table 1). <br />The bottom surface elevations of soil with radium -226 values at the 7 pCi /g threshold was estimated <br />for 8 TR- and DP -series sample locations using linear interpolation between soil sample depths <br />having known radium -226 analytical results. The bottom surface elevations for 22 TR- and DP- <br />series sample locations where elevated radium -226 analytical results extended to the bottom of the <br />excavation were estimated based on field observations (Table 1). Again. concentrations at five <br />sample locations did not reach the threshold value (Table 1). <br />The bottom elevation of soil containing radium -226 values above the 7 pCiig threshold for soil <br />observation locations (TR -OBS series) were estimated using a qualitative comparison of gamma <br />scan values to soil sample locations with known radium -226 analytical results (Table 1). <br />2.4 Estimation of the Surface of the Natural Alluvium <br />The surface of the natural alluvium was estimated using data from monitoring well logs. a calculated <br />unit gradient of the historical 1937 (pre - disturbed) Ralston Creek channel. and a boundary condition <br />(with an alluvial thickness of zero) at the margins of the valley floor. The approach assumes that the <br />historical stream channel occurred at the deepest point in the canyon and thinned to zero at the valley <br />boundaries. with control points obtained from the geologic contacts provided on monitoring well <br />logs. <br />The gradient of the historical 1937 Ralston Creek channel was calculated using the elevations at two <br />undisturbed points. above and below the re- routed stream reach. The slope between these two points <br />was distributed equally along the length of the historical channel. It was assumed that the height of <br />the alluvial overbank sediments was two feet above the stream channel elevation. Therefore. the <br />location points along the historical 1937 Ralston Creek channel were assigned an elevation of the <br />calculated 1937 channel elevation plus 2 feet to approximate the elevation of the natural alluvitmn on <br />the historical floodplain. <br />2.5 Estimation of Horizontal Extent of Above Threshold Soil <br />The horizontal extent of above - threshold material was estimated based on the five soil sample <br />locations that did not reach the radium -226 threshold value of 7 pCiIg (bottom of Table 1. Figure 1). <br />qualitative analysis of gamma scan results. and field observations. Two base zones of estimated <br />horizontal extent are shown outlined in yellow in Figure 1. <br />For calculation of a contingency soil volume. horizontal extent of areas where total depth of above <br />threshold soil is uncertain were estimated based on locations where above threshold radium -226 <br />values extended to the full depth of the soil sample excavation. Three contingency zones of <br />estimated horizontal extent are shown outlined in red in Figure 1. <br />2.6 Generation of Surfaces <br />Three grid surfaces were generated using Surfer from Golden Softwarr: a surface representing the <br />top surface of above - threshold materials (TopRa7pCiig). a surface representing the bottom surface <br />of above - threshold materials (BotRa7pCi'g). and a surface representing the approximate boundary <br />between natural alluvium and fill (AlatAlluviuun). The surfaces were each generated using the <br />4109C.110427 3 <br />36 <br />