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National Water-Quality Assessment Program
The Quality of Our Nation's Waters
Nutrients in the Nation’s Streams and Groundwater, 1992–2004
By Neil M. Dubrovsky, Karen R. Burow, Gregory M. Clark,
Jo Ann M. Gronberg, Pixie A. Hamilton, Kerie J. Hitt, David K. Mueller,
Mark D. Munn, Bernard T. Nolan, Larry J. Puckett, Michael G. Rupert,
Terry M. Short, Norman E. Spahr, Lori A. Sprague, and William G. Wilber
National Findings and Their Implications
Although the use of artificial fertilizer
has supported increasing food
production to meet the needs of a
growing population, increases in
nutrient loadings from agricultural
and, to a lesser extent, urban sources
have resulted in nutrient concentrations
in many streams and parts of aquifers
that exceed standards for protection
of human health and (or) aquatic life,
often by large margins.
Do NAWQA findings substantiate national concerns for aquatic and
human health?
National Water-Quality Assessment (NAWQA) findings indicate that
nutrient concentrations in streams and groundwater in basins with significant
agricultural or urban development are substantially greater than naturally
occurring or “background” levels. For example, median concentrations
of total nitrogen and phosphorus in agricultural streams are about 6 times
greater than background levels. Findings also indicate that concentrations in
streams routinely were 2 to 10 times greater than regional nutrient criteria
recommended by the U.S. Environmental Protection Agency (USEPA)
to protect aquatic life. Such large differences in magnitude suggest that
significant reductions in sources of nutrients, as well as greater use of
land management strategies to reduce the transport of nutrients to streams,
are needed to meet recommended criteria for streams draining areas with
significant agricultural and urban development.
Nitrate concentrations above the Federal drinking-water standard—or
Maximum Contaminant Level (MCL)—of 10 milligrams per liter (mg/L,
as nitrogen) are relatively uncommon in samples from streams used for
drinking water or from relatively deep aquifers; the MCL is exceeded,
however, in more than 20 percent of shallow (less than 100 feet below the
water table) domestic wells in agricultural areas. This finding raises concerns
for human health in rural agricultural areas where shallow groundwater is
used for domestic supply and may warn of future contamination of deeper
groundwater pumped from public‑supply wells.
Are levels of nutrients in water increasing or decreasing?
A decadal assessment of trends in concentrations of nitrogen and
phosphorus from about 1993 to 2003 shows minimal changes in those
concentrations in the majority of studied streams across the Nation, and more
upward than downward trends in concentrations at sites with changes. These
findings underscore the need for reductions in nutrient inputs or management
strategies that would reduce transport of nutrients to streams. Upward trends
were evident among all land uses, including those only minimally affected
by agricultural and (or) urban development, which suggests that additional
protection of some of our Nation’s most pristine streams
warrants consideration.
The median of nitrate concentrations in groundwater from 495 wells
also increased significantly from 3.2 to 3.4 mg/L (6 percent) during about the
same period, and the proportion of wells with concentrations of nitrate greater
than the MCL increased from 16 to 21 percent. Nitrate concentrations in
water in deep aquifers are likely to increase during the next decade as shallow
groundwater with elevated concentrations moves downward. The potential
for future contamination of the deep aquifers requires attention because these
aquifers commonly are used for public water supply, and because restoration
of groundwater is costly and difficult.
Long-term and consistent monitoring of nutrients, improved accounting
of nutrient sources, and improved tracking and modeling of climatic and
landscape changes will be essential for distinguishing trends in nutrient
concentrations, understanding the causes of those trends, and accurately
tracking the effectiveness of strategies implemented to manage nutrients.
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First posted September 23, 2010
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Suggested citation:
Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., Hamilton P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., Nolan, B.T.,
Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, The quality of our Nation’s
waters—Nutrients in the Nation’s streams and groundwater, 1992–2004: U.S. Geological Survey Circular 1350, 174 p.
Additional information about this study is available at http://water.usgs.gov/nawqa/nutrients/pubs/circ1350
Contents
Chapter 1. National Findings and Their Implications
Chapter 2. Nutrient Primer
Chapter 3. NAWQA’s Approach to Nutrient Assessment
Chapter 4. Occurrence and Distribution of Nutrients in Streams and Groundwater
Chapter 5. Exchange of Nutrients between Surface Water and Groundwater
Chapter 6. Potential for Effects on Human Health
Chapter 7. Potential for Effects on Aquatic Life
Chapter 8. Changes in Nutrient Concentrations: Past and Predicted
References Cited
Glossary
Abbreviations, Acronyms, and Units of Measure
Additional Information
Supplemental Technical Information (available on-line only)