Nitrates ( NO3- ) are part of the nitrogen cycle. The main nitrogen reservoir is the atmosphere.
In natural water, nitrogen comes essentially from rain ( 1 - 3 mg/l ) and from soil drainage.
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Micro-organisms decompose organic matter present in the soil, and the mineral nitrogen thus produced is then transformed into nitrates by aerobic bacteria.
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This transformation, called nitrification, has two stages : nitritation, which produces nitrites ( NO2- ) and nitratation, which transforms the nitrites into nitrates ( NO3- ).
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If the conditions of the environment permit it, the nitrates are used as a source of energy by anaerobic bacteria, and transformed into gaseous nitrogen.
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Nitrates can also be absorbed and become fixed on clays and humus.
The content of nitrates in soil, an later in water, is strongly linked to the quantity of organic matter and to the environmental conditions.
Contributions from human activities ( use of nitrate fertilisers and manure ) are therefore significant.
Also, waste from sewage-treatment plants, latrines and septic tanks contribute organic matters capable of producing nitrates.
Nitrates and nitrites are considered dangerous to health. Nitrates ingested with water are reduced to nitrites in the intestines and become fixed to haemoglobin, thus decreasing oxygen transfer: this known as methaemoglobinaemia, which affects children in particular ( especially under 6 moths ).
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The scale of nitrate concentration in water is very wide.
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Groundwater, with an environment that facilitates denitrification, can be nitrate free, whereas water "polluted" by a significant contribution of organic matter, fertilisers or residual water may contain up to several hundreds of mg NO3 / litre.
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AEP standard upper limits are 50 mg/l for NO3- and 0,1 mg/l for NO2-
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WHO guidelines ( health related ) are 50 mg/l ( nitrate ) and 3 mg/l ( nitrite ).
