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1.0 INTRODUCTION A groundwater model was prepared for SW-TKO Joint Venture, LLC's (SW-TKO) Lupton Lakes Project (Site) to evaluate the potential impacts of gravel mining and the final reclamation of the Site to the shallow alluvial aquifer and surounding wells. 1.1 Site Description The Site is comprised of several properties (Wiant, Gurule, Villano, Sasaki, Adams, Watada, and Yokooji) located within parts of Sections 18 and 19 of Township 1 North, Range 66 West of the 6th Principal Meridian. The project is located approximately 600 to 2000 feet east of the South Platte River and is generally bounded on the west by U.S. Highway 85, Weld County Road 6 (WCR) on the south, and WCR 27 to the east in Weld County, Colorado, as shown on Figure 1. The Site slopes north, northwest toward the South Platte River and is underlain by the Broadway Alluvium and the Denver/Arapahoe Formation claystone (Colton, 1978). The average land slope is 0.1 percent and the groundwater is typically 25 feet below the surface during most of the year. The Site is surounded by primarily agricultural land with domestic wells and irigation wells to the north, south and east. Several commercial buildings exist near the central eastern and southwestern project boundaries. The model was built with available data to evaluate the potential effects to the shallow alluvial groundwater aquifer resulting from the proposed mining activities at the Site. This groundwater model was calibrated to pre-mining observed conditions and then used to approximate the probable affects of mining and final site reclamation. 2.0 GROUNDWATER MODEL 2.1 Description The MODFLOW program within Groundwater Vistas Version 4 utilizes afinite-difference mathematical model to simulate groundwater flow developed by the United States Geological Survey (USGS). The model has a total area of approximately 23 square miles, extending approximately 2.5 miles to the north and south; 1.5 miles to the west; and 3.5 miles to the east (refer to Figure 1). The grid size is 500 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 138 rows and 77 columns. The following boundary conditions were applied to the model as presented in Figure 2 and a typical cross section can be seen in Figure 2a: - South Platte River to the west (River Package within MODFLOVI~; - No flow boundary to the east to represent noted bedrock outcroppings, - Constant head boundaries located on the northern and southert margins of the model and, - Caaystone bedrock - aquitard forming the bottom of the aquifer. The shallow alluvial aquifer was modeled as a single homogeneous aquii'er (sand and small gravel) underlain by impervious claystone bedrock. The model parameters such as, site topography, bedrock elevations, geologic conditions, recharge and hydraulic conductivities are discussed in the following sections. Groundwater Model Report - SW TKO Water Storage Project - t - February 2005