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H OM;~'A K E nv~N o. 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 March 16, 1998 RE: Bulldog Mine, 9360 Adit Seal Installation Response to Comments on Technical Revision No. 7 Dear Mr. Shuey: ~ ~~ ~ ~S ~~C ~~1 V ~~ ~~~ 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 letter (attached as Appendix A). Our responses are numbered consistently with your four comments. As specified in TR-07, the concrete for pouring the adit plug will be Fosroc-Terracem, 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 B. 2. The required adit 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 Adit, the rationale for installing a plug, and the anticipated benefits. The monitoring plan associated with the adit plug needs to consider the specific site conditions and objectives of the adit 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, Homestake made a decision to re-open the adit to observe water conditions and to mitigate surficial seepage. Driving reports written during adit construction indicate that much of the adit 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 adit flow is most likely the result of shallow seepage from the Creede Formation into the adit. This is consistent with the conceptual hydrogeological model of the site. The Creede Formation is largely asoil-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 94108~2]BB (<I5) 9B1 8150