2018-05-18_PERMIT FILE - C2009087A (3)

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RULE 2 - PERMITS Creek mines, provide information on mine - affected groundwater quality. The TDS in groundwater . pumped from those mines rantes from 2,700 to 6,000 mg/L. However, two factors differ between those mines and the proposed PSCM. First, both of those mines operated with longwall methods that resulted in mine roof subsidence. The mining proposed at the PSCM will use room and pillar methods, which will not result in significant roof collapse and will therefore leave groundwater in the mine in contact with relatively undisturbed overburden, underburden and coal adjacent to the mine workings. Second, the gob at both of those mines was primarily marine shale with inherently higher soluble salt content than the overburden materials in the PSCM area. Because the overburden materials in the PSCM area are continental sediments, they have much lower soluble salt content than the marine shales predominant at the other mines mentioned. Based on the duration of higher TDS concentrations in groundwater pumped from underground mines in the vicinity, elevated TDS concentrations could occur for decades after mining at the proposed PSCM is completed. Robson and Stewart (1990) modeled the movement of mining- affected groundwater away from surface mine spoils aquifers at an inactive former underground mine in the area. Their simulations of the underground mine water quality were based on transport under steady -state conditions after potentiometric levels throughout the mined area had recovered to pre- mining conditions, and under transient conditions assuming recovery of potentiometric levels in the mined area to the pre- mining levels at the perimeter of the mined area. The latter conditions are more representative of a mined area that has an extremely large hydraulic conductivity due to water flow through uncollapsed mine workings (Robson and Stewart, 1990), which are the conditions expected in the PSCM area. The results of their 30 -year simulations for both conditions are not significantly different. Both predicted • that TDS concentrations in groundwater away from the mined area would increase by 300 mg /L out to a distance of about 1000 feet downgradient and by 30 mg /L out to a distance of about 2,400 feet downgradient of the mined area. Dispersion effects resulted in simulated TDS increases in areas upgradient and cross - gradient of the mined areas as well, but to much smaller distances from the mined area. The modeled TDS changes did not account for geochemical reactions in the aquifers outside the mined area. Additionally, the TDS changes predicted by Robson and Stewart are based on groundwater with 4,500 mg /L TDS leaving the flooded mined areas. That TDS concentration is similar to the quality of groundwater in surface mine spoils aquifers in the area, but it is much higher than that expected in the flooded workings in the PSCM area. The modeling by Robson and Stewart (1990) and the above discussion of geochemical conditions in the flooded mine area suggest that changes will occur to the groundwater quality in the vicinity of the PSCM. Groundwater flow and transport velocities may be different in the PSCM area than in the area modeled by Robson and Stewart, because hydrogeologic conditions differ between the two areas. The macroscopic groundwater flow velocity of 0.014 to 11 ft /yr calculated for the PSCM area (see Section 2.04.7) overlaps with the lower par of the range of 1 to 30 ft /yr cited by Robson and Stewart (1994) for the areas they modeled. The velocity differences are likely because of differences in hydraulic conductivities and gradients between the areas. The smaller groundwater flow velocity in the PSCM area would result in slower migration of mining - affected groundwater away from the mined area than that modeled by Robson and Stewart. Consequently, applying the Robson and Stewart modeling results to the PSCM area would predict TDS increases that are larger and extend farther from the mined area than actually would be expected at the PSCM area. • Vertical (normal to the bedrock bedding planes) migration of mining- affected groundwater could also occur, with subsequent discharge of the affected groundwater into the alluvium along Grassy PSCM Permit App. 2.05 -83 Revision 03/05/10