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1.0 INTRODUCTION <br />A groundwater model was prepared for Hall-Irwin's Parker-Dersham Mine Site (Site) located adjacent to Hall-Irwin's <br />Brighton Pit Gravel Mine to evaluate the potential impacts of gravel mining and the final reclamation of the Site to the <br />shallow alluvial aquifer and surrounding wells. <br />1.1 Site Description <br />The Site is comprised of two properties and one gravel mine (Parker Property to the North, Dersham Property to the <br />south and the Brighton Pit to the east) located in Sections 36, Township 1 North, Range 67 West of the 6th Principal <br />Meridian. The area encompassing the Site and the Brighton Pit is approximately 300 acres and is located approximately <br />one half mile west of the South Platte River. The Site is generally bounded on the west by Weld County Road 23.5, <br />Weld County Road 2 (WCR) on the south, and pasture immediately to the east. As presented in Figure 1, the Site is <br />located in Weld County, Colorado just northwest of Brighton, Colorado. <br />The Site slopes north, northeast toward the South Platte River and is underlain by the Piney Creek and Past Piney Creek <br />Alluvium and the Denver/Arapahoe Formation claystone (Colton, 1978). The average land slope is 0.5 percent and the <br />groundwater is typically 10 feet below the surface during most of the year. <br />The Site is surrounded by primarily agricultural land with domestic wells and irrigation wells to the north, south and east. <br />Wells change from shallow to deep (non-tributary) as you move further west from the Site indicating the loss of <br />productive shallow alluvial aquifer. Active gravel pits surround the site to the south and reclaimed gravel pits are located <br />to the northeast of the site. The model was built based on available data to evaluate the potential effects to the shallow <br />alluvial groundwater aquifer resulting from the proposed mining activities at the Site. This groundwater model was <br />calibrated topre-mining observed conditions and then used to approximate the probable affects of mining and final site <br />reclamation. <br />2.0 GROUNDWATER MODEL <br />2.1 Description <br />The MODFLOW program utilizes afinite-difference mathematical model to simulate groundwater flow developed by the <br />United States Geological Survey (USGS). Groundwater Vistas (V 4.0) provided the graphical interface for entering and <br />reviewing the MODLFOW data. The model has a total area of approximately 10 square miles, extending approximately <br />2.5 miles to the north and south; 2.0 miles to the west; and half a mile to the east (refer to Figure 1). The grid size is 500 <br />feet by 500 feet squares outside of the Site and reduced to 100 feet by 100 feet squares on the Site, with a total of 96 <br />rows and 58 columns. <br />The following boundary conditions were applied to the model as presented in Figure 2 and a typical cross section can be <br />seen in Figure 2a: <br />- South Platte River to the east (River Package within MODFLOW); <br />- No flow boundary to the southwest to simulate bedrock outcropping; <br />- Dry Creek to the northwest (River Package within MODLFOW); <br />- Constant head boundaries located on the southern margins of the model; and, <br />- Claystone bedrock - aquitard forming the bottom of the aquifer. <br />Groundwater Model Report -Hall-Irwin Parker-Dersham Site - t - April 2006 <br />