Michigan
State University Extension
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06/24/03
Nitrates in Drinking Water
Although nitrate occurs naturally in drinking water, elevated levels in
groundwater usually result from human activities such as overuse of
chemical fertilizers and improper disposal of human and animal wastes.
These fertilizers and wastes are sources of nitrogen, containing
compounds which are converted to nitrates in the soil. Nitrates are
extremely soluble in water and can move easily through soil into the
drinking water supply.
High levels can build up over time as nitrate accumulates in the water,
but even at elevated levels, they are not likely to be a health hazard
for most adults. However, the ingestion of excessive amounts of nitrate
can cause adverse health effects in very young infants and susceptible
adults. Consequently, the federal government has established a maximum
acceptable level, known as the Maximum Contaminant Level (MCL), for
nitrate in public drinking water supplies. This level is 10 milligrams
per liter (mg/l)--often expressed as 10 parts per million
(ppm)---measured on the basis of the nitrogen content of nitrate.
Nitrate in the Environment
The most common sources of nitrate are municipal and industrial
wastewaters, refuse dumps, animal feed lots, and septic systems. Once
nitrate is formed, its movement in soil and potential for contamination
of ground, water depend on several factors including the soil
characteristics, location and characteristics of the underground water
formations (aquifers), and climatic conditions. Potential for nitrate
contamination of drinking water also depends on the depth and
construction of wells.
Identifying the source of nitrates for an individual well is often very
difficult. Because nitrates move with the flow of groundwater, the
source may be located a considerable distance from the well. In many
cases, the time needed for nitrate to pass through the soil into
groundwater is difficult to predict due to many variables including
application rate, the soil type, and the depth to the water table.
Human Exposure
Human exposure to nitrates occurs primarily through the diet because
nitrate is a natural substance found in both water and plants. In the
United States, the average dietary intake of nitrate is about 75 to 100
mg per day.About 80 to 90 percent of this amount comes from vegetables.
Some common vegetables with high nitrate content are beets, celery,
lettuce, and spinach. People following a vegetarian diet may have
nitrate intakes of up to 250 mg per day.
Drinking water generally accounts for 5 to 10 percent of nitrates
consumed. However, where drinking water is contaminated to a level of
50 mg/l (5 times the MCL), it may supply as much as half of the total
daily intake Ingested nitrate is the major source of nitrite in the
body, high levels of nitrate in drinking water are generally
responsible for high levels of nitrite in the body. Nitrate intake
depends on a variety of factors, including diet, and amount and quality
of water consumed.
Health Effects
The most significant health effect associated with nitrate ingestion is
methemoglobinemia in infants under six months of age. This condition
results from the presence of high nitrite levels in the blood.
Untreated, severe methemoglobinemia can result in brain damage and even
death. Infants in the first six months of life are particularly
susceptible to nitrite induced methemoglobinemia. Finally, infants have
a higher intake of water for their weight than adults, so consequently,
they ingest a relatively higher amount of nitrate. In addition to small
infants, some adults may be susceptible to the development of nitrite,
induced methemoglobinemia.These include pregnant women with a
particular enzyme deficiency, adults with reduced stomach acidity, and
those with a deficiency in the enzyme needed to change methemoglobin
back to normal hemoglobin, a condition which can be hereditary.
Fortunately, methemoglobinemia is easily recognized by the medical and
public health communities and can be readily diagnosed and treated.
Another concern about nitrate ingestion is the possibility that
nitrites in the stomach and intestines may contribute to the
development of some cancers. Nitrate in groundwater is of concern not
only because of its toxic potential, but also because it may indicate
contamination of the groundwater. If the source of contamination is
animal waste or effluent from septic tanks, bacteria, viruses, and
protozoa may also be present. Contamination of groundwater by
fertilizers may also indicate the presence of other agricultural
chemicals such as pesticides. The source of the nitrate may be a clue
as to which other contaminants may be present.
Remedies
Drinking water containing more nitrates than the maximum Contaminant
Level of 10 mg/l should not be consumed by infants or other susceptible
individuals. Water that is bottled or taken from another safe source
should be used. Simple in-line filters do not remove nitrates; but
deionization, reverse osmosis, or distillation can be effective in the
removal of nitrate. However, these treatments are expensive and require
careful maintenance. In some cases drilling a deeper well extending
into a noncontaminated water source may be the best, and in the long
run, the least expensive remedy.
References
This information comes from Michigan State University Extension
bulletin WQ19, Nitrate-A Drinking Water Concern.
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June 30, 1914, in cooperation with the U.S. Department of Agriculture.
Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This
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