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UPPER ARKANSAS GROUND -WATER STUDY In 2000, The Upper Arkansas Water Conservancy District and the United States Geological Survey embarked as partners on a three -year study of ground -water resources in the Upper Arkansas Area between Buena Vista and Salida. The purpose of the study was to provide a scientific understanding of the ground -water resources in the Upper Arkansas Area. Reference to this study will be made for illustrations throughout this booklet. In addition, much of the material in this booklet is excerpted from a similar booklet that was prepared by the Jefferson County Zoning and Planning Department. Additional funding for this study was provided by Chaffee County and the Colorado Water Conservation Board. Unconsolidated alluvial deposits in the Leadville Basin, Buena Vista - Salida Basin, Wet Mountain Valley, and eastern Fremont County primarily are clay, silt, sand, gravel, and boulders that were deposited by streams and rivers. Generally, these deposits are less than 100 feet thick and occur along streams and rivers or as terraces. They are the youngest rock type in the area and may overlie any of the other rock types. They are exposed at the surface in about 6 percent of the area. Where saturated, well yields in these deposits range from 50 to 1,000 gallons per minute. Because these deposits are relatively shallow they are susceptible to contamination from the surface. Glacial deposits, which are unconsolidated to poorly consolidated, extend along valleys from the surrounding mountains into the Leadville Basin and from the mountains on the west into the Buena Vista - Salida Basin and Wet Mountain Valley. The glacial deposits consist of poorly sorted sand, gravel, cobbles, and boulders, with layers of silt and clay. Thickness of the glacial deposits ranges from about 100 to about 500 feet. They are exposed at the surface in about 3 percent of the area. They may underlie younger alluvial deposits in the Buena Vista - Salida and Leadville Basins and the Wet Mountain Valley. Where saturated, well yields from these deposits range from less than 10 to 1,500 gallons per minute. Because these deposits also are relatively shallow, they also are susceptible to contamination from the surface. Basin -fill deposits are poorly consolidated rocks that occur in the down- faulted Buena Vista - Salida and Leadville Basins, and Wet Mountain Valley. They consist of sand, gravel, and cobbles, with beds of siltstone, sandstone, and volcanic ash. Thickness is estimated to be as much as 4,000 feet in the Leadville Basin; 4,600 feet in the Buena Vista - Salida Basin; and 6,700 feet in the Wet Mountain Valley. In some areas they underlie alluvial and glacial deposits. They may overlie consolidated rocks or crystalline rocks. These rocks are exposed at the surface in about 5 percent of the area but underlie unconsolidated deposits in the upper basins. Well yields from the basin -fill deposits range from about 10 to 1,200 gallons per minute. Volcanic rocks occur primarily in northwestern Fremont County, in Custer County near Westcliffe, and locally in the mountains around the Buena Vista - Salida and Leadville Basins. Thickness may be as much as 1,500 feet. Water occurs in pores in the volcanic rocks and interbedded sand and gravel. Well yields generally are less than 10 gallons per minute, but may be as much as 30 gallons per minute in the Wet Mountain Valley. Consolidated rocks include some strata (shale and shaley limestone), which generally do not yield significant quantities of water to wells. Maximum combined thickness of these rocks is about 5,000 feet. These rocks are exposed at the surface in about 20 percent of the area and generally overlie water - bearing consolidated rocks. Some wells may obtain water from fractures and possibly solution openings in limestone strata. Well yields range from 2 to 100 gallons per minute. Consolidated rocks also include porous rocks (conglomerate, sandstone, siltstone, limestone, and coal), which commonly yield water to wells. Maximum combined thickness of these rocks is more than about 7,000 feet. They are exposed at the surface in about 15 percent of the area but probably underlie about 35 percent of the area. However, few wells have been completed in some of these formations. Well yields from the consolidated rocks are reported to range from 3 to 1,600 gallons per minute, but generally are from 10 to 500 gallons per minute. Crystalline rocks consist primarily of igneous rocks (granite, diorite, and