Elevated concentrations of nitrogen and phosphorus, nutrients that can negatively impact aquatic ecosystems and human health, have remained the same or increased in many streams and aquifers across the nation since the early 1990s, according to a new national study by the U.S. Geological Survey (USGS).
USGS report provides the most comprehensive national-scale assessment to date
of nitrogen and phosphorus in our streams and ground water," says Marcia
McNutt, USGS director. "For years, we have known that these same nutrients
in high concentrations have resulted in 'dead zones' when they reach our
estuaries, such as during the spring at the mouth of the Mississippi, and now
we have improved science-based explanations of when, where and how elevated
concentrations reach our streams and aquifers and affect aquatic life and the quality
of our drinking water."
major federal, state and local efforts and expenditures to control sources and
movement of nutrients within our nation's watersheds, national-scale progress
was not evident in this assessment, which is based on thousands of measurements
and hundreds of studies across the country from the 1990s and early 2000s,"
explains Matthew Larsen, USGS associate director for water.
to the U.S. Environmental Protection Agency (EPA), nutrient pollution has
consistently ranked as one of the top three causes of degradation in U.S.
streams and rivers for decades.
findings show that widespread concentrations of nitrogen and phosphorus remain
two to 10 times greater than levels recommended by the EPA to protect aquatic
life. Most often, these elevated levels were found in agricultural and urban
streams. These findings show that continued reductions in nutrient sources and
implementation of land-management strategies for reducing nutrient delivery to
streams are needed to meet EPA recommended levels in most regions.
occur naturally in water, and are needed for plant growth and productive
aquatic ecosystems; however, in high concentrations nutrients often result in
the growth of large amounts of algae and other nuisance plants in streams,
lakes and estuaries. The decay of these plants and algae can cause areas of low
dissolved oxygen, known as hypoxic, or dead, zones that stress or kill aquatic
life. Some forms of algae release toxins that can result in health concerns.
also found that nitrate is a continuing human-health concern in many shallow
aquifers across the nation that are sources of drinking water. In agricultural
areas, more than one in five shallow, private wells contained nitrate at levels
above the EPA drinking water standard. The quality and safety of water from
private wells – which are a source of drinking water for about 40 million
people – are not regulated by the Federal Safe Drinking Water Act, and are the
responsibility of the homeowner.
can persist in ground water for years and even decades, nitrate concentrations
are likely to increase in aquifers used for public drinking-water supplies
during at least the next decade, as shallow ground water with high nutrient
concentrations moves downward into deeper aquifers.
designed to reduce nutrient inputs on the land will improve the quality of
water in near-surface parts of aquifers; however, decades may pass before
quality improves in deeper parts of the aquifer, which serve as major sources
for public-supply wells," says Neil Dubrovsky, USGS hydrologist and lead
scientist on this study. "Unfortunately, similar delays for improvements
are expected for streams that receive substantial inputs of ground water."
of sources can contribute nutrients to surface and ground water, such as
wastewater and industrial discharges, fertilizer and manure applications to
agricultural land, runoff from urban areas, and atmospheric sources. USGS
findings show that nutrient sources and resulting concentrations vary across
the nation. For example, concentrations of nitrogen generally are highest in
agricultural streams in the Northeast, Midwest
and the Northwest, which have some of the most intense applications of
fertilizer and manure in the nation.
in concentrations across the nation also are due to natural features and human
activities. For example, concentrations of nitrogen in streams draining parts
of the agricultural Midwest are increased by
contributions from artificial subsurface tile drains that are used to promote
rapid dewatering of poorly drained soils. Conversely, concentrations of nitrate
in streams draining parts of the Southeast appear to dissipate faster as a
result of enhanced natural removal processes in soils and streams.
nationwide assessment of sources and natural and human factors that control how
nutrients enter our streams and ground water helps decision-makers anticipate
where watersheds are most vulnerable to contamination and set priorities and management
actions in different geographic regions of the country," says Dubrovsky.
Elevated Nitrogen, Phosphorus Still Widespread in U.S. Ground Water
September 28, 2010