2021-05-06_HYDROLOGY - M1977344 - Laserfiche WebLink

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2021 Groundwater Monitoring Report www.arcadis.com 2021 Holcim FINAL GWMR 6 3 Results 3.1 Groundwater Elevations and Flow Characteristics Measured depths to the groundwater surface (Table 1) were used to develop groundwater elevation contours (Figure 2). The water table is encountered in the Codell Sandstone Member of the Carlile Formation (RGI 1999), which underlies the quarried rock. All of the monitoring wells are screened in the Codell Sandstone. The groundwater flow pattern is similar to what has been observed in the past. Groundwater generally flows south toward the Arkansas River, which is the major groundwater discharge zone in the area, as reported by previous investigators (i.e., BBL 2002; RGI 1999). Based upon an average measured horizontal groundwater gradient of 0.02 foot per foot, a Codell Sandstone hydraulic conductivity of approximately 4.5 x 10-6 centimeters per second (RGI 1999), and a range of effective porosities for sandstone of 5 to 30 percent (Freeze and Cherry 1979), the resulting groundwater velocity is 0.3 foot to 1.9 feet per year. The resulting groundwater velocity, when including wells NP-1 and DP-1 in the average hydraulic gradient calculation, is 0.3 foot to 2 feet per year. This range remained the same in 2021 and similar to previously reported values by RGI (1999) and previous groundwater monitoring reports for this site. 3.2 Analytical Results Laboratory analyses of the 2021 groundwater samples were conducted by ACZ. Results of the laboratory analyses are provided in Table 2. The laboratory report is provided as Appendix D. Historical groundwater quality data are provided in Appendix E (updated with results through 2021). Concentrations of metals and other constituents measured in groundwater at well MW-7 were compared to the NPLs approved by the DRMS in February 2009 and the potassium to sodium ratio NPL (0.5) approved by the DRMS in February 2015 (see Section 1.3.5). The concentration of total iron exceeded the approved NPL by more than 10 percent. However, no other exceedances were observed. Therefore, pursuant to the DRMS (2009) GMP approval letter, resampling of MW-7 is not required. Sampling of MW-7 will remain on an annual schedule. In addition to MW-7, groundwater monitoring wells MW-12 (upgradient well) and MW-13 (compliance well) were sampled as part of the annual groundwater monitoring program. Analytical results are presented in Table 2. As discussed in Section 1.3.4 of this report, the DRMS set NPLs for MW-13 in a letter dated November 27, 2012 (Appendix A). All analytes measured in MW-13 were below their respective NPLs. Time series graphs of the potassium to sodium ratio, TDS, sulfate, manganese, total iron, and dissolved iron for MW-7, MW-12, and MW-13 were prepared (Figures 3, 4, 5, 6, 7, and 8 respectively) to evaluate potential impacts of CKD disposal to the groundwater. In general, the following trends area observed:  Potassium to Sodium Ratio: As shown in Table 2 and on Figure 3, the potassium to sodium ratio in all monitoring wells continues to be generally less than 0.10, indicating that there is no discernible impact from the CKD disposal area. The K:Na ratios have stabilized or are declining in the last few years.