2024-03-21_REVISION - M2022009 - Laserfiche WebLink

Home Browse Search

3.0 PREDICTED IMPACTS TO HYDROLOGIC BALANCE The Site is approved for a sequence of soil bentonite slurry wall (slurry wall)lined cells that will encompass the majority of the Site. The slurry walls will be constructed in stages starting in the south and progressing to the north. The slurry walls will be constructed of low permeability soil and bentonite three(3)feet thick and keyed into bedrock approximately four(4)feet. Slurry walls typically cause a rise in groundwater elevation on the upgradient side(mound)and a drop in groundwater elevation on the downgradient side(shadow). The impacts of the Site on the hydraulic balance are discussed in detail in the"Gilcrest Area Alluvial Groundwater Model" Report by Civil Resources dated January 2023(Groundwater Model Report). The model demonstrated that upgradient of the Site,the groundwater table is expected to rise up to 5.7 feet above the baseline level of the model,without any mitigation. Depth to groundwater in this area after construction of the slurry walls ranges from 18.4 to 20.5 feet below ground. There is one(1)well on the north side(downgradient)side of the site between the South Platte River and the slurry walls. The drop in groundwater elevation at this well with all slurry walls in place is expected to be 2.5 feet below the baseline period of the model. The modest shadow effect on the downgradient side of the Site is likely mitigated by the Site's close proximity to the South Platte River. 4.0 PREDICTED IMPACTS TO WATER QUALITY The majority of mining at the Site will occur inside slurry walls keyed into low permeability bedrock. Groundwater will not be exposed in these cells prior to construction of the slurry walls. After construction of the slurry walls a sump will be dug and trapped groundwater will be pumped from the cell to one of the permitted discharge points. A leak test will be performed once the water level inside of the pit is ten (10)feet below the water table outside of the slurry wall creating a gradient where any slurry wall leakage will be into the pit. Comingled discharge from the lined pits will be made under a NPDES discharge permit from CDPHE and the Site will operate in accordance with a Stormwater Management Plan. This combination of factors will significantly decrease the likelihood of groundwater contamination to the area surrounding the slurry wall lined cells. There will be three(3)unlined cells located on northeastern side of the Site, parallel to Highway 60. These cells will act as both a freshwater source and siltation ponds. Groundwater will be pumped from the Freshwater Pond to the plant where the aggregate will be washed. This water will then be collected and pumped back to the Siltation Pond where the suspended silt will be allowed to settle out. No hazardous chemicals are used in Site operations or processing. Fuel or other fluids used in mining equipment could be a potential source of chemical contaminates. The risk associated with the mining operation are similar to any operation utilizing heavy equipment such as farming or construction. There is a potential of accumulating existing contaminates during the wash process and as such,the area with the highest risk of potential contamination is the plant and unlined pond areas. 5.0 GROUNDWATER QUALITY BASELINE MONITORING Baseline Groundwater Quality Sampling will be conducted for five(5)quarters starting in January 2024 and ending in the second quarter of 2025. One (1)upgradient well (PZ-20)and two(2)downgradient wells(PZ-04 and PZ-09)were selected for chemical analysis. 5.1 Sample Collection Methods Samples will be collected starting in the upgradient well and progressing towards the downgradient well. PZ-20 will be sampled first followed by PZ-09 and then PZ-04. Samples will be collected on the same day. Depth to water will be determined using a water level indicator probe and volume of water inside the well will be calculated and recorded. Monitoring and Analysis Plan—Monarch DENM Gravel Mine -3- March 2024