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Grassy Valley <br /> • Page 17: Was the title supposed to read"...on Surface Water Flow in Grassy Valley" <br /> rather than"...Surface Vegetation in Grassy Valley"? If so,please correct this title to <br /> prevent any confusion it may cause. <br /> RESPONSE. The title is correct. The section of the report does refer to the lack of impact <br /> of mine drainage (if any) on Grassy Valley vegetation. <br /> • Page 17: The text lists two reasons why there will be no impact due to drainage to the <br /> mine, including the head elevation of the water in the Precambrian wall rock being lower <br /> than the base of the mine,and the up to 1,000 feet of unsaturated rock between the base of <br /> the Grassy Valley aquifer and the top of the zone of saturation in the diatreme will prevent <br /> any impact to Grassy Valley due to water pressure changes. Considering that up to 700 <br /> feet of Precambrian rock in the headwaters of Grassy Valley will be mined out in the <br /> WHEX pit, please expand on your answer as to how this will not cause inflow into the <br /> mine from surrounding rock, reduction in head in the Grassy Valley aquifer,and/or <br /> impacts due to reduction in head in the surrounding valley. <br /> RESPONSE: The Precambrian rock in the headwaters of Grassy Valley is, in general, <br /> unsaturated(or at least depressurized) to depths greater than the proposed depth of WHEX, <br /> as shown on Plate 16 of the report. This desaturation is the result of the presence of the <br /> dewatered diatreme immediately adjacent to the steep edge of the Precambrian rock on the <br /> eastern side of WHEX, which has caused long-term drainage of what has become the wall <br /> rock of WHEX. There is currently no observed groundwater inflow to the WHEX mine, and <br /> this condition is expected to continue. Accordingly, no inflow is expected from the <br /> Precambrian rock to WHEX or any other mine area. <br /> • Plate 16 - Impact Potential of Drainage to WHEX shows a significant portion of the <br /> Grassy Valley headwaters being mined out. Is the location where the southern edge of <br /> the WHEX pit intercepts the perched aquifer the same location where the mined wall <br /> recently experienced slope stability issues? If so,could this be attributed in any way <br /> to the presence of a saturated zone on this west-facing pit wall where the Grassy <br /> Valley aquifer was mined out? If so, might the operation experience similar slope <br /> stability problems along the northern and eastern mined walls of the WHEX where <br /> the aquifer will also be intercepted? <br /> RESPONSE: Note, the WHEX pit was permitted under Amendment 10 and no <br /> modifications to the extent or envelope of WHEX are proposed in Amendment IL The <br /> information on the 'perched aquifer"presented on Plate 16 refers to pre-mining <br /> conditions. The referenced slope failure occurred on the western side of the WHEX, and is <br /> not considered to be associated with the perched aquifer. <br /> It should be noted that the use of the term 'perched aquifer" is not intended to convey that <br /> it constitutes a significant source of water. It is not; rather, it is made up of low <br /> permeability clay, silt, and fine sand(which is why it retains moisture in the near surface <br /> that sustains plant growth), and is largely incapable of conducting lateral flow. <br /> Accordingly, the "perched aquifer"represents a unit that restricts or prevents water <br /> pressure buildup in the underlying materials, and thus improves rather than degrades <br /> slope stability. <br /> DRMS's fundamental point that water is not good for slope stability is noted and care is <br /> being and will continue to be taken to ensure surface water and groundwater is managed <br /> Page 6 of 28 <br />