2019-11-21_REVISION - M2004044 (44)

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model. There is one remaining domestic alluvial water well in this strip of land. Potential impacts to the well will be addressed by the Ground Water Mitigation Plan. Groundwater Mitigation Plan Groundwater monitoring and evaluation of potential mining or reclamation impacts shall be conducted as part of the Groundwater Monitoring and Mitigation Plan. The Monitoring Plan was reviewed and approved by the Division with the original permit and will be used during mining and reclamation activities. Historic and current water level measurements in the monitoring wells are included as Attachment G1. The Monitoring Plan is outlined below. Al acknowledges that the wells near the South Mine Area will likely be impacted by the dewatering operations. The preemptive plan to address the potential impact will include the installation of cisterns on each of the nearby wells. Figure G-1 presents the preliminary design developed by Tetra Tech and a local driller familiar with the wells in the area.Al will bear the cost of installing and operating the cisterns. If the wells are completely dewatered and it is not practical to replace the existing wells with deeper or more efficient wells,Al will deliver potable water to the cisterns for the duration of the impacts.As indicated above Al may improve the existing wells to provide water to the cistern rather than continue to haul water for an extended period of time. In any event Al will provide potable water to the well owner.Al Response: Please see the revised Exhibit G Sections 2(b), and Potential Mining Impacts. Al is committed to installing an underdrain system to minimize disturbances to the prevailing hydrologic balance of the ground water regime near the mine. Groundwater modeling indicated mounding up to approximately six feet will likely occur along the west and south west margins of the West Mine Area.A preliminary design is shown in plan view and cross sections on Figure G-2 through G-5. Please note that this design is preliminary and may change after the final determination of the location of the slurry wall. The drain system will consist of three sections, South Side Drain,West Drain South Reach and West Drain North Reach. Each drain has a"receiving"section and a"discharging"section. In the"receiving' sections the drain will be installed at approximately the pre-mining ground water level.As the mound builds it will flow into the drainpipe. In non-mounding areas the"discharging"sections the drainpipe will be at or approximately one foot above the pre- mining ground water levels. In the areas of the ground water mound the ground water will flow into the drain then flow down gradient through the drainpipe and discharge back to the ground water. The entire length of the drainpipe will be perforate. The slope of the drainpipe and seasonal high and low water levels are shown on the drain profiles. This design concept has been utilized at the Challenger Pit just north of the West Mine Area. One dimensional steady state ground water flow equations commonly used in construction dewatering were used to size the drainpipe. The calculations are presented in Appendix G-1. The analysis indicated a drain pipe diameter of six inches would be adequate. To establish a baseline,Al installed 17 monitoring wells at the mining site in late 2003. The frequency of water level monitoring was approximately monthly from December 2003 to July 2005.After July 2005, monitoring continued quarterly through 2017. The monitoring frequency increased to monthly in January 2018. Monitoring in the wells will continue on a monthly basis for the life of the mine and for one year after mining ends. (Note: MW-11 was removed during Todd Creek Metropolitan District's construction in 2017 and MW-2 on the Bloom property can no longer be found). MW-2 was I replaced with a new well in roughly the same location in 2018. MW- 11 will not be reinstalled as the area of the well is adjacent to the river and Todd Creek has monitoring wells that they monitor as part of their alluvial wellfield. Al will conduct weekly water level monitoring for the monitoring wells around the south mine area during dewatering and until groundwater levels have recovered once dewatering ends. For the wells around the East and West Mine area, monthly water monitoring would continue to be appropriate. If sufficient data is collected during the life of the mining operation, and a demonstration can be made that impacts to the groundwater system have been minimized, Aggregate Industries Tucson South Resource Permit#M-2004-044112 Permit Amendment Page G-6