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1/7(98 15:31:13 FROM TODD Gilmer TO MIIGrI AlLerl t OF S <br />eliminate imgation of the hav fields. which then would reduce or elntlirtate a recharge sourx to the <br />• ouhvash aquifer. Assuming 60 acres irrigated I.5 feet of irrigation water requirement, and 50°b <br />lrrigation efficiency. about 90 acre-feet of recharge might be eliminated. Such reduction of irrigation <br />can be expected to reduce water table elevatiotu beneath the irrigated fields and irtltnediately adjacent <br />areas to the south and east. Such an action b}' Dyl'R could negatively impact the Steer's welVspring. <br />Bedrock Aquifers <br />Bedrock aquifers in the Project area include Gactured Precambrian crystalline rocks, the Dakota <br />Group (Dakota Sandstone and Purgatoire Formation), and fractured portions of the Greenltom, <br />Carlisle, and Niobrara Fomations. The Graneros Shale and Pierre Shatz are not conventionally <br />deemed as bedrock aquifers, although small volumes of water may occur as springs and seeps, or flow <br />into wells. <br />Fracturing associated with faulting and folding provides porosity and enhances permeability in <br />bedrock units. These fractures, when occumng in normally "impermeable" rocks such as granites or <br />low-permeability sedimentary rocks such as limestones and silty shales, allow ground water to occur <br />and move. <br />Springs and wells in the Proje~~t region tap fractures in the Precambrian rocks, and in the Greenhorn, <br />Carlisle, and Niobrara Formations. Other springs and wells tap intergranular porosity and <br />permeability in parts of the Dakota Group, which may be augmented by fractures. <br />Ground water flow in the bedrock units begins with deep percolation of a part of annual precipitation, <br />• plus infiltration of surface waters where streams cut these aquifers, in upland areas. Ground water is <br />driven by gravity down flow paths (in fractured aquifers, in the more intensely fractured volumes) <br />toward discharge areas such as springs, seeps, and subcrop areas brneath outwash deposits. <br />Fast, shallow flow paths may be characterized by low TDS, relatively low temperature ground water. <br />Conversely, slow, deep flow paths may be characterized by high TDS and relatively high <br />temperatures. Several types of flow paths may converge in one discharge area. Tf a predominantly <br />horizontal flow path is interrupted b}• a relativel}• impermeable fault, horizontal ground water Clow <br />may stop or be slowed sign~cantly, and vertical flow or leakage may occur in fractures located away <br />from the main fault. <br />Faults that occur in the Project area appear to limit lateral ground water flow within bedrock aquifer <br />units to fault blocks. Johnston (1952) notes that most faults in the Pazkdale area have throws <br />exceeding 60 feet which is suffcirnt to hydraulically separate individual aquifer units within the <br />Mesozoic sedunrnts. Faults do juxtapose relatively permeable rock (such as the Dakota Sandstone) <br />against relatively impermeable rock (say, Graneros Shale and Carlisle Formation); an example ofiltis <br />is located about two miles south of US 50, on the Copper Canyon road just south of the turnoff to the <br />south entrance of the Royal Gorge. Slow vertical irrteraquifer water movement may occur in fracnues <br />located adjacent to faults. <br />Southwest and southeast of the Project azea, faults mapped by the USGS (shown on Figure 1-1) serve <br />to hydraulically isolate the Project area bedrock block from areas to the south. These boundaries, <br />combined with the Arkansas River as a constant-head boundary, localize Project impacts to the <br />N~•drogeo(opy Report--Parkdala Prorrcr Paga3ofJ <br />