2016-07-28_REVISION - M1980244 (2)

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vertical seepage capacity of the underlying clay till. This may be the explanation of the observed 201 S flow. However none of this indicates that "the ECOSA underdrain system is not functioning properly to drain ECOSA infiltrated water east into the diatreme'; as stated by DRMS. The ECOSA underdrain system is designed to accommodate and direct to the diatreme infiltration to ECOSA after reclamation, when the surface cover of clay till that will be emplaced at reclamation-time is in position. As there is no evidence of flow from the ECOSA toe in 2016 in any location except that immediately adjacent to the pipe break location, under normal infiltration rates the system appears to be functioning adequately, despite being unreclaimed. Thus, Newmont will "work to minimize seepage and potentially impacted groundwater flow from ECOSA into Grassy Valley"by collection and treatment, if necessary, during operation as discussed above, and by installation of the reclamation clay till surface at closure. At the end of Item#l,the text states "Seeps from the toe of ECOSA, if any, will be sampled, characterized, and treated if necessary."Please provide more detail. RESPONSE.Sampling will be triggered by the observation of water flow to and retention within the ECOSA toe drain. Routine inspections are conducted as a standard operating practice for ECOSA. Sampling and characterization of seepage is conducted in accordance with Newmont's Best Management Practices and in alignment with the Spill Prevention Containment and Countermeasures Plan and the Stormwater Management Plan on a case by case basis. Impacted waters would be diverted to the EMP stormwater ponds around the base of the ECOSA and prevented from flowing into Grassy Valley. What is the current estimated volume of seepage from ECOSA to outside of the diatreme? RESPONSE. The current estimated volume of seepage from ECOSA to outside of the diatreme is zero. The only observed past flows from the ECOSA toe is the reported flow from the toe in 2015 due to high springtime precipitation, and the observed flow from the toe in 2016 due to breakage of a freshwater pipe that was located under the toe of ECOSA. There is no showing that water from either of those two events exited the diatreme, although there is also no direct measurement that supports that they did not. As a result of both of these seepage events, the toe seepage collection system at ECOSA has been upgraded to intercept future seepage, and direct it into the diatremal rock via the colluvium and clay till overlying the diatreme. In the event that the capacity of this system is observed to be inadequate, Newmont will capture excess flow and direct it into the mine water system,preventing any seepage from ECOSA to outside of the diatreme. Where and how often will sampling occur? What parameters will be measured? How will the seeped water be treated if necessary? Will the seeped water be captured and/or diverted in any way? RESPONSE.Sampling would occur on a case by case basis. If toe seepage was discovered, the intent would be to keep the seepage within the footprint and to flow into the diatreme or evaporate. Surface flows would be directed to EMPs situated around the toe of ECOSA. Samples are tested for a suite of constituents and include multiple metals; acidity; alkalinity; cyanide ; nitrates; suspended solids; as well as field parameters that include pH, conductivity; dissolved oxygen;flow (gpm); and temperature. As the ECOSA is loaded to the AM-11 proposed 10,960 ft amsl height,will this increased load intensify seepage from the ECOSA toe? RESPONSE.No. The increased thickness of ECOSA provides a greater attenuation potential for infiltration, and so tends to reduce, rather than intense, seepage. Page 9 of 28