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~~ i Mr. Tom Kaldenbach December 18, 1998 Page 2 TDS from the non-mined area, therefore, an increaze has not been defined." Assuming this statement refers to the non-mined S-1 site and the mined NPDES 001 and 002 sites, is the conclusion still valid given that flows at S-I have greatly declined over the years, while flows at NPDES 00] and 002 have shown little change? Could it be that the TDS increase at S-1 was caused by diminishing flows, while the T'DS increases at 001 and 002 are caused by a local influx of dissolved solids? Response 4.) It is probably not entirely accurate to attribute meazured TDS "increases" at site S-1 to diminishing flows in the Flume Gulch drainage. During the past several years, measurements at Site S-l have occurred during periods of low flow and it is reasonable to conclude that higher TDS values have been measured as a result. The TDS values recorded have been within the range previously established for the site. The flows associated with the recent measurements have been low but do not necessarily indicate that overall flows from Flume Gulch have diminished. It appears as though the low flow TDS measurements at Site S-] may provide a good indication of the upper bound natural values occurring in Flume Gulch. The measurements at Site S-1 compare favorably with the TDS values recorded at Site 002 which receives drainage from a mined area (No Name drainage). On the other hand, the prevailing TDS values recorded at Site 001 (Johnson drainage), which also drains a mined area, have been noted to be relatively higher over the past several years. A discussion noting this trend in the Johnson drainage appears in Permit C-81-010 on page 4-237. Trapper will continue to monitor TDS trends in surface water runoff in the future and will provide more specific conclusions regarding these trends in future Annual Reports. Comment 10.) Reference to well GE-1 on page 6-2, 2n0 paragraph, and page 6-3, 1" paragraph. The reference on page 6-2 indicates that mining affected the water level in this well through pit dewatering, but the reference on page b-3 says that the increase in TDS in this well was "probably not caused by mining". Please explain your basis for concluding that the elevated TDS was not caused by mining. Response ]0.) As discussed on page 4-226j of Permit No. C-81-010, mine dewatering from adjacent underground operations, and dcwatering from previous C pit pumping, are thought to have affected the water levels at this location. TDS concentrations az well GE-1 are noted to have gradually increased over the last few years but the amount of increase has occurred within the natural range of TDS wncentrations documented elsewhere within the QR aquifer. Sulfate concentrations at well GE-1 do appear to have increased relative to baseline levels at this site. Recently recorded sulfate values are similar to those measured at site GD-3, completed in the Derringer pit backfill aquifer. Pronounced TDS and sulfate increases have not, however, been documented to date at sites GD-2 (completed in the R coal seam) or GF-6 (completed in the Q coal seam) situated downgradient and in close proximity to previously mined areas. Given the proximity of wells GD-2 and GF-6 to surface mining operations in D pit and E pit respectively, it is reasonable to anticipate that groundwater quality changes would manifest themselves at these sites following the recovery of water levels in these wells. The trends noted at sites GD-2 and GF-6 are not consistent with the observations at site GE-l which suggests that other factors may be influencing the water quality at this site which are not related to surface mining. Correspondence to the Division dated April 10, 1997, pertaining to the TR-75 proposal to modify the groundwater sampling program, estimates 0.3 ft/day as the maximum groundwater velocity projected for the area. This assessment included estimates for backfill aquifers which likely exhibit higher velocities