2009-05-19_REVISION - M1981021 (32)

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Section 4 Potential for Rock to be Toxic-Forming The bioavailability estimate for one sample, SVWR-1_MAG, was 136 percent, likely reflecting difficulties with precision for low levels of arsenic. Bioavailabihty for the whole soil for this sample was 63 percent, the highest for any of the re-run samples. For another sample, SJWR-1- NON, arsenic was not detected in the simulated stomach fluid and no bioavailability estimate could be estimated. Lack of detection of arsenic in this sample suggests low bioavailability. In the companion sample, SJWR-I-MAG, bioavailability was low (27 percent) and thus consistent with this conclusion. Overall, the highest bioavailability not subject to methodological problems that was observed in the re-analysis of development rock samples from Sunday Group mines was 63 percent. This value is considered a conservative estimate of arsenic bioavailability. The use of this value for arsenic in development rock is further discussed below using the results of the electron microprobe analysis. A complete report on the results of the microprobe analysis is provided as Attachment 1. In general, arsenic was found in development rock samples within iron sulfide and uranium minerals at fairly high concentrations (up to 10,000 mg/kg). However, because these minerals are rare, the overall concentration of arsenic is much lower - not exceeding 45 mg/kg in any of the development rock samples collected. Most of the iron sulfides are likely pyrite in crystalline forms. These forms are likely to be stable in stomach fluids and thus have limited bioavailability. Some framboidal forms of pyrite also exist, but are relatively less abundant. These forms could dissolve in stomach fluid and thus arsenic associated with these forms would be available for uptake into the body. In addition, some iron oxyhydroxides which • would also be more soluble in stomach fluid than crystalline sulfides; arsenic associated with these forms may also be available for absorption. Dr. John Drexler with the Laboratory for Environmental and Geological Studies at the University of Colorado maintains a data table used by EPA Region 8 as a "library" of mineral forms and there relative stability in stomach fluid. Based on telephone conversations with Dr. Drexler (CDM 2009b), the library contains no samples with sulfides and no samples with uranium or vanadium minerals which could be used as relative RBA comparison to the Sunday Mines Group data. Samples containing arsenic associated with iron oxyhydroxides are represented in the table. A sample with 69 percent of the arsenic in iron oxyhydroxides had an RBA value of 38 percent. A sample with 40 percent of the arsenic in iron oxyhydroxides had an RBA value of 51 percent. The weight-of-evidence protocol, using in vitro assay, mircroprobe analyses to determine mineralogy of arsenic in developmental rock, and comparison of values in the EPA Region 8 library, appears to support an RBA value of 63 percent or less for the Sunday Mines Group development rock. Much of the arsenic is associated with crystalline iron sulfide (probably pyrite) which would be stable in stomach fluid. The highest concentration of arsenic found in any soil particle examined was found in associated with crystalline iron sulfide. Other forms of arsenic were encounter less frequently. These more soluble forms suggest that some arsenic would be available of uptake into the body. However, even when a high percentage of arsenic was associated with iron oxyhydroxides - a form associated with relatively high availability - bioavailability was not more than 51 percent. Thus, an estimate of 63 percent for development . rock from the Sunday Group can be anticipated to adequately (and conservatively) represent potential uptake of arsenic after accidental soil ingestion. cm 4-5 TAS4986 3eniaon MineslTask Order 3 - DMO Sampling and Analysis Plan%Task 3.12 - Soil, Ore, Rack Data Assessment ReporOSunday Rack Reporiltinal reporttReporAFinat Sunday Mines Gmp Soil Ore Rock Date Assessment Repott_051509Aoc