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<br /> <br />Approach: <br />Task 34.1 Field Data Collection <br />` __~ <br />An intensive investigation of streambed properties has been conducted by the USGS on the <br />mainstern South Platte River between Denver and Ft. Lupton, where they investigated 30 cross- <br />sections. The results of these investigations will be incorporated into Hydrobase and will reduce <br />the number of field investigation locations required to meet this data requirement. Additional <br />field investigations will be conducted at up to 301ocations, with 5 of these locations including <br />paired shallow monitoring wells and SEO stream gages, as described below. The field studies will <br />develop and use empirical correlations between grain size distribution and hydraulic conductivity. <br />• Data from the USGS stream gain/loss investigation from Denver to Ft. Lupton will be <br />obtained and incorporated into this effort since the goals and methods of-that study are similar <br />to those of this task. <br />• The consultant will identify locations for field investigations and obtain necessary access <br />agreements for sampling and testing of up to 30 locations along the mainstem of the South <br />Platte River and in selected tributaries. These locations will be selected to represent the range <br />of stream channel conditions (width, depth, braiding, etc.) and will be approximately evenly <br />distributed on the main stem from Ft. Lupton to the state line and on selected tributaries. <br />Many of these locations will likely require access to private property, though public <br />easements will be used, if suitable. <br />• All sites will be characterized by installing a transect of up to three core holes to a depth of 3 <br />feet below the water-sediment interface across the channel and collecting up to 6 samples for <br />grain size distribution. Five of these sites will be located. adjacent to the existing SEOlUSGS <br />gaging stations (at Henderson, Kersey, Baizac, Weldona and Julesburg) where shallow wells <br />were installed during Phase 2. At these five locations up to two additional cores will be <br />sampled upstream of the transverse transect and up to 4 samples will be collected for grain <br />size distribution to help define local spatial variability. The materials retrieved from the core <br />holes will be described by the field geologist in a bound field notebook. Up to 200 core <br />samples will be submitted for geotechnical grain size testing using wet sieve methods for the <br />size fraction larger than 200 mesh. <br />At one of the core locations at each site (up to 30 locations total) a temporary piezometer will <br />be installed with a 10 - 20 cm screen length corresponding to a depth interval where a grain <br />size sample is collected. Field testing will be conducted on each temporary piezometer to <br />estimate hydraulic conductivity of the streambed materials, using either the constant head <br />injection method or a recovery method. This will allow development of an empirical <br />relationship between grain size distribution and hydraulic; conductivity that will be used at the <br />other locations that only have grain size data available to estimate the hydraulic conductivity. <br />All sites will be used to calculate vertical hydraulic gradients. The vertical gradient beneath <br />the streambed will be determined at the time of field testing using the difference between the <br />stabilized water level in the temporary piezometer and the stream stage. In addition, at the 5 <br />locations adjacent to the SEO/USGS gaging stations, the water level in existing monitoring <br />wells and the stage height on the SEO/USGS staff gage will be recorded to provide additional <br />SPDSS Groundwater SOW, Phase 3 <br />