2008-03-05_REVISION - M1997054 (24)

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1 1 1 F [7 1 Using a numerical model approach and defining the river not as a constant head boundary, but as a river with a rate of flux between the river and aquifer can simulate the effects of dewatering on the opposite side of the river. That approach is not obtainable here due to the lack of available data on the existing ground water flow, which is required to effectively calibrate such a model. The USGS report from 2003, entitled Analytical and Numerical Simulation of the Steady-State Hydrologic Effects of Mining Aggregate In Hypothetical Sand-and-Gravel and Fractured Crystalline- Rock Aquifers provides the results of numerical modeling on hypothetical situations such as that presented in the present study. The results of the USGS numerical modeling show that the effects of dewatering on the opposite side of the river have a small but noticeable effect opposite the pit at distances of less than 1,000 feet from the river. The drawdown on the opposite side of the river is equal to that on the side of the pit, but confined to about a tenth of the area. Applying the USGS's results for small, homogeneous, isotropic aquifers to the present study, the drawdown on the opposite side of the Arkansas River will be at most 1-foot at a distance of about 150 feet from the river. The drawdown opposite the river will decrease radially from the river. POTENTIAL GROUNWATER IMPACTS FROM THE GRANITE QUARRY The Parkdale Granite Quarry, located north-east of the sand and gravel pit occupies a pre-Cambrian granitic hillside just south-west of Cactus Mountain. As described within the Parkdale Pit Reservoir Feasibility Study, prepared by J.A. Cesare and Associates, Inc. in June of 2007, the rock type of the quarry "...has been mapped as granodiorite by Taylor and others (1975) and is described as pinkish-gray, massive, coarse-grained, porphyritic rock chiefly made up of plagioclase, microcline, biotite, and quartz." (J.A. Ceasar and Assoc., Inc., 2007) In the Colorado Geological Survey's (CGS) publication, Ground Water Atlas of Colorado from 2003, Section 7.5 details general hydrogeologic conditions of Precambrian Crystalline and Tertiary Rock Aquifers. As stated on page 194 within Section 7.5, "The porosity of Precambrian crystalline rocks is very low - as a general rule, less than 1 percent. Snow (1968 and 1973) reports primary porosity of 0.05 to 0.005 percent and secondary porosity of 0.1 to 0.001 percent. Fractures provide the only significant porosity and flow conduits within the unweathered crystalline rocks of Colorado." (CGS, 2003) As previously noted, the primary material of composition of the granite quarry is Precambrian crystalline rock. Furthermore, in April of 2004 J.A. Cesare and Associates, Inc. prepared the Aggregate Resource Investigation for the Parkdale Aggregate Mine, which included the drilling of two core samples within the existing granite quarry. From the samples, it was determined that "the core can generally be described as moderately hard to very hard and slightly weathered to fresh." (J.A. Cesare and Assoc., 2004) the report goes on to state that " groundwater was not encountered during the drilling of the two JAC (Joseph A. Cesare and Associates, Inc.) core holes in the granite quarry area. The presence of groundwater was not reported in the CTL/Thompson test holes either." (J.A. Cesare and Assoc., 2004) The bore holes performed by J.A. Cesare and Associates, Inc. extended to depths of 76 feet and 278 feet. Parkdale Aggregate Mine I Potential Grounwater Impacts from the Granite Quarry M