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SPDSS_Task42-2_Phase2_DenverBasinAquiferConfiguration
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SPDSS_Task42-2_Phase2_DenverBasinAquiferConfiguration
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Last modified
4/17/2013 9:20:31 AM
Creation date
6/11/2008 8:54:36 AM
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Decision Support Systems
Title
SPDSS Task 42.2 - Denver Basin Region Aquifer Configuratioin - Phase 2
Description
This Technical Memorandum was undertaken under Task 42.2, and summarizes the compilation, analysis and mapping of existing published aquifer configuration data for the Denver Basin Region.
Decision Support - Doc Type
Task Memorandum
Date
2/13/2006
DSS Category
Groundwater
DSS
South Platte
Basin
South Platte
Contract/PO #
C153953
Grant Type
Non-Reimbursable
Bill Number
SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
Prepared By
CDM
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were observed, and therefore, control points were not justified. Control points were <br />gridded with the net sand thickness picks to produce the final grids. A list of the control <br />points is contained in Appendix C. <br />1.2.6 Creating paleochannels in the Bedrock Outcrop Surfaces <br />The density of picks used to grid the tops of the bedrock surfaces in the outcrop areas is <br />not sufficient to reflect the major paleochannels that were carved over time into the <br />bedrock and that now contain some of the alluvial aquifers that overlie the Denver Basin <br />bedrock aquifers. Therefore, the Task 42.3 TM base of alluvium grid was used to provide <br />control for the appropriate surface of each of the bedrock aquifers. Grid Math was used <br />in Surfer® to evaluate the intersection of each of the bedrock surfaces with the base of the <br />alluvium by comparing the top or bottom elevation of each bedrock layer with the base <br />of alluvium elevation. In those cases where the base of alluvium was lower than a <br />bedrock surface, the bedrock surface elevation was lowered to match the base of the <br />alluvium. This process allowed the alluvial paleochannels that have incised into the <br />bedrock units to be seen in locations where the bedrock surface data did not <br />independently depict the alluvial incision. <br />In the northeast corner of the Denver Basin, the base of alluvium surface was lower than <br />both the top and bottom of the Laramie-Fox Hills Aquifer, indicating that the Laramie- <br />Fox Hills Aquifer has been eroded away in this region. The area in question covers <br />about 50 square miles, mostly in the west-central part of Morgan County along the <br />South Platte River. Review of alluvial aquifer data suggests that the bedrock surface <br />picks and the aquifer extent currently assigned to the Laramie-Fox Hills Aquifer should <br />be reevaluated in this area. There were other similar but much smaller areas of <br />disagreement around the perimeters of all the aquifers, where modification of the <br />aquifer outline may be warranted based upon further investigation. Currently, the <br />elevation contours in these areas are identical for the top and bottom surfaces. <br />1.2.7 Addition of Ground Surface Control <br />In some bedrock outcrop areas the density of picks was not sufficient to prevent an <br />aquifer's top surface from being estimated above ground surface. To correct for this Grid <br />Math was used in Surfer®to set the aquifer top surface equal to ground surface in areas <br />where the contouring algorithm estimated the top elevation to be greater than ground <br />surface as defined by the USGS 30-meter NED. <br />1.2.8 Accuracy of Grids <br />Table 3 provides a statistical summary of the measure of goodness of fit between the <br />interpolated grids, without paleochannel and ground surface carving, and the picks <br />used for gridding. (Artificial control picks were not used in this analysis). The measure <br />of fit in this case is defined by the observed range of residuals. A residual is the <br />difference between the grid cell value and the pick value or values that fall in that cell. <br />Cells containing more than one pick, where the values of the picks vary significantly, <br />will have a higher residual calculated at each pick because the grid cell value reflects an <br />averaging of all the picks within that cell. The table reports the median, 10th percentile, <br />and 90th percentile for the absolute value of the residuals. The 90 to 90th percentile rank <br />tells one that 90 percent of the picks have a residual of less than the number of feet <br />SPDSS Phase 2 Task 42.2 TM -Final 10 <br />2/13/06 <br />
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