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July 19,2019 18107649 <br /> Group 1 (MW-1 and MW-5): The monitoring wells in Group 1 have screened intervals that are located less than <br /> 70 feet below ground surface(ft bgs)and are the furthest west and north of the Site wells. MW-1 is screened <br /> across the Niobrara/Codell contact near CKD area A2 and MW-5 is screened in the overburden above bedrock <br /> north of the CKD disposal areas. groups. <br /> Group 2 (MW-2 and MW-3): The monitoring wells in Group 2 are screened from approximately 100 to 200 ft bgs <br /> across the Niobrara/Codell contact. <br /> Group 3(MW-4, MW-6 and MW-7): Monitoring wells in Group 3 are screened at depths greater than 180 ft bgs, in <br /> the Niobrara Formation above the Niobrara/Codell contact'. <br /> The monitoring depth and screened lithology influences the observed water chemistry of the groups. The observed <br /> chemistry of each group is discussed below and summarized in Figure 8 through Figure 11. <br /> Group 1 (MW-1 and MW-5): The laboratory pH of samples collected in 2018 from these wells ranges between <br /> 7.5 and 8.3. The field pH of samples collected in 2018 from these wells ranges between 6.8 and 7.6. The <br /> groundwater in this group is sodium-sulfate type water, as shown by the Piper diagram on Figure 8, with 2018 <br /> TDS concentrations between 3,020 and 7,240 mg/L, which is generally lower than the other groups. The <br /> groundwater in this group exhibits higher concentrations of calcium, magnesium, sulfate, fluoride, and nitrate and <br /> lower alkalinity than the other groups. Except for selenium and iron, dissolved trace metals concentrations for this <br /> group are lower than the other groups. <br /> Group 2 (MW-2 and MW-3): The groundwater in this group is sodium-chloride type water(Figure 8)with 2018 TDS <br /> concentrations between 4,020 and 6,280 mg/L. The laboratory pH of samples collected from these wells in 2018 <br /> ranges between 8.0 and 8.4. The field pH of samples collected in 2018 from these wells ranges between 7.5 and <br /> 8.0. The groundwater in this group of wells exhibits higher alkalinity and lower magnesium concentrations than <br /> ' that of the other groups. Dissolved trace metals concentrations for this group are generally equal to or lower than <br /> Group 3. <br /> Group 3(MW-4, MW-6 and MW-7): The laboratory pH of samples collected from these wells ranges between <br /> 7.7 and 8.1. The field pH of samples collected in 2018 from these wells ranges between 7.2 and 7.9. The <br /> groundwater in this group is sodium-chloride type water with TDS concentrations that are greater than <br /> 10,000 mg/L. The signatures for groundwater from these three wells plot closely together on a piper diagram <br /> (Figure 8)as the groundwater in this group exhibits higher sodium, chloride and trace metals concentrations than <br /> the other groups. <br /> Results from groundwater samples collected from the Group 3 wells indicate that constituent concentrations have <br /> taken longer to reach equilibrium with the surrounding formations than those for the wells in the other groups. For <br /> example, concentrations of chloride and sodium (both relatively conservative constituents2) in MW-6 and MW-7 <br /> increased following installation of the wells and within approximately one year of well installation reached <br /> relatively stable concentrations similar to those in MW-4(Figure-7 and Figure 12 of Appendix D, respectively). <br /> ' Screened lithology of MW-4 is unknown. MW-4 is assumed for this discussion to be part of Group 3 based on the <br /> well depth, hydraulic characteristics and chemical signature. <br /> 2 Conservative constituents are relatively unreactive, and their concentrations are not expected to be changed by <br /> geochemical processes such as mineral precipitation and/or sorption along a flow path. Therefore, their <br /> concentrations are relatively conservative along a flow path. <br /> ,> GQLDER 5 <br />