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RESPONSE. After reclamation, water infiltrating the area overlain by ECOSA and WHEX <br /> is expected to enter the diatremal bedrock, and proceed through the diatreme to the Carlton <br /> Tunnel and thence to Four Mile Creek. The recent evaluation of the water levels in the <br /> overflow apron of the diatreme indicates that some water infiltrating at the extreme eastern <br /> edge of the diatreme, in particular in the thalweg of Grassy Valley, is now and has always <br /> been exiting the diatreme via upwelling into Grassy Valley at the location where the <br /> Precambrian rockmass outcrops. This water is not captured by the diatreme. The area where <br /> this capture has always taken place, and is expected to continue to take place in the future is <br /> shown on Plate 9(Rev.1), which is included in Attachment 1. As no Amendment 11 or prior <br /> amendment mining or overburden placement is anticipated in the indicated area, no impacted <br /> water from the operation is expected to enter the Grassy Valley(surface) drainage system. <br /> 3.4.3 Grassy Valley Perched Water Table <br /> • Page 12: The text states"The existence of the perched aquifer in Grassy Valley is an <br /> important aspect of the valley,because it holds groundwater near the ground surface, and <br /> prevents all the water in the alluvium from draining into the underlying bedrock." This <br /> perched aquifer is said to have a water table between 0-50 feet of the ground surface. <br /> Considering this information,could the perched aquifer be impacted by mining <br /> activities(e.g.,ECOSA,WHEX)? <br /> RESPONSE. Perched conditions exist where there is saturated groundwater system <br /> overlying an unsaturated zone (locations as shown in Plate 11 of the original report). <br /> Under these conditions there is always flow from the saturated(perched)groundwater <br /> system vertically through the unsaturated zone beneath. The rate of flow out of the <br /> saturated groundwater zone is ind Wndent of the thickness of the unsaturated zone <br /> beneath, as long as there is one. Accordingly, the perched aquifer cannot be impacted by <br /> mining activities in WHEX. <br /> With respect to the potential impact of ECOSA on the perched aquifer, there will likely be a <br /> short-term change in infiltration to the perched aquifer prior to reclamation, which may be <br /> an increase or a decrease depending on the re-saturation behavior of the overburden, the <br /> precipitation, and the infiltration rate during that period. Following reclamation, using <br /> native material that was reserved and stored from the footprint of ECOSA, infiltration into <br /> ECOSA is expected to return to approximately pre-mining levels, and conditions within the <br /> perched aquifer also will be restored. Accordingly, the perched aquifer will not be <br /> impacted by mining activities at ECOSA. <br /> How will you sufficiently monitor Grassy Valley for impact? <br /> RESPONSE. Grassy Valley surface water flow and quality will continue to be monitored <br /> at GV-03. In addition, Newmont proposes to resume monitoring for surface flow at GV-04, <br /> the monitoring point at approximately the outcrop of the eastern edge of the diatreme in <br /> Grassy Valley. These locations are downstream of arty Amendment 11 or prior amendment <br /> mining disturbance that can affect Grassy Valley surface water flow or quality. Grassy <br /> Valley groundwater level and quality will be monitored at GVMW-22A and GVMW-22B, <br /> which are also located at the "overflow"point in the Grassy Valley thalweg(adjacent to <br /> monitoring location GV-04), so will respond to any change in groundwater level or quality <br /> resulting from the mining operation. Locations of these surface water monitoring locations <br /> are shown, along with Amendment 11 facility locations, in Plate 17(Rev.1), which is <br /> included in Attachment 1. Locations of groundwater monitoring wells are shown on <br /> Figure 2-la, which is included in Attachment 2. <br /> If impacted,what remediation is anticipated? <br /> Page 4 of 28 <br />