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
Nation’s 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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