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Request#6: 3.6.6 Impact of East Cresson Overburden Storage Area on the Hydrology of Grassy Valley The Operator's response indicates the ECOSA underdrain system is functioning property to route infiltration from disturbed areas located outside of the diatreme, to the diatreme. The Operator's response also indicates the ECOSA underdrain system is designed to conduct water which will seep through areas underlain by Precambrian rock to areas underlain by diatremal rock. Pursuant to Rule 3.1.5(11), no unauthorized release of pollutants to groundwater shall occur from any materials mined, handled, or disposed of within the permit area. The purpose of the ECOSA underdrain system is to direct potentially impacted water(infiltrating ECOSA materials) to the diatreme. Therefore, please specify the maximum flow rate of the ECOSA underdrain system (based on the as-constructed survey), and the maximum infiltration rate and volume of areas drained by the system. Response: The underdrain system was designed and installed prior to the construction of the ECOSA in an area of the ECOSA that is underlain by Precambrian bedrock. It addresses a possible concern that the groundwater system in this area may not have sufficient infiltration capacity to accept the infiltration that will take place during construction, operation and/or closure in this northernmost section of ECOSA. As the capacity of the area was sufficient to accommodate the infiltration that occurred prior to mining, it is anticipated that it will continue to do so after coverage by ECOSA, rendering the underdrains superfluous. The purpose of the underdrain system is to ensure that seepage from ECOSA reaches the diatremal seepage treatment system that underlies almost the entire development of the Cresson Mining project, and which is integral to the permitted operation of the mine. The capacity of each underdrain in the system to convey water is computed to be approximately 0.25 cfs (Plate 33). The average annual amount of seepage that is computed to impinge on the surface serviced by the drains is (Plate 33): Seepage from portion of ECOSA underlain by Precambrian rock......0.017 cfs (-8 gpm) Seepage from portion of ECOSA covered by the drains..................0.046 cfs (-20 gpm) Both of these flows are far short of the capacity of a single drain, so the system will accommodate all infiltration to either area without the flow causing overflow of even one drain. Note that the above computations assume that none of the incident seepage from the ECOSA infiltrates into the surface below the storage area, which is considered unlikely. In fact, it is expected that all or most of the seepage will infiltrate, bypassing the drain system. With respect to the requirements of Rule 3.1.5 (11), the entire Cresson Mining project is designed, authorized, and operated with the understanding that precipitation water will: • infiltrate into the surface of the mining facilities during operation and after closure, • proceed through the mined materials present on the project, • be directed into the diatremal rockmass, • seep predominantly vertically through the underlying vadose zone, • flow laterally in the saturated rockmass within the project boundary to enter the Carlton Tunnel, and • flow for approximately 7 miles to the southwest and exit under permit at the portal of the Carlton Tunnel at Four Mile Creek. For the ECOSA underdrains, the maximum average annual quantity of water that is expected to pass out of the underdrains into the subsurface and pass ultimately through the Carlton Tunnel is 0.046 cfs (20 gpm). Page 8 of 16