2008-03-05_REVISION - M1997054 (24)

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If2m 16:31:13 FROM Todd Gibner TO MMah Alberl ' eliminate irrigation of the hay fields. which then would reduce or eliminate a recharge source to the outwash aquifer. Assurning 60 acres irrigated. 1.5 feet of irrigation eater requirement, and 50%n irrigation efficiency. about 90 acre-feet of recharge might be eliminated. Such reduction of irrigation can be expected to reduce water table elevations beneath the irrigated fields and immediately adjacent areas to the south and east. Such an action by DWR could negatively impact the Steer's well/spring. ' Bed rock Aquifers Bedrock aquifers in the Project area include fractured Precambrian crystalline rocks, the Dakota ' Group (Dakota Sandstone and Purgatoire Formation), and fractured portions of the Greenhorn, Carlisle, and Niobrara Formations. The Graneros Shale and Pierre Shale are not conventionally deemed as bedrock aquifers, although small volumes of water may occur as springs and seeps, or flow into wells. Fracturing associated with faulting and folding provides porosity and enhances permeability in bedrock units. These fractures. when occurring in normally "impermeable" rocks such as granites or ' low-permeability sedimentary rocks such as limestones and silty shales, allow ground water to occur and move. Springs and wells in the Project region tap factures in the Precambrian rocks, and in the Greenhom, Carlisle, and Niobrara Formations. Other springs and wells tap intergranular porosity and permeability in parts ofthe Dakota Group, which may be augmented by fractures. ' Ground water flow in the bedrock units begins with deep percolation of a pad of annual precipitation, • plus infiltration of surface waters where streams cut these aquifers, in upland areas. Ground water is driven by gravity down flow paths (in fractured aquifers, in the more intensely fractured volumes) toward discharge areas such as springs, seeps, and subcrop areas beneath outwash deposits. Fast, shallow flow paths may be characterized by low TDS, relatively low temperature ground water. Conversely, slow, deep flow paths may be characterized by high TDS and relatively high temperatures. Several types of flow paths may converge in one discharge area. If a predominantly horizontal flow path is interrupted by a relatively impermeable fault, horizontal ground water flow may stop or be slowed significantly, and vertical flow or leakage may occur in fractures located away ' from the main fault. Faults that occur in the Project area appear to limit lateral ground water flow within bedrock aquifer units to fault blocks. Johnston (1952) notes that most faults in the Park-dale area have throws exceeding 60 feet, which is sufficient to hydraulically separate individual aquifer units within the Mesozoic sediments. Faults do juxtapose relatively permeable rock (such as the Dakota Sandstone) against relatively impermeable rock (say, Graneros Shale and Carlisle Formation); an example of this ' is located about two miles south of US 50, on the Copper Canyon road just south of the turnoff to the south entrance of the Royal Gorge. Slow vertical interaquifer water movement may occur in fractures located adjacent to faults. J 1 Southwest and southeast of the Project area. faults mapped by the USGS (shown on Figure 1-1) serve to hydraulically isolate the Project area bedrock block from areas to the south. These boundaries, combined with the Arkansas River as a constant-head boundary, localize Project impacts to the HjdrogeologyReport ParkdaleProlecr Page 3 of 0 I OFS