2009-05-19_REVISION - M1981021 (32)

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Section 4 Potential for Rock to be Toxic-Forming IRs = Age Adjusted Soil Ingestion Rate SFi = Inhalation Carcinogenic Potency Slope Nci = Number of COCs with Inhalation Slope Factor 114 (mg-yr/kg-day) 15 (mg/ kl; day)-1 2 01.1 IR = Inhalation Rate PEF = Particulate Emission Factor 2.5 m3/da:y 4.63X 10-9 m3/kg This equation, when used with the above exposure assumptions for workers, yields a criterion for arsenic of 37 mg/ kg for protection from carcinogenic health effects; an upwards :revision from the guidance document criteria of 12 mg/kg. Bioavailability: Past and ongoing studies at mine sites throughout the American west have repeatedly shown that arsenic in mine wastes is less bioavailable than is typically assumed in calculations for exposure and risk (EPA 1997). BLM RMCs do not consider relative bioavailability (RBA); instead the RMC values are based on a toxicity study using exposure via dissolved arsenic in drinking water; an exposure where arsenic is nearly completely absorbed into the body. Accordingly, RMCs for arsenic assume that all arsenic in development rock is available for uptake into the body; this is not consistent with studies of arsenic exposure at mine sites. As discussed in the BLM guidance (BLM 2004, p.13), RBA can be used to adjust the RMC screening value upwards for arsenic to account for differences in arsenic absorption into the body from development rock. To examine the bioavailability of arsenic in development rock from Sunday Group mines, two complementary tests were performed. First, the fine fraction from development rock samples (< 250 micron particle size) were used as substrate for an in vitro assay that uses simulated stomach acid to estimate the bioavailable fraction of arsenic, and :second, these same samples were examined in the electron microprobe to determine the types and amounts of different arsenic containing mineral forms. Additional information regarding these laboratory methods is provided in Attachment 4. The U.S. Environmental Protection Agency (EPA) Region 8 has developed a bioavailability assessment protocol which uses the rE'sults of these two tests to provide a quantitative estimate of bioavailability (the in vitro assay) and to show that mineral forms present in the samples are consistent with the observed in vitro assay results. EPA refers to this bioavailability protocol as a weight-of-evidence approach (see Attachment 4). Bioavailability data are shown in Table 4-2. Initially, aliquots from development rock: samples were sieved to obtain the 250 micron and smaller particle fraction and this fine fraction was run in the in vitro assay. Due to low concentrations of arsenic in these samples, results of the assay were considered unreliable. The range of bioavailability results was from 64 to 232 percent. Since bioavailability cannot exceed 100 percent, the results were unduly influenced by difficulties in achieving consistent sample representativeness between the two analyses (e.g., total metals and in vitro assay). To address this problem, several samples were magnetically separated to enrich the samples in common mineral forms and the samples, both separated and whole were re-run in the in vitro assay. Except for two samples, the results provided reasonably consistent estimates for bioavailability (range of 20 to 63 percent). Some of this variation in estimates can be explained by differences in mineralogy (see discussion below). LJ cm 4-4 7:66906-Deni-n Mines%Task Order 3 - DMO Sampling and Ana"is Plen\Task 3.12- Soil, Ore, Rxk Data Assessment ReporltSunday Rock Relornlinal reportlReporttFinal Sunday Minns Group Shc OreRock Data Assessment Repat_051509.6oc