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• III IIIIIIIIIIIIIIII • 999 INTEROFF/CE MEMORANDUM DIVISION OF MINERALS AND GEOLOGY TO: Jim Dillie FROM: Harry Posey S~~Q~~ DATE: February 19, 1997 SUBJECT: Cyanide in soil samples; BMRI, M-88-112 I looked over your calculations converting ug/g soil values to mg/L values for water in the soil and concur with your methods. As we discussed, WAD and total cyanide $ccurs in the samples above the clay layer and evidently lower quantities below the clay layer. At a minimum, it is clear that the clay layer did not contain the leak, though whether it was meant to do so is not obvious from the records. For several reasons, cyanide can degrade or attenuate in soil. Such attenuation .increases with distance traveled. Factors that promote the degradation or attenuation of cyanide species include: • ~H; any pH below about 9 will favor the formation of HCN from CN-, and HCN volatilizes readily; • enhanced vapor path; unsaturated soils have more air spaces than saturated soils or solutions, and these enhance vaporization of HCN; • biolo ical degradation; soil enzymes in contact with soil oxygen and HCN tend to form cyanate (HCNO), which then hydrolizes to ammonia and carbon dioxide; • adsorption and precipitation; a complex cyanide solution can attenuate due to adsorption on the surfaces of minerels and organic carbon, or cau precipitate as an relatively insoluble cyanide species such as ferrocyanide; • hydrolysis; HCN in the presence of soil water can form ammonium forntate of formic acid and ammonia. The risk posed by the loss of cyanide solution from the tanks should be consideted in terms of measured attenuation and degradation in the soils, and of attenuation and dilution in groundwater. It is not practical to assess the rate of leakage or the effectivenes$ of cyanide attenuation by any means other than direct measurement from backhoe trenches or perhaps drilling. cc: Bruce Humphries