2009-03-02_PERMIT FILE - C1980004 (14)

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4~ 15 Alluvial Aquifers • The relatively small areal extent and the lithological character of the alluvial materials indicate that the alluvial aquifers will not produce significant amounts of water. Since the proposed mine is located above the alluvial fills, no direct disturbance of the alluvial aquifer will result from the actual mining operation. To ascertain groundwater conditions in the alluvial aquifer, a total of six baseline monitoring wells were installed in this potential aquifer. Of the six wells in the alluvium associated with East Salt Creek (See Figure 4.2.2), three were dry on December 30, 1977. Well No. 3 never realized water sufficient to collect and analyze. Other wells were frequently dry during sample periods. During the other periods the depth to water level in the monitoring wells where water was present was between 7 and 17 feet below the ground surface. The total depth of alluvial sediments is about 80 feet in the central parts of the valley, of which only about 50 to 70 feet is saturated most of the year. The exact quantity of water flowing in the alluvial aquifer underlying East Salt Creek is not known; however, a rough approximation is possible. Grand Valley's monitoring wells indicate a • hydraulic gradient of about 80 feet per mile (= 0.015 ft/ft). The saturated thickness of the alluvium is estimated to be 40 to 60 feet. The average width of the alluvial valley is about 400 feet. The area of alluvial flow is then between 16,000 and 24,000 square feet. Using a transmissivity of 1,000 square feet per day the hydraulic conductivity (K) is estimated to be between 16.67 and 25 feet per day. These values can be used with Darcy's Law to calculate a flow of 31.5 gpm in the alluvial sediments in the East Salt Creek Valley. Bedrock Aquifers Within the Mesaverde Formation as exposed in the study area, there are no known major aquifers with substantial yields of ground water as can be determined from the absence of seeps and springs. Aquifers with a low yield to wells are developed in sandstone strata and locally in the coal seams. The permeability of the sandstone strata is a combination of primary (ntergranular) and secondary, (fracture) permeability. The primary permeability of the Mesaverde sandstone is minimal because of their lenticular nature, and the interstitial clays inhibiting permeability. The • primary permeability in the coal seams is always very low. Significant secondary permeability occurs only locally where fracturing and faulting are the recharge mechanism. M` Volume 1 4 - 29 - 96