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Western Slope Carbon, Inc. Page 7 <br />December 15, 1981 <br />I <br />municipal and private wells in the vicinity likely obtain water from this <br />source. While specific information is not readily available, we suspect <br />11 that the average depth to groundwater in the valley is likely on the order <br />of 10 to 20 feet. Recharge likely occurs over much of the valley floor <br />and results from direct precipitation, infiltration of stream runoff from <br />the adjacent uplands and possibly through the channel of the Gunnison River. <br />In the Pit No. 6 area, groundwater was encountered in two of the five borings, <br />Borings 4 and 5. Both of these were drilled west of the existing embankment <br />in the bottom of the shallow unnamed ephemeral drainage channels discussed <br />-~ above. Stabilized water levels were measured 13'~ and 8~ feet below the ground <br />surface in Borings 4 and 5, respectively. In both borings, seepage was first <br />encountered in the soil directly overlying the relatively unweathered Mancos <br />shale. It appears that surface infiltration is seeping down through the soil <br />1 column to the surface of the relatively impermeable Mancos shale and forming <br />a thin perched groundwater layer. Since groundwater was not encountered in <br />the other three borings which also penetrated the Mancos shale, this perched <br />water table is not continuous over the site area. Based on the site topography <br />and soil conditions, it is our opinion that free groundwater likely does not <br />exist in significant quantities over much of the site. <br />It is not clear whether hydraulic continuity exists between the groundwater <br />encountered at the site and the groundwater basin of the Gunnison River Valley. <br />Assuming that continuity does exist, it is unlikely that significant quantities <br />of groundwater reach the Gunnison River Basin from the site area by underground <br />flow because of the extremely low permeabilities (approximately 10-6 centi- <br />meters per second) of the site soils. As noted in the borings, the native <br />' surf ace .soils, as well as, the embankment materials which are constructed of <br />native soils,.were saturated in the top 6 to 12 inches from recent precipitation, <br />but were dry or only slightly moist for a considerable distance below that depth. <br />This suggests that initial precipitation tends to saturate and seal the near <br />surface soils and that little infiltration occurs after the initial saturation. <br />7 As such, it is likely that the major portion of the precipitation incident upon <br />the site area is carried from the site as surface runoff. <br />