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As shown in Figure 11, the Laramie-Fox Hills Aquifer contains several areas with <br />distinct groupings of K values. One is an area of relatively high K values, located south <br />of the Denver area in northern Douglas and southwest Arapahoe Counties. This area is <br />similar to the region of high K values observed in the overlying Lower Arapahoe <br />Aquifer (Figure 10) and could indicate a common source area was present to the west of <br />the Denver Basin during the time in which the sediments for both aquifers were <br />deposited. <br />A clear trend of decreasing K values is seen in this area, from Range 68W to 66W, and <br />supports the concept of a decrease in grain size to the east as the distance from the <br />source area increases. Another area with a distinct grouping of K values is a region with <br />low K, located north of Denver in western Adams and northeastern Jefferson counties. <br />Relatively high K values are found throughout the eastern portion of this aquifer. This <br />suggests the aquifer sediments are consistent over large areas. A possible explanation is <br />the environment of deposition, thought to be laterally extensive near shore beach <br />deposits, was migrating eastward during this time as the ancestral Rocky Mountains <br />were being uplifted to the west and the shoreline was receding to the east (Romero <br />1976). <br />3.2 Laboratory Specific Yield (Sy) Data <br />The laboratory Sy values of each aquifer, which are presented below, were compiled and <br />analyzed for trends in geographic and statistical distribution. Table 5 summarizes the <br />number of data points analyzed, statistical groupings, and the range of values observed <br />by aquifer. As described in Section 1.0, the term "specific yield" refers to laboratory <br />results from samples collected from the bedrock aquifers regardless of whether the <br />aquifer is under confined or unconfined conditions. The values comprising this data set <br />are derived from lab sample analyses reported in two main sources (McConaghy et al. <br />1964, Robson 1983), which include compilations of data from other previous <br />publications. The original samples may have been derived from materials that have <br />undergone weathering or other conditions leading to bias in the results as described in <br />Section 1.1. <br />The significance of specific yield in this case is twofold: it indicates the amount of water <br />that will drain from a unit volume of aquifer due to gravity (normally used in <br />unconfined aquifers), and it is also used in a regulatory setting to determine how much <br />water a well operator is entitled to extract from a given aquifer. It should be noted Sy <br />data are sparsely distributed across the Denver Basin Region and only storage coefficient <br />data are more scarce in the number of point values available. <br />3.2.1 Upper Dawson Aquifer Laboratory Specific Yield Data <br />Ten Sy values were obtained from this aquifer. The values range from 0.04 to 0.34 with <br />an average value of 0.18 and a median value of 0.17. Senate Bi115 assigned a specific <br />yield value of 0.20 for this aquifer. The average and median values determined by the <br />SPDSS are 10% and 15% lower than the SB-5 values for this aquifer, respectively. This <br />difference indicates there may not be as much water available for withdrawal as <br />determined by SPDSS compared to the SB-5 value from 1985. Specific yield values <br />generally decrease from west to east, consistent with a trend of increasing clay content in <br />SPDSS Phase 2 Task 43.2 TM -Final 22 <br />2/ 13/ 2006 <br />