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~ III IIIIIIIIIIIII III ~, 1A1~E OF COLORADO DIVISION OF MINERALS AND GEOLOGY Depanmenl of Natural Resources ~I~~ I S I S Sherman 51., Runm '_ I i Denver, Colorado A11207 Phnnc' IJOJI AbhJ 567 FAX: 11(171 AJ2~AIIlIi DEPARTMENT OF NATURAL RESOURCES September 16, ]997 c;venorr dames 5. Luchhrarl Mr. Alan D. Cox Evecmrve D~redor Homestake Mining Company Michael A. Lunp Division Director 650 California Street San Francisco, CA 94108 RE: Evaluation of the Upper and Lower Tailings Ponds, Bulldog Mine, Permit No. M-77-215 Deaz Mr. Cox: The Division has completed its evaluation of the above captioned report prepazed for Homestake Mining Company by Shepherd Miller, Inc. Please find enclosed an evaluation of the geochemical aspects of the Shepard Miller report prepazed by the Division's staff geochemist, Dr. Harry Posey. In his evaluation, Dr. Posey expresses certain reservations about some of the conclusions reached in the Shepard Miller report, but concurs with the overall conclusion that seepage from the Bulldog Mine tailings impoundments is unlikely to cause an unacceptable adverse impact on ground water quality. Please address the two questions raised in Dr. Posey's evaluation memorandum, i.e., how was the water: rock ratio measured for the attenuation tests measured? And, were samples rinsed prior to paste pH analysis? The Division's review of the geotechnical aspects of the Shepazd Miller report fell to me, and my evaluation is provided here. The basic conclusions found in the Shepazd Miller report aze that the tailings in both impoundments will remain saturated, and.that the rate of seepage from the impoundments is such that a minimum dilution rate of three parts unaffected ground water with one part seepage will occur immediately below each impoundment. The Division concurs with the derivation of the minimum dilution rate, and with the position stated in the Shepazd Miller report that this dilution rate is based on conservative assumptions making it likely that seepage will be diluted by more than the 3:1 working estimate. The Division further agrees that the tailings are likely to remain saturated or nearly saturated over the long term. However, it is the Division's position that the potential for the tailings to drain to field capacity does exist. The potential for the tailings to drain would be increased as additional vegetation establishes in the impoundment, if the area is subjected to a prolonged drought, or if the tailings surface were to crust over thus reducing infiltration.