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H. Evaluation of Hydrologic Impacts <br />The Division has reviewed the hydrologic impacts caused by the Chimney Rock Mine. The results <br />of this review are summazized below. <br />Leachate Formation -Coal spoil leachate probably is forming in the backfilled pits ofthe reclaimed <br />Chimney Rock Mine. The leachate forms as slightly acidic meteoric water infiltrates from the <br />ground surface through spoil. As the water percolates through the spoil, it dissolves and oxidizes <br />minerals in the spoil, resulting in a leachate high in dissolved solids. Sulfate would be a main <br />component of the leachate. The TDS concentration of the leachate can be expected to reach an <br />ultimate maximum in the range of 3000 to 4000 mg/1. This concentration may continue for several <br />hundred years, as predicted for the Seneca II Mine in Routt County, Colorado (USGS Water- <br />Resources Investigations Report 92-4187). <br />Leachate Discharge -Kaiser's backfilled mine pits probably discharge coal spoil leachate into the <br />Stollsteimer Creek alluvial aquifer where the pit was excavated in the alluvium. The discharge is <br />probably fairly continuous over time. The following two pathways aze predicted for the dischazge: a) <br />leachate may flow in the subsurface from the east side of the backfilled Barren Ridge pit into the <br />alluvium at the base of Barren Ridge in the center of Section 30-T34N-R4W; and b) leachate may <br />flow in the subsurface from the southwest comer of the backfilled East pit into the alluvium in Nl /2 <br />SE1/4 Section 30-T34N-R4W. The amount ofleachate discharging into sandstones ofthe Fruitland <br />Formation in the walls of the pits is probably small because the preferred flowpath for leachate <br />would be towazd the much more permeable alluvium of Stollsteimer Creek in the walls of the pits, <br />rather than the low permeability sandstones in the walls of the pits. <br />Impacts to Bedrock Aquifers -The Fruitland Formation is the only bedrock aquifer that is in the <br />ground water flow path of the previously-described spoil leachate. There is nopre-mining monitoring <br />data from bedrock wells. Kaiser monitored the Fruitland formation downgradient from the Barren <br />Ridge pit in well W-9. Monitoring was conducted from 1485 through 1991. It showed no impacts <br />from leachate. Kaiser monitored the Fmitland underneath the Barren Ridge pit in well W-10. <br />Monitoring spanned 1985 through 1991. This well showed a significant TDS increase (on the order <br />of 2,000 mg/1) when it was last monitored in 1991. This increase is consistent with degradation of <br />ground water in the Fruitland at this location from an influx of leachate from the pit in the Revision <br />1 azea. That pit is upgradient from W-10 and had been reclaimed a few years earlier. If leachate <br />caused this degradation, the impact would be considered a destruction of a small, localized aquifer <br />that is not used; this is a ma}or hydrologic impact, but does not rise to the level of material damage. <br />A plume of degraded water resulting from such degradation can be expected to not migrate much <br />beyond the permit boundary due to the low permeability of Fruitland Formation sandstones. The <br />salinity of Fruitland Formation ground water naturally increases downgradient from the Chimney <br />Rock Mine as shown by gas field brines in the San Juan Basin. Wells W-9 and W-10 have shown no <br />impacts to the potentiometric surface of the Fruitland Formation from mining at Chimney Rock. <br />Impacts to Alluvial Aquifers. Kaiser collected baseline water level and water quality data at the <br />upstream alluvial monitoring well W-1 in late 1980 and early 1981. The well is completed in alluvial <br />deposits of Stollsteimer Creek. From 1985 through 2002, Kaiser monitored alluvial water levels and <br />water quality upstream from the Chimney Rock Mine in well W-1, and downstream from the mine in <br />11 <br />