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A and D Pit and E Pit have been investigated as two other options for disposal of <br />Craig ash and sludge in [he mine. Attenuation studies have been conducted to <br />develop a quantitative comparison of [heir re spec[ive capacities for attenuating <br />contamination in the ash leachate. Disposal in the spoils bench was studied in <br />the comparison. Key leacliing/attenuation parameters are summarized in Tables <br />4.3-21 and 4.3-23. Attenuation capacities and attenuated concentrations of the <br />two overburdens are compared in Tables 4.3-24 and 4.3-25 respectively. <br />No significant difference in either their attenuation capacities or concentra- <br />tions of barium, aluminum, boron, chromium, molybdenum, vanadium or manganese was <br />observed. Approximately four times as much sulfate was leached from the E Pit <br />overburden as the A and D Pit overburden. <br />Potential to Effect Down-Gradient Users of Ground Water <br />The hydrology and geology of the Trapper Mine and surrounding areas are discussed <br />in detail in Section 2.7. Much of the work done in the past three years has been <br />to determine the potential for groundwater contamination from disposal of Craig <br />Station waste in the Trapper Mine. <br />The Trapper ?Sine and surrounding areas are located within a complex hydrogeologic <br />setting with lithologic and hydrologic properties which are extremely variable <br />laterally and vertically. The coal mined at the Trapper Nine is in the top 20 to <br />160 fee[ of the approximately 680-foot [hick Upper Williams Fork member of the <br />Williams Fork formation. Topography and geologic structure primarily control <br />groundwater movement from the mine area and create the theoretical potential for <br />discharge of mine-area groundwater to the Yampa River through its alluvial aqui- <br />fer. <br />Groundwater a[ the Trapper Mine site occurs in both bedrock and alluvial aqui- <br />fers. The best aquifer in [he Trapper ?fine area is the Twenty Mile sandstone <br />which is also in the Williams Fork formation and generally underlies [he Williams <br /> Fork member. It probably has a permeability near 50 gallons/day/ft2 and [rans- <br /> missivi[y values of 5000 gallons/day/ft. The upper unit of the Williams Fork <br />4-89 <br />