by James C. Adamski, James C. Petersen, David A. Freiwald, and Jerri V. Davis
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The Ozark Plateaus study is 1 of 20 National
Water-Quality Assessment (NAWQA) studies initiated
by the U.S. Geological Survey in 1991 to
describe the status and trends in the quality of the
Nationís water resources. When the NAWQA program
is fully implemented, a total of 60 study
units in the United States will be investigated on a
rotational basis. Study-unit investigations will
include 5 years of intensive assessment activity
followed by 5 years of low-level monitoring.
The environmental and hydrologic setting of the Ozark Plateaus National Water-Quality Assessment study unit and their factors that affect water quality are described in this report. The primary natural and cultural features that affect water-quality characteristics and the potential for future water-quality problems are described. These environmental features include physiography, climate, population, land use, water use, geology, soils, and surface- and ground-water flow systems.
The Ozark Plateaus study unit has an area of approximately 48,000 square miles and includes parts of Arkansas, Kansas, Missouri, and Oklahoma. The study unit contains most of the Ozark Plateaus Province and parts of the adjacent Osage Plains section of the Central Lowland Province and the Mississippi Alluvial Plain section of the Coastal Plain Province. The Ozark Plateaus Province consists of three sections--the Springfield Plateau, the Salem Plateau, and the Boston Mountains. Topography in the study unit is mostly gently rolling, except in the Boston Mountains and along the escarpment separating the Springfield and Salem Plateaus, where it is rugged. Karst features such as springs, sinkholes, and caves are common in the Springfield Plateau and abundant in the Salem Plateau.
The study unit has a temperate climate with average annual precipitation ranging from about 38 to 48 inches and mean annual air temperature ranging from 56 to 60 degrees Fahrenheit. Population in the study unit was about 2.3 million people in 1990 and increased 28 percent between 1970 and 1990. Land use in the study unit is predominantly pasture and cropland in the northwestern part, and forest and pasture in the southeastern part. Poultry farming is a major industry in the southwestern part of the study unit. Mining, primarily in the four major lead-zinc mining districts, has been an important part of the local economy in the past. Total water use averaged 1,053 million gallons per day in the study unit in 1990. Ground water accounted for about 58 percent of the water withdrawn for all uses; surface water accounted for 42 percent.
Basement igneous rocks of Precambrian age are overlain by as much as 5,000 feet of gently dipping sedimentary rocks throughout much of the study unit. The igneous rocks, which include granite, rhyolite, and diabase, are exposed only in the St. Francois Mountains of southeastern Missouri. The sedimentary rocks include rocks of Cambrian through Ordovician age, which consist of dolomite, sandstone, and limestone with minor amounts of shale; rocks of Mississippian age, which are mostly cherty limestones; rocks of Pennsylvanian age, which consist mostly of shale, sandstone, and limestone; and Post-Paleozoic sediments, which consist of sands, gravels, and clays. The igneous and sedimentary rocks that underlie the study unit are extensively fractured and Environmental and Hydrologic Setting of the Ozark Plateaus Study Unit, Arkansas, Kansas, Missouri, and Oklahoma faulted. Alfisol and ultisol soil types underlie most of the study unit. These soils are moderately to deeply weathered and have a wide range of hydraulic properties.
All or part of seven major river basins are within the study unit. These basins include the White, Neosho-Illinois, Osage, Gasconade, Meramec, St. Francis, and Black River Basins. Many of the rivers are impounded to form reservoirs. Stream gradients are steepest in the Boston and St. Francois Mountains and least steep in the Osage Plains and Mississippi Alluvial Plain. Streambed material ranges from clay and silt in the Osage Plains to sand, gravel, boulders, and bedrock in most of the Ozark Plateaus Province. Mean annual runoff ranges from 9 to 10 inches in the Osage Plains to 14 to 20 inches in the Boston Mountains. Minimum monthly streamflows generally occur from July through October, and maximum monthly streamflows occur from March through May. Surface- and ground-water interactions are greatest in the Springfield and Salem Plateaus and least in the Boston Mountains and Osage Plains. The ionic composition of surface water generally is calcium or calcium magnesium bicarbonate in the study unit. Dissolved-solids concentrations in water from streams range from about 40 milligrams per liter in the Boston Mountains to as much as 280 milligrams per liter in the Osage Plains, but generally are less than 200 milligrams per liter. Streams in the Boston Mountains generally are the least mineralized and those in the Osage Plains generally are the most mineralized in the study unit.
The study unit contains eight hydrogeologic units that consist of three major aquifers--the Springfield Plateau, Ozark, and St. Francois aquifers-- interbedded with four confining units. The unconsolidated sediments of the Mississippi Alluvial Plain are a very productive aquifer, but are of limited areal extent in the study unit. The Springfield Plateau and Ozark aquifers are formed from thick sequences of limestones and dolomites. Rocks in both of these aquifers have secondary porosity as a result of fracturing and dissolutioning and these aquifers are used extensively for sources of water supply. Where the Springfield Plateau aquifer is unconfined, it is extensively used as a source of water for domestic purposes. Well yields in this aquifer generally are less than 20 gallons per minute. The Ozark aquifer is used throughout much of the study unit as a source of water for public and domestic supply. Yields of wells completed in this aquifer commonly range from 50 to 100 gallons per minute but can be as much as 600 gallons per minute. The St. Francois aquifer consists of sandstones and dolomites of Cambrian age. Although well yields in this aquifer can be as much as 500 gallons per minute, the aquifer is rarely used except where it crops out. The ionic composition of ground water in most of the aquifers in the study unit is calcium or calcium magnesium bicarbonate, but locally it can be a calcium sulfate or sodium chloride where the aquifers are confined. Dissolved-solids concentrations generally range from 200 to 300 milligrams per liter, but can be as much as 10,000 milligrams per liter in the deeper aquifers along the western boundary. Ground water in the study unit has a pH of 5.2 to 8.3, locally can contain fecal bacteria, and in some areas has elevated concentrations of radionuclides and nitrates.
Factors that affect water quality in the study unit include geology, land use, and population density. The geochemical processes of mineral dissolution, ion exchange, and oxidation-reduction reactions are the dominant natural factors that affect water quality on a regional scale. Agricultural and mining land-use activities can increase the concentrations of nutrients, bacteria, dissolved solids, sulfate, and trace elements in the surface and ground water of the study unit. Increased population density can result in increased discharges of nutrients, trace elements, bacteria, suspended sediment, and organic compounds.
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