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'~ <br />DISCUSSION AND ANALYSIS <br />In reviewing the subject report, the following main themes or question areas were <br />considered to be the most important: <br />Is there a potential for acid generation in the Cresson overburden materials? <br />Is there a potential for metals mobilization from the Cresson overburden materials? <br />How might release of acids or metals from this material impact the receiving <br />environment? <br />What steps might be taken to mitigate acid generation, metals mobilization, or release <br />of acid and metals to the receiving environment? <br />The organization of this section of the report follows generally the order of subjects as <br />they are introduced in the reviewed document and as they relate to the above questions. <br />Background Chemistry <br />The section of the report outlining the chemistry which may be involved with the exposure <br />of overburden materials to atmospheric moisture and oxygen identifies the reactions of <br />major consequence. <br />The oxidation of pyrite to produce sulfuric acid and hydroxy iron species would account <br />for both acidity and sulfate ion in waters contacting the overburden materials. The role of <br />bacterial agents in accelerating this oxidation is not discussed although this may be a factor <br />that impacts rate of oxidation, particularly at more acid conditions (below a pH of about <br />3). The report correctly notes that the rate of oxidation is proportional to the surface area <br />of pyrite exposed for reaction and that reaction products need to be transported from the <br />reaction surface to avoid loss of this active area. <br />The hydrolysis of jarosite can, as noted, also give rise to both acid and soluble sulfate <br />generation from the overburden materials. In this case, atmospheric oxygen is not a <br />reactant and only the presence of a liquid water phase is necessary to hydrolyze exposed <br />jarosite and transport soluble reaction products from the reacted surface. <br />The reaction of basic carbonates and aluminosilicates in the overburden with sulfuric acid <br />is correctly identified as an acid-base neutralization reaction and, as noted, would serve to <br />remove free acid from solutions contacting the basic mineral in the overburden materials. <br />As in the case of the pyrite oxidation and jarosite hydrolysis, only the fraction of basic <br />minerals which are exposed to the contacting acid solution can participate in reactions and <br />so neutralization effectiveness is also a function of exposed basic minerals surface area. <br />OMLR951DEH <br />