Nitrate (NO3-) is a water-soluble molecule made up of nitrogen and oxygen. It is formed
when nitrogen from ammonia or other sources combines with oxygenated water.
Nitrogenous materials are rare in the geological record; therefore, occurrence of nitrate in groundwater is an anthropogenic pollutant contributed by nitrogenous fertilizers, industrial effluents, human and animal wastes through biochemical activity of nitrifying bacteria, such as nitrosomanas and nitrobacter. The groundwater resources contaminated with high level of nitrate (greater than 45 mg/l) are an environmental hazard. Nitrate has been linked to agricultural activities due to excessive use of nitrate fertilizers.
The nitrate is taken up by plants during their growth and used in the synthesis of organic nitrogenous compounds. Surplus nitrate readily moves with groundwater. Under aerobic conditions, it percolates in large quantities into the aquifer because of the small extent to which degradation or denitrification occurs. Under anaerobic conditions, nitrate may be denitrified or degraded almost completely to nitrogen.
In surface water, nitrification and denitrification may also occur, depending on the temperature and pH. The uptake of nitrate by plants, however, is responsible for most of the nitrate reduction in surface water.
Concentrations of nitrate in rainwater of up to 5 mg/litre have been observed in industrial areas. In rural areas, concentrations are somewhat lower.
Nitrogen compounds are formed in the air by lightning or discharged into it from industrial processes, motor vehicles, and intensive agriculture. These are removed by wet and dry deposition.
Air pollution appears to be a minor source. In general, vegetables will be the main source of nitrate intake when levels in drinking water are below 10 mg/litre. When levels in drinking water exceed 50 mg/litre, drinking water will be the major source of total nitrate intake, especially for bottle-fed infants. In the Netherlands, the average population exposure is approximately 140 mg of nitrate per day(including the nitrate in drinking water).
Vegetables and cured meat are in general the main sources of nitrate and nitrite in the diet, but small amounts may be present in fish and dairy products. Most vegetables and fruits contain 200-2500 mg of nitrate per kg. The nitrate content of vegetables can be affected by the processing of the food, the use of fertilizers, and growing conditions. Vegetables such as beetroot, lettuce, radish, and spinach often contain concentrations above 2500 mg/kg, especially when they are cultivated in greenhouse.
The nitrate concentration in surface water is normally low (0-18 mg/litre), but can reach high levels as a result of agricultural run-off, refuse dump runoff, or contamination with human or animal wastes. The concentration often fluctuates with season and may increase when the river is fed by nitrate-rich aquifers. Nitrate concentrations have gradually increased in many European countries in the last few decades and have sometimes doubled over the past 20 years. In the United Kingdom, for example, an average annual increase of 0.7 mg/litre has been observed in some rivers.
The natural nitrate concentration in groundwater under aerobic conditions is few milligrams per litre and depends strongly on soil type and on the geological situation. In the USA, naturally occurring levels do not exceed 4-9 mg/litre for nitrate.
Several researches in India have reported nitrate incidence of groundwater related to different sources, such as leaching of organic and inorganic fertilizers from agricultural land by infiltration and percolation of rainwater, irrigation water, animal waste, leakage from sewers and by subsurface flow from up gradient areas.
Somasundram et al. (1993) reported nitrate concentrations ranging from 12 to 999 mg/litre in Madras (now Chennai) urban area, and found that source of nitrate concentrations are due to sewage effluents. In Varanasi area of Uttar Pradesh in India, high nitrate concentrations are due to the direct leaching of domestic sewage effluents.
Nitrate is high in groundwater in some pockets of Jharkhand State. Concentration varies from less than one ppm to 100 ppm. In some parts of Godda the concentration level has reached up to 168ppm. In few parts of Sisai area, near the capital of Jharkhand State the concentration is above 100ppm.
