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F. Geotechnical A series of geotechnical subsurface explorations were completed by Earth Engineering Consultants and are included in Appendix A. The initial field exploration consisted of shallow borings extending to a maximum depth of 6 feet. A total of 13 shallow borings were completed within and adjacent to the proposed site. The main purpose of this initial exploration was to determine the location and extent of clay overburden on the property and if the clays were suitable materials for construction of a dam. The report describes the underlying material as consisting of 3 -6 inches of topsoil and vegetation underlain by a dark gray brown lean clay ranging in depth from approximately 3 -6 feet. The lean clays were underlain by granular fine to medium sands with varying amounts of silts and clays. The depth and consistency of the underlying granular material was not evaluated in the initial evaluation. After further review of the initial evaluation, a second series of borings that extended down into the bedrock formation was obtained to evaluate the underlying alluvial material. It was also considered important to confirm that the underlying material was of the same composition described by the USGS. A series of six borings were completed to a depth sufficient to encounter the underlying bedrock, which generally is found at a depth of 50 feet in this area. Two of the six borings completed to the bedrock were used to perform field slug tests for comparison with published USGS geologic information. The slug tests indicated permeability's ranging from 1x10-5 — 1x10'3 cm/s. The slug tests were performed in the lean fine clays and the underlying fine granular material. The site is underlain by the Brule formation ranging from a depth of 42 -50 feet. The field data from the borings indicated bedrock was encountered at approximately 3,520' msl, which corresponds to USGS published data for this area. Groundwater levels at the time of testing varied from a depth of 2 feet on the north side of the site, to approximately 10 feet in the southern most portions. Fluctuations in groundwater levels can occur depending on hydrological and seasonal conditions. The location of groundwater will be significant in construction of the reservoir. A high groundwater table will require the use of dewatering trenches to allow for the effective keying of the dam core. Additional information will need to be developed during the preliminary design phase to more accurately locate the normal groundwater table. This information is also important for the 404 permit phase of the project. It is recommended that the current routing of irrigation tailwater upgradient of the property be evaluated to see if any reductions in flows entering the property are possible. G. Recharge Potential/Stream Depletion Factor Analysis The effects of recharge to the South Platte River were analyzed to evaluate contributions made to the river through natural seepage or enhanced recharge. The proposed reservoir was modeled using Stream Depletion Factors developed by the U.S.G.S. The U.S.G.S. has established Stream Depletion Factors (SDF) to help describe the hydrogeological characteristics of alluvial aquifers. A SDF is defined as the number of days that elapses between the time a certain volume of water is removed from or added to the alluvial aquifer and when 28% of that volume is realized at the active stream. SDF values are determined based on the hydrogeological characteristics of each reach of the stream including the transmissivity, specific yield, distance from the stream, hydraulic connection to the stream, and the distances from the alluvial boundary or other impermeable boundaries. The U.S.G.S. Hydrogeological Maps for the Julesburg reach of the South Platte River valley indicates a SDF for the proposed site as varying between 40 -200 days. The SDF increases as the distance from the stream is increased. Due to the size of the proposed site the SDF varies as the distance from the stream increases from approximately ;/a of a mile to 1 %z 11