2020-12-28_GENERAL DOCUMENTS - M1977300 (22)

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PERMIT#: M-1977-300 INSPECTOR'S INITIALS:AME INSPECTION DATE: October 8,2020 ,mponents will be keyed in one foot deep upstream of the seal wall. After installation of the Geoweb components is complete, the Geoweb will be filled with concrete. The operator anticipates the concrete pour will begin approximately one week after the date of this inspection, and will occur over a period of approximately two weeks. Therefore, the concrete placement is expected to be completed by the end of October. After the concrete has been installed, the riprap energy dissipation basin and road crossing will be installed at the base of the channel. Channel flows will discharge into the riprap basin then flow through a riprap-lined channel to a Geoweb-lined, concrete-filled, low-flow road crossing before discharging into Ralston Creek. The operator anticipates construction of the dissipation basin and road crossing will occur the first week of November, depending on weather conditions. After all components of the diversion channel have been installed, disturbed areas (e.g., top of NWRP, side slope of new access road) will be seeded at 16 pounds/acre with Jefferson County's native conifer seed mixture. The operator estimates the project will be completed in early November. The Division approves the Phase 2 construction work observed duringthe he inspection and authorizes the operator to proceed with construction as planned. The next inspection must be scheduled after the NWRP diversion channel project is completed. The Division observed the borrow area located up valley from the NWRP where rock and fill material was derived for the diversion channel project. The material was excavated from an alluvial fan deposited at the base of an ephemeral drainage during the September 2013 flood events. This area includes a pit, some material stockpiles, and a relatively flat equipment storage/turnaround area. Material processing in this area consists of greening only. Mr. Ray indicated this area will be graded and seeded for reclamation. The Division did not -)serve any boundary markers in this area, so it was not clear whether this borrow area is included within the approved permit area. Typically, if an operator extracts construction material on their land to maintain roads, etc. on their land, and the material does not leave the site, then the disturbed area is not required to be included within the permit area. The Division will discuss this matter further with the operator to determine whether or not the disturbed area is considered affected lands which must be permitted. During the inspection, Ralston Creek was dry upstream from the upgradient cutoff wall/headgate. The creek channel downstream from the upgradient cutoff wall was also dry. The water treatment plant was online and the treated water was being discharged into Ralston Creek below the mine site (under the NPDES permit). The mine pool level was estimated to be at approximately 288 feet below the Steve Level. The exact level was not known at the time due to a transducer malfunction. According to Mr. Ray, the malfunction was due to the transducer not being suspended deep enough in the water column to allow it to accurately monitor water levels. With the mine pool being pumped down to lower levels, the transducer needs to be lowered to cover the new range of depth. The operation had plans to lower the transducer another 50 feet or so inside the mine and to also run a tape down Shaft 1 inside the Steve adit to obtain an actual depth to water measurement. After the inspection(on 10/12), the operator informed the Division the mine pool was measured (on 10/8) at 315 feet below the Steve Level, which is 165 feet below the required 150 foot depth. The operator monitored the transducer readings over the weekend and confirmed it is functioning properly at its new depth. Current plans are to continue lowering the mine pool through the rest of the month to enable a 6-month shutdown of the plant (anticipated for November 2020 through May 2021). The alluvial excavation project was not underway during the inspection. Mr. Ray indicated this project has been .7 hold to some extent with minimal activities while the NWRP diversion channel project is underway. However, the operator has plans to complete the excavation project over winter once the channel project is complete and AM-5 has been approved. Given the limited capacity remaining in the Minnesota Mine for disposal of the excavated material, the operator will not be able to complete the excavation project until another Page 3 of 19