2024-12-16_REVISION - M1977410 (2)

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Cross Gold Mine December 2024 G-5 contribute to flow. Flow is confined to secondary porosity; joints, fractures, and faults in the rock units. As previously described and illustrated, the fracture and fault density in the Front Range was caused by regional tectonics. Mines in fractured terrain are often located in the most fractured portion of the terrain; this is the case for the Cross Gold Mine. The mapped fault and fracture density at the mining district is higher than areas immediately adjacent (Holland, 1994; Gabel, 1969). Because most veins and associated fractures set trend east-west or northeast-southwest it is expected that these are preferred flow directions causing significant anisotropy in transmissivity. With enough fracture and joint density, fractured bedrock hydrology may behave hydrogeologically as a granular aquifer, except the ‘grains’ are fist to boulder sized. This “representative porous media” (RPM) when present in leached rock and fractured bedrock reduces fracture-based anisotropy that simplifies understanding of the system. As the Cross Mine is located in a high fracture density area and leaching near the surface the RPM approach may be usable in the conceptual hydrogeologic model for the site. The water table is influenced by mine dewatering and stopes from legacy mining between the Cross and Idaho Tunnel/Caribou resulting in large stopes connected to shafts on the surface. The Cross Winze is a near-vertical (70° incline) internal shaft within the Cross Mine that intercepts the Cross Adit (tunnel) at the point projected to the surface in Appendix G-4, Figure 6. The water level in the Cross Winze will quickly rise to the level of the tunnel in snowmelt season if the winze is not pumped to the Coon Track Creek elevation after treatment. The bottom of the Winze is approximately 235 feet below the floor of the tunnel. Pumping the winze has been noted to influence the water level in the Cross Well. A non- pumping condition was assumed for the July 2021 water table map and the water level at the Cross Winze set to the tunnel floor (9,700 feet above mean sea level- amsl). The full influence of pumping and water chemistry will be determined over time. The shallow ground water system is also seasonally dynamic, being strongly influenced by annual snowmelt. Much of the observed flow within the mine comes from fractures, veins, faults, and legacy stopes/workings changing from just-damp to fully flowing streams during snowmelt. In the snowmelt season the ground water flow increases greatly and the water table rises. Casual observations of the Cross Mine Winze show tens of feet of water table rise in the snowmelt season. According to the conceptual model the large increases in streamflow flow and water table rise will be forced by young water from snowmelt. The mine workings lie within this shallow flow zone. A substantial portion of water in this zone will be displaced by each snowmelt and will have a lithogenic signature that roughly corresponds with its residence time. The time from infiltration to discharge can be roughly estimated to be from 1 month to 100 years for shallow ground water (Frisbee et al., 2013). Residence time is primarily controlled by transmissivity and transmissivity decreases with depth in fractured rock aquifers. Deep ground water circulation in alpine basins can approach 7000 feet in depth and still return to surface within a basin but may take over 5000 years to do so. Transmissivity estimates are difficult in fractured bedrock due to the discontinuous nature of hydraulic conductivity as compared to granular aquifers. Point estimates from single well tests conducted for well rehabilitation may not be representative if applied over large areas. A vertically and horizontally averaged estimate of bulk transmissivity is possible using historic dewatering records. Long ago dewatering of the Caribou Mine could be accomplished by pumping 100 gpm, six hours per day (36,000