2016-07-28_REVISION - M1980244 (2)

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be imported to the mine for operations. With respect to surface water management,please see Appendix 10: Design Report for Storm Water Management at the Chicago Tunnel Site, which requires retention of 2 times the 10 year 24 hour storm event. 3.6.6 Impact of East Cresson Overburden Storage Area on the Hydrology of Grassy Valley • Page 18: Under Item#1 —Seepage from Toe of ECOSA,the text states"the northern portion of the AM-I I ECOSA is located over the boulder till infill of Grassy Valley. This material is of low(vertical)permeability,and creates a partial barrier to downward flow into the underlying unsaturated bedrock. As a result, under high infiltration conditions, some of the infiltrating precipitation water will flow along the surface of the boulder till,and emerge at the northern downhill tow of the ECOSA." In footnote#4 (on the same page),you further explain"this condition is currently occurring,particularly following the high intensity thaw/precipitation event in May, 2015. It is considered that this event provided sufficient water to bring the thin toe-area overburden in ECOSA to its field capacity, and allowed the balance of the water to flow to the base of the pile and shed downhill to exit the toe. The underdrain system installed prior to the construction of the ECOSA under AM-10 was designed to transport water to the east away from the area of ECOSA underlain by both the boulder till and low permeability Precambrian rock and re-infiltrate it through the till to the diatreme beneath. However, it appears that the till permeability is too low to allow at least this high infiltration rate water to pass vertically to the diatreme, causing the toe seepage." • Plate 17—Impact Potential of ECOSA on Grassy Valley shows the northern portion of ECOSA to be located outside of the diatreme. Considering that the ECOSA underdrain system is not functioning properly to drain ECOSA infiltrated water east into the diatreme,the potential for impact to Grassy Valley appears to be high. How will you work to minimize seepage and potentially impacted groundwater flow from ECOSA into Grassy Valley? RESPONSE: Seepage from the toe of ECOSA is and will continue to be collected in a ditch that extends the entire length of the toe. Flow so collected will be tested and directed to infiltration or treatment, as appropriate. This procedure has already been implemented in response to afresh water pipe failure beneath ECOSA, which occurred in 20162. The ECOSA ditch and EWs have been effective at protecting Grassy Valley from impact. The fresh water pipe break most probably occurred in 2016, based on water supply system information, which suggests that the seepage observed by Newmont personnel in 2015 had a different cause. It is possible that this cause was the high precipitation observed in the spring of that year, which resulted in the highest observed surface water flows in Grassy Valley(GV-03, which is normally dry, recorded a 15 day period of flow in March with a peak of 565 gpm recorded on March 17, 2015, and a 49 day period of flow in May and June peaking at 325 gpm on May 21, 2015). This is indicative that there was "seepage" from the entire basin in that unusually wet spring period, and so it would not be surprising if there were corresponding "seepage"from ECOSA.At the time, ECOSA was not reclaimed, so the conditions existed to allow for high infiltration through the overburden materials placed on it. At and near the toe the thickness of overburden material was small, and the grainsize of the overburden is large, so it would not be surprising if the overburden was at or reached its field capacity at that time, allowing infiltration through the highly permeable ECOSA toe material at a rate exceeding the z Letter from CC&V/Newmont to Timothy Cazier dated June 9, 2016, Re: Permit No.M-1980-244:Cripple Creek and Victor Gold Mining Company;Cresson Project; Fresh Water Pipe Line Release Update. Page 8 of 28