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1.0 INTRODUCTION The Monarch DENM Gravel Mine(the Site)is located west of Colorado State Highway 85 with Weld County Road 40.5 to the south and is bounded by the South Platte River to the west and north. More specifically,the Site is comprised of portions of Section 18, Section 19,and Section 30,Township 4 North, Range 66 West of the 6th Principal Meridian. The Site is permitted through the Colorado Division of Reclamation, Mining,and Safety as a construction aggregate material mine.The approved mine plan calls for four(4)soil bentonite slurry wall (slurry wall)lined cells that will encompass most of the Site excluding approximately 15 acres along the eastern side of the Site along Highway 60 which will be wet mined and backfilled with wash fines and waste material. Currently,the Site is utilized for pivot irrigated and dry land farming. A groundwater model was prepared for Monarch Mountain Minerals and Aggregates' (MMMA) Monarch DENM Gravel Mine 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.The regional geology map is depicted in Figure 1. Civil Resources analyzed and mapped bedrock from 177 well logs from the Division of Water Resources as well as 27 borings that were drilled at the site by the operator. As shown on Figure 1, 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 4,697 feet in the southeast corner of the Site to an elevation of 4,664 feet around the Town of Gilcrest. On the southeastern side of Highway 85 the bedrock rises to an elevation of 4,719 feet on the southeast side of Highway 85. Bedrock elevation contours are depicted in Figure 1. 3.0 SITE GEOLOGY A total of twenty-seven (27)borings were drilled to bedrock at the Monarch DENM Site. Depth to bedrock ranged from 13 feet in H-13 on the lower terrace approximately 800 feet west of the site boundary to 58 feet in H-02 in the northeast corner 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. Borehole locations and depth to bedrock are included in Figure 1. Groundwater Model Report—Monarch DENM Gravel Mine Site -1 - January 2023