2020-02-05_REVISION - M1998013 (27)

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January 31, 2020 Mr. Mark Johnson, Compliance Manager Burnco Colorado, LLC 301 Centennial Drive Milliken, Colorado 80543 RE: Shores Gravel Mine Vogl Amendment AM03-Mitigation Plan for Potential Groundwater Impacts Dear Mr. Johnson: The purpose of this memo is to describe the existing groundwater regime in the vicinity of the Shores gravel mine including new mine cells on the Vogl property. Potential groundwater impacts that could result from dewatering of unlined cells and from soil-bentonite slurry wall(slurry wall) lining are also addressed herein. The Vogl Amendment is located west and north of the intersection of Weld County Roads 11 1/<and 26, immediately north of the Town of Firestone, Colorado. More specifically,the mine is within parts of Section 1, Township 2 North, Range 68 West and Section 36,Township 3 North, Range 68 West of the 6th Principal Meridian. The site is 200 feet south of the Saint Vrain River at its closest point. Land uses in the area include irrigated agricultural,oil and gas production,active gravel mines,gravel mines reclaimed as below grade reservoirs, and residential housing. The mine plan is shown on Figure 1. The Shores site was mined in five lined cells. With the addition of the Vogl property,four more cells referred to as Cell 1, Cell 2a, Cell 2b, and Cell 3 will be mined. The Cells 2a and 3 will be lined with soil-bentonite slurry walls and will be reclaimed as below grade reservoirs. Cells 1 and 2b will be unlined and will receive wash fines from the processing of mined sand and gravel. Existing Groundwater Conditions The near surface groundwater is part of an alluvial aquifer in which permeable sand and gravel alluvium overlies relatively impermeable bedrock of the Upper Transition Member of the Pierre Shale and the Fox Hills Sandstone. Groundwater is measured in eight(8)site monitoring wells occurs at depths usually ranging from approximately 5 to 10 feet blow existing ground. However,groundwater depths in one well on the easternmost side of the mine, occur at a deeper depth generally in the range of 18 feet below the ground. This deeper depth is likely due to dewatering activities at the LG Everist Mine located on the east side of Weld County Road 13. The prevailing groundwater flow direction at the Vogl Gravel Mine area is northeasterly roughly reflecting the site topography. Groundwater in the area is tributary to the Saint Vrain River located north of the site. Locally the groundwater levels and flow directions are likely influenced by: ➢ The Saint Vrain River is north of the site. For most of the year,the river likely acts as a drainage way maintaining groundwater levels at elevations greater than water elevations in the river. In shorter periods of high run off, usually in the spring, river water levels will locally recharge the groundwater table. ➢ The Last Chance Ditch traverses the Vogl property from west to east between Cells 1 and 2a. The ditch may act like a drain during the non-irrigation season maintaining water levels at or above water levels in the ditch. During the irrigation season,the ditch may serve as a source of recharge to the water table.