2023-05-25_PERMIT FILE - M2022048 (9)

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ENVIRONMENT, INC. PAGE 8 L.G. EVERIST, INC.-WEST FARM RESERVOIR ADEQUACY RESPONSE 01 MAY 23, 2023 Attached is a Water Quality Monitoring Plan prepared by Molen and Associates. The plan includes installing 7 additional monitoring well outside the mining area and a list of compounds that will be tested for in the first 5 quarters. 23) The Mining Plan as submitted only includes a 15 foot setback from the permit boundary/utility corridor to the slurry wall on the south side of Phase 1, and does not include any provisions such as a French drain, to allow groundwater to flow unimpeded around the proposed slurry wall. No groundwater modeling data has been provided to demonstrate that the construction of this slurry wall will not result in excessive groundwater mounding on the adjacent property to the south of Phase 1, or adverse impacts to the irrigation return flows to the South Platte River which currently flow through Phase 1. The actual setback is 15 feet from the AN& gas line 50' ROW and 40 feet from the actual gas line. The south line of the ROW is 18 feet north of the property line so there is 80 feet from the permit line to the liner. The ROW agreement does not limit the land owner's use of that area, so the necessary precautions will need to be taken to install a ground water elevation drain if needed and/or the irrigation return flow ditch. Please provide a detailed groundwater model illustrating the projected impacts to groundwater levels adjacent to the site after installation of the proposed slurry walls. This should include a groundwater contour map showing projected depths to groundwater and any mounding or shadowing on properties adjacent to the proposed permit area. If the modeling predicts excessive groundwater mounding on adjacent property, the applicant will need to provide a plan to minimize the impacts and maintain the prevailing hydrologic balance during and after mining activity. Attached is a detailed ground water model prepared by Schnabel Engineering addressing your suggested parameters. Based on the historic data available for the site, please propose trigger levels for groundwater level mitigation and where the monitoring points will be located. For example: excessive mounding requiring mitigation could be a groundwater level increase of over 2 feet from baseline, or groundwater levels within 4 feet of the ground surface measured at the property boundary. A mitigation plan was addressed in Exhibit G - Water, on pages 29 & 30. It was based on the average LGE has seen over many years mining gravel in the South Platte River valley. It has been revised using information from the Ground Water Model prepared by Schnabel that is site specific. The Schnabel model predicts a change of as much as 3 feet along the southwest and west sides of the Permit area in an area where it is 16 plus feet to the water table and a decrease of as much as 1.5 feet immediately along the river in the north central part of the permit area.