Nitrogen (N) is added to soil naturally from rainfall, plant debris, animal residues and microbial fixation of N2 from atmosphere whereas anthropogenic nitrogen comes through nitrogenous fertilisers, organic manures and human, animal and industrial wastes. Apart from nitrate (NO3-) form, nitrogen is also applied as ammonium (NH4+) and amide (NH2-) fertilisers in agriculture, which generate NO3- in soil, primarily via micro-organism mediated processes. Nitrogen in organic debris, manure and waste also undergoes mineralisation to form NO3- in soil in due course of time.
High solubility of NO3- in water and its low soil retention by predominantly negatively charged soil particles makes it prone to downward movement leaching to subsoil layers and ultimately to the groundwater. Nitrate in soil is used by plants and microbes for their metabolism; NO3- also undergoes conversion to NO2-, NH4+, N2O and N2 via various biochemical pathways but the unutilised NO3- is what enters groundwater eventually (Fig 1). So, the excessive usage of nitrogenous fertilisers in agriculture is one of the primary reasons of high NO3- in groundwater as it often exceeds its utilisation potential in soil.
The rate of NO3- leaching is mainly governed by soil properties such as texture and ion exchange potential, presence of vegetation and amount of water present in the soil system. The arrival of NO3- to groundwater is enhanced by shallow groundwater table, excessive application of N-fertlisers, manures, irrigation or abundant rainfall, livestock urination in pastures and presence of unlined septic tanks. In and around areas of high urbanisation and industrialisation, municipal and industrial wastes may contribute high levels of nitrate to the groundwater.
- EEC countries are entitled to the above rules in special situation
- EEC Directive
- Chief Medical Officer’s recommendation
*Source: Lunkad (1994)
When NO3- rich groundwater is pumped out and used for drinking, it may cause a number of health disorders in humans. Different organisations and countries have set standards for NO3- in potable water (Table 1). The anthropogenic element in the nitrate pollution of groundwater can well be perceived from the fact that terrestrial waters in uninhabited and less polluted regions like high altitude lakes, glaciers have negligible nitrate content. Studies reveal that in the Central Himalayan snow and ice, NO3- content is about 0.5 mg/l (S K Lunkad 1994, Rising nitrate levels in groundwater and increasing N-fertiliser consumption, Bhujal News 9) while average river water may contain about 1.0 mg/l NO3- and the ultimate sink of terrestrial waters, the oceans, on an average, have 0.67 mg/l NO3-.
Blue Baby Disease: NO3- itself is not toxic. NO3- becomes a problem only when it is converted to nitrite (NO2-) in human body and causes Methaemoglobinemia. Nitrite produced from nitrate in drinking water enters the bloodstream and oxidises haemoglobin to methaemoglobin, a Fe (III) compound with reduced oxygen transporting capacity. As different parts of the body get deprived of oxygen, clinical symptoms of oxygen starvation start to appear, the main being cyanosis. Since infants are more susceptible to health complications due to biochemical and behavioural reasons, the disorder takes the name of ‘Blue Baby Disease’.
Gastric Cancer: Results have shown that there may not be a straightforward cause and effect relationship between nitrate exposure and cancer risk. It is hypothesised that nitrate reacts with a variety of organic compounds in human body to form nitrosamines and nitrosamides through nitrosation reactions in the stomach. Various research claims that nitrosamines and nitrosamides are carcinogenic as they can be metabolised to potent electrophilic alkylating agents.
Other Health Effects: Reports also relate high nitrate intake and increasing incidences of goitre. It has been suggested by scientists that high nitrate reduces assimilation of iodine by human body causing goitre. It is also suggested that high nitrate intake with drinking water may lead to the birth of a malformed child – although animal experiments have failed to prove any association. There are reports of other health disorders namely non-Hodgkin’s lymphoma, increased infant mortality and hypertension.
