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Groundwater <br /> Groundwater hydrology information can be found in section 2.04.5 of the permit application. A <br /> "hydrostatigraphic unit" is a group of strata with hydraulic continuity. Hydrostatigraphic units in <br /> and adjacent to the permit area that may serve as aquifers are(in ascending stratigraphic order): the <br /> Trout Creek Sandstone member of the Iles Formation, coal and sandstone beds in the lower member <br /> of the William Fork Formation,the Twentymile Sandstone member of the Williams Fork <br /> Formation, alluvial bodies in local drainages, and coal spoil backfill in reclaimed surface mine pits <br /> from the Seneca II Mine. <br /> The sandstone and coal hydrostratigraphic units are referred to in this discussion as bedrock units. <br /> Of the bedrock units, the Trout Creek and Twentymile Sandstone have the greatest potential for <br /> serving as regional aquifers because individual sandstone beds in these units are thicker and more <br /> homogenous over wider areas than individual sandstone or coal beds in the lower member of the <br /> Williams Fork Formation. All the bedrock units have low hydraulic conductivity values, resulting <br /> in initial well yields in the vicinity of the PSC permit area of less than 20 gallons per minute. <br /> Alluvium in Grassy Creek and Fish Creek may yield water to wells in limited usable quantities. <br /> Backfilled surface mine pits on lands of the Seneca II Mine that were transferred to the PSC <br /> discharge coal spoil leachate to the land surface at five locations in the Little Grassy Creek <br /> drainage. Sample data from the springs from 2005 through 2008 showed flows in the range of <br /> 0.4 to 290 gallons per minute. Flows varied seasonally, peaking in the spring and diminishing <br /> to a minimum by late summer. Spoil leachate also seeps to the Phase III released, Pond 004 in <br /> the Fish Creek drainage. Coal spoil leachate flowing into Ponds 002, 004, and the PeCoCo <br /> pond in the PSC permit area maintain permanent pools in the ponds and can cause the ponds to <br /> discharge. <br /> Groundwater in the non-coal bedrock units is characterized by total dissolved solids in the 500 to <br /> 2,400 mg/I range,pH between 7 and 9,and a naturally high magnesium content. Groundwater in <br /> coal has characteristics similar to the non-coal bedrock units, but TDS is as much as 3,670 mg/I. <br /> The groundwater in the bedrock units is primarily a sodium bicarbonate-type,with local gradation to <br /> calcium magnesium sulfate. Groundwater in Grassy Creek and Fish Creek alluvium is an alkaline <br /> calcium magnesium sulfate water with TDS that ranges between 300 and 4,000 mg/I range. Spoil <br /> leachate is an alkaline calcium-magnesium sulfate type water,with total dissolved solids in the 2,600 <br /> to 4,300 mg/I range. <br /> Bedrock hydrostratigraphic units in the vicinity of the PSC permit area are likely recharged by <br /> meteoric waters wherever a significant quantity of snowmelt or rainfall accumulates on bedrock <br /> outcrops at high elevations around the perimeter of the Hayden Syncline. The water seeps into the <br /> rock through intergranular porosity and fractures. Immediately down dip from a recharge area, the <br /> bedrock is under water table conditions. From a recharge area, bedrock groundwater flows under <br /> the force of gravity generally down dip, toward the deeper part of the Sand Wash Basin (or <br /> Twentymile Park east of the permit area) where the ground water is under confined conditions. <br /> The hydraulic pressure at a location in a basin is determined by the height of the recharge areas <br /> above that location and the proximity to discharge areas. Bedrock units discharge ground water at <br /> low elevations in a basin wherever the units crop out or have a subcrop underneath the younger <br /> Page 7 of 27 <br />