2020-08-20_REVISION - C2009087 (41)

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are locally present in stream valleys. Recent colluvial slides and slumps may also be present in canyons and gulches throughout the area. The Seneca II Mine extracted coal on approximately 1,800 acres of land that was transferred to the PSCM. Coal seams were surface mined at the Seneca II Mine from the lower member of the Williams Fork Formation. The lower member of the Williams Fork Formation is approximately 300 feet thick in the permit area. Approximate thicknesses of the coal seams, in ascending stratigraphic order are: the Wolf Creek seam, 15 ft; the Wadge seam, 10 ft; and the Lennox seam, 4 ft. Shale, siltstone, and sandstone separate the coal seams. The Wadge seam is approximately 150 above the Wolf Creek seam and approximately 40 feet below the Lennox seam. The PSCM will mine coal from the Wadge seam in the Hayden syncline. Groundwater Groundwater hydrology information can be found in section 2.04.5 of the permit application. A "hydrostatigraphic unit" is a group of stata which has hydraulic continuity. Hydrostatigraphic units in and adjacent to the permit area that may serve as aquifers are (in ascending stratigraphic order): the Trout Creek Sandstone member of the Iles Formation, coal and sandstone beds in the lower member of the William Fork Formation, the Twentymile Sandstone member of the Williams Fork Formation, alluvial bodies in local drainages, and coal spoil backfill in reclaimed surface mine pits from the Seneca II Mine. The sandstone and coal hydrostratigraphic units are referred to in this discussion as bedrock units. Of the bedrock units, the Trout Creek and Twentymile Sandstone have the greatest potential for serving as regional aquifers because individual sandstone beds in these units are thicker and more homogenous over wider areas than individual sandstone or coal beds in the lower member of the Williams Fork Formation. All the bedrock units have low hydraulic conductivity values, resulting in initial well yields in the vicinity of the PSCM permit area of less than 20 gallons per minute. Alluvium in Grassy Creek and Fish Creek may yield water to wells in limited usable quantities. Backfilled surface mine pits on lands of the Seneca II Mine that were transferred to the PSCM discharge coal spoil leachate to the land surface at five locations in the Little Grassy Creek drainage. Sample data from the springs from 2005 through 2008 showed flows in the range of 0.4 to 290 gallons per minute. Flows varied seasonally,peaking in the spring and diminishing to a minimum by late summer. Spoil leachate also seeps to the Phase III released, Pond 004 in the Fish Creek drainage. Coal spoil leachate flowing into Ponds 002, 004, and the PeCoCo pond in the PSCM permit area maintain permanent pools in the ponds and can cause the ponds to discharge. Groundwater in the non-coal bedrock units is characterized by total dissolved solids in the 500 to 2,400 mg/l range,pH between 7 and 9, and a naturally high magnesium content. Groundwater in coal has characteristics similar to the non-coal bedrock units, but TDS is as much as 3,670 mg/l. The groundwater in the bedrock units is primarily a sodium bicarbonate-type, with local gradation to calcium magnesium sulfate. Groundwater in Grassy Creek and Fish Creek alluvium is an alkaline calcium magnesium sulfate water with TDS that ranges between 300 and 4,000 5