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COLORADO LEGACY LAND precipitation in Colorado.They show a range of possible outcomes from a 5 percent decrease to a 6 percent increase by mid-century(2050).The risk of decreasing precipitation appears to be higher for the southern parts of the state." Kennedy,Caitlyn.Future Temperature and Precipitation Change in Colorado.NOAA.Published August 9,2014, Updated July 3,2021.https:Ilwww.climate.gov/news-features/featured-images/future-temperature-and- reci itation-chan e- of r do Accessed May 2,2022. 3 No contingency plans were offered if the current strategy were to fail,and CLL indicates that they will CLL notes that it has previously stated that it will operate the WTP so that the mine pool elevation remains below the regulatory continue with their current approach through 20 years with no description of a plan beyond that time. limit.Within the 20-year period of operation,sufficient data will be collected to evaluate any contingency lan,if needed. We appreciate the pumping groundwater contour figure that was provided with Revised Amendment 6. o The mine workings are a network of adits and shafts that can be viewed as interconnected"pipes." The shafts run Please provide: vertically through the entire thickness of the mine workings,and to our knowledge,there are no installed bulkheads or o The basis for the assumption that the mine workings are a fully connected infinite hydraulic seals to restrict vertical flow within the shafts. The diameter of the adits and shafts are typically greater than 6 feet, conductivity feature,and which means that water can flow through these features with very little hydraulic head variation. The consequence is o Reasoning for selecting the elevation of 6,434 feet for water in the mine workings as a constant that water within the workings will have a uniform hydraulic head that is controlled by the free surface(water table) head feature. elevation in the upper workings. 4 It is not appropriate to describe the mine workings as an"infinite hydraulic conductivity feature"because the flow mechanism is more similar to pipe flow rather than fracture or porous media flow. o The mine workings are not a constant head feature. The mine is a"uniform"head feature that varies seasonally. The stipulated maximum allowable head in the mine is 6,452 ft amsl. However,seasonal pumping is typically initiated when the mine water level reaches about 20 ft below the allowable maximum. The assumed head of 6,434 ft msl is considered the operating maximum water level that will typically occur in late spring before the pumps are turned on. If we were to conservatively set this value to the maximum allowable water level,the contour map would change slightly,but the interpretation of a groundwater divide between the mine workings and Ralston Creek to the southeast would remain. A stagnation zone in bedrock may exist between the downgradient end of the mine pool and the Creek such The feature discussed in this comment is better described as a"groundwater divide"rather than a stagnation zone. On the that a portion of impacted groundwater in bedrock(not necessarily the mine pool)could discharge to groundwater contour map(Figure E-4),this feature is located southeast of the mine workings and its interpretation is supported 5 Ralston Creek;we continue to recommend an additional monitoring well and continued environmental by the configuration of contours and measured water levels in wells MW-15 and MW-16. Groundwater northwest of the divide monitoring(see CLL response to Denver Water Comment#15). flows towards the workings,so there does not exist a hydraulic mechanism by which water in or near the mine workings can migrate southeast to Ralston Creek. This situation will be maintained as long as the water level in the mine is depressed by pumping. CLL's argument on stable discharged uranium concentrations does not sufficiently address the overall Comment noted. However,at this time CLL has no further explanation to provide. CLL will continue with the in-situ injections increasing uranium concentrations in the mine pool over the past few years.See the graph and Mann- and collection of data to evaluate future trends. The concentration of uranium in the mine pool is stable with respect to the 6 Kendall trends provided as Attachments A and B,respectively(see CLL response to Denver Water current water treatment technology at the site. The RO-IX system can treat these concentrations and discharging water below Comments#2a,20,21,23). the USEPA drinking water standard(0.03mg/L). Therefore,CLL has achieved the necessary physical and chemical stabilization re uirements outlined in the succession of operator's letters. CLL has not sufficiently addressed that the most recent in situ treatment did not decrease uranium Comment noted. However,at this time CLL has no further explanation to provide. CLL will continue with the in-situ injections concentrations(see CLL's response to Denver Water Comments#2a, 17, 18).CLL's response to Denver and collection of data to evaluate future trends. The concentration of uranium in the mine pool is stable with respect to the 7 Water Comment#17 states that"CLL is not relying on in-situ treatment for the operations of the RO current water treatment technology at the site. The ROAX system can treat these concentrations and discharging water below treatment systems"and that the mine pool is chemically stable.However,mine pool uranium the USEPA drinking water standard(0.03mg/L).Therefore,CLL has achieved the necessary physical and chemical stabilization concentration trends are increasing Attachment A),despite increased frequencies of in situ treatments. requirements outlined in the succession of operator's letter. CLL argues that the bulk total dissolved solids(TDS)of the mine water has not changed,indicating that the Comment noted. However,at this time CLL has no further explanation to provide. CLL will continue with the in-situ injections 8 RO reject is not impacting the concentrations within the mine and that the mine is chemically stable.TDS and collection of data to evaluate future trends. concentrations are not an indicator of uranium trends within the mine pool and this argument does not address the chemical stabilization of the mine pool for key constituents such as uranium. CLL has not established a contingency plan if uranium levels continue to rise in the mine pool and in situ CLL notes that it has previously stated that it will operate the WTP so that the mine pool elevation remains below the regulatory treatments are no longer effective and/or the RO system is less effective(see CLL responses to Denver limit.Within the 20-year period of operation,sufficient data will be collected to evaluate any contingency plan,if needed.At this Water Comments#2a,17, 18). juncture there is no evidence to suggest that either the in-situ treatments will no longer be effective or that the RO system will be 9 less effective.In fact,as stated in the report(Section E.5.2.4),and provided in the revised Figure E-6 (formerly Figure E-4),the TDS concentrations have been generally stable,which supports CLL's assertion that the RO technology appears to remain viable as long as it can be projected by the current data set,and no trend of loss of use or efficiency of RO can be projected from these data. 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