Janice Woodard, Kathleen Parrott, Blake Ross
Virginia Cooperative Extension
Nitrate
(NO3) is the primary source of nitrogen (N) for plants; it is
a nutrient they cannot live without. Nitrate contamination of
a water supply occurs when there is more nitrate in the soil
than plants can use and when water can move easily through the
soil and underlying rock. The excess nitrate is carried through
the soil into groundwater supplies by irrigation water, rainwater,
and snowmelt. This is most likely to occur when the soil is
sandy, or gravelly and/or shallow water tables exist. Excess
nitrate can accumulate in the soil from several sources, including
fertilizer, manure, and sewage.
Return
to Table of Contents
High
levels of nitrate in household water supplies can be of grave
concern, especially to families with infants. Human babies are
extremely susceptible to acute nitrate poisoning because of
certain bacteria that may live in their digestive system during
the first few months of life. These bacteria change nitrate
into toxic nitrite (NO2). The nitrite reacts with hemoglobin
(which carries oxygen to all parts of the body) to form methemoglobin,
which does not carry oxygen. The level of oxygen being carried
throughout the body decreases in proportion to the amount of
hemoglobin converted to methemoglobin. As the oxygen level decreases,
the baby is gradually suffocated. This condition is called methemoglobinemia
commonly referred to as "blue baby" disease.
Around
the age of three months, as the baby's digestive system develops,
stomach acid kills most of the bacteria that convert nitrate
to toxic nitrite. By the time a baby is about six months old,
its digestive system is fully developed, and none of the nitrate-converting
bacteria remain. In older children and adults, nitrate is
absorbed and excreted, and methemoglobinemia is no longer
a concern.
Return
to Table of Contents
Drinking
water quality standards have been set by the Federal government
to offer the greatest protection to infants. The standard of
ten milligrams per liter (mg/1) nitrate-nitrogen (NO3-N) or
45 milligrams nitrate (NO3) has a small margin of safety built
into it. Because of this safety factor, some individuals can
drink water exceeding the standard and show no adverse effects.
However, levels above this standard are considered to be potentially
hazardous.
Because
nitrate is tasteless and odorless, water must be chemically
tested to determine if it is contaminated. If you are on a
municipal or public water system, it is the responsibility
of the water authority to test and treat the water supply
to prevent nitrate contamination. If nitrate contamination
occurs in a public water supply, customers may be warned not
to feed the water to infants until the problem is corrected.
However, if you are on a private or individual water system,
it is your responsibility to monitor for nitrate.
Return
to Table of Contents
Water
testing for nitrate needs to be done at an independent, certified
laboratory. For more information about the location of these
laboratories, contact the local Cooperative Extension office
or county or city health department. Contact the laboratory
for specific instructions required for collecting a water sample.
Accurately
determining the nitrate level in a well can be difficult since
nitrate levels vary according to the time of year. For this
reason, spring is the best time to test, since snowmelt and
rains will leach any excess nitrate into the groundwater.
Also, commercial fertilizer usage is generally greatest at
this time of year.
When
your report comes back from the lab after the water test,
the nitrate concentrations can be reported either as nitrate
(NO3) or as nitrate-nitrogen (NO3-N). Be sure to know which
reporting system is being used since the acceptable concentrations
of each are considerably different. If the lab reports its
results as nitrate-nitrogen (NO3-N), the drinking water quality
standard is 10 milligrams per liter. The standard is 45 milligrams
per liter if the results are reported as nitrate (NO3). A
milligram per liter (mg/l) is also equal to a part per million
(ppm). If you are unsure of how to interpret the report, contact
the lab, the local Extension office, or health department.
It is important to check the lab report carefully because
the two systems of reporting are frequently interchanged.
If your
water supply has been tested and the report shows that the
water has a high nitrate level, you must immediately stop
feeding it to infants. As alternatives to contaminated water,
you can plan to breast feed, feed formula prepared with bottled
distilled water, or feed premixed infant formula.
