How to use this page: This is an indicator page. Examine this page for detail on the indicator and use that information to establish a metric for the indication of water quality. This is the final step in examining a sustainable community for its environmental attributes, water sub-attributes, nitrogenous compounds sub-sub-attributes. After completing this page, please go back and review other indicators and see discern metrics and weights for the AHD process.
Why are Nitrates and Nitrites indicators of sustainability?
Nitrate and nitrite are compounds that contain a
nitrogen atom joined to oxygen atoms, with nitrate containing three oxygen
atoms and nitrite containing two. In nature, nitrates are readily converted
to nitrites and vice versa. Both are anions, or ions with a negative charge.
They tend to associate with cations, or ions with a positive charge, to
achieve a neutral charge balance.Nitrates are naturally present in soil, water, and food. In the natural nitrogen cycle, bacteria convert nitrogen to nitrate, which is taken up by plants and incorporated into
tissues. Animals that eat plants use the nitrate to produce proteins. Nitrate is returned to the environment
in animal feces, as well as through microbial degradation of plants and animals after they die.
Microorganisms can convert nitrate or the ammonium ion (which is a nitrogen atom
combined with four hydrogen atoms) to nitrite; this reaction occurs in the environment
as well as within the digestive tract of humans and other animals. After bacteria
convert (reduce) nitrate to nitrite in the environment, the nitrogen cycle is completed
when they then convert the nitrite to nitrogen. Normally, this natural cycling process
does not allow excessive amounts of nitrates or nitrites to accumulate in the
environment. However, human activities have increased environmental nitrate concentrations, with
agriculture being the major source. This includes increased use of nitrogen-containing fertilizers as well
as concentrated livestock and poultry farming; the latter two produce millions of tons of nitrate-containing
manure each year. Nitrate and nitrite compounds are very soluble in water and quite mobile in the
environment. They have a high potential for entering surface water when it rains, as nitrates in applied
fertilizers can dissolve in runoff that flows into streams or lakes; they also have a high potential for
entering groundwater through leaching. The concentration associated with soil particles has been
estimated to be about half the concentration in interstitial water (the water in the pore spaces between the
What are some health effects of Nitrates and Nitrite?
Nitrates themselves are relatively nontoxic. However, when
swallowed, they are converted to nitrites that can react with hemoglobin in the blood, oxidizing its
divalent iron to the trivalent form and creating methemoglobin. This methemoglobin cannot bind oxygen,
which decreases the capacity of the blood to transport oxygen so less oxygen is transported from the lungs
to the body tissues, thus causing a condition known as methemoglobinemia.
Normal individuals have low levels (0.5 to 2%) of methemoglobin in
their blood. When this level increases to 10%, the skin and lips can take
on a bluish tinge (cyanosis), and levels above 25% can cause weakness
and a rapid pulse. At levels above 50 to 60%, a person can lose
consciousness, go into a coma, and die. Infants are much more sensitive than adults to nitrates/nitrites, and essentially all deaths from nitrate/nitrite poisoning have been in infants. Long-term exposure to
lower levels of nitrates and nitrites can cause diuresis (an increase in the
amount of urine, and starchy deposits and hemorrhaging of the spleen).
What level of Nitrates and Nitrites should be a community striving for community?
The U.S. Environmental Protection Agency
(EPA) has developed toxicity values to estimate the risk of non-cancer
health effects from ingesting nitrates and nitrites. The toxicity value
used to estimate a non-cancer effect following ingestion is called a
reference dose (RfD). An RfD is an estimate of the highest dose that
can be taken in every day without causing an adverse effect.
The RfD for nitrate was developed considering the concentration at which methemoglobinemia was
indicated at levels above 10% for 0- to 3-month-old infants. This was based on a daily intake of formula
made with water containing 10 mg per liter (mg/L) of nitrate as nitrogen. (This reflects the amount of
nitrogen within a nitrate molecule, where 1 mg nitrate as nitrogen = 4.4 mg nitrate as measured in the
water.) Most cases of infant methemoglobinemia are associated with exposure to formula prepared with
drinking water at nitrate-nitrogen levels at least two times higher, or exceeding 20 mg/L nitrate-nitrogen.
For nitrite, the RfD is based on a 10-kg (22-pound [lb]) child drinking 1 liter, or about 1 quart, of water
every day. The RfD represents a “safe daily dose” and so is
compared to the amount an individual is estimated to take in
every day, as a ratio. No adverse effects have been linked
with inhaling nitrates or nitrites, so reference concentrations
(RfCs), which are used to assess inhalation toxicity, have not
been developed. The contribution of nitrites, and indirectly
nitrates, to potential human carcinogenicity and the
magnitude of the associated risk are unclear. Nitrites react with secondary amines in food to form nitrosamines, many of which are carcinogenic in experimental animals and exert other toxic effects. While the EPA has not developed slope factors (toxicity values used to estimate cancer risk) for nitrates
or nitrites, some are available for nitrosamines.
What Are Current Limits for Environmental Releases and Human Exposure?
The EPA requires that sodium nitrite releases of more than 100 lb (45.4 kg) and nitrate releases of more than
10,000 lb (4,540 kg) be reported immediately and that normal releases be reported annually for inclusion
in the nationwide Toxics Release Inventory. The limits for nitrosamines range from 1 to 10 pounds. The EPA primary drinking water standards for nitrate and nitrite are 10 and 1 parts per million (ppm),
respectively. The Food and Drug Administration allows these compounds to be used as food additives as
long as they are of food grade and are added only in the amount needed. The maximum amount of nitrite
allowed in smoked and cured fish and meat is 200 ppm
Where to go now that you
have reviewed an indicator:
Now that you have reviewed an indicator you should
create a metric (see
Step 4 of the
Analytical Hierarchy Processes) that indicates the importance of
this indicator in your decision process. Keep this step in mind as
you go through one of the other indicators below. Once you have
chosen a metric for each of your indicators, you should decide how
they collectively measure the sustainability of water by weighting
each indicator (see
Step 5 of the
Analytical Hierarchy Process). These will be applied in an
Step 6 of the Analytical Hierarchy Process) to give you the
final measurement for Water and Sustainability.