2012-09-06_REVISION - M2008070 (22)

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SUMMARY AND CONCLUSIONS The White River alluvial aquifer in the study area ranges in width from 0.1 mi at each of several locations in the western part of the study area to about 1.5 mi in Agency Park and Powell Park; average width is about 0.5 mi. In general, the aquifer is wide where underlain by the Mancos Shale or the Wasatch Formation and narrow where underlain by the Mesaverde Group or the Green River Formation. Saturated thickness ranges from zero along the valley sides at each of several locations to more than 140 ft in Agency Park; average saturated thickness is about 22 ft. Test -hole data at the Meeker municipal well field in Agency Park established a local saturated thickness of at least 114 ft. Ver- tical electrical- resistivity soundings indicate that the alluvial thickness exceeds this value nearby, and perhaps is 295 ft in one area. The area east of and adjacent to Agency Park, in the drainage areas of Little Beaver and Coal Creeks, may be underlain by 80 ft of White River alluvium in places; however, useful data are scarce and the limit of the White River alluvial aquifer is uncertain in this area. Preliminary test -hole data indicate that saturated thickness is greater than 90 ft in the western part of Powell Park. Near Rangely, the aquifer is about 1 mi wide, but saturated thickness averages only 14 ft. Alluvium in the White River valley consists of fragments of rocks origi- nating on the White River uplift, transported downstream by the White River; and of material from nearby sources to the north and south, transported into the valley via tributaries, draws, and gullies. In the eastern part of the study area, the alluvium consists predominantly of coarse - grained sand and gravel with alternating thin layers of fine - grained material such as clay, silt and sand. In the western part of the study area, the alluvium consists of clay, silt, and fine - grained sand from land surface to several feet in depth, underlain by coarse sand and gravel containing interstitial fine - grained sedi- ments. West of the Grand Hogback monocline, the fine - grained alluvium over- lying the sand and gravel aquifer forms a semi - confining layer that causes the aquifer to be under artesian conditions in many places. Wells drilled through this dry to very moist fine - grained layer encountered water in the underlying saturated sand and gravel aquifer, which rose under artesian pressure up into the fine - grained layer. The interstitial fine - grained alluvium present in the western part of the study area clogs the openings between grains of the sand and gravel deposits and reduces their ability to transmit water. Yields of existing domestic and stock wells in the study area usually are less than 25 gal /min. Most of these wells are completed in a relatively inex- pensive and inefficient manner so that the well inhibits the maximum yield obtainable more than the transmissive properties of the aquifer do. Relatively efficient and expensive wells, completed with stainless - steel, wire- wrapped screens designed specifically for the well and aquifer material at the site, produce water from the White River alluvial aquifer at the Meeker municipal well field in Agency Park and at the Deserado coal -mine site about 7 mi north- east of Rangely. The municipal wells in Agency Park reportedly could yield more than 1,000 gal /min each. At the Deserado coal -mine site, the wells were designed for a long -term yield of 130 to 145 gal /min each. 30