The El Dorado aquifer (lower part of the Sparta Sand) is the basal freshwater unit in Union County and the major source for municipal and industrial water supplies. Since 1960, chloride concentration in water from the aquifer has increased in the vicinity of El Dorado, the center of greatest withdrawal. The aquifer is approximately 300 feet thick and is confined by the Cane River Formation below and the middle confining unit of the Sparta Sand above. These Tertiary rocks dip gently east-southeast in Union County and are affected locally by a graben to the south and east of El Dorado. From the onset of development in the 1920's the center of greatest withdrawal from the El Dorado aquifer has remained at El Dorado. Pumpage has increased from less than 0.5 million gallons per day in 1921 to 16 million gallons per day in 1982, reaching a high of 19 million gallons per day in 1965. In response to the withdrawal, the potentiometric surface at El Dorado has been lowered more than 300 feet, increasing the hydraulic gradient toward the center of pumping. Chloride concentrations generally increase in the El Dorado aquifer to the east-southeast, conforming to the dip of the rocks and the original direction of water movement. North and west of the graben, chloride concentrations range from 25 to 150 milligrams per liter. Wells with the highest concentrations are located at or near the west end of the graben. Estimates based on interpretation of electric logs for two wells in the graben indicate chloride equivalents may be as high as 2,500 milligrams per liter in the El Dorado aquifer. Analyses of water from aquifers above the El Dorado aquifer preclude surface brines as a source of this contamination. Saltwater-bearing formations below the El Dorado aquifer are eliminated as sources of contamination on the basis of geocheraical analyses and hydrostatic pressures. Geologic, hydrologic, and chemical data indicate that the primary source of chloride is the El Dorado aquifer within the graben.

An appproximate area of 56 square miles of the alluvial aquifer has been contaminated by saltwater (chloride concentration equal to or greater than 50 milligrams per liter) intruded from underlying aquifers. The contamination was mapped from water quality data for 217 wells. Saltwater problems appear to have spread rapidly in the alluvial aquifer since the late 1940 f s. Chemical comparisons indicate that the alluvial aquifer was contaminated by water from the Sparta aquifer which in turn was contaminated by the underlying Nacatoch aquifer. The possibility of intrusion into the alluvial aquifer through abandoned oil and gas test wells was investigated but no evidence could be found to support this possibility. Upward movement into the alluvial aquifer from the underlying Sparta aquifer through the thinned or absent Jackson confining unit appears to be the principal reason for saltwater in the alluvial aquifer.

No abstract

NOTE: This map is the product of collaboration of State geological surveys and the U.S. Geological Survey and is designed for both scientific and practical purposes. It was prepared in two stages. First, separate maps and map explanations of the parts of States included in the quadrangle were prepared by the State compilers. Second, these maps were integrated and locally supplemented by the editors; map unit symbols were revised to a uniform system of classification; and map unit descriptions were prepared from information received from State compilers and from additional sources. Diagrams accompanying the map were prepared by the editors. Differences in mapping or interpretation in different areas were resolved by correspondence to the extent possible. Most simply reflect differences in available information or in philosophies of mapping and should encourage further investigation. Less than forty percent of the surficial deposits of the United States have been mapped and described. Traditionally, mapping of surficial deposits has focused on glacial, alluvial, eolian, lacustrine, marine, and landslide deposits. Slope and upland deposits have been mapped in detail only in restricted areas. However, an enormous amount of engineering construction and many important problems of land use and land management are associated with regions that have extensive slope and upland deposits (colluvium, residuum, and saprolite, for example). These materials have many different physical characteristics. Therefore, an effort has been made to classify, map, and describe these deposits, based in large part on unpublished interpretations, published and unpublished subsoil data, and the distribution of bedrock parent materials. The classification is crude, but represents a first step toward a more refined and useful product. For scientific purposes, the map differentiates Quaternary surficial deposits on the basis of a combination of criteria, such as lithology, texture, genesis, stratigraphic relationships, and age, as shown on the correlation diagram and indicated in the map unit descriptions. Some geomorphic features, such as end moraines, are distinguished as map units. Erosional features, such as stream terraces, are not distinguished, and differentiation of sequences of alluvial deposits of different ages is rarely possible at a scale of 1:1,000,000. Most landslide deposits are too small to be shown at this scale. For practical purposes, the map is a surficial materials map. Materials are distinguished on the basis of texture, composition, and local specific characteristics such as swelling clay. It is not a map of soils as soils are recognized and classified in pedology or agronomy. Rather it is a generalized map of soils as recognized in solifluction, mudflow, frost heave, and, locally, landslide. 2 DECOMPOSITION RESIDUUM, for purposes of this map, is defined as material derived primarily by in-place chemical decay of clastic rock with no appreciable subsequent lateral transport. 3 SOLUTION RESIDUUM, for purposes of this map, i...

This map is the fourth in a folio of maps of the Harrison I°x2° quadrangle, Missouri and Arkansas, prepared under the Conterminous United States Mineral Assessment Program (CUSMAP). Previously published maps in this folio relate to the geochemistry of the subsurface carbonate rocks (Erickson and others, 1988), the geophysics of the basement terranes (McCafferty and others, 1989), and the mineralization and carbonate lithofacies in the Caulfield district in the eastern part of the Harrison quadrangle (Hayes and others, 1992). Additional maps showing other geologic aspects of the Harrison quadrangle will be published as U.S. Geological Survey Miscellaneous Field Studies Maps bearing the same serial number but with different letter suffixes (MF-1994-E, -F, and so on).