PERMFILE49399

• Permit No: m2004044
• IBM Index Class Name: Permit File
• Doc Date: 8/4/2004
• Doc Name: Groundwater Model
• From: Tetra Tech RMC
• To: DMG
• Media Type: D
• Archive: No

1.0
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 aloes-permeability clay sluny 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 esfimate 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 pazameters
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.
21 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 imgation. 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
1: V919_019\TS GW ModeRTS Rcport\Tucson South_Rpt Drefl.dx