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Mr. Alan D. Cox <br />Corporate Manager -Environmental Affairs • <br />September 22, 1997 <br />Page 3 <br />including iron oxide rinds, do provide evidence of the oxidation of sulfide minerals. However, <br />we agree with Dr. Posey that the lack of [his evidence alone does not prove a complete lack of <br />oxidation within the tailings. <br />Attenuation Tests. Dr. Posey considers that the distribution coefficient (Kd) values determined in <br />the attenuation tests aze "wildly" divergent, and that the water:rock ratio for the test (2:l) was too <br />low. Sample descriptions in the SMI report indicate that the soiUsediment samples used in the <br />attenuation testing were highly variable in grain size, organic content, clay content, and <br />mineralogy. This suite of samples was taken because of the variability of the Creede Formation <br />materials that underlie and surround the impoundments. Given the variability of the samples, it <br />is acceptable, and expected, that the ICd values determined in the tests aze variable. <br />The 2:1 liquid:solid ratio of the test was determined on a weight basis and is considered by SMI <br />to be higher than a liquid:solid ratio that would actually occur in natural conditions. SMI <br />believes that the tests aze environmentally conservative and protective. The primary basis for <br />this conclusion is that water leaving the impoundments would travel through several tens of feet <br />of Creede Formation materials before leaving the property azea. The total particle surface azea <br />of the sediments available for attenuation processes that an equivalent amount of water would <br />travel through at the site is certainly greater than the particle surface azea of materials used in the <br />testwork. <br />SUMMARY <br />The results from three different lines of testing (water chemistry, oxygen measurements, and <br />petrographic analyses) indicate that sulfide oxidation and acid generation aze not occurring to any <br />appreciable degree at the Bulldog Mine Tailings Impoundments. This conclusion is not based on <br />any one test, but on the preponderance of evidence from the three independent investigations <br />presented in the report. We concur with Dr. Posey that the unsaturated portions of the <br />impoundment aze open to atmospheric oxygen; however, the physico-chemical conditions of the <br />impoundments are appazently not conducive to sulfide oxidation and acid generation. <br />The attenuation testing results give an estimate of the range of ICd values that aze expected to <br />occur should water seep out of the impoundment and into Creede Formation materials. Even at <br />the lowest ICd values determined from the tests, significant attenuation of manganese, lead, and <br />antimony are predicted to occur at the site. The measured attenuation capacity of the sub-strata, <br />in combination with the physical process of dilution as outlined in the SMI report, indicates that <br />the magnitude of potential seepage impacts to ground water from Bulldog Tailings <br />Impoundments would be minimal. <br />