Laserfiche WebLink
Cresson Project – AM-11 Adequacy Review <br />Page 3 <br />May 03, 2016 (Revised May 31, 2016) <br /> <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 <br />of the WHEX pit intercepts the perched aquifer the same location where the <br />mined wall recently experienced slope stability issues? If so, could this be <br />attributed in any way to the presence of a saturated zone on this west-facing pit <br />wall where the Grassy Valley aquifer was mined out? If so, might the operation <br />experience similar slope stability problems along the northern and eastern <br />mined walls of the WHEX where the aquifer will also be intercepted? <br />3.6.3 Impact of Underground Exploration and Mining <br /> Page 17: The text states that the proposed underground mining operation will “be <br />located entirely in the vadose zone above the water table, and will only intersect <br />meteoric water that infiltrates immediately above the mine workings”, estimated to be <br />approximately 3 gpm. Does this require additional water rights? If so, have they <br />been obtained? <br /> <br /> Plate 11 – Perched and Bedrock Water Table Depths shows the bedrock groundwater <br />depth from ground surface in the region of the proposed underground operation to <br />range from approximately 300 - 1,000 feet. What is the maximum expected depth <br />of disturbance for the underground mining operation? Is there potential for the <br />underground mine operation to intercept the water table in this area? <br /> <br /> Does the area of the proposed underground operation (located in Precambrian <br />rock separating the eruptive centers) have sufficient vertical hydraulic gradient <br />to drain into the diatreme fast enough as to not have a significant impact on the <br />underground operation? Will the new activities associated with the proposed <br />underground mining operation have an impact on the Cripple Creek drainage <br />system? Plate 12 Infiltration Capture by Diatreme shows the Chicago Underground <br />Tunnel trending northwest into Poverty Gulch, and terminating directly into or below <br />the stream. How will the operation work to minimize impact to Poverty Gulch, <br />and thus, to the Cripple Creek drainage system? <br />3.6.6 Impact of East Cresson Overburden Storage Area on the Hydrology of Grassy Valley <br /> Page 18: Under Item #1 – Seepage from Toe of ECOSA, the text states “the northern <br />portion of the AM-11 ECOSA is located over the boulder till infill of Grassy Valley. <br />This material is of low (vertical) permeability, and creates a partial barrier to <br />downward flow into the underlying unsaturated bedrock. As a result, under high <br />infiltration conditions, some of the infiltrating precipitation water will flow along the <br />surface of the boulder till, and emerge at the northern downhill tow of the ECOSA.” <br />In footnote #4 (on the same page), you further explain “this condition is currently <br />occurring, particularly following the high intensity thaw/prec ipitation event in May, <br />2015. It is considered that this event provided sufficient water to bring the thin toe - <br />area overburden in ECOSA to its field capacity, and allowed the balance of the water <br />to flow to the base of the pile and shed downhill to exit the toe. The underdrain <br />system installed prior to the construction of the ECOSA under AM-10 was designed <br />to transport water to the east away from the area of ECOSA underlain by both the <br />boulder till and low permeability Precambrian rock and re -infiltrate it through the till