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By
Pixie A. Hamilton and Timothy L. Miller
Societal concerns for
the quality of our water resources continue, as many of the Nation's
streams and coastal waters do not meet water-quality goals. States
report that 40 percent of the waters they surveyed are too contaminated
for basic uses, such as fishing and swimming. Some progress has been
made since passage of the Clean Water Act in 1972, much of which has
been directed toward municipal and industrial point sources. Although
some violations still occur, this legislation has had a positive effect
on limiting contaminants from point sources entering streams.
Progress
in cleaning up contamination from point sources has not yet been matched
by control of contaminated runoff from nonpoint sources, such as fertilizers
and pesticides applied in agricultural and urban areas, and nutrients
from human and animal wastes. The National Water-Quality Assessment
(NAWQA) Program of the U.S. Geological Survey (USGS) recently has
released findings about the prevalence of nonpoint pollution based
on findings from studies completed in 20 of the Nation's most important
river basins and aquifers systems. Some NAWQA highlights are:
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Average annual
concentrations of phosphorus in three-fourths of streams in urban
and agricultural areas were greater than the USEPA desired goal for
preventing nuisance plant growth in streams.
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Nitrate was
detected in nearly three-quarters of shallow ground-water samples;
about 15 percent of all samples exceeded the USEPA drinking-water
standard.
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At least
one pesticide was found in almost every water and fish sample collected
from streams and in more than one-half of shallow wells sampled in
agricultural and urban areas.
Close Links to Land Use and
Chemical Use
The types and concentrations of nutrients and pesticides
found in streams and groundwater are closely linked to land use and
to the types of nutrients and pesticides used in each watershed. For
example, some of the highest concentrations of nitrogen and herbicides,
including atrazine, metolachlor, alachlor, and cyanazine, were detected
in samples collected from agricultural areas with the highest chemical
use. But high levels of chemical contamination are not just an agricultural
problem. For example:
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Insecticides
� most commonly diazinon, carbaryl, malathion, and chlorpyrifos �
occurred more frequently and usually at higher concentrations in urban
streams than in agricultural streams.
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Nearly all
urban streams in the study had concentrations of insecticides exceeding
at least one guideline established to protect aquatic life.
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Concentrations
of total phosphorus generally were higher in urban streams than in
agricultural and other settings.
Significance to human and aquatic
health
The
good news is that concentrations of individual pesticides in samples
from wells and as annual averages in streams were almost always lower
than current USEPA drinking-water standards and guidelines. Standards
and guidelines have been established for 46 pesticides and breakdown
products. Effects of pesticides on aquatic life, however, are a concern
based on U.S. and Canadian aquatic-life guidelines, which have been
established for 28 pesticides measured. More than one-half of agricultural
and urban streams sampled had concentrations of at least one pesticide
that exceeded a guideline for the protection of aquatic life.
Potential
risks to humans and aquatic life implied by NAWQA pesticide findings
can be only partially addressed by comparison to established standards
and guidelines for several reasons. First, many pesticides and their
breakdown products do not have standards or guidelines. Second, streams
and ground water in basins with significant agricultural or urban
development almost always contain complex mixtures of pesticides and
nutrients. More than one-half of all stream samples in the study contained
five or more pesticides, and nearly one-fourth of all groundwater
samples contained two or more. Existing water quality standards and
guidelines typically are based on tests of individual concentrations
and do not account for commonly found mixtures or byproducts of chemical
degradation. Third, standards and guidelines usually are based on
long-term exposure to constant concentrations of contaminants, rather
than lengthy periods of low concentrations punctuated by brief, seasonal
pulses of high concentrations that emerged in almost every basin sampled.And
finally, many possible impacts on aquatic organisms have not been
tested. Potential effects on reproductive, nervous, and immune systems,
as well as on chemically sensitive individuals, are not yet well understood.
For example, many of the 20 most frequently detected pesticides in
this study, although detected at relatively low concentrations, are
suspected endocrine disrupters that have potential to affect reproduction
or development of aquatic organisms or wildlife by interfering with
natural hormones. A top priority for future research should be to
enhance the understanding of risk to humans and aquatic life by collecting
information that reflects these complexities in contaminant occurrence
and the nature of exposure and potential effects.
Nutrients
generally do not pose a health risk for residents whose drinking water
comes from streams or from aquifers buried relatively deep beneath
the land. Health risks from elevated nitrate increase in those aquifers
located in geologic settings, such as in sand, gravel, or karst (weathered
carbonate rock), that enable rapid movement of water. The most prevalent
nitrate contamination found in this study was in shallow ground water
(less than 100 feet below land surface) beneath agricultural and urban
areas. Because of its proximity to the land surface, the shallow groundwater
is younger and more vulnerable to contamination from human activities
than deep groundwater. For example, about 15 percent of all shallow
groundwater sampled beneath agricultural and urban areas exceeded
the drinking-water standard for nitrate. These findings are of particular
importance in rural areas where shallow water commonly is used for
domestic supply. Homeowners may not be aware of possible contamination
because domestic wells are not monitored regularly, as is required
for large public-supply wells. In addition, many homeowners in newly
residential areas that rely on domestic wells may not know that chemicals
leached from previously farmed land can remain in shallow, slow-moving
groundwater for decades.
Seasonal and geographic patterns
Land and chemical use are
not the sole predictors of water quality. Concentrations of nutrients
and pesticides vary considerably from season to season, as well as among
watersheds with differing vulnerability to contamination. Natural features,
such as geology and soils, and land-management practices, such as tile
drainage and irrigation, can affect the movement of chemicals over land
or to aquifers and can thereby exert local and regional controls on water
quality. Understanding the national, regional, and local importance of
land and chemical use, natural features, and management practices on water
quality increases the effectiveness of policies designed to protect water
resources in diverse settings.
This
article is based largely on the first report in a U.S. Geological Survey
series, �The Quality of Our Nation's Waters,� which addresses regional
and national water-quality issues of concern. Copies of the report are
available from Sarah Laible at 1-703-648-5716. Requests for copies of
the report can be sent to Laible by mail at U.S. Geological Survey,
413 National Center, Reston, VA 20192 or by e-mail at nawqa_info@usgs.gov.
The report is also available online at: http://water.usgs.gov/pubs/circ/circ1225/.
A companion 4-page fact sheet is available online at http://water.usgs.gov/pubs/FS/FS-116-99.
Visit the NAWQA website, http://water.usgs.gov/nawqa,
where you can also directly access other publications, national data
sets, and national maps of chemical use and occurrence.
For
more information, contact:
Timothy
L. Miller, Chief, National Water-Quality Assessment (NAWQA) Program;
email: tlmiller@usgs.gov; or
Pixie Hamilton, Staff Hydrologist for NAWQA; email: pahamilt@usgs.gov.
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