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.