2014-04-16_REVISION - C1982056 - Laserfiche WebLink

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4/16/2014 State.co.us Executive Branch Mail - Fwd: FW: Wolf Creek Reserve -Response to CDRMS Preli mi nary Comments and Modification of Groundwater Invest... Subsequent to review and discussion of the Division's concerns relative to well location and number of wells, Twentymile (TC) reviewed mapping of the Wadge Coal Seam structural contours (which generally mirror the Wolf Creek Structure, based on drilling to date) to assess the probably direction of movement for any groundwater in either the Wolf Creek Seam or the overlying Interburden. The structural axis runs NE /SW along the western edge of the WCR, as shown on the attached figure, with the structural low point just north of the mapped WCR mining area, and within the area previously mined in the Wadge Seam. Note that the Wolf Creek Seam contours would, on average, be approximately 150 feet lower than the Wadge contours shown, so the structural low point for the Wolf Creek Seam would be at an elevation of approximately 4,875 feet, and any groundwater movement within the WCR area would tend to be from SE to NW. Given this configuration and these considerations, Drillhole WC008 on the attached figure, would represent a good location for an upgradient monitoring well, which is both outside the previous and planned mining areas, and would be accessible year- round. Options for a potential downgradient monitoring well include Drillhole locations WC003, WC013, and WC014 (see attached figure). While any of these potential sites would be at a lower elevation relative to WC008, they are not truly downgradient (given the structural axis) and would be from 400 to 475 feet above the structural low point for the Wolf Creek Seam. Because the structural low point falls within the area previously mined in the Wadge Seam, it would be impractical to drill and complete a monitoring well through the Wadge Seam gob areas which overlie this structural low. A "downgradient well" at Drillhole WC013 would potentially provide some information, but given its location and elevation relative to the structural low, its utility and value would be somewhat questionable. A possible alternative would be to monitor groundwater (flows and quality) at a location near the bottom of the rock -slope access, as shown on the attached figure, which would be near the structural axis, and at an elevation of approximately 5,200 feet, or 75 feet below the lowest borehole available. The combination of an upgradient groundwater monitoring well at the Drillhole WC004 location, and monitoring at a location near the bottom of the rock -slope access appears to be the most practical option given the groundwater hydrologic configuration and the constraints imposed by the previous mining in the area. It should also be noted that, as a closed basin, any water quality impacts in the Wolf Creek Coal Seam or Interburden would generally be confined to a relatively small basin area, with a difference of roughly 550 feet between the structural low point (at approximately 4,875 feet) and the potential "spill- over" elevation of 5,425 feet to the NE or W. While acknowledged as not ideal, this approach provides a reasonable means of characterizing and monitoring the groundwater environment for the WCR, consistent with the very limited potential for hydrologic impacts. As part of the drilling program for all of the WCR surface delineation boreholes, a 6 -inch diameter surface casing was placed and grouted for each hole. TC proposes to complete the WC004 monitoring well by placing a 4 -inch stainless steel screen with conventional sand -pack attached to a 2% -inch stainless steel tubing, which would then be grouted in the hole. It is necessary to go with the smaller 23/ -inch casing to assure that a tremmie line can access the annulus so the well can be properly grouted. TC currently has several similar 2 -inch wells in place at similar depths , which are successfully monitored on a regular basis. We submit that this proposed well construction would meet the applicable SEO requirements, and TC would comply with all applicable SEO /DWR monitoring well permitting and construction requirements. Given the considerations previously noted (ie: no appreciable water encountered in either the Wolf Creek Interburden or Coal Seam, and a closed groundwater basin) and that TC would be monitoring any groundwater inflows to the mine workings, both at the base of the rock - slope, and as part of a regular annual inflow study, we question the utility and value of specific coal seam monitoring wells in characterizing and monitoring the groundwater environment. https: / /mail.g oog le.conVmai I /u /0 / ?ui =2 &i k= e29129fcb5 &viev�-- pt &search =i nbox&th= 1456bl4cO8d4b9ff &si ml= 1456bl4cO8d4b9ff 7/9