2018-07-10_PERMIT FILE - M2018037 (4)

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May 30, 2018 Mr. Mark Johnson, Compliance Manager Burnco Colorado, LLC 301 Centennial Drive Milliken, Colorado 80543 RE: Inouye Gravel Mine—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 proposed Inouye Gravel Mine(Site)and the groundwater impacts that could result from soil-bentonite slurry wall (slung wall) lining and mining at the proposed Inouye Gravel Mine. The site is located at and in the vicinity of 12210 Weld County Road (WCR) 14.5 in Weld County, Colorado. More specifically,the site is within parts of Section 31, Township 2 North, Range 66 West; and Section 36, Township 2 North, Range 67 West of the 6th Principal Meridian. 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 Laramie Formation and/or Denver/Arapahoe Formations. Groundwater measured in five site monitoring wells occurs at depths ranging from approximately 5 to 9 feet blow existing ground. The prevailing groundwater flow direction at the Inouye Gravel Mine area is north- northeasterly roughly reflecting the site topography. Groundwater in the area is tributary to the South Platte River located east and northeast of the site. Locally the groundwater levels and flow directions are likely influenced by: ➢ The South Platte River is east 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 Lupton Bottom Ditch traverses the site from south to north between the northwest and northeast cells. Where the ditch enters the site on the south,the ditch is raised relative to the surrounding land. In this south area, seepage from the ditch likely recharges the groundwater table. Progressing to the north, the elevation difference between the ditch and surrounding land becomes more subdued and the ditch may behave more like a drain maintaining groundwater elevations at or above the ditch water levels. ➢ An unnamed drainage way is present on the west part of the site. This drainage enters the site in the southwest area and traverses the site in an east-northeasterly orientation before turning a more northerly direction and exiting the site on the north. This drainage likely maintains groundwater levels at or above the water levels in the drainage. This drainage will be relocated to the west boundary of the site.