2022-11-07_PERMIT FILE - M2022001 (11)

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1.0 INTRODUCTION The Section 20 Gravel Mine(Site)is located on Colorado State Highway 60 to the west,Weld County Road 44 to the north,Weld County Road 29 to the east and Weld County Road 42 to the south. More specifically,the Site is comprised of the majority of Section 20,Township 4 North, Range 66 West of the 61h Principal Meridian. The Site is planned to be a construction aggregate material mine to accommodate growth in the area.The current mine plan calls for eight(8)soil bentonite slurry walled cells that will encompass most of the Site excluding approximately 20 acres in the northwest comer of the Site which will wet mined and backfilled with wash fines and waste material. Currently, overburden is being mined in the future Cell 8 under an approved Division of Reclamation Mining and Safety(DRMS) 111 Special Operations Permit. No groundwater is planned to be exposed during this operation. A groundwater model was prepared for Red Tierra Equities, LLC's Section 20 Gravel Mine Site (Site)to investigate the impacts of the final construction of slurry walls at the Site on the surrounding alluvial aquifer. This report details the model inputs and construction,the findings of the model and recommendations to mitigate alluvial aquifer impacts. 2.0 AREA GEOLOGY The Site is located approximately 25 miles east of the eastern flank of the Rocky Mountain Front Range. Younger sedimentary strata dip eastward off the Pre-Cambrian igneous and metamorphic rocks that form the core of the Front Range into the Denver Structural Basin. The Denver Basin is an asymmetrical downwarp of sedimentary strata with a steeply dipping west limb and a gently dipping east limb. Bedrock does not crop out at the Site, however regional geologic mapping of the area(Colton, 1978)indicates the near surface bedrock at the Site is most likely the Laramie Formation. Colton (1978)describes the Laramie Formation as mostly claystone, shale, sandy shale, and lenticular sandstone(refer Figure 1). The regional mapping indicates the bedrock is overlain by the Broadway Alluvium. Colton (1978)describes these alluvial deposits as sand and gravel deposited by the South Platte River and its tributaries. In addition to the sonic borings drilled by Cascade, Civil Resources analyzed and mapped bedrock from 177 well logs from the Division of Water Resources. As shown on Figure 2, bedrock in the area generally slopes to the northeast following the route of the South Platte River. Approximately one and a half(1.5)miles southeast of the South Platte River the bedrock drops in elevation dramatically from an elevation of 4696 feet in the northwest corner of Section 20 to an elevation of 4664 feet around the Town of Gilcrest. On the southeastern side of Highway 85 the bedrock rises to an elevation of 4719 feet on the southeast side of Highway 85. 3.0 SITE GEOLOGY A total of 23 borings were drilled to bedrock at the Section 20 Site. Depth to bedrock ranged from 39.5 feet in BH-38 in the northeast comer of the Site to 97.5 feet in BH-41 in the southeast comer of the Site. The bedrock encountered at the Site consisted of wet brown weathered claystone in the first 6 inches to 1 foot which transitioned in to moist to dry, olive to grey claystone with further depth. The claystone had a high plasticity and minimal fracturing. The bedrock unit was not sampled. Depth to bedrock is mapped out in Figure 1. The alluvial aquifer at the Site consist mainly of gravelly sand which grades to sandy gravel, usually becoming more gravelly with depth.Some cobble units(grain size 4"or greater)were encountered at the Site; mainly consisting of a few feet overlaying bedrock. Aggregate unit thickness varied from 35 feet in BH-36 in the northern side of the Site to 72.5 feet in BH-27 in the eastern side of the Site. Groundwater Model Report—Section 20 Gravel Mine Site -1 - August 2022