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Knight Piesold <br /> CONSULTING <br /> All of these values fall within the"uncertain range"according to GARD guidance. However as can be seen <br /> from the photos in Attachment 2 and as stated above,the piles have been exposed to the atmosphere since <br /> the late 1800s with no visible signs of significant acid rock drainage emanating from the piles (i.e., no <br /> significant evidence of"yellow boy")which is the reddish-brown precipitate generated as shown in Figure 1 <br /> below: <br /> V <br /> AIJ <br /> Xor <br /> 71 <br /> 71 <br /> 't � r <br /> ao <br /> Figure 1 Example of Acid Rock Drainage Discoloration <br /> Question 5: While the waste rock samples were tested for leachability. Samples should also be <br /> tested using EPA Method 6010 and the results compared to the EPA Regional Screening Levels <br /> (RSLs) Summary Table used by CDPHE (May 2020) to determine potential levels of soil <br /> contamination at the site. This analysis will help guide what happens to the tailings that come <br /> out of the mill. <br /> KP Response: <br /> EPA Method 6020b is an analytical method that uses inductively coupled plasma — mass spectrometry <br /> (ICP-MS) to evaluate metal concentrations that could leach from the existing piles during precipitation or <br /> snow melt events. We used ICP-MS (i.e., EPA Method 6020b) - instead of ICP- Optical Emission <br /> Spectrometry(OES)in the EPA Method 6010—because this method has lower detection limits and allows <br /> the evaluation of leachate chemistry at lower constituent concentrations. Table 1 below compares the <br /> results of the synthetic precipitation leaching procedure (SPLP) and toxicity characteristic leaching <br /> procedure (TCLP)and to the EPA RSLs. As indicated in the report, none of the materials have TCLP and <br /> SPLP leachate concentrations that exceed standards. <br /> June 29,2021 <br /> DV-21-0775 3 of 5 <br />