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DEERS & A U L T 600 South Airport Road, Suite A-205 <br /> Longmont, Colorado 80503 <br /> 303-651-1468 <br /> �°� � �'• x � schnabel-eng.com I deereault.com <br /> TECHNICAL MEMORANDUM <br /> TO: Matthew Noteboom DATE: May 19, 2023 <br /> COMPANY: L.G. Everist, Mountain Division SUBJECT: West Farm Groundwater Model <br /> ADDRESS: 7321 E. 88th Ave , Suite 200, PROJECT L.G. Everist West Farm <br /> Henderson, CO 80640 NAME/NO.: 20C26026.02 <br /> FROM: Naveen Kumar CC: <br /> Victor deWolfe, P.E., P.G. <br /> This memorandum discusses the groundwater impact analysis at the proposed future West Farm mine site. The <br /> purpose of the analysis is to provide information related to the potential groundwater mounding upgradient of the <br /> proposed slurry walls at the site. The site is located approximately two miles west of the town of Platteville, <br /> Colorado The property is bordered on the east by the South Platte River and on the west by Weld County Road <br /> (WCR)21. The mine plan for the site consists of two main mining areas (North and South) divided by WCR 32.5. <br /> In each area there are two mine cells planned: one on the lower portion of the property (to the east), and a <br /> second along the upper terrace on the western portion of the site. Slurry walls will be constructed around the <br /> perimeter of each of the four planned cells, as shown on Figure 1. A standard offset of 200 feet from the river, <br /> and 15 feet from property lines, rights-of-way or utilities was used for the proposed slurry wall alignment. <br /> A groundwater model was constructed to evaluate the impact of the slurry walls on groundwater levels. The <br /> objectives of the groundwater model are to: <br /> 1. Approximate the pre-slurry wall groundwater hydrology using available data <br /> 2. Simulate the hydraulic effect of the slurry walls by predicting potential groundwater mounding <br /> upgradient of the property and shadowing downgradient <br /> 3. Simulate the conditions if a drain was installed to mitigate high groundwater <br /> To satisfy these objectives, steady state groundwater models were constructed for: 1) pre-slurry wall conditions, <br /> 2) post-slurry wall construction conditions and 3) mitigated slurry wall conditions. <br /> The model domain was set within the alluvial floodplain of the South Platte River just upstream from the <br /> confluence with Saint Vrain Creek. The South Platte is the primary surface hydraulic feature in the area. The <br /> topography slopes gently from the south along the valley with alluvial terraces immediately above the floodplain. <br /> The near surface bedrock is the Cretaceous Laramie Formation. Land use in the area consists of mining and <br /> agricultural uses. <br /> This modeling memorandum presents a general conceptual model of the aquifer system followed by a discussion <br /> of the groundwater modeling software; construction of the model; calibration of the model in terms of target <br /> residuals and mass balance; and finally, a discussion of the predictive simulations and conclusions. The <br /> groundwater modeling was conducted in general conformance with ASTM standards for groundwater modeling. <br />