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Section 4 <br />Pofentia/forRockfo be Toxic Forming <br />To examine the bioavailability of arsenic in development rock from Van 4 Mine, two <br />complementary tests were performed. First, the fine fraction from a development rock <br />sample (< 250 micron particle size) was used as substrate for an in vitro assay that <br />uses simulated stomach acid to estimate the bioavailable fraction of arsenic, and <br />second, these same sample was examined in the electron microprobe to determine the <br />types and amounts of different arsenic containing mineral forms. Additional <br />information regarding these laboratory methods is provided in Attachments 1 and 4. <br />The U.S. Environmental Protection Agency (EPA) Region 8 has developed a <br />bioavailability assessment protocol which uses the results of these two tests to <br />provide a quantitative estimate of bioavailability (the in vitro assay) and to show that <br />mineral forms present in the samples are consistent with the observed in vitro assay <br />results. EPA refers to this bioavailability protocol as a weight-of-evidence approach <br />(see Attachment 4). <br />Bioavailability data are shown in Table 4-2. Initially, a sample of Van 4 development <br />rock was sieved to obtain the 250 micron and smaller particle fraction and this fine <br />fraction was run in the in vitro assay. This test indicates that the arsenic in the sample <br />is only 64 percent bio available based on the in vitro assay. <br />A complete report on the results of the microprobe analyses is provided as <br />Attachment 1. In general, arsenic was found in development rock within iron sulfide <br />and uranium minerals at fairly high concentrations (up to 10,000 mg/kg). However, <br />because these minerals are rare, the overall concentration of arsenic is much lower (i.e. <br />not exceeding 45 mg/kg in any of the development rock samples collected. Most of <br />the iron sulfides are likely pyrite in crystalline forms. These forms are likely to be <br />stable in stomach fluids and thus have limited bioavailability. Some framboidal forms <br />of pyrite also exist, but are relatively less abundant. These forms could dissolve in <br />stomach fluid and thus arsenic associated with these forms would be available for <br />uptake into the body. In addition, some iron oxyhydroxides may be present which <br />would also be more soluble in stomach fluid than crystalline sulfides; arsenic <br />associated with these forms may also be available for absorption. <br />Dr. John Drexler with the Laboratory for Environmental and Geological Studies at the <br />University of Colorado maintains a data table used by EPA Region 8 as a "library" of <br />mineral forms and there relative stability in stomach fluid. Based on telephone <br />conversations with Dr. Drexler (CDM 2009b), the library contains no samples with <br />sulfides and no samples with uranium or vanadium minerals which could be used as <br />relative RBA comparison to the Van 4 mine data. Samples containing arsenic <br />associated with iron oxyhydroxides are represented in the library. A sample with 69 <br />percent of the arsenic in iron oxyhydroxides had an RBA value of 38 percent. A <br />sample with 40 percent of the arsenic in iron oxyhydroxides had an RBA value of 51 <br />percent. <br />The weight-of-evidence protocol, using in vitro assay, mircroprobe analyses to <br />determine mineralogy of arsenic in developmental rock, and comparison of values in <br />the EPA Region 8 library, appears to support an RBA value of 64 percent or less for <br />the Van 4 mine development rock. Much of the arsenic is associated with crystalline <br />4-5