2010-12-17_REVISION - C1981019 (153)

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Groundwater 63 Bedrock water quality Water quality samples were collected by WMC from all the bedrock monitoring wells in December 2004. A second sampling round of the H wells was performed by KEC in May 2005. Field parameters have been measured more frequently by KEC in some wells. Water samples were also collected from C -04 -1613, to characterize the groundwater that will be produced during dewatering operations. Additional data collected by KEC in 1996 and 1997 from UL- 95 -01, W- 95 -11, and UL- 95 -40, were integrated into the conceptual hydrogeologic model. The locations of these wells are shown on Plan 5.1. A summary of the pre - feasibility sampling history and results (through April 2005) are provided in Table 5.8 for the bedrock wells. Table 5.8 also lists the Federal MCLs and secondary drinking water quality criteria. Although these criteria are not specifically applicable, they provide a basis for comparison. The complete groundwater quality database is provided in Appendix 5.K. The inorganic chemistry of the bedrock groundwater changes as a function of depth and distance downdip (Figure 5.15). Groundwater samples from the F sandstone are all calcium /magnesium— bicarbonate waters and fall within region 1 in Figure 5.15. Within Region 1, the downdip wells show a gradual evolution towards a sodium - bicarbonate water. Samples from the F/G sequence contain higher percentages of bicarbonate and occupy region 2. These wells also evolve towards a more sodic chemistry with distance down dip. Samples collected from C- 04 -16B during the pumping test fall into to this C region and most closely resemble the groundwater from MWC- 04 -13FG. Groundwaters in the H sandstone are all sodium bicarbonate solutions, with the sodium content a function of their distance down dip. These groundwaters generally plot in Region 3 on Figure 5.16. The inorganic chemistry of the samples collected from MWC 04 33H and MWC- 04 -13H plots outside of Region 3. As discussed in Section 5.2.3, MWC- 04 -033H is actually screened in the lower part of the G sequence, and therefore plots in Region 2 of Figure 5.15. MWC- 04 -13H and MWC- 04 -13FS both plot in Region 2 indicating a hydraulic connection or sample handling issue. Future samples from MWC -04 -13 wells should be carefully evaluated against the existing data. Historic data from UL- 95 -01, which is located a short distance northeast of well cluster MWC -04 -33 and completed in the Fab, produces groundwater that plots in Region 2 on Figure 5.15. The chemistry of this groundwater is very similar to a majority of samples from upgradient F sandstone wells. Samples from UL -95 -01 collected between September 1996 and August 1997 show relatively little variation over time. Samples from well W- 95 -11, located on the ridge crest between Jubb and Wilson Creeks, plot in Region 2 of Figure 5.16, indicating that the well is screened in the F/G sequence. The samples show little variation between October 1996 and August 1997. Total dissolved solids (TDS) ranged between 500 and 1,000 mg /I, which exceeds the secondary drinking water MCL of 500 mg /I. The pHs of the samples are slightly alkaline, ranging from 7.6 to 8.1. Concentrations of dissolved metals were generally low. Problematic metals (cadmium, lead, mercury and selenium) were not detected above the laboratory reporting limit. Concentrations of iron and manganese exceeded their respective drinking water criteria of 0.3 and 0.5 mg /i in all bedrock groundwater samples except those from C -04 -1613, MWC -04 -1 FG, MWC- 04 -05H, and MWC- 04 -13FG (Table 5.8). 2572 -R2 Colowyo Coal Company Water Management Consultants