2019-03-05_REVISION - M1977348 (5)

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' March 1,2019 18107649 ' 3.2 Hydrogeology The Site is located in the Colorado Piedmont subprovince. The Dakota-Cheyenne aquifer is the principal aquifer of the Colorado Piedmont subprovince, and is comprised of the Early Cretaceous Dakota Sandstone and the ' Cheyenne Sandstone Member of the Purgatoire Formation (Topper and others 2003), and is overlain by the Carlile Formation and Niobrara Formation. The Niobrara Formation and Carlile Formation are not considered to be part of a principal aquifer system, but portions of these units can yield some water to a few stock wells in parts of the Colorado Piedmont (Topper and others 2003). Previous mining activities and the existing monitoring wells at the Site are limited to the Niobrara Formation and the top of the Carlile Formation (the Codell sandstone), and ' do not penetrate into the Dakota-Cheyenne aquifer. Groundwater at the Site has been thought to occur along the contact of the Codell sandstone and the Fort Hayes limestone (A band) members. The groundwater flow direction has been observed to generally flow easterly. Hydraulic testing was performed on six of the monitoring wells at the Site. Hydraulic rising and falling head slug tests were performed on MW-1, MW-2, MW-3, MW-4 and MW-5 (Secor 1998, Golder 2010 and Golder 2012). Hydraulic packer injection testing was performed during the advancement of borehole MW-7 (Golder 2013). ' Hydraulic testing results are summarized in Table 9. 3.3 Background Geochemical Conditions There is not a Site groundwater monitoring well located outside the areas of CKD disposal screened similarly to the downgradient Site wells. Therefore, a literature search was conducted to characterize naturally occurring barium and total dissolved solids (TDS)concentrations in groundwater. In Colorado the Niobrara Formation and ' its surrounding formations are not generally considered good sources of groundwater, due to their low production rates (Pearl 1974). Consequently, information regarding groundwater quality is sparse for the Niobrara in ' Colorado. The Wyoming State Geological Survey published the Green River Basin Water Plan II, Groundwater Study in 2010 (Surdam 2010). This study assessed groundwater resources in the Greater Green River Basin (GGRB)of southwestern Wyoming and adjacent areas in Colorado and Utah. This study divided the GGRB basin into sub- basins and characterized the aquifers' groundwater chemistry characteristics for water quality purposes. Two of the sub-basins identified are the Washakie and Sand Wash basins. These basins straddle the Wyoming/Colorado border and are approximately 80 miles west and northwest of the Site. The Niobrara Formation occurs as part of the Cody shale unit of the Washakie and Sand Wash basins. Five groundwater chemistry samples of produced water were collected from production well heads from the Niobrara Formation. The samples had concentrations of barium between 0.19 and 42 milligrams per liter(mg/L), with an average concentration of 15.5 mg/L. TDS concentrations in these samples ranged between 2,270 mg/L and 45,600 mg/L, with an average concentration of ' 22,200 mg/L. These findings indicate that elevated barium and TDS concentrations are naturally occurring in Niobrara aquifer groundwater. Li (2013) characterized produced water quality within the Wattenberg Field in Colorado from wells screened along ' the interface of the Niobrara Formation and the Codell Sandstone, and solely in the Codell. For the tested production waters across 66 wells, the average TDS concentration was 18,285 mg/L and the average barium concentration was 18.2 mg/L. Li attributes the presence of barium within the formation water to the surrounding ' limestone and shale, noting average whole rock barium concentrations of 10 mg/kg and 580 mg/kg for shale and limestone, respectively. ' ' GOLDEP 3