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Mine Tai[inzs Ponds Evaluation <br />• <br />U <br />of six tests, and manganese in two of six tests. State standazds for fluoride, sulfate, and thallium <br />were not exceeded in any of the SPLP test leachates. <br />Cyanide was a constituent of concern in seepage from the Lower Pond prior to the cessation of <br />milling in 1985. However, weak acid dissociable (WAD) cyanide concentrations in all of the <br />tests summarized above were below the analysis detection limits of 0.01 mg/I. WAD cyanide is <br />the toxic form of cyanide and includes free cyanide and most aqueous metal/cyanide complexes. <br />Because cyanide was a constituent of concern in the past, a brief discussion of cyanide chemistry <br />and stability is presented below so that the likely pathways of cyanide degradation within the <br />tailings and soils around the impoundments are understood. <br />Dissolved and solid forms of cyanide include (Smith. 1994): <br />• free cyanide <br />• weak aqueous complexes <br />• moderately strong aqueous complexes <br />• strong aqueous complexes <br />• solid compounds <br />CN" and HCN <br />Zn(CN)QZ-, Cd(CN)3-, Cd(CN)4z- <br />Cu(CN)Z , Cu(CN)~'"-, Ni(CN)a'-, Ag(CN)Z~ <br />Fe(CN)6°~, Co(CN)b'~~, Au(CN)z", Fe(CN)6}_ <br />a) readily soluble salts <br />b) neutral insoluble salts <br />NaCN, KCN, Ca(CN)z. Hg(CN), <br />Zn(CN)Z, Cd(CN)Z, CuCN, Ni(CN), AgCN <br />Free cyanide consists of the cyanide ion (CN-) and gaseous hydrogen cyanide (HCN). The <br />relative proportion of these two forms depends upon the pH of the pore water they aze in contact <br />with. The reaction between the cyanide ion and water is expressed by the following equation: <br />CN- + HZO <_> HCN + OH~ <br />At pH values below about 8.3, cyanide is present predominantly (about 90 percent) as HCN gas. <br />In the natural environment, the primary mechanisms of cyanide degradation and attenuation <br />include (California Mining Association, 1992 and Smith, 1994): <br />• volatilization <br />• biodegradation <br />• • complexation <br />Homeslake .tlining Company Shrphrrd lli!lrr, lnc. <br />e.u-.~rnn~,nd„s..p, 30 dprrl l~. 1997 <br />