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Mr. Alex Schatz March 3Q 2007 Page 2 of 7 The alluvium in Fountain Valley consists of gravels and sand containing minor amounts of silt and clay, ranges in thickness from a few feet up to 100 feet thick, and is underlain by the relatively impermeable Pierre Shale. Mesa gravel can be found along the side of the Fountain Creek valley. The mesa gravel is interbedded with numerous sandy layers and ranges in thickness but averages about 10 feet thick. Hydrogeologic data specific to the Fountain Valley alluvium were available in Water- Supply Paper 1583 (Jenkins, 1964), and typical hydrogeologic data for sands and gravels have also been published. Values for hydraulic conductivity range from 100 to 10,000 feet per day. Values of specific yield range from 18.6 to 35.3 percent (Jenkins, 1964). Water levels reported in nearby wells and in site boring logs indicate the water level varies from a few feet below the ground surface to 28 feet below ground surface. MODFLOW MODEL To allow for estimation of the potential impact on the local water table in response to dewatering of the pits, a numerical ground water flow model was constructed. The mode] code utilized was the United States Geological Survey "MODFLOW" model ("Modular Finite Difference Ground Water Flow Model"). The proprietary pre- and post-processing software, Groundwater Modeling System ("GMS"), was used to develop the model files, to assist in model grid layout, and to graphically present the water table and drawdown configurations following each run. MODFLOW and GMS are both widely used and accepted modeling software packages that have become standards in the industry. Discretization The model domain was rotated 22 degree clockwise in order to better align the model grid and the proposed mining cells. The model was then bounded by constant head cells on the top and bottom edges of the model domain. The model was bounded on the left side by a combination of no flow cells and constant head cells approximately 1.4 miles from the edge of the proposed gravel pits and bounded on the right side by no flow cells and constant head cells approximately 0.7 miles from the edge of the proposed gravel pits. The model cells were sized to be approximate 100-foot squares. The model domain is presented in Figure 2. The model was created as a one-layer model representing the alluvial layer in which the gravel pits will be mined. The depth of the alluvium was determined from site boring logs provided by Lafarge, data from the State Engineer's well database, and ground surface elevations at each boring or well location. The elevations of the bottom of the alluvium were then contoured using the Kriging method to develop the topography of the base of the alluvium. S:\536 -Banks and Gesso, LLC~536.4 - Sundance Pit\Report\Final Letter Report.doc Martin and Wood Water Consultants, Inc.