2011-09-09_REVISION - M1990057 (3)

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CDRMS question 3. Page 5, first paragraph: The last sentence indicates groundwater was encountered at 25 feet in boring TH -2. In contrast, the last paragraph of the first page of the "TAILINGS DISPOSAL / Operating History" dated June 6, 2011 submitted to DRMS by the applicant, indicates the aquifer is 80 to 100 feet below the surface based on communication with the Colorado State Engineers, Ground Water Section. The second bullet on the following page states "The pond excavation at a maximum 21 ft depth does not impact the known aquifers at 80 -100 feet below the surface. " Based on the water level encountered in TH -2, the separation between bottom of the pond and groundwater is on the order of four feet. Please address this discrepancy, the gradient of the water table encountered in TH -2, and how the applicant can ensure groundwater separation. CTL Thompson Response (September 1, 2011 letter) The site was covered with snow at the time of our field investigation in May and spring melt was in progress. There was a pool of water in the pond adjacent to the exploratory boring. The water found in our boring may be indicative of temporary perched water conditions which develop during snowmelt and the influence of the water within the existing, unlined pond, rather than the "aquifer." In 1995, we drilled an observation hole south of the existing pond and found no water to approximate elevation 9685, or about 9 feet below the proposed excavation level (see Appendix C of July 8 letter). UMC Response UMC concurs with CTL that the TSF is in an area of a temporary perched water condition, prevalent during spring melts and wet years. As discussed in CTL Thompson's April 1990 Report, Permeability Study Tailing Pond Area Leadville Mining and Milling Corporation Mill near Leadville, Colorado, the tailings disposal area soils are colluvial. There appear to be two colluvial types, clays with lower permeabilities and gravel lenses with higher permeabilities. Most of the tailings disposal area is over the lower permeability clays (k= 2ft/yr to 7ft/yr). The northern TSF area appears to be over a 1.5ft thick gravel lens with a permeable of 5000ft/yr at 8 feet of depth. Other gravel lenses have permeabilities of 230ft/yr. A monitoring well and several permeability test holes from 20yrs ago still exist within the existing TSF area. UMC samples /observes the monitoring well quarterly. Additionally, UMC has sporadically observed permeability test wells over the last several years. All but one of the permeability test wells will become part of the TSF impoundment. Monitoring well MW -1 (OH -1 in CTL Thompson reports) shows minor water levels in the spring, and then is dry, or at best damp following precipitation events. MW -1 is completed to a depth of 9685 (9ft below the bottom of the impoundment) and is completed in a layer described as "sand, dense, very clayey, gravelly, moist, red -brown (SC) ". UMC monitors this well quarterly and reports the results in the annual report. All but one sample period has reported no water. Permeability test holes PT -2, located just south of the existing TSF disturbance and in the northern portion of the proposed TSF disturbance, has shown water levels during spring melts with the water dissipating over time. PT -2 is completed to a depth of 9693 feet (lft below the bottom of the impoundment) and is completed in a layer described as "gravel, dense, very clayey, very sandy, moist, red -brown (GC) ". PT -2 averages a permeability of 2ft/yr. Based on random observations over 3 years, UMC's concludes water infiltrates this hole in the spring and dries out in the summer. PT -3, located east of MW -1 and to the southeast of the proposed TSF disturbance, has not shown water levels during spring melts. PT -3 is completed to a depth of 9701 feet (7ft above the bottom of the impoundment) and is completed in a layer described as "gravel, dense, very clayey, very sandy, moist, red -brown (GC) ". PT -3 averages a permeability of 7ft/yr. TR3AdequacyReview.Question 19UMCResponse.Sep9.jme.004 M1990 -057 September 9, 2011