May 26, 2006
Nitrate ions
found in freshwater samples result from a variety of natural and manmade sources.
Nitrates are an important source of nitrogen necessary for plants and animals
to synthesize amino acids and proteins. Most nitrogen on earth is found in the
atmosphere in the form of nitrogen gas, N2. Through a process called
the nitrogen cycle, nitrogen gas is changed into forms that are useable
by plants and animals. These conversions include industrial production of fertilizers,
as well as natural processes, such as legume-plant nitrogen fixation, plant
and animal decomposition, and animal waste.
Sources of Nitrate Ions |
Urban run off |
Agricultural run off |
Municipal and industrial wastewater |
Automobile and industrial emissions |
Decomposition of both plants and animals |
Animal feedlots and barnyards |
Although nitrate
levels in freshwater are usually less than 1 mg/L, manmade sources of nitrate
may elevate levels to above 3 mg/L. These sources include animal feedlots, runoff
from fertilized fields, or treated municipal wastewater being returned to streams.
Levels above 10 mg/L in drinking water can cause a potentially fatal disease
in infants called methemoglobinemia, or Blue-Baby Syndrome.
In this disease, nitrate converts hemoglobin into a form that can no longer
transport oxygen.
High nitrate
concentrations also contribute to a condition in lakes and ponds called eutrophication,
the excessive growth of aquatic plants and algae. Unpleasant odor and taste
of water, as well as reduced clarity, often accompany this process. Eventually,
dead biomass accumulates in the bottom of the lake, where it decays and compounds
the problem by recycling nutrients. If other necessary nutrients are present,
algal blooms can occur in a lake with as little as 0.50 mg/L NO3–-N.
Nitrate pollution
of surface and groundwater has become a major ecological problem in some agricultural
areas. Although fertilizer in runoff is most often blamed, there is evidence
that concentration of livestock in feedlots is now the major source of agricultural
nitrate pollution. Runoff from fertilized fields is still a significant source
of nitrate, although fertilizer use peaked in 1981 and has remained fairly constant
since.
Expected Levels
The nitrate
level in freshwater is usually found in the range of 0.1 to 4 mg/L NO3–-N.
Unpolluted waters generally have nitrate levels below 1 mg/L. The effluent of
some sewage treatment plants may have levels in excess of 20 mg/L.
The percentage of various land types reporting greater than 1 mg/L of nitrate
were
range land <5%, forested land ~10%, urban areas ~30%, and agricultural land
~40%.