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Peabody Sage Creek Mine (C-2009-087) MT- 2 <br />alluvial bodies in local drainages, and coal spoil backfill in reclaimed surface mine pits from the <br />Seneca II Mine. <br />The sandstone and coal hydrostratigraphic units are referred to in this discussion as bedrock units. Of <br />the bedrock units, the Trout Creek and Twentymile Sandstone have the greatest potential for serving <br />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 in <br />initial well yields in the vicinity of the PSCM permit area of less than 20 gallons per minute. Alluvium <br />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 transfered to the <br />PSCM 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 0.4 to <br />290 gallons per minute. Flows varied seasonally, peaking in the springtime and diminishing to a <br />minimum by late summer. Spoil leachate also seeps to Pond 004 in the Fish Creek drainage. Coal spoil <br />leachate flowing into Ponds 002, 004, and the PeCoCo pond in the PSCM permit area maintain <br />permanent pools in the ponds and can cause the ponds to 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 coal <br />has characteristics similar to the non -coal bedrock units, but TDS is as much as 3,670 mg/I. The <br />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/1 range. <br />Bedrock hydrostratigraphic units in the vicinity of the PSCM 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 the <br />force of gravity generally down dip, toward the deeper part of the Sand Wash Basin (or Twentymile <br />Park east of the permit area) where the ground water is under confined conditions. The head <br />(hydraulic pressure) at a location in a basin is determined by the height of the recharge areas above <br />that location and the proximity to discharge areas. Bedrock units discharge ground water at low <br />elevations in a basin wherever the units crop out or have a subcrop underneath the younger <br />colluvium and alluvium. The dissolved solids content of ground water in bedrock units generally <br />increases basinward from the recharge areas. <br />Page 5 of 12 <br />