2011-09-29_REVISION - M1977215 - Laserfiche WebLink

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Formation groundwater is separate from the mine pool groundwater in the underground workings. The 9360 adit was plugged and abandoned in 1999 as part of mine closure activities. Prior to closing the plug, monitoring well HW -3 was drilled and completed to better characterize upgradient ambient water quality, and the local groundwater gradient between the Creede Formation and the 9360 adit. Prior to plugging the 9360, the adit behaved as a groundwater sink for the Creede Formation as gradients were towards the tunnel from the formation Appendix A (WMC, 1999a). Historically, adit discharge rates ranged from 10 to 30 gpm, and were reported to be originating primarily from the contact between the Creede and Campbell Mountain Formations. Upon sealing of the adit plug the tunnel began to fill, and within 60 days leveled out at approximately 9,383 ft msl (WMC, 2001). The 9,383 ft elevation corresponds to the adit invert elevation near the A59 spad, the high point in the adit from which water drains to the underground mine workings pool within the 9200 level. The adit invert at the A59 spad is the primary hydraulic control on the Creede Formation at this elevation. Plugging of the adit has stabilized the groundwater gradient in the area created by the former 9360 adit as evidenced by recent water level readings from monitoring well HW -3 at 9,381 ft msl and BD -MW installed by RGS which tagged the 9360 adit and resulted in a water level elevation of 9,381 ft msl. These two data points indicate the gradient is essentially flat and the Creede Formation groundwater level is controlled by the invert elevation at the A59 spad. Further support of the groundwater conditions within the Creede Formation can be drawn from the drilling logs and reports for monitoring wells HW -3 and BD -MW. During the drilling of these wells no groundwater was reported to have been encountered for the first several hundred feet. In the case of HW -3, water production via airlifting was first encountered during drilling at around 305 feet below ground surface (9,330 ft msl), indicating a significant portion of the Creede Formation is relatively dry or unsaturated. After well completion and development, the water level stabilized in the well around 9,375 ft msl. Development of HW3 via airlifting methods produced 3 to 5 gpm over the course of a 4 -hour development period, indicating overall yields from the formation are low. Recent drilling of BD -MW is consistent with what was observed during the first 200 feet of drilling of HW -3. Figure 3.1 presents a snapshot of water levels in relationship to adit seepage rates during the fall of 1998 prior to sealing the plug on the 9360 adit. Although the data range is of limited duration, an interesting relationship appears to exist between adit seepage rates and water levels in HW -3. The seepage rate is observed to decrease in proportion with declining head at the well. Although this relationship was observed prior to sealing the 9360 plug, sufficient evidence exists to support similar groundwater hydraulic characteristics exists between HW- 3 and the 9400 decline. This supports the conceptual model construction indicating that the source of historic adit discharge, and water that will be encountered during the 9400 decline drive, is Creede Formation groundwater. Estimates of groundwater inflow to the proposed 9,400 decline The basis for estimating groundwater inflow rates to the 9400 decline is supported by the conceptual model presented. The 9400 decline is planned to start from elevation 9,400 ft msl, offset from the 9360 adit by approximately 100 feet to the north. Based on the conceptual model presented minimal amounts of groundwater will be encountered for the first several hundred, and perhaps even the first 1,000 feet of tunnel drive planned. TR -18 Respo/ise to PAR LeiieiA &5. doe 9/26/2011