Poisonous encounters: Nitrate contamination of groundwater in Rajasthan

Groundwater in arid and semi arid regions continues to be a valuable and often the single most source of freshwater and conservation of this resource remains crucial for the survival of communities residing in the area. At the same time, a serious decline in the quality of groundwater is adding pressure on these water resources.

No where is this more obvious than in arid and semiarid regions of states such as Rajasthan in India where rapid urbanisation, industralisation, human induced anthropogenic activities, use of fertilisers, improper sewage disposal, human and animal waste are increasingly contributing to groundwater pollution, posing health risks to populations.

This paper titled 'Anthropogenic nitrate in groundwater and its health risks in the view of background concentration in a semi arid area of Rajasthan, India' published in Nature Scientific Reports presents the findings of a study that explores the extent of nitrate contamination of groundwater, a growing but unexplored problem in Rajasthan, where the only source of freshwater is groundwater.

The study aimed at assessing the natural and anthropogenic nitrate (NO3) concentrations in groundwater in semi arid areas of Rajasthan and the risks it posed to health among children and adult men and women.

Nitrate contamination of groundwater

Nitrate (NO3), is an end product of the biological nitrification process and is very commonly found in aquifers throughout the world. Recent evidence shows that nitrate concentrations in aquifers are increasing.

The distribution of nitrate in groundwater is influenced by a number of factors such as source availability, thickness and composition of the vadose zone, precipitation, irrigation, ground water flow, aquifer heterogeneity, dissolved oxygen concentrations and electron donor availability and dispersion. Nitrogen is commonly found in the natural water as ammonia (NH3), nitrite (NO2-), nitrate (NO3-), and the ammonium ion (NH4+).

Most of the groundwater samples with high nitrate concentrations are found to be associated with decaying plant or animal material, agricultural fertilisers, domestic sewage or geologic materials containing soluble nitrogen compounds.

Excessive nitrate buildup happens in shallow aquifers due to the downward leaching of nitrogen from surface water, while deep wells (>150 m) show little or no evidence of nitrate contamination.

In India, higher NO3 concentration in groundwater has been reported from different hydrogeological terrains and has been extensively studied in the Gangetic plains.

Health risks of drinking nitrate contaminated groundwater

Consumption of drinking water contaminated with nitrogen can lead to methemoglobinemia in infants under 6 month of age. This disease is caused by the bacterial reduction of nitrate to nitrite in the intestinal tract. The nitrite then enters the blood-stream and combines with the haemoglobin to form methemoglobin, which reduces the blood's capacity to transport oxygen. Methemoglobinemia  is also known as a 'blue baby syndrome'.

Severe methemoglobinemia can result in brain damage and death. Prolonged intake of high levels of nitrate is also linked to gastric problems and stomach cancer in extreme cases.

The World Health Organisation (WHO) has defined the maximum permissible nitrate level in drinking water as 50 mg/L. In the Indian context, the Bureau of Indian Standards (BIS) recommends 45 mg/L as the permissible level of NO3 in drinking water.

The study found that:

• The natural background concentration (concentration that shows minimal influence by human anthropogenic sources) of nitrate was found to be 7.2 mg/L. As high as 93 percent of nitrate samples in the groundwater exceeded this background limit, while 28 percent of the groundwater samples were beyond the permissible limit of 45 mg/L as per the BIS standards.

• The nitrate contamination in majority of the cases was due to anthropogenic activities such as use of agricultural fertilisers and contamination due to human and animal wastes.

• Oral exposure to nitrate was very high as compared to dermal contact among populations, which increased the contamination risk even more.

• The nitrate health risk assessment found that about 38 percent, 46 percent, and 49 percent of the samples posed non-carcinogenic health risks to males, females, and children. Children were found to be more prone to health risks due to high nitrate content in groundwater as compared to adult men and women.

• The spatial distribution of nitrate background values revealed that the health hazard was high at the places where there was a significant difference between NBL (natural background concentration) and total nitrate concentrations.

The study recommends:

• Inclusion of natural background concentration (NBL) in health risk assessment for gaining a better understanding of geogenic and anthropogenic factors contributing to contamination of groundwater by chemicals.
• Monitoring the changes in NO3 content in groundwater to be able to control the future spread of the pollutants. This can be done by using sub-surface physical and hydraulic barriers designed to prevent or to control the flow of polluted water into the aquifers.