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CIVIL RES"*URCES, LLC <br /> ENGINEERS & PLANNERS <br /> June 111, 2021 <br /> Mr.Joshua Oliver <br /> Ready Mixed Concrete Company <br /> 2500 E Brannan Way <br /> Denver, CO 80229 <br /> RE: Nix Gravel Mine Amendment AM01 —Mitigation Plan for Potential Groundwater Impacts <br /> Dear Mr. Oliver <br /> The purpose of this memo is to describe the existing groundwater regime in the vicinity of the Nix gravel mine including <br /> the potential groundwater impacts of the new soil-bentonite slurry wall(slurry wall) installations and wet mining of the <br /> Siltation and Freshwater Ponds. The Nix site is bounded by Colorado State Highway 66 to the north,Weld County Road <br /> 19 to the east, Weld County Road 28 to the south and Weld County Road 17 to the west. More specifically, the site is <br /> within parts of Section 28, Township 3 North, Range 67 West, 6th P.M., County of Weld, State of Colorado. The site is <br /> 3,200' south east of the Saint Wain River at its closest point. Land uses in the area include irrigated agricultural, oil and <br /> gas production, active gravel mines, mines reclaimed as below grade reservoirs, and low-density residential housing. <br /> The Nix site will be mined in 3 stages comprising 7 cells. Stage 1 will consist of 2 unlined cells referred to as the <br /> freshwater pond and the siltation pond. The siltation pond will receive wash fines from the processing of mined sand and <br /> gravel. Stage 2 will be lined with a slurry wall and will contain Cell 3 and 4. Stage 3 will also be lined with a slurry wall <br /> and will contain Cells 5, 6, and 7. <br /> Existing Groundwater Conditions <br /> The near surface groundwater is part of an alluvial aquifer in which permeable sand and gravel alluvium overlies <br /> relatively impermeable bedrock of either the Fox Hills Sandstone and/or the Laramie Formation. Groundwater, measured <br /> in 8 piezometers, occurs at depths usually ranging from 5'to 26'feet with shallower groundwater to the north of the site. <br /> The prevailing groundwater flow at the site is to the north west reflecting the site topography. Groundwater in the area is <br /> tributary to the Saint Wain River located north of the site. Locally the groundwater levels and flow directions are likely <br /> influenced by: <br /> ➢ The Saint Wain River is north of the site. For most of the year, the river likely acts as a drainage way <br /> maintaining groundwater levels at elevations greater than water elevations in the river. In shorter <br /> periods of high run off, usually in the spring, river water levels will locally recharge the groundwater <br /> table. <br /> ➢ A lateral of the Last Chance Ditch traverses the Nix property from south to north between Cell 1 and <br /> Cell 7 and then follows the west permit boundary until it flows underneath Highway 66. The ditch may <br /> act like a drain during the non-irrigation season maintaining water levels at or above the water levels in <br /> the ditch. During the irrigation season, the ditch may serve as a source of recharge to the water table. <br /> ➢ There is a pond south of County Road 28. This pond likely causes elevated groundwater levels at the <br /> south of the site during the irrigation season. <br /> ➢ The Varra Pit to the northwest of the site likely causes some groundwater mounding (higher water <br /> levels)on the downgradient side of the site. <br /> 323 FIFTH STREET• FREDERICK, COLORADO 80530• PHONE: 303.833.1416 9 FAX: 303.833.2850 <br />