Unintended Consequences of Nitrates
Eutrophication of lakes occurs due to the denitrification of nitrate,
where nitrates are reduced, and produce N2 gas as the end
product. The nitrogen produced in this process is available for
biological nitrogen fixation. A few genera of blue-green algae (such
as Anabaena, Nostoc and Gloeotrichia) are able to fix
nitrogen and do so at the surface of the lake. The oxygen produced by
algae is prevented from diffusing into the blue-green algae by
structures called heterocyst. The heterocyst consume the oxygen and
prevent nitrogen from entering the cells. This fixation process can
speed up eutrophication. Eutrophication results in low dissolved
oxygen content, high BOD (biochemical oxygen demand) and reduced
sunlight penetration reaching the aquatic macrophytes. Entropic lakes
lack diversity and generally contain only a few thriving species many
of which are bacteria and aquatic worms. The ultimate end result is
the destruction of fish habitat. The most distinct characteristic is
usually a thick blanket of blue-green algae covering the surface of
the pond or lake.
The major concern affecting human health pertains to infants less than
six months of age. In sufficient quantities, at nitrate
concentrations exceeding 10 mg/L, the possibility of a health hazard
is significant towards infants. This health hazard is due to a
bacterium that exists in their gastrointestinal tract that converts
nitrate to nitrite (NO2). The nitrite produced then reacts
with haemoglobin to form methemoglobin, which does not carry oxygen.
As more and more hemoglobin is converted, the infant receives less
oxygen to the brain resulting in slate blue skin, vomiting, diarrhoea,
mental retardation and/or suffocation leading to death. This is a
syndrome known as methemoglobinemia or “blue baby” syndrome.
After six months of age, nitrate is absorbed and secreted without
conversion to toxic nitrite. There is evidence that other health
problems are associated with nitrate including stomach cancer, birth
defects, hypertension, enlarged thyroid and lymphoma, but studies are
conflicting and inconclusive.
Nitrite had been found to react with amines and amides to form
nitrosamines and nitrosamides, which have been found to induce cancer
in rodents. There is no other group of carcinogens known that have
the ability to induce such a wide variety of tumours in organs,
ranging from lung, oral, brain, skin, leukaemia, bladder, to name a
Nitrate Contamination Prevention
There are several ways to prevent nitrate from entering drinking water
reserves. For example, wells need to be isolated from possible
sources of contamination and situated on groundwater recharge zones
compared to groundwater discharge zones to eliminate collection of
run-off. Abandoned wells need to be sealed to prevent entry of
nitrates into the groundwater. In addition, sinkholes are direct
routes to an aquifer and should never be used as garbage dumps.
For some communities it may be possible to find a new source of water
from a new well, a deeper well or obtain water from a nearby waterway
that has a lower concentration of nitrate. Another possibility is to
install a more effective treatment system. However, this is not
easily implemented in many parts of the world, so the best
preventative measure is to improve fertilization management.
There are several ways to control the source of contamination
including reducing the residual nitrate level at the completion of the
growing season when the potential is high for nitrate loss. In
addition, the quantity and timing of water and nitrogen applications
should be linked closely with actual crop requirements. There is also
nitrogen in the soil, irrigation water and manure that need to be
taken into account and irrigation should be minimized.
Only now are we beginning to see the resulting contamination from
agricultural chemical usage of thirty to forty years ago, the impact
of the previous fifteen years is yet to be seen. Therefore,
preventative measures will not present immediate ramifications and
nitrate removal is the only option.
With the increasing concern of high concentrations of nitrate in
drinking water, many methods have been developed in attempts to
efficiently remove nitrate. Nitrate is a stable and highly soluble
ion and has a low potential for adsorption or co precipitation. These
properties are what make this ion difficult to remove using
conventional water treatment processes including lime softening and
filtration. The methods include reverse osmosis, ion exchange,
biological denitrification, ion exchange with denitrification,
catalytic reduction, chemical denitrification and electro dialysis.
Ecotech Systems International Pakistan
uses an exclusive Ion Exchange Media regenerated with common salt
for selective removal of nitrites. This media is effective, robust
and has over ten years of life. Plants installed in Rawalpindi
Islamabad where nitrites are unusually high are running efficiently
for past ten years.