Laserfiche WebLink
During mining of Phase 1 the most affected groundwater level was approximately 0.5 miles northwest of the Site. This <br />groundwater level was drawn down approximately seven feet. Mitigation measures at this location may be implemented <br />and are discussed below. Groundwater levels directly south of the Site mounded approximately 0-2 feet. Generally <br />groundwater levels surrounding the Site mounded as a result of slurry lining the Site as presented above in Table 9. <br />4.2.1 Miti ation <br />Mitigation measures are proposed to reduce the extent of groundwater drawdown due to mining. Measures that are <br />recommended to help alleviate the effects of mining include: 1) pumping dewatering water to a recharge pond near <br />irrigation wells on the injured sites or close to the injured sites, 2) possibly replacing the wells or drilling wells deeper, 3) <br />providing a pipeline with an uninterruptiblesupply of water. <br />As discussed in Section 4.1 a recharge pond is presently being used to mitigate the Sakata Farms well. Hall-Irwin has <br />sent a letter to Mr. Sakata committing to address well production problems caused by the mining operation. If <br />necessary, Hall-Irwin will excavate a recharge pond on the Dersham properly near the irrigation well. Dewatering water <br />that has been tested for total suspended solids (TSS) and pH will be delivered to the pond which will then percolate <br />down into the aquifer. Refer to Figure 9 for a schematic of a proposed recharge pond. The aquifer is conducive to <br />recharge ponds because the groundwater table is approximately 10 feet below existing ground and the ponds can be <br />constructed to hold a sufficient volume of water. <br />4.3 Post -Construction (Reclamation) Simulation <br />The proposed Site reclamation after mining consists of surrounding the Dersham Property to the south with a low <br />permeability bentonite soil slurry wall keyed into the low permeability claystone bedrock . The Parker Property 1o the <br />north will be backfilled to existing grade and revegetated with native grasses. Figure 10 shows the proposed Site <br />reclamation. The slurry wall will act as a hydraulic barrier between the water inside the storage vessel and the natural <br />groundwater table. This model was used to evaluate the long-term effects on the groundwater regime of the post <br />construciion project. <br />Cells (100 tt by 100 ft) along the perimeter of the proposed slurry wall were modeled by assigning them a hydraulic <br />conductivity of 0.0045 ft per day (1.6E-6 cm/sec). Recharge inside the storage vessels was eliminated and surrounding <br />Site conditions used in the model were; 1) Aggregate's Pit to the south was fully slurry lined, 2) Gravel Pit west of Tucson <br />Road's water surface elevation was set to approximately 5 -10 feet below existing ground to simulate an unlined <br />reclaimed pit, 3) the recharge area (mitigating Sakata Farm's Well) to the north of Baseline Road was eliminated and 4) <br />the settling pong onsite was eliminated. <br />Little to no decrease (shadow) in groundwater levels on the down gradient (east and north) side of the Site were <br />modeled due to the east, northeast, groundwater gradient. No change in the groundwater levels east of the Site were <br />modeled due to the South Platte River's strong hydrologic connection to the Site. Approximately 0-2 feet of mounding <br />did occur on the western and southern flank of the Site as a result of slurry lining the pit and the strong east, northeast <br />hydraulic gradient (the Site's proximity to the ridge located 2 miles to the west and the western slurry wall contributed <br />heavily to the western groundwater mounding). <br />Groundwater Model Reporl-Hall-Irwin Parker-Dersham Site - 9 - April 2006 <br />