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1.0 The Hazeltine sand and gravel resource is located approximately three miles south of the Town of Henderson in the south half of Section 9, Township 2 South, Range 67 West in Adams County, Colorado. As shown in Figure 1, the site is bordered by 104th Avenue on the south, Brighton Road on the east, the South Platte River on the west, and the existing Howe-Haller gravel mine on the north. Aggregate Industries-WCR, Inc. plans to install aloes-permeability clay slurry wall, keyed to bedrock, around the primary mining area at the Hazeltine site. The slurry wall will both reduce the need for dewatering during mining and will provide a flow bamer so that the excavation area can be used as a lined water storage reservoir after mining. A small area on the east side of the property outside the slurry wall is also proposed for mining. The east mine cell will be dewatered during mining and then backfilled with unmarketable fine-grained material. This report presents the results of a groundwater modeling investigation of the Hazeltine site conducted by Tetra Tech RMC. The model was developed to simulate the alluvial aquifer in the South Platte Valley in the vicinity of the Hazeltine mine excavation as well as other nearby and lined and unlined reservoirs constructed by others. The model was used to estimate relative effects on groundwater levels due to construction of the slurry wall around the primary Hazeltine extraction area and dewatering of the east side extraction area. The effects simulated by the model represent the estimated additive effects of the Hazeltine site when considered in conjunction with current lined water storage areas created by others upstream and downstream of the site from sand and gravel excavations. 2.0 MODEL DEVELOPMENT Described in this section are the basic framework for how the model was constructed, how geologic and hydrologic features were represented in the model, and the basis for assignment of aquifer parameters and other hydrologic factors in the model. As needed, final aquifer parameters and hydrologic factors were derived during the calibration phase (Section 3.2). General hydrological data were obtained from the Groundwater Atlas of Colorado (Topper, et al., 2003), Robson (1996), and our experience with similar projects in the area. Well water level information was taken from the Rocky Mountain Arsenal well database (transmitted from the Department of Army on October 15 and 22, 2003), existing wells on the property just south of this site, wells along the Fulton Ditch, and monitoring wells installed by Tetra Tech RMC in 2004. 2.1 Model Framework A numerical groundwater mode] was used for the study because such a model can simulate: 1) the alluvial groundwater system taking into account variable aquifer properties and hydrologic stresses; 2) hydrologic influences of the South Platte River and seepage from or to ditches, creeks, and ponds; and 3) variable recharge from precipitation and irrigation. Also, this type of model can simulate the positioning of the lined reservoirs in relation to the direction of groundwater flow and groundwater sources/sinks, which is critical to assess the degree of effects on groundwater levels. The U.S. Geologic Survey's modular finite-difference groundwater flow code, MODFLOW (McDonald and Harbaugh, 1988), was used for the modeling study. The mode] was developed using the pre- and post-processing software Groundwater Vistas, version 4.08 (Environmental Simulations, Inc., 2004). The model encompasses an approximate eight square mile area around the Hazeltine site (Figure 1). A finite-difference grid containing 110 rows and 91 columns was used to model the area. Grid cell - 1 - July 2004 I V919_01 %\GW ModeNtcpottVGZeltine_Rpt_] 2].doc