2019-03-05_REVISION - M1977348 (5)

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March 1,2019 18107649 ' 4.0 GROUNDWATER MONITORING RESULTS ' 4.1 Routine Sampling Water quality sampling results for Site wells are presented in Table 2 through Table 8. The analyte list has varied ' over the monitoring history; time series graphs though 2018 for routinely monitored constituents are presented in Appendix C. Based on these results and the location and depth of the wells, groundwater beneath the Site can be classified into three distinct groups. These groups are distinguished based on water chemistry, monitoring depth, ' and screened lithology. The characteristics of each group are discussed below and summarized in Figure 4. Group 1 (MW-1 and MW-5): The monitoring wells in Group 1 have screened intervals that are located less than 70 feet below ground surface (ft bgs)and are the furthest west and north of the Site wells. MW-1 is screened t across the Niobrara/Codell contact near CKD area A2 and MW-5 is screened in the overburden above bedrock north of the CKD disposal areas. The pH of samples collected in 2018 from these wells ranges between 7.5 and ' 8.3. The groundwater in this group is sodium-sulfate type water, as shown by the Piper diagram on Figure 4, with 2018 TDS concentrations between 3,020 and 7,240 mg/L, which is generally lower than the other groups. The groundwater in this group exhibits higher concentrations of calcium, magnesium, and sulfate and lower alkalinity ' than the other groups. Barium concentrations in this group are below 1 mg/L, which is lower than that of the other groups. Group 2 (MW-2 and MW-3): The monitoring wells in Group 2 are screened from approximately 100 to 200 ft bgs across the Niobrara/Codell contact. The wells in this group are located less than 100 feet downgradient to the east of CKD disposal area A2. The groundwater in this group is sodium-chloride type water(Figure 4)with 2018 TDS concentrations between 4,020 and 6,280 mg/L. The pH of samples collected from these wells in 2018 ranges ' between 8.0 and 8.4. The groundwater in this group of wells exhibits higher alkalinity concentrations than that of the other groups and barium concentrations in this group are between 1 and 2 mg/L. ' Group 3 (MW-4, MW-6 and MW-7): Monitoring wells in Group 3 are screened at depths greater than 180 ft bgs, in the Niobrara Formation above the Niobrara/Codell contact'. The wells in this group are located greater than 500 feet downgradient or side-gradient from CKD disposal area A2. The pH of samples collected from these wells ' ranges between 7.7 and 8.1. The groundwater in this group is sodium-chloride type water with TDS concentrations that are greater than 10,000 mg/L. The signatures for groundwater from these three wells plot closely together on a piper diagram (Figure 4) as the groundwater in this group exhibits higher sodium and ' chloride concentrations than the other groups. Barium concentrations in this group are above 2 mg/L, which are higher than those of the other groups. Results from groundwater samples collected from the Group 3 wells indicate that constituent concentrations have , taken longer to reach equilibrium with the surrounding formations than those for the wells in the other groups. For example, concentrations of chloride and sodium (both relatively conservative constituents2) in MW-6 and MW-7 ' increased following installation of the wells and within approximately one year of well installation reached relatively stable concentrations similar to those in MW-4 (Figure-7 and Figure 12 of Appendix C, respectively). 1 ' Screened lithology of MW-4 is unknown. MW-4 is assumed for this discussion to be part of Group 3 based on the ' well depth, hydraulic characteristics and chemical signature. 2 Conservative constituents are relatively unreactive, and their concentrations are not expected to be changed by geochemical processes such as mineral precipitation and/or sorption along a flow path. Therefore, their concentrations are relatively conservative along a flow path. ' 4 GOLDER 4 '