2009-05-19_REVISION - M1981021 (32)

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Section 4 Potential for Rock to be Toxic-Forming Comparison to Refined Site Specific Criterion for Arsenic to Site Concentrations: Wh.enlthe RMC estimated for arsenic (37 mg/kg) is adjusted assuming a bioavailability factor of 63%, the resulting site-specific RMC is 59 mg/kg. The site-specific criterion for arsenic of 59 mg/kg is higher than any total arsenic reported from development rock samples. The highest concentration of arsenic found in the development rock at the Sunday-mines, 18.81 mg/ kg, which is less than one third of the site-specific criterion. Therefore, based on the'measured concentrations of arsenic and the calculated criteria protective of workers, development rock at the Sunday Group mines is not expected to be a toxic-forming material. 4.1.3 Evaluation of Inorganic Constituents not Addressed by BUVI BLM RMCs have not been identified for several metals detected alt the Sunday Mines Group including uranium. In order to assess the potential toxicity of these metals in development rock and ore, EPA Regional Screening Levels (RSLs; EPA 2009) for a commercial/ industrial exposure scenario were utilized as screening level concentrations for the following metals to assess if development rock and ore are a potential toxic producing material: aluminum, barium, beryllium, boron, chromium, cobalt, iron, molybdenum, thallium, tin, titanium, urardum and vanadium. Although the commercial/ industrial exposure scenario is not the same as the expected post- mining land use of the property (e.g., non-motorized recreation), RSLs provided for this scenario are conservative; that is, the scenario provides additional human health exposure to metals on the site than what is anticipated given the anticipated post-mining land use. For example, commercial/ industrial workers are exposed 250 days per year compared to 40 days per year for the BLM worker. No maximum concentrations of additional metals evaluated using RSLs (Table 4-1) exceed screening criteria, indicating that constituents not considered by BLM pose no toxic-forming threat for direct contact. 4.1.4 Background Soil Concentrations All inorganic constituents detected in development rock at Sunday Group mines occur naturally in soil and rock. Thus, some level of these constituents will be found in development rock that is unrelated to mining activity. Background soil concentrations for soils in the area near the mine and shown in Table 4-3 were developed based on guidance provided in the Proposed Soil Remediation Objectives Policy Document (CDPHE 1997). The guidance identifies the creation of Tier 1 soil remediation objectives based on a statistical analysis of background soil samples. A total of nine background soil samples were collected within the vicinity of the Sunday Mines Group but in areas that were either upwind or upslope of development rock and ore piles. Table 3-3 shows background concentrations of total metals in soil. These data were analyzed initially using basic statistics (average, standard deviation, and coefficient of variation). As defined in the guidance (CDPHE 1997), constituents with a coefficient of variation less than one can be considered normally distributed. For analytes that met this criterion, background soil concentrations for comparison with measured concentrations in development rock were calculated as the average plus two times the standard deviation. For barium, the coefficient of variation was greater than one and estimate for background soil concentration was determined from log-transformed data. • cm 4-6 7:164986-Denison Mira3slTask Order 3 - DMO Sampling and Anaysis PlaMTask 3.12 - Soil, Ore, Rock Data Assessment Report%Sunday Rock ReporMinel report\Reporffinal Sunday Mine. Group Soil Ore Rock Data Assessment Reporl_051509.doc