2021-07-21_PERMIT FILE - C1981019 (5)

Home Browse Search

Point of Compliance Well Investigation AECOM 9 on Figure 11a, the average sulfate concentration is lower than the upgradient background UTL at five temporary wells in the Good Spring Creek and Wilson Creek drainages. This figure also shows that average sulfate concentrations tend to be higher farther downgradient in both drainages than they are closer to the Mine site. Finally, using the pre- 1994 baseline UTL for comparison (Figure 11b) results in eight of the temporary well locations falling below the higher threshold value (997 mg/L). Average sulfate concentrations at NGSW, MT-95-02, and Gossard Well are also below the pre-1994 baseline UTL. • Figures 12a and 12b – TDS: Based on data from A-6 and A-8, the upgradient background UTL for TDS is 1,360 mg/L. Per Regulation 41, the groundwater standard for TDS was calculated as 1.25 times the site background value (1,360 * 1.25 = 1,700 mg/L). AECOM used the 1,700 mg/L TDS standard as the basis for comparison on Figure 12a. This figure illustrates that like sulfate, average TDS concentrations tend to be higher farther downgradient from the Mine than they are near the permit boundary. A trend of increasing TDS downgradient was also documented by baseline sampling data presented in the Mine Permit, which found that TDS increased by 40 to 50 mg/L per mile due to natural factors such as agricultural drainage, dissolution of soluble minerals, and the concentrating effect of evaporation (Colowyo Coal Company 1981 et seq.). The current spatial trend of TDS concentrations is therefore consistent with baseline results. The temporary well locations most likely to meet the 1700 mg/L TDS standard include POC-1 and POC-2 in the Wilson Creek drainage, and POC-8, POC-9, POC-10, and POC-11 in the Good Spring Creek drainage. Using pre-1994 baseline data to calculate background increases the TDS threshold even further to 1,840 mg/L, but does not change the temporary wells that would meet the threshold value. Overall, the comparative analysis provided in Tables 3 through 14 and depicted on Figures 2 through 12 indicates that manganese, sulfate, and TDS are the constituents most likely to exceed standards in future point of compliance wells. The probability of meeting standards for these constituents will be highest when background UTLs calculated based on pre-1994 baseline data are used for comparison. This occurs because the pre-1994 baseline UTLs for manganese, sulfate, and TDS are higher than the most stringent Regulation 41 standard, as well as the upgradient background UTLs (Table 2). 3.3 Point of Compliance Well Recommendations Based on the analysis presented above, AECOM recommends installing two permanent point of compliance well locations in the downgradient site area. Our recommended well locations are shown on Figure 13. The first permanent compliance well should be installed in the Wilson Creek drainage near the present location of POC-2. A permanent well in this area could serve as the groundwater point of compliance for the both the Wilson Creek and Taylor Creek alluvial groundwater systems. The recommendation to install a permanent well in this area is based on the relatively low average concentrations of dissolved manganese (0.39 mg/L), sulfate (597 mg/L), and TDS (1,427 mg/L) compared to other temporary monitoring well locations in the Wilson Creek drainage, including some that are farther downgradient of the Mine Permit boundary. This location is also consistent with the recommended location for a permanent compliance well in the Division’s adequacy review of Permit Revision 02 (Division of Reclamation, Mining and Safety 2006, 2007), which states: …the Division is recommending two locations for additional alluvial monitoring wells to be utilized as points of compliance for alluvial groundwater as follows: one should be located in the Quaternary alluvium below the juncture of East Taylor Creek and Wilson Creek, and the other should be located in the Quaternary alluvium along Good Spring