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HON.#KE • ALAN D. COX CORPORATE MANAGER, ENVIRONMENTAL AFFAIRS Mr. Steve Shuey Division of Minerals and Geology Durango Field Office 484 Turner Drive Building F, Room 101 Durango, Colorado 81301 iiiiiiiuiiniiiiii RECEIVED ~a~z 31 ~sse Division of Minerals 8 Geology l,I, March 16, 1996 A~~. ®EiVVE~i OFFICE PUBLtC FILE COPY FILE:__ t~~ ~~ 2 t S- SITE: 3,~ 1 ~ ,.~ e TR-a'I ~ 2 61'9 pg~ngo Fle1d OHS logy oNiak,n of Marerals RE: Bulldog Mine, 9360 Adit Seal Installation Response ro Comments on Technical Revision No. 7 Dear Mr. Shuey: Thank you for your timely review and comment on Homestake Mining Company's application for Technical Revision No. 7 to Permit M-77-215. This letter provides responses for your comments as provided to Homestake in your February 20, 1998 fetter (attached as Appendix A?. Our responses are numbered consistently with your four comments. As specfied in TR-07, the concrete for pouring the edit plug will be Fosroc-Terrecem, a modified Type 5 cement, which offers acid resistance, a high ground slag content, and no free residual lime. The gradation of aggregate will be as shown in Appendix 8. 2. The required edit plug water drain pipe will be a 3-inch diameter, 316 stainless steel, Schedule 40 pipe. Pipe weight is 7.6 Ibs/ft. ID 3.068-in, wall thickness 0.216-in. 3. As part of the response to item 3, it is important to review the hydrogeological conditions in the 9360 Adis, the rationale for installing a plug, and the anticipated benefits. The monitoring plan associated with the edit plug needs to consider the specific site conditions and objectives of the edit plug. Prior to the most recent re-opening the 9360 Adit in late 1997, minor seepage was observed outside the portal during the abnormally wet spring of 1995. As a result, Homesteke made a decision to re-0pen the edit to observe water conditions and to mitigate surficial seepage. Driving reports written during edit construction indicate that much of the edit inflow may occur near the contact between the Creede Formation and the Bachelor Mountain Rhyolite. During operation of the mine, seasonal vertical seepage was also observed from the Creede Formation nearer to the portal. These observations indicate that edit flow is most likely the result of shallow seepage from the Creede Formation into the edit. This is consistent with the conceptual hydrogeological model of the site. The Creede Formation is largely asail-like material and is expected to have a higher hydraulic conductivity than the underlying rhyolite. Therefore, seasonal recharge to the Creede would be anticipated to percolate vertically until reaching the rhyolite contact and then flow along the contact. HOMESTAKE MINING COMPANY 650 CALIFORNIA STREET • SAN FRANCISCO • CA 911062]!8 II15) 9BI-0~50