University of California, Center for Water Resources has proposed a groundwater nitrate Pollution Hazard Index with the purpose of optimising resources for management practices that might help achieve highest reduction in nitrogen contamination potential of groundwater by identifying fields of highest intrinsic vulnerability (http:ucanr.org). They used overlay and index method for the development of the hazard index. The overlay consisted of soil maps, crop and irrigation system distributions. Each of soil, crop and irrigation systems was indexed. A hazard index would indicate the potential risk of groundwater nitrate contamination from a crop, soil or irrigation system. Higher the hazard index, higher would be the risk of groundwater NO3- contamination. For example, lettuce has a hazard index of 4 because it is shallow rooted with low nitrogen intake, is harvested at the time of peak nitrogen uptake rate and much of the N in the tops remains in the field, elevating the risk of NO3- leaching and contamination in groundwater below this crop field. Conversely, alfalfa has a hazard index of 1 because it is deep rooted, taking up higher soil nitrogen and nitrogen fertiliser application is not required.
Groundwater Nitrate: Indian Scenario
In India, many studies on NO3- in groundwater have been undertaken in last two to three decades and in many areas groundwater has been found to contain more than 45 mg l-1 NO3- (D Majumdar et. al., 2000, Nitrate pollution of groundwater and associated human health disorders, Indian Journal of Environmental Health, NEERI, Nagpur; D C Jhariya et al., 2012, Nitrate pollution in the groundwater around Sagar Town – Madhya Pradesh, International Conference on Chemical, Ecology and Environmental Sciences (ICEES), Bangkok; N C Mondal et al., 2008, Occurrence of elevated nitrate in groundwaters of Krishna delta, African Journal of Environmental Science and Technology). Groundwater NO3- concentration, however may vary with time, depending on season, rainfall, changes in farming practices etc. Most researchers have attributed nitrate pollution in shallow and moderately deep aquifers to intense agriculture practices, improper sewerage and organic waste disposal. Researchers in last decade itself had suggested immediate and urgent actions to tackle groundwater NO3- issue in India.
Water containing unsafe levels of NO3- can be recorded by regular monitoring. Shallow wells might often get contaminated by floodwater runoff from NO3- contaminated agricultural fields. Poorly constructed wells too can get contaminated by nearby animal feedlots and septic tanks. It is suggested that wells more than 30 m in depth (100 ft) are likely to be fairly safe. Boiling drinking water is discouraged in areas of high groundwater NO3-, as it further increases concentrations by removing pure water by evaporation. For NO3- removal, processes such as distillation, reverse osmosis, electrodialysis and ion exchange are recommended. More than 90 per cent of the nitrate-nitrogen can be removed by distillation. Manufacturers claim that reverse osmosis can remove 85-95 per cent of the NO3-. For NO3- removal by ion exchange, special anion exchange resins are used that exchange chloride ions for nitrate and sulfate (SO4=) ions in water as it passes through the resin. Most anion exchange resins have a higher selectivity for removing sulfate than nitrate. Thus, the level of sulfate in the water is an important factor in the efficiency of an ion exchange system. To avoid resin saturation by NO3-, ion exchange resins should be regenerated frequently. Ion exchange is not commonly used for household water treatment and is more applicable for large commercial or community water system installations.
Remedial measures for minimising groundwater NO3- pollution range from managing farming practices to waste management. Measures that might prove beneficial are:
- Application of optimum amount of N-fertilisers and water depending on crop need, crop stage and soil type;
- Avoidance of flood irrigation methods;
- Avoidance of nitrate fertiliser application;
- Avoidance of wanton application of inorganic manure on agricultural fields;
- Encouragement of spilt application of
- Application of coated urea and nitrification inhibitors;
- Avoidance fallow land practice;
- Reduction in unnecessary tillage;
- Inclusion of perennial and deep rooted crops wherever possible; and,
- Stoppage in the construction and usage of unlined septic tanks.
In a study by J Sumelius et al. 2002, CEESA Discussion Paper, Central and Eastern European Sustainable Agriculture, it was found that about 76 per cent reduction in NO3- leaching can be achieved by either a tax on optimal N doses or a product tax or a fertiliser quota.
A good number of reports on groundwater NO3- in various regions in Indian states are available. Though fluctuating in concentration over the years, in an agriculture intensive state like Punjab, groundwater NO3- was found to have an increasing trend. Similar conditions may prevail in several other regions of India. Serious measures should be undertaken by farmers, agricultural extension workers, NGOs, policy makers and general public to minimise the worsening of groundwater NO3- pollution in India.