2025-11-14_PERMIT FILE - M2025056 (14)

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Whetstone Associates Technical Memorandum 3. HYDROGEOLOGIC REGIME The JD-8 Mine portal is located on the escarpment of Monogram Mesa. The workings extend south and west from the portal, penetrating the Morrison Formation below the mesa. As such, the mine facilities occupy two different hydrogeologic regimes: the waste rock dump and surface facilities arc located on the escarpment while workings are located in tabular sedimentary units below the mesa. The hydrogeologic regime for surface facilities on the escarpment is described in detail in the EPP. The geologic structure of the escarpment is dominated by major faults that parallel the Paradox Valley. The mine is located on a rotated fault block of Mesozoic sedimentary rocks (Figure I). As described in more detail in the EPP, the escarpment has no upgradient groundwater recharge area. The only potential source of groundwater recharge is direct precipitation onto the escarpment, and the rate of recharge from direct precipitation is low to negligible because potential evaporation exceeds precipitation. The lysimeter located at the toe of the JD-8 Mine waste rock pile has always been dry, as is the abandoned Black Diamond Mine, located two hundred feet below the waste rock pile. No seepage has been detected at the toe of the waste rock pile, in the lysimeter below the toe of the waste rock pile,or in any faults or fractures in the vicinity of the JD-8 Mine facilities. The hydrogeologic regime for the mine workings was described generally in the EPP and is described in more detail in this memo. The mine workings occur within the Mesozoic-age sedimentary units that comprise the stratigraphy of Monogram Mesa and the adjacent Davis Mesa. These strata dip toward the south and west. The recharge area for these units is limited; a groundwater divide occurs at the crest of the mesa so the recharge is limited to direct precipitation on top of the mesa. The sediments dip toward the southwest and crop out to the south and west, where the mesa is incised by ephemeral drainages. For example, the Entrada sandstone crops out in Bull Canyon,approximately 5.1 miles downdip(southwest)of the JD-8 Mine. 4. HYDROGEOLOGY OF THE UNDERGROUND MINE The underground mine workings at the JD-8 mine have been dry since they were initially excavated. This is notable because some of these workings have been open for more than 30 years. Future development is expected to remain dry, although limited groundwater may be encountered at the southwestern extent of the workings. Exploration drill hole records were compiled for the JD-8 Mine to further define the hydrogeologic system in the ore horizon below Monogram Mesa. Data compiled from 559 exploration drill holes confirms that the strata penetrated by the 1D-8 Mine are not saturated. Drill hole locations are shown in Figure 4 and provide comprehensive coverage of the existing and planned mine workings. The groundwater information from these exploration drill holes is considered to be reliable, but when drilling in a regime with downward hydraulic gradients, it is possible to encounter lenses of perched groundwater over low-permeability zones that are disconnected from an underlying saturated aquifer. Therefore, the water noted by the geologists during drilling did not always represent the level of the saturated aquifer. Only 29% of the exploration drill holes encountered water (Table 2), and in most cases, the groundwater occurred at or near the contact of the Brushy Basin Member of the Morrison Formation and the underlying Summerville Formation. It is likely that groundwater is perched at the contact N because the low-permeability rocks fo the Summerville Formation impede the downward migration of groundwater to the underlying Entrada Sandstone. 4122A.120413 a