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Colorado Division of Mining Reclamation and Safety February 17, 2015 Eric Scott 9 1400159 Outside the context of nodule formation, multiple authors have noted the presence of barium as a significant trace metal within various portions of the Niobrara and associated formations. Kulp and Pratt (2004) found a range of barium concentrations in samples from South Dakota and Wyoming of Smoky Hill Chalk (an end - member of the Niobrara Formation) and Pierre Shale (the formation overlying the Niobrara) of 11.45 ± 28.04 mg /kg and 503.87 ± 237.38 mg /kg, respectively. Arthur et al. (1985) found regional barium maxima above 1,000 mg /kg in the Bridge Creek Limestone near Pueblo, CO, and above 2,500 mg /kg within the Fort Hays and Smoky Hill Members of the Niobrara as sampled at the former Ideal Cement Quarry Site near Fort Collins — now known as the Boettcher Quarry. Arthur et al. (1985) noted that the highest barium concentrations identified within the Niobrara Formation are thought to occur in oxidized zones directly adjacent to strata with increased organic carbon contents. Other representative examples of barium concentrations within the Niobrara and associated surrounding formations can be found in Dean and Arthur (1998) and Landis (1959). 6.0 SUMMARY AND RECOMMENDATIONS Site groundwater characterization data support the conclusion that concentrations of barium and TDS observed in the Site groundwater monitoring well samples are attributed to local geology, and do not represent influence from CKD disposal in area A2. This conclusion is based on the following observations. 1. If CKD in area A2 was an ongoing source for barium and TDS loading to the groundwater, higher concentrations of barium would be expected in the Group 2 wells that are closer to CKD disposal area A2, however the data indicates the opposite. The highest barium and TDS concentrations are in groundwater samples from the Group 3 wells (MW -4, MW -6 and MW -7), which are located greater than 500 feet from CKD disposal area A2. Group 2 wells (MW -2 and MW -3) are located less than 100 feet from the CKD disposal area and have lower concentrations of barium and TDS. 2. Based on leaching tests of CKD solids and groundwater from other CKD impacted sites, elevated pH values and concentrations of potassium (and other constituents) would be expected for CKD impacted waters. Groundwater pH values at the Site are between 7 and 9, which are similar to values observed in the wells prior to CKD placement in area A2, and do not show an increase in pH values or constituent concentrations over time that would be expected if the waters were being influenced by CKD leachate. Potassium was not measured prior to CKD placement; however, current concentrations are < 25 mg /L and show no increasing trends. 3. Dissolved metals concentrations elevated above benchmark levels are only consistently observed for barium. Elevated or upward trending concentrations of other constituents would be expected if CKD was influencing groundwater concentrations in the Site wells. 4. The tritium results indicate that groundwater sampled in MW -1 recharged relatively recently, and that tritium levels for groundwater in the other site wells represent predominantly pre -1952 water. 5. Naturally occurring barium and TDS background concentrations are regionally elevated in the Niobrara aquifer groundwater providing a likely explanation for elevated values of those analytes that is not related to CKD. Il Golder is \14\ 1400159\ 0100 \0122 \tm- boettcherlimeslone uar fnl- 17febl5 \1400159 tm- boettcherlimestone ua fnl- 17febl5.docx Associates Q rY_ Q m'_