National Water-Quality Assessment Program
Nutrients in the Nation's Waters--Too Much of a Good Thing?
U.S. Geological Survey Circular 1136
HOW LARGE ARE NATURAL CONCENTRATIONS OF
NUTRIENTS IN WATER?
(5K GIF)
"Background" concentrations of ammonia, nitrate, and total phosphorus
in streams and ground water
These are the concentrations that can be expected in the absence of
significant human influence. Concentrations exceeding these values
generally were found in samples from streams and wells in agricultural
or urban areas, or in areas potentially affected by atmospheric
deposition or phosphate mining.
Whenever concentrations of any chemical in water are discussed, one
of the first questions asked is, "How much is there naturally?"
One way to answer this question is to measure concentrations in
relatively pristine waters. Such waters are difficult to find. In
our analysis, we used land-use classifications to identify areas of
the Nation that were mostly undeveloped, and so minimally impacted by
agriculture, cities, and associated human activities. Samples from
wells in forest areas and from streams draining predominantly forests
and rangelands were selected to evaluate the natural occurrence, or
"background levels," of nutrients in water.
Typical background sites downstream from forested areas:
tributary to the Chattahoochee River near Columbus, Georgia
(photograph by Dan Hippe),
(93K GIF)
and
Rockwell Falls on the Hudson
River near Hadley, New York (photograph by Elizabeth Flanary).
(45K GIF)
Ammonia concentrations usually were less than 0.1 mg/L in
ground-water and stream samples from background sites. Ammonia
is not a stable nutrient in most environments. It is easily
transformed to nitrate in waters that contain oxygen and can be
transformed to nitrogen gas and released to the atmosphere in waters
that are low in oxygen. So it is not surprising that natural
concentrations of ammonia are low.
Nitrate concentrations in samples from background sites generally were
less than 2 mg/L for ground water and less than 0.6 mg/L for streams.
Concentrations in streams were higher in the Northeast than in other
parts of the Nation. Atmospheric deposition has been cited by past
studies as providing more nitrogen in rainfall to land in the
Northeast than in other parts of the Nation, and our analysis supports
that conclusion.
(7K GIF)
Median nitrate concentrations in streams draining undeveloped areas
(forest and rangeland) in NAWQA study units where sufficient data were
available. "Acid rain" might be contributing to the higher nitrate
concentrations in parts of the Northeast.
Concentrations of total phosphorus usually were less than 0.1 mg/L in
stream samples from background sites. Information was sparse about
phosphorus in ground water, but where available, background
concentrations also were less than 0.1 mg/L.
(80K GIF)
Samples from wells in undeveloped areas, such as this one in Grand
Teton National Park, Wyoming, provide data on background
concentrations of nutrients in ground water (photograph by Michael
Rupert).
(93K GIF)
(45K GIF)
(7K GIF)
(80K GIF)