Geoscience Reference
In-Depth Information
Jury
1984
). In addition, not all the applied nitrogen enters the soil; some is washed
off fields in the form of runoff and flows into surface waters, such as streams and
rivers.
Although various nitrogen forms are present in subsurface environments,
environmental risks usually are linked to nitrate concentrations. Nitrates can have
adverse effects on humans, animals, and plants. High nitrate concentrations (i.e.,
more than a few tenths of mg/L) may promote eutrophication in surface waters.
Over 30 years ago, nitrate concentration criteria for irrigation water were sug-
gested by Ayers and Westcott (
1976
), differentiating ''no problems'' (\5 mg/L),
''increasing problems'' (5-30 mg/L), and ''severe problems'' ([30 mg/L) for the
production of sensitive crops.
The major sources of nitrate for human intake are food (e.g., vegetables and
meat) and water, although polluted air, cigarette smoking, and certain medications
also contribute to nitrate ingestion. Specifically, the contribution from nitrogen-
containing fertilizers to high levels of nitrate in food and drinking water has been
identified as an environmental health concern.
In general, exposure to nitrate is not of particular interest with respect to human
health. However, about 5 % of the total dietary nitrate can be reduced endoge-
nously (within the human body) to nitrite through bacterial and other reactions;
and nitrite can be further reduced to N-nitroso compounds (NOCs). Methemo-
globinemia (''blue baby syndrome'') in infants up to six months old, various
cancers, and birth defects have been listed as possibly being associated to exposure
to elevated nitrate levels in drinking water. A maximum (allowable) contaminant
level (MCL) of 10 ppm for nitrates in drinking water was established by the US
Environmental Protection Agency (EPA) on the basis of concerns about methe-
moglobinemia. This syndrome is expressed as an inability of the blood to effec-
tively transport oxygen and carbon dioxide and may result in acute distress to the
system; in severe cases, it can cause a bluish tinge in the skin color, hence the term
blue baby. The World Health Organization (WHO
1985
) refers to several cases of
nitrite poisoning due to use of drinking water with high concentrations of nitrates,
in the range of 44-88 ppm. Other health outcomes usually are sporadic and
debatable. However, in some cases, exposure to nitrate concentrations in drinking
water has been associated with an increased incidence of hyperthyroidism (goiter)
(Seffner
1995
; van Maanen et al.
1994
). Also, an increased risk of central nervous
system malformations in infants whose mothers consumed drinking water with
high nitrate levels has been reported (Dorsch et al.
1984
; Arbuckle et al.
1988
). A
different study suggested a link between consumption of water with high nitrate
levels and risk of delivering malformed children, relative to an area where nitrate
concentration in the water supply was \5 ppm (Scragg et al.
1982
). A significant
increase in the mean number of chromatid/chromosome breaks in children exposed
to nitrate concentrations that exceeded 70.5 mg/L was detected, indicating that
chronic elevated concentrations of nitrate in drinking water are capable of
inducing cytogenetic effects (Tsezou et al.
1996
). Another study suggested that
consumption of drinking water, especially well water, with high nitrate levels can
imply a genotoxic risk
for humans as indicated
by increased hypoxanthine
