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associated with residual effects of irrigation-season well pumping. Tl1e lack of a <br />detectable difference between the stream stage and groundwater head at many of the <br />test locations could be due to the lack of glower-permeability streambed material <br />within the depth interval sampled (3 ft), allowing a high degree of hydraulic <br />communication between t11e surface water and shallow groundwater. <br />Additional information on vertical gradients was obtained through analysis of stream <br />stage and adjacent piezometer water levels collected at stream gage locations at t11e <br />Henderson, Kersey, Balzac and Julesburg gages. At each location the Head difference <br />between the aquifer and stream stage was calculated as the groundwater level <br />elevation nunus the stream stage elevation for each site. Tl1e difference between <br />stream stage and groundwater levels at t11e Kersey, Balzac and Julesburg Gages is <br />plotted on the y-axis in Figure 11. Positive values indicate a gaining stream while <br />negative values indicate a losing stream, the same sign convention as used wit11 the <br />temporary piezometers discussed in t11e preceding paragraphs. Discussions with the <br />operator of a gravel pit near the Henderson Gage indicate that groundwater elevation <br />data from this location is affected by gravel pit dewatering resulting in groundwater <br />elevations approximately 7.5 ft below stream stage, therefore the data from this site is <br />not used in this analysis. <br />Figure 11 illustrates that the South Platte River appeared to transition from a ga>lz>lzg <br />to a losing stream in the downstream direction during autunu12005. This is <br />consistent with results from a previous study, conducted in the fall of 1993, in which <br />gaining stream conditions in monitoring wells installed adjacent to the South Platte <br />River in t11e vicinity of Henderson were observed (McMahon et al. 1995). Results <br />from Task 34 indicated that t11e upstream location (Kersey Gage) shows an average <br />upward gradient of approximately 0.4 ft. McMahon et al. (1995) observed an upward <br />gradient of 0.3 to 0.6 ft. Tl1e noddle location (Balzac Gage) shows an average upward <br />gradient of approximately 0.2 ft. The downstream location (Julesburg Gage) shows a <br />downward gradient of approximately 0.1 ft, indicating a losing stream (Figure ll). <br />There is some noise in the stream stage data associated with high flow events that <br />make correlations between the three gage locations imperfect. However, t11e average <br />gradients shown on Figure 11 s11ow a consistent trend in the upstream to downstream <br />direction. These general relationships are useful and will be incorporated into the <br />Task 48 modeling. Tl1e magnitude and location of the vertical gradients also will be <br />used >IZ groundwater flux estimations performed under Tasks 46 and 48. <br />3.5 Site Survey Results <br />The results of the site survey are presented in Appendix D. The ground surface <br />elevations at t11e sampling sites ranged from 6,087 ft above mean sea level (ft MSL) at <br />site SC-20 near the headwaters of Bijou Creek, to 3,465 ft MSL at the benchmark for <br />the Julesburg Gage. The stream stage data were adjusted to absolute elevation MSL <br />and are presented in the cross sections provided uz Appendix A. <br />Fnlal T~zsl: 3~TM 06-28-07.doc 12 <br />