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Technical Revision No. 79 -PAR Responses Apri118, 1997 Page 6 23. Locally available sandstone within the West Elk Mine site will be utilized for the rock drain. The primary source will be from numerous boulders and cobbles within the RPE site. A secondary source of rock is the lazge rock fall neaz MB-5. The rock will be of sufficient durability as is demonstrated by its historic performance in numerous ditches at the mine, which have been in place for over ten years. Sizing wil] be performed by the contractor using hydraulic breakers and screening equipment, with field verification of suitable material and gradations conducted by MCC construction management personnel. 24. The bottom of the RPE will be lined with PVC. 25. Figure L-1 has been revised to show that the bottom and down gradient side of the sand drain will be fined with 30-m1 PVC. The liner will be utilized in the sand drains outside the limits of Phase IA-iD construction. 26. Figure L-1 has been revised to include a 12 ounce per square yazd of non-woven geotextile felt above the finer. This geotextile will also be placed below the liner in the rock drains. 27. The Division is correct in their observation that refuse pile leachate and water from seeps and springs will comingle in the rock drains and sedimentation pond, and would be discharged together with stortnwater runoff to the North Fork of the Gunnison River. The Division has raised numerous issues in question 27, and MCC would like to respond to each issue individually as follows: In paragraph two the Division raises a concern that undiluted seep effluent and refuse leachate will be discharged to the North Fork. This will not occw. Water from the rock drain flows into the pond as shown on the Rock Drain Inlet Tower Detail on Figure L-3. It will then mix with stormwater in the pond. Water from the rock drain cannot be directly discharged through the outlet. The hydrologic calculations presented in Appendix L shows a maximum seep leachate flow of 40 gallons per minute. At this design flow rate, the two cell pond (9.09 AF capacity) has approximately 51 days of storage, without stormwater inflow. MCC believes that a flow rate of 40 gpm is extremely conservative, and will unlikely occur. However, it is reasonable to expect that maximum seepage would only occur in response to storm events thus providing adequate dilution for any leachate produced. In addition, MCC is expected to discharge water which meets effluent limitations established in our NPDES permit. MCC is very aware of water quality issues and is committed to complying the NPDES permit. In the third paragraph the Division raises the issue of potential saturation of the waste refuse if the rock drains were to backup due to their capacity being exceeded or being plugged. As previously stated, MCC has created a drain sizing table to ensure the drains can handle 200 percent of observed or design flows. In addition, Figure L-1 has been revised to include liner material under and around the sand drains. MCC believes that because the springs are within the RPE footprint rather than outside the footprint, the best engineering solution is to carry this water to the rock drain rather than to cover the springs with a liner and risk