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• Use of grails-size distribution data in the Shepherd equation is an appropriate <br />method of estimatuzg K of South Platte Basuz sediments. <br />Based on grain-size data and the S1epherd equation, the geometric mean of <br />streambed hydraulic conductivities along the South Platte and Cache la <br />Poudre Rivers is approximately 184 ft/day, roughly five times the geometric <br />mean value for the tributaries (37 ft/day). These values, while considered <br />High for streambed sediments, are reasonable for the coarse-grained size of <br />materials present in the streambed at many test locations. <br />• At most field sites there was no measurable head difference between the <br />stream stage and groundwater 3 ft below the stream-sediment interface. This <br />suggests a high degree of hydraulic communication between surface water <br />and shallow groundwater. However, at t11e paired stream gage and <br />monitoring well locations, there was a trend in t11e Sout11 Platte River of losing <br />water in the downstream direction at the time of testing in autumn 2005. The <br />losing nature of the South Platte River could be a result of lowered <br />growldwater levels associated with residual effects of irrigation season well <br />pumpmg. <br />5.0 Recommendations <br />Below are recommendations from the activities performed under Task 34: <br />• K and channel geometry data obtained from this study should be used to help <br />establish initial inputs for the stream parameters used >11 the alluvial <br />growldwater model being developed under Task 48. <br />Although some outliers exist, the grain-size based K values are consistent with <br />K values of coarse-grained aquifer sediments based on literature values. <br />Should t11e groundwater modeling performed under Task 48 indicate the need <br />for more refined values of streambed K, then additional grain-size samples <br />should be considered for collection and analysis (as described in this TM) to <br />satisfy this need. <br />Stream stage and paired aquifer water level data should be compared over the <br />entire period of record for which both types of data exist at the stream gage <br />locations. This should be performed as part of Task 46 to enhance t11e <br />understanding of stream ga>11/loss i11 the South Platte River. <br />6.0 References <br />ASTM (American Society for Testing and Materials), 2002. Standard Test Method for <br />Particle-Size Analysis of Soils (D-422). <br />Fnlal T~zsl: 3~TM 06-28-07.doc 14 <br />