Ground Water Monitoring Activities Arkansas River Valley Alluvial Aquifer 94-95

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I ` 0295 SAMPLING AREA DESCRIPTION AND CHARACTERISTICS SAMPLING AREA LOCATION AND DESCRIPTION The sampling area includes the Arkansas River valley from just east of Pueblo, Colorado to the Colorado - Kansas border inProwers County. The area is approximately 150 miles in length and occupies about 400 square miles. AGRICULTURAL HISTORY AND WATER USE IN THE AREA The agricultural economy of the Arkansas river basin is based on irrigated and dry land farming, and livestock production. The principal irrigated crops are alfalfa, corn, sorghum, winter wheat, melons, onions, and dry beans. Small grains, chiefly wheat, are the principal dry land farm crops. The livestock consists mostly of beef and dairy cattle although some hogs and sheep are raised. Several large canals for diverting water for irrigation were constructed in the late 1800's. Water for irrigation is also diverted from several reservoirs. Because shortages of surface water occur during years of low runoff and the timing of runoff does not coincide with irrigation requirements, irrigation wells have been drilled in the valley to supplement the surface -water supply. In some areas groundwater pumped from wells is the sole supply for irrigation. Many of the towns and some industries in the area also obtain water from wells. Although nearly all the large - capacity wells are in the valleys, small - capacity domestic and stock wells have been constructed both in the valleys and on the uplands since early settlement of the region. The upland areas that are without adequate surface or groundwater supplies for irrigation total about 250,000 acres and are used largely for dry land farming and for grazing. The principal industries in the area are related to the processing of agricultural products; they include vegetable processing plants, alfalfa mills, vegetable warehouses, and confined animal feeding. Deposits of sand and gravel are exploited for the fabrication of concrete products and building roads. GEOLOGY M M The rocks that crop out in the lower Arkansas River Valley are sedimentary and range in age from Early Cretaceous to Recent. The oldest rocks, which are of Early Cretaceous age, are the Dakota sandstone, the Carlile shale, and the Niobrara formation. Most of the major valleys contain Quaternary deposits of alluvium, terrace deposits, and dune sand. The alluvium in the Arkansas River valley was deposited in a channel eroded into the underlying bedrock and consists mainly of heterogeneous mixtures of clay, sand, and gravel, or lenses of these materials. Pebbles, cobbles, and boulders occur as erratics. The particles generally are well rounded to subrounded and range from well sorted to poorly sorted. Extensive lenses of clay are present within the alluvium. These clay lenses are most prevalent in the tributary valleys