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3.0 Monitoring Well Surveying <br />The objective of Task 39.3 was to survey the horizontal and vertical location of selected wells in <br />the DWR monitoring program that have not been surveyed by a licensed surveyor. The purpose <br />of the surveying was to obtain more accurate location and vertical datum information on the <br />measuring points for these wells to improve the accuracy of the water level elevation data. <br />The selected areas for alluvial well surveying include the downstream portions of Lost Creek, <br />Bijou Creek, and on the main stem of the South Platte River just upstream of Greeley. These <br />locations were chosen to assist with groundwater flux calculations in Phase 4 Task 48 of the <br />SPDSS and to assist with groundwater model calibration. Wells with longer periods of <br />measurement records were given preference when selecting the candidates to be surveyed. <br />The CCWCD measures water levels in wells located in the SPDSS study area and provides data <br />from a subset of their wells to the DWR alluvial water level monitoring program. Any wells <br />measured by CCWCD with multiple historic water level data measurements were considered <br />candidates for surveying. <br />From the DWR's and CCWCD's monitoring programs, a candidate list of wells to be surveyed <br />within the target areas was created. Approximate horizontal coordinates for each well were <br />known, so county assessor's parcel data were used to identify the landowner for each well. Each <br />landowner was then contacted to obtain permission to survey the horizontal and vertical <br />location of their well. Afollow-up letter summarizing the survey program was then mailed to <br />each owner. In total, 15 wells in the DWR program and three CCWCD wells, from which data <br />had not previously been shared with the DWR, were selected to be surveyed. <br />Additionally the four bedrock wells discussed in Section 2.0 were selected to be surveyed, for a <br />total of 22 wells identified for surveying. <br />In June 2006 a Colorado licensed surveyor obtained the horizontal coordinates and vertical <br />elevations of each well's measuring points for 22 wells. The results of the surveying and <br />photographs of the measuring points surveyed are presented in Appendix E, and locations of <br />the surveyed wells are shown in Figure 4. <br />Of the 221ocations surveyed, 15 had elevation data available from the DWR for their measuring <br />point; all of these are alluvial wells. Table 3 compares the surveyed measuring point against the <br />existing DWR measuring point elevation. Of the 151ocations where previous elevation data <br />were available, the average difference between the survey and DWR elevation is 4.6 feet and <br />two locations had surveyed elevations greater than ten feet compared to the previous elevation <br />value. Using a statistical quartile analysis, 25 percent of the data have differences between <br />existing and surveyed elevations less than or equal to 3.2 feet, 50 percent of the data have <br />differences less than or equal to 3.9 feet, and 75 percent of the data have differences less than or <br />equal to 5.6 feet. These differences are large with respect to the very low hydraulic gradients <br />present in the study area. As an example of the potential effect this measuring point error has, <br />consider that typical gradients observed in the alluvium of the South Platte River range from 10 <br />to 20 feet per mile. If the groundwater level elevation from just one of two wells a mile apart is <br />SPDSS Phase 3 Task 39 Technical Memarandum -Final 11 <br />09/27/2006 <br />