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&dldag Mine Tailings Ponds Evaluation <br />• adsorption <br />• oxidation to cyanate (CNO~ <br />Volatilization is one of the main attenuation mechanisms of cyanide in the unsaturated zone <br />(Smith and Mudder, 1991). Volatilization will occur if the soil acts to buffer the solutions to pH <br />values below approximately 8.3. Under these conditions, the limiting factor concerning <br />attenuation time is the rate at which HCN gas migrates through the soil pore spaces. <br />Biodegradation is another attenuating factor for free cyanide in the unsaturated zone. <br />Biodegradation occurs mainly under aerobic conditions and generally involves the conversion of <br />cyanide to nitrate. Biodegradation rates depend on the amounts of oxygen, organic matter, <br />nutrients, and cyanide-degrading bacteria available in the soil. In azeas with significant organic <br />material in the soil, attenuation may occur over shallow depths of only a few feet. <br />The chemical analyses of the tailings pore fluids collected for this evaluation (shown in Table 5.4 <br />and 5.5) show that these waters have pH values between 7.? and 7.8. These pH values and the <br />• lack of cyanide in the tailings pore water samples in both the Lower and Upper Ponds indicate <br />that volatilization of cyanide since the cessation of operations at the Bulldog mine is mostly <br />complete. The alternating fine grained slimes interspersed with coazser tailings have appazently <br />provided adequate migration pathways for degassing of cyanide within the tailings. <br />5.4 Attenuation of Lead, Antimony, and Manganese in Creede Formation Soils and <br />Sediments <br />Based on water testing results, manganese, lead, and antimony aze the only constituents that may <br />be released at concentrations above drinking water standazds should precipitation infiltrate <br />through the Bulldog tailings. To determine the fate of these constituents, laboratory attenuation <br />tests were performed using tailings pore water from the Upper Pond and Creede Formation soils <br />from slopes outside of the impoundments and cut banks immediately adjacent to the <br />impoundments. The tailings pore water was used because it provided appropriate concentrations <br />of the constituents of concern and allowed competition for sorption sites from other ions in the <br />• pore waters to occur. <br />Homesrake Mining Comparty~ Shepherd ,1frller. !nc <br />pv--une„um,F p~ 31 dpnll J, 1997 <br />