National Water Quality Assessment Program (NAWQA)
Study Unit Environmental Setting
The following information is based on USGS
Water Resources Investigations Report 94-4022
The Ozark Plateaus study unit has an area of approximately 48,000 mi2 and includes parts of Arkansas, Kansas, Missouri, and Oklahoma. Major water quality concerns in the study unit include elevated concentrations of nutrients in surface and ground waters; elevated concentrations of bacteria, trace elements, dissolved solids, and radionuclides in ground water, and saline ground-water encroachment.
The study unit has a temperate climate with average annual precipitation ranging from about 38 to 48 in., and mean annual air temperature ranging from 56 to 60o F. Evapotranspiration rates range from 30 to 35 in/yr.
Population in the study unit was approximately 2.3 million people in 1990 and increased 28 percent between 1970 and 1990. Northwestern Arkansas and southwestern Missouri are the fastest growing areas in the study unit. Springfield, Missouri, with a 1990 population of 140,494, is the largest city in the study unit.
Land use in the study unit is predominantly pasture and cropland in the northwestern part of the study unit, and forest and pasture in the southeastern part. Forests consist mostly of oak and hickory trees mixed with some pine trees. Pasture is mostly fescue and Kentucky blue grass. 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. Coal has also been mined in the northwestern part of the study unit.
Stream gradients are steepest in the Boston and St. Francois Mountains and least 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. Streams in the Osage Plains are turbid, with long pools separated by poorly defined riffles. Streams in the Ozarks Plateaus Province are mostly clear, with pools separated by riffles, and in places, cascading waterfalls.
Mean annual runoff ranges from 9 to 10 in. in the Osage Plains to 14 to 20 in. 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. Ground water discharging through springs contributes significantly to low flows in the Springfield and Salem Plateaus.
Surface water in the study unit generally is a calcium or calcium magnesium bicarbonate type water. Dissolved-solids concentrations in water from streams ranged from about 40 mg/L in the Boston Mountains to as much as 280 mg/L in the Osage Plains, but generally were less than 200 mg/L. 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 ground-water system in the Ozark Plateaus study unit is divided into seven hydrogeologic units consisting of three major aquifers and four confining units. These units, from youngest to oldest, are as follows: the Western Interior Plains confining system, the Springfield Plateau aquifer, the Ozark confining unit, the Ozark aquifer, the St. Francois confining unit, the St. Francois aquifer, and the Basement confining unit.
The Springfield Plateau and Ozark aquifers are formed from thick sequences of limestones and dolomites that have secondary porosity as a result of fractures and dissolution. Where the Springfield Plateau aquifer is unconfined, it is extensively used as a source of domestic water. Yields of wells completed in this aquifer generally are less than 20 gal/min. The Ozark aquifer is used throughout much of the area for public supply and domestic use. Well yields commonly range from 50 to 100 gal/min but are as much as 600 gal/min in some areas.
The St. Francois aquifer consists of sandstones and dolomites of Cambrian age. Well yields in the aquifer can be as much as 500 gal/min. The aquifer is little used except where it crops out.
Ground water in most of the aquifers in the study unit is a calcium or calcium magnesium bicarbonate type water, but locally it can be calcium sulfate or sodium chloride water where the aquifers are confined. Dissolved- solids concentrations generally ranged from 200 to 300 mg/L, but can be as much as 10,000 mg/L where the aquifers are confined along the western boundary. The pH of ground water in the study unit ranged from 5.2 to 8.3. The Springfield Plateau aquifer locally can contain fecal bacteria. The Ozark aquifer has elevated concentrations of radionuclides in some areas where it is confined. Elevated nitrate concentrations are present in ground water from unconfined parts of the Springfield Plateau and Ozark aquifer in some areas.
U.S. Department of the Interior , U.S. Geological Survey
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Last Modified: Wednesday, 09-Jan-2013 17:35:30 EST btj
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