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<br />T880161 <br />' A ditch around that southern quarry slope and along the gravelfshale interface will <br />intercept seeps and surface water, direct flows around the slope, along the road and to <br />the pond. Precipitation falling into the pit will collect in an impoundment sump at the <br />northeast corner of the quarry and be discharged in accordance with NPDES Permit <br />requirements. <br />Ground li'aterHydrology <br />Ground water occurs in several geologic units near the quarry site. A major regional <br />aquifer which is exposed and recharged in the area is the Fox Hills Sandstone. The <br />Fox Hills Sandstone here dips steeply to the east (50° to 67°, Spencer, 1961). The <br />' formation is exposed as a resistant ridge about .3S miles east of the quarry. The <br />intervening interval (from the quarry to the ridge) is Pierre Shale. Because of the <br />Pierre's very low permeability (i. e, on the order of 10-6 cm/sec hydraulic conductivity), <br />' the quarry operations will have no effect on the water level or quality in the Fox Hills <br />aquifer. <br />' Two other confined aquifers occur to the west of, and stratigraphically down-section <br />from, the quarry. The Hygiene Sandstone Member of the Pierre Shale forms a possible <br />aquifer about S65 feet thick. It is exposed along Doudy Draw and the interchanne] area <br />towards Goal Creek. The outcrop is approximately 3,500 feet west of the quarry. The <br />t Hygiene clips 4S°-SS° to the east, which would cause it to occur at a depth greater than <br />3,000 feet under the site. The 3,000 foot interval is composed of extremely low <br />permeability shales and claystones of the Pierre Shale which makes any percolation from <br />the quarry impossible. The recharge areas are up gradient from the quarry so the <br />~ possibility of affecting the Hygiene aquifer is nil. <br />The last confined aquifer in the area is the sandstones of the Dakota Group which form <br />the first resistant hogback along the mountain front, approximately 1.S miles west of <br />the site. Because the dip of the Dakota averages 45°, these beds would be very deep <br />' (> 6,000 :"eet) below the site and contain highly mineralized, non-potable water. It is <br />cwlcluded ;vo confined aquifers will be effected 6y the quany operation. <br />1 <br />-25- <br /> <br />