Return
to Table of Contents
Prevention
of nitrate contamination of the water supply is critical. Wells
need to be isolated from possible sources of contamination and
to be protected from surface damage. Abandoned wells need to
be sealed to prevent contamination. Sinkholes are direct routes
to groundwater and should never be used as garbage dumps. Good
nitrogen management is needed when applying fertilizers, both
on the farm and in the home yard and garden.
The
surest way to deal with a contaminated water supply is to
find a new, clean water supply. If you do not have access
to a new water supply, the contaminated water can be treated
to remove the nitrate.
Return
to Table of Contents
Nitrate
is a very soluble substance, easily dissolved in water and extremely
hard to remove. Treatment for nitrate is, therefore, very complicated
and expensive. However, only water used for drinking and cooking
needs to be treated. The three methods of reducing or removing
nitrate are: 1) demineralization by distillation or reverse
osmosis; 2) ion exchange; and 3) blending.
Demineralization
Demineralization removes nitrate and all other minerals from
the water. Distillation is one of the most effective types
of demineralization. The distilling process has only three
steps: 1) the water is boiled; 2) the resulting steam is caught;
and 3) the steam is condensed on a cold surface, turning the
water back into liquid form. The nitrate and other minerals
remain concentrated in the boiling tank.
Reverse
osmosis is another way to demineralize water. In a reverse
osmosis system, the water is put under pressure and forced
through a membrane that filters out minerals and nitrate.
Both of these demineralization systems require a lot of energy
to operate efficiently. They are also low-yield systems, and
storage space for treated water is required.
Ion
Exchange The second type of water treatment for nitrate
contamination is ion exchange, the same process used for water
softening. Most often chloride is exchanged for nitrate. The
ion-exchange unit is a tank filled with special resin beads
that are charged with chloride. As water containing nitrate
flows through the tank, the resin takes up the nitrate and
replaces it with chloride. In time, all the chloride will
be exchanged for nitrate. The resin can then be recharged
by backwashing with a brine solution (sodium chloride) and
reused.
Ion-exchange
systems can treat large volumes of water. However, in addition
to exchanging nitrate, the resin beads will also take up sulfate
in exchange for chloride. Therefore, if sulfates are present
in the water supply, the capacity of the resin to take up
nitrate is reduced. Second, the exchange process may also
make the water corrosive. For this reason, the water must
go through a neutralizing system after going through the ion-exchange
unit. Finally, the backwash brines, which are high in nitrate,
must be disposed of properly so they do not recontaminate
the groundwater supply.
Blending
The third and most common way to reduce nitrates is to dilute
the nitrate-polluted water by blending it with water from
another source that has low nitrate concentrations. Blending
the two waters produces water that is low overall in nitrate
concentration. However, blended water is not considered safe
enough for infants.
There
is no simple way to remove all nitrate from your water. Although
it is common to think of boiling, softening, or filtration
as a means of purifying water, none of these methods reduce
nitrate contamination. Boiling water is, in fact, the worst
thing to do because it actually concentrates the nitrate.
Softening and filtration do nothing at all to remove nitrate.
Disclaimer
and Reproduction Information: Information in NASD does not represent
NIOSH policy. Information included in NASD appears by permission
of the author and/or copyright holder. More
NASD Review: 04/2002
For more
information on water treatment, contact your local Virginia
Cooperative Extension office
Janice Woodard, Retired Extension Specialist, Home Management
and Equipment
Kathleen Parrott, Extension Specialist, Housing
Blake Ross, Extension Specialist, Agricultural Engineering
Remember
that nitrate is an essential nutrient for plant growth. Only
when there is too much nitrate in the soil does it become
a problem in water. Routine water testing is important to
protect the health of families. If nitrate levels in a water
supply exceed the present water quality standards, low-nitrate
water must be provided for infants to drink. Nitrogen management,
the only long-term solution to nitrate contamination, requires
consideration of all aspects of nitrogen use. The problem
of nitrate contamination is not a simple one, but it must
be faced to protect families, animals, and the environment.
Portions
of this publication are adapted with permission from Nitrate
in Drinking Water by Karen M. Mancel, Ohio Cooperative
Extension Service.
Publication
Number
356-484
,
October 1996
|