2018-11-21_REVISION - M2004044 (21)

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1.0 INTRODUCTION The Tucson South sand and gravel resource is located approximately one-half mile west of the City of Brighton. It is bordered by 160th Avenue(Colorado Highway 7)on the south,Tucson Road in the center,the South Platte River on the east,and the existing Tucson North(Rogers Pit)gravel mine on the north.The proposed site will occupy approximately 290.7 acres in the South Half of Section 1 and Northwest Quarter of Section 12,both within Township 1 South,Range 67 West of the 6th Principal Meridian in Adams County,Colorado.Figure 1 shows the site and vicinity.Aggregate Industries-WCR, Inc.plans to install a low-permeability clay slurry wall,keyed to bedrock,around the primary mining area at the Tucson South site. The slurry wall will both reduce the need for dewatering during mining and will provide a flow barrier so that the excavation area can be used as a lined water storage reservoir after mining. A small area on the southwest side of the property outside the slurry wall is also proposed for mining. The current plans call for the southwest cell to be wet mined(no dewatering)and then backfilled with fine-grained material. This report presents the results of a groundwater modeling investigation of the Tucson South 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 Tucson South 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 Tucson South extraction area and dewatering of other mines in the vicinity. The effects simulated by the model represent the estimated additive effects of the Tucson South site when considered in conjunction with current and proposed 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 2000),and our experience with similar projects in the area. Specific site water level information was taken from monitoring wells installed by Tetra Tech RMC in 2003. 2.1 Model Framework A numerical groundwater model 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 model was developed using the pre-and post-processing software Groundwater Vistas,version 4.09 (Environmental Simulations,Inc.,2004). The model encompasses an approximate 4 square mile area around the Tucson South site(Figure 2). A - 1 - August 2004 LA3919 019M GW ModeATS Reporffumn Soutfi Rpt_DnAdot