Nitrate Pollution in Ground Water
Dr. S.C. Kaushik

Although nitrate ion, the end product of oxidation process of nitrogen, is a plant nutrient, but beyond certain limits in the ground water, it is a potential threat to human health. It is perceived mainly as a chemical fertilizers used by farmers, but much of nitrate found in soil is produced by the microorganisms that decompose plant residues and other nitrogen containing residues present in the soil. Nitrate is highly soluble and very mobile. Consequently, the potential sources of nitrate contamination in ground water are leaching of soluble chemical fertilizers, untreated effluents of nitrogenous industries and sewage disposal, decomposing of organic matters in soils and human and animal excreta.

Nitrate pollution in ground water is rising at an alarming rate in India. In alluvial aquifers of U.P., there is an alarming level of nitrate concentration. Similarly, in the highly percolating light textured soils of Punjab, there is alarming level of deterioration by nitrate. According to the current public health standards for safe drinking water, the maximum contamination level should not exceed nitrate concentration of 45 mg/l as nitrate (NO3-). World Health Organisation (WHO) has prescribed a maximum limit of 3.65 mg nitrate per kg body weight for total intake in a day. The nitrate content of piped public water ranges from less than 1 mg/l to around or above the statutory limit of 50 mg/l. Water from private wells can have a much broader range of nitrate concentration, values up to some hundreds of mg/l, and even 1000 mg/l.

Nitrate itself is harmless but when it is reduced to nitrite, it causes a rare disease in infants known as Methemoglobinemia (Blue Baby Syndrome) and stomach cancer in adults. The Blue Baby Syndrome can occur when an infant ingest a high level of nitrate. Microorganisms present in the stomach convert the nitrate to nitrite. Subsequently, nitrite reaches in the blood where t reacts with hemoglobin to form methaemoglobin in which the iron is in the ferric state or iron (III). Normal oxyhaemoglobin contains iron in the ferrous state or iron (II). The resultant diminished capacity of blood to carry oxygen owing to Methaemoglobin causes chemical suffocation. Foetal haemoglobin has a high affinity for nitrate than normal haemoglobin. Foetal haemoglobin persists in the blood stream for a while in the very young children. Also their stomachs are not acidic to inhibit the microorganisms that convert nitrate to nitrite. That's why infants of less than about one year are most susceptible.

Nitrite can react in the stomach with a secondary amine to produce an N-nitroso compound that is carcinogenic resulting in the stomach cancer. It has been reported that a high nitrate intake can reduce the body assimilation of iodine, thus causing goiter. Some of the health problems associated with nitrate toxicity are anoxia, dipsnia, oral cancer, cancer of colon, rectum or other gastrointestinal concerns, absorption and secretive functional disorders of the intestinal mucous etc.

Treatment of groundwater for the removal of nitrate can be performed by physical methods (application of reverse osmosis and nanofiltration), chemical methods (catalytic removal, abiotic degradation of nitrate using zero-valent ion and electro-kinetic processes) and biotechnological methods use of vegetable oil for denitrification, reduction of agricultural nitrate loading through microbial processes using sulphur and limestone autotrophic denitrification).
Although technologies are available for decontamination nitrate containing drinking water, but these are very costly. So, it is better and necessary to minimize nitrate leaching from agricultural and non-agricultural activities. By adopting the following improved methods of nitrogen fertilizers use, nitrate leaching can be minimized:

• Adopting integrated management system i.e. substitute part of inorganic fertilizer with
organic fertilizers
• Use of slow release fertilizers
• Use nitrification inhibitors
• Use safe chemicals
• Use of improved crop management system
• Choose the right cropping system
• Intercept nitrate by means of trees or deep-rooted, nitrate minimizing crops such as
alfalfa, or by digging ditches

Finally, as serious health effects are associated with excess nitrate and therefore, it is necessary to generate an adequate database to investigate nitrate pollution problems. Some ground water quality management programmes should be implemented to avoid the hazards of nitrate concentration.

About Author / Additional Info:
Dr. Suresh Kaushik
A Biotechnogical Professional from India