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residual values in feet. The 90th percentile rank indicates 90 percent of the picks have a residual <br />of less than the number of feet indicated in the table. The counts in Table 2 are different than <br />those in Table 1 because only the picks within the defined alluvial extent are included in Table <br />2. <br />Table 2: Summary of Absolute Values of Residuals between Picks and the Bottom of <br />Alluvium Grid <br /> Count 10th Percentile (ft) Median (ft) 90th Percentile (ft) <br />Bjorklund and <br />Brown (1957) 315 1 5 20 <br />DW R 235 1 7 19 <br />Smith et al. (1964) 1,009 1 5 17 <br />SPDSS 17 2 29 58 <br />Weist (1964) 1 2 2 2 <br />All Data Sources 1,577 1 6 18 <br />The residual values for all data sources show that the median residual is six feet and the 90th <br />percentile residual is 18 feet. These values are relatively low, less than the 20-feet contour <br />interval of the historic reports, indicating the grid is in good agreement with the picks. <br />The residuals for the SPDSS picks are higher than the picks from other sources. SPDSS picks are <br />derived from geophysical logging from direct-push drilling, which is a less accurate measure of <br />the bedrock contact than the use of discrete cores samples and drill cuttings used by other <br />sources. Another explanation is that the grid does not accurately reflect the base of the <br />alluvium and therefore differs from the SPDSS pick value. Neither option can be validated <br />because the SPDSS picks with high residuals mostly reside in areas with few to no picks nearby. <br />Because of the lack of neighboring picks, there was not sufficient physical evidence to justify <br />modifying the grid based on one pick of questionable confidence. <br />For example, in the Julesburg area, the SPDSS picks suggest the primary paleochannel could <br />possibly be shifted slightly to the northwest. It was determined inappropriate to modify the <br />bottom of alluvium grid based on only a few picks, because it is presumed the Hurr and <br />Schneider (1972) contours in this area were developed with a larger pick data set, which is not <br />available for evaluation. Furthermore additional pick data from other sources were not <br />available in this area to evaluate a modification to the grid. For regional scale modeling, it is <br />believed the current configuration sufficiently captures the overall trend and geometry of the <br />base of the alluvium. If a more localized study is conducted in this area, it is recommended <br />more data be gathered to evaluate the bottom of alluvium grid. <br />1.2.8 Comparison of 2005 Groundwater Level Data and the Groundwater Level Grid <br />March 2005 groundwater level data from 32 SPDSS alluvial monitoring wells were selected to <br />provide a representation of current groundwater levels spanning the reach from Henderson to <br />Julesburg. These measurements were used to assess how well groundwater levels digitized <br />from the historical reports represent current levels. The time period of water level measurement <br />for each historic source are shown in Table 3. Using GIS, a residual was calculated as the <br />difference between the groundwater level grid value and the March 2005 groundwater level <br />from the SPDSS wells. Ninety percent of the March 2005 measurements differed by less than 10 <br />SPDSS Phase 3 Task 42.3 TM -Final <br />11/30/2006 <br />