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this source. Probable leaching mechanisms include variable cycles of oxidation and formation of soluble <br />salts on waste rock surfaces, combined with seasonally fluctuating alluvial groundwater tables. <br />The degree of uranium mineralization present in mine waste rock and its location within the alluvial fill <br />may vary depending on original in -situ proximity to highly mineralized veins in the mine, and when <br />during the historic sequence of mine development the material was brought to the surface. Subsequent <br />construction of site facilities, as well as later remediation of areas involving the ore sorter and water <br />treatment ponds, likely mixed and redistributed mine waste rock materials. <br />The purpose of this study was to characterize current spatial distributions (both horizontal and vertical) of <br />radiologically elevated mine waste within the alluvial fill pad, and to assess remedial criteria and options <br />for disposal of this material in order to permanently mitigate associated impacts to water quality in <br />Ralston Creek. Investigation of other potential sources of impacts to water quality in Ralston Creek was <br />beyond the scope of this study. This project was approved by the Colorado Division of Reclamation, <br />Mining, and Safety (DRMS) under Technical Revision 14 (TR -14) to Mining Permit M- 1977 -300. <br />Methods <br />Field investigation methods were based primarily on depth profile sampling of alluvial fill materials in <br />test pits and open trenches along with shielded measurements of gamma radiation emissions from these <br />samples in an onsite field laboratory to provide quantitative estimates of subsurface distributions of <br />radium (Ra -226) and natural uranium (U -nat). Unshielded gamma scanning in the field was also <br />conducted as a qualitative, yet spatially comprehensive method to identify radiologically elevated <br />materials in -situ, guide selection of sampling locations and depth profile increments, augment direct <br />subsurface sampling information, and expand upon previously available information from a prior surface <br />gamma survey at the site (Tetra Tech, 2007). Specifics of these methods are detailed below. <br />Gamma Surveys <br />Conceptually, radiological site characterizations generally begin with surveys of the spatial distribution of <br />terrestrial sources of gamma radiation residing at or near the ground surface. Advanced GPS -based <br />gamma scanning systems with automated electronic data collection have become widely used in the field <br />for such surveys (Meyer et al., 2005a; Meyer et al., 2005b; Johnson et al., 2005; Whicker et al., 2008). <br />These systems are consistent with radiological survey guidelines outlined in MARSSIM (NRC, 2000). <br />Associated methods for application of this technology have been refined to meet various analytical <br />objectives and regulatory requirements (Whicker et al., 2008). <br />Cotter leased special software from Tetra Tech (Fort Collins, Colorado) to enable an automated gamma <br />scanning system. This system, comprised of a GPS receiver and NaI -based gamma detection equipment <br />(a Ludlum 44 -10 detector coupled to a Ludlum 23 -50 rate meter), was mounted on a backpack (Figure 1) <br />for walking surveys across areas of interest. The gamma detector was unshielded and positioned at about <br />3 feet above the ground surface. Up to 60 individual gamma readings and corresponding GPS <br />measurements per minute were recorded on a shoulder- harnessed mini - tablet PC, providing a detailed <br />record of gamma exposure rate conditions across scanned areas. <br />2 <br />