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2020 GROUNDWATER MONITORING REPORT <br />arcadis.com 6 <br />3 RESULTS <br />3.1 Groundwater Elevations and Flow Characteristics <br />Measured depths to the groundwater surface (Table 1) were used to develop groundwater elevation <br />contours (Figure 2). The water table is encountered in the Codell Sandstone Member of the Carlile <br />Formation (RGI 1999), which underlies the quarried rock. All of the monitoring wells are screened in the <br />Codell Sandstone. The groundwater flow pattern is similar to what has been observed in the past. <br />Groundwater generally flows south toward the Arkansas River, which is the major groundwater discharge <br />zone in the area, as reported by previous investigators (i.e., BBL 2002; RGI 1999). <br />Based upon an average measured horizontal groundwater gradient of 0.02 foot per foot, a Codell <br />Sandstone hydraulic conductivity of approximately 4.5 x 10-6 centimeters per second (RGI 1999), and a <br />range of effective porosities for sandstone of 5 to 30 percent (Freeze and Cherry 1979), the resulting <br />groundwater velocity is 0.3 foot to 1.9 feet per year. The resulting groundwater velocity, when including <br />wells NP-1 and DP-1 in the average hydraulic gradient calculation, is 0.3 foot to 2 feet per year. This <br />range remained the same in 2020 and similar to previously reported values by RGI (1999) and previous <br />groundwater monitoring reports for this site. <br />3.2 Analytical Results <br />Laboratory analyses of the 2020 groundwater samples were conducted by ACZ. Results of the laboratory <br />analyses are provided in Table 2. The laboratory report is provided as Appendix D. Historical <br />groundwater quality data are provided in Appendix E (updated with results through 2020). <br />Concentrations of metals and other constituents measured in groundwater at well MW-7 were compared <br />to the NPLs approved by the DRMS in February 2009 and the potassium to sodium ratio NPL (0.5) <br />approved by the DRMS in February 2015 (see Section 1.3.5). The concentration of total iron exceeded <br />the approved NPL by more than 10 percent. However, no other exceedances were observed. Therefore, <br />pursuant to the DRMS (2009) GMP approval letter, resampling of MW-7 is not required. Sampling of MW- <br />7 will remain on an annual schedule. <br />In addition to MW-7, groundwater monitoring wells MW-12 (upgradient well) and MW-13 (compliance <br />well) were sampled as part of the annual groundwater monitoring program. Analytical results are <br />presented in Table 2. <br />As discussed in Section 1.3.4 of this report, the DRMS set NPLs for MW-13 in a letter dated November <br />27, 2012 (Appendix A). All analytes measured in MW-13 were below their respective NPLs. <br />Time series graphs of the potassium to sodium ratio, TDS, sulfate, manganese, total iron, and dissolved <br />iron for MW-7, MW-12, and MW-13 were prepared (Figures 3, 4, 5, 6, 7, and 8 respectively) to evaluate <br />potential impacts of CKD disposal to the groundwater. <br />In general, the following trends area observed: <br />• Potassium to Sodium Ratio: As shown in Table 2 and on Figure 3, the potassium to sodium <br />ratio in all monitoring wells continues to be generally less than 0.10, indicating that there is no