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Mr. Steve Shuey <br />March 16. 1998 Page 4 <br />• Introduction of tracer into the adit would not be uniform since only the outer <br />section of the adit is accessible. Thus, all potential flow paths from the <br />upstream side of the plug to Windy Gulch could not be evaluated. <br />Minor seepage, estimated to be several gallons per minute or less, may occur <br />in the natural formation around the plug and flow back into the portal side of <br />the adit. It is proposed to use the existing infiltration trench to land apply any <br />such seepage back into the Creede Formation. Therefore, it would not be <br />possible to determine whether a tracer was being introduced to Windy Gulch <br />via the infiltration trench or directly by flow from behind the plug. <br />The selection of a reliable tracer is problematic. Tracers such as chloride and <br />bromide may interact with clay minerals that are likely to occur in the Creede <br />Formation and can negatively affect water quality. Environmental tracers <br />such as fluorescent dyes le.g., Rhodamine WT) may be affected by <br />suspended solids, temperature and pH. They also experience photochemical <br />decay when exposed to light. Lower Windy Gulch is characterized by <br />intermittent small pools of standing water in which a fluorescent tracer would <br />be subject to photochemical decay. <br />If the proposed approach to monitoring adit plug performance and water quality in Windy <br />Gulch is considered appropriate by the DMG, Homestake will prepare a detailed monitoring <br />plan to incorporate the specific protocols involved. Quarterly monitoring reports would be <br />submitted to DMG with a comparison of water quality results with the current baseline <br />conditions. It is considered that approximately three years of water quality monitoring in <br />Windy Gulch will be sufficient to establish the plug performance and any impacts on water <br />quality. <br />4. During the recent rehabilitation of the 9360 Level Adit at the Bulldog Mine, it was estimated <br />that the adit at Daylight Curve had caved up to about 30 feet above the track to form a <br />relatively stable opening. The side walls of this opening tended to taper out away from the <br />ribs of the original adit before it arched over to form the roof. This caved area extended a <br />total of about 30 feet along the drift alignment. The downstream end of the caved area is <br />therefore about 310 feet from the 9360 Portal. At this location the ground cover above the <br />cave, up to ground surface, is about 102 feet. Figure 2 shows a cross section of the 9360 <br />Adit extending from the portal to Daylight Curve. <br />After the proposed plug is in place, the maximum head of water impounded behind the plug <br />will be about 25 feet. Consequently, the maximum height of water behind the plug will be <br />about 5 feet below the top of the caved area. <br />If the present stability of the caved area is influenced adversely by the resaturation, it is <br />anticipated that the opening will continue to grow in the same manner in which it has <br />started. Failure will occur along a line that angles out from the sides of the adit at <br />approximately 30° to 35° from the vertical, with arching occurring across the roof. The <br />fallen material will fill in the space currently existing in and above the adit. Assuming that <br />the ground swells by 20% when it is loose, compared to its solid state, it is calculated that <br />the cave might progress upwards another 50 feet (approximately), i.e., through 50% of the <br />existing ground cover. At this point, the roof of the opening would be re-supported by the <br />broken muck such that caving would cease. <br />