Ranchi city is coming under the threat of nitrate poisoning as city is becoming the dumping ground of the municipal wastes and household wastes. At present the concentration of nitrate has reached threshold level in groundwater of Ranchi city but it may increase in summer season because low level of groundwater increases the concentration of metals.
The accumulation of high nitrate in cracks, fissures in the hard rocks of the Jharkhand State, is obvious due to the relatively thin soil cover and occurrence of groundwater at shallower depth, pollutants find their way to the groundwater body. Other than anthropogenic sources metamorphic rocks present in Jharkhand Plateau can be the other source of nitrate to ground water because nitrogen is found in the metamorphic rocks.
In Bihar State other than Rafiganj and Bhagalpur area concentration of nitrate in groundwater is below toxic level. In Rafiganj the concentration of nitrate varies from 100 ppm (parts per million) to 300ppm. Whereas in Bhagalpur it varies from 90 to 160 ppm. In some parts of Motihari district the concentration of nitrate has reached up to the level of 155 ppm. In Valmiki Nagar few places have been identified where the concentration of nitrate has crossed the toxic level.
The primary health effect from high nitrate levels is methemoglobinemia which affects infants up to 6 months of age and can ultimately result in death. The condition is also called "blue baby syndrome" because the skin appears blue-gray or lavender in color. This color change is caused by a lack of oxygen in the blood. . Bacteria that live in the digestive tracts of newborn babies convert nitrate to nitrite (NO2). Nitrite then reacts with hemoglobin, which carries oxygen in blood, to form methemoglobin. Infants suffering from "blue baby syndrome" need immediate medical care because the condition can lead to coma and death if it is not treated promptly.
Within several months after birth, the increasing level of hydrochloric acid in a baby's stomach kills most of the bacteria which convert nitrate to nitrite. By the age of six months, the digestive system is fully developed, and the risk of nitrate-induced methemoglobinemia is greatly reduced.
Increased mortality rates are recorded from gastric and prostate cancer with increasing exposures to nitrates in the drinking water as found in Spain. A cluster of spontaneous abortions was identified in Indiana among women consuming nitrate contaminated well waters. Some scientific studies have found evidence suggesting that women who drink nitrate-contaminated water during pregnancy are more likely to have babies with birth defects. Nitrate ingested by the mother may also lower the amount of oxygen available to the fetus.
The toxicity of nitrate to humans is thought to be solely the consequence of its reduction to nitrite. The major biological effect of nitrite in humans is its involvement in the oxidation of normal haemoglobin to methaemoglobin, which is unable to transport oxygen to the tissues. The reduced oxygen transport becomes clinically manifest when methaemoglobin concentrations reach 10% of that of haemoglobin and above. The haemoglobin of young infants is more susceptible to methaemoglobin formation than that of older children and adults. Other groups especially susceptible to methaemoglobin formation include pregnant women and people deficient in glucose-6-phosphate dehydrogenase or methaemoglobin reductase.
People who have heart or lung disease, certain inherited enzyme defects, or cancer may be more sensitive to the toxic effects of nitrate than others. In addition, some experts believe that long-term ingestion of water high in nitrate may increase the risk of certain types of cancer.
Grant, W., Steele, G., Isiorho, S.A. 1996. Spontaneous abortions possibly related to ingestion of nitrate-contaminated well water-LaGrange County, Indiana,1991-1994. Morb. Mortal Wkly Rep., v.45, pp 569-572.
Guidelines for drinking-water quality 1999. Health criteria and other supporting information. World Health Organization, Geneva, 2nd edition, v.2.
Raju, N.J., Prahlad, R.Dey,S. 2009. Groundwater quality in the Lower Varuna River Basin, Varanasi District, Uttar Pradesh. Jr. of Geological society of India, v.73 (2) pp. 178-192.
Somasundaram, M.V., Ravindran, G., Tellam, J.H. 1993. Groundwater pollution of the Madras urban aquifer, India. Groundwater,v.31 (1) pp 4-12.