High levels of uranium found in groundwater from Rajasthan

India relies heavily on groundwater for crop irrigation and for drinking in many regions. Studies have shown high levels of fluoride, nitrate, and arsenic in groundwater in some parts of the country.

Now, a new study sampling groundwater from wells in Gujarat and Rajasthan as well as analysing previous reports from other parts of India has revealed that groundwater in many parts of Rajasthan has high concentrations of uranium.

“The results were surprising in recognising the magnitude of uranium occurrence and the high levels we measured,” said Avner Vengosh, professor at Duke University’s Nicholas School of the Environment and senior author of the study. “Uranium is major issue in some areas and should be monitored and avoided from drinking water,” he added.

“Uranium in this form is not of radioactive risk, but is chemically toxic with kidney damage being the main problem,” explained Sunderrajan Krishnan, executive director at India Natural Resource Economics and Management (INREM) Foundation, a research institute based in Anand, Gujarat, who was not involved in the study.

The causes were mainly due to natural factors but human activities such as overpumping leading to declining groundwater tables and nitrate pollution may aggravate the situation.

Vengosh’s team examined the chemistry of water samples from 324 wells, which included 226 from Rajasthan and 98 from Gujarat. They measured the uranium isotope ratios in a subset of the samples. In addition, they combined water quality data from 68 past studies that covered these states as well as 14 others in their analysis to get an overview of the extent of contamination.

Rajasthan emerged with the highest level of contamination with uranium concentrations in a third of the water wells tested in the state exceeding the World Health Organization’s (WHO) provisional guideline value of 30 micrograms per litre in drinking water; only 5 percent of the water wells from Gujarat had uranium concentrations higher than the WHO guideline.

The previous studies showed that aquifers from 26 districts in northwestern India and nine in southern and southeastern India had levels of uranium above 30 micrograms per litre.

Uranium absent from India’s drinking water standards

Currently the Bureau of Indian Standards’ Drinking Water Specifications does not include uranium in its list of contaminants monitored.

Brindha Karthikeyan, a hydrogeological researcher at the Free University of Berlin, who was not involved in the study, said that the central and state governments do “not include uranium monitoring in groundwater in the routine sampling and chemical analysis which is carried out 2 to 4 times in a year.”

According to Krishnan, Mumbai’s Bhabha Atomic Research Centre has been monitoring uranium over the past five years and has found the presence country-wide, “but the results have not been shared as yet.”

Abhijeet Mukherjee, associate professor of hydrogeology at the Indian Institute of Technology in Kharagpur, who was not part of the study, said that apart from Gujarat and Rajasthan, “which has been long known” for uranium contamination, “some parts of Bihar and Orissa would have that pollution.” “Beyond that, in my knowledge, the problem is not that severe.”

Natural and human-aggravated causes

Groundwater aquifers are composed of underground porous rocks. Some rocks have naturally higher levels of uranium than others such as granite.

Rachel M. Coyte, the lead author of the study, a PhD student in Vengosh’s lab, said there are several factors involved leading to high uranium concentrations in groundwater. These “include the amount of uranium contained in an aquifer’s rocks; the interaction between water and the rocks that cause uranium to be extracted; oxidation conditions that enhance the extracted uranium’s solubility in water; and the interaction of the extracted uranium with other chemicals in the groundwater, such as bicarbonate, which can further enhance its solubility.”

Most of Rajasthan’s groundwater with high concentrations of uranium came from either alluvial aquifers, which are composed of clay, sand and gravel, or from aquifers containing uranium-rich rocks. Both of these types of aquifers are widespread throughout India.

The team observed that bicarbonate levels were associated with uranium in most aquifers. Notably, almost half of the wells from the alluvial and high-uranium rock aquifers that had high bicarbonate concentrations also had uranium concentrations above the WHO’s provisional guideline.

Although the alluvial aquifers in both Rajasthan and Gujarat were composed of similar rocks, those in Gujarat were deeper and had clay confining layers that delay the seepage of water, which might explain why Gujarat had lower concentrations of uranium.

Intensive pumping of groundwater, which is widely used in India for irrigating crops, could worsen the contamination.

“The decline in groundwater level causes oxidation conditions that increase uranium solubility in the groundwater. The most of uranium mobilization would occur near the interface of the saturated-unsaturated zone. Deeper groundwater would have more reducing conditions and less uranium solubility,” explained Vengosh.

The researchers suspected that nitrate pollution due to fertiliser application on crops might contribute to the high levels of uranium as they found that 42 percent of the wells tested also had nitrate levels exceeding those stated in the drinking water guidelines by the WHO. However, they did not detect any significant association between nitrate and uranium.

Remedial measures and the way forward

Vengosh suggested including “uranium to the drinking water standard in India, modify[ing] the monitoring program to include trace elements measurements including uranium and initiat[ing] projects that link human health to uranium occurrence. The team recommends that uranium monitoring should be targeted in areas where the groundwater has high bicarbonate levels. Treating water with reverse osmosis will remove uranium from the water.”

As groundwater depletion due to over-pumping is more severe in the states of Rajasthan, Gujarat, and Punjab, “groundwater quality deterioration cannot be dealt as a single issue, but a combined management strategy is required,” Karthikeyan said.

She stressed that “drinking water standards should be established based on the toxicological effects of uranium to humans.”

Also, she stated that the “public water supply system by the government should be made the only source” to the entire population. “Closing of existing private bore wells and prohibiting construction of new bore wells are some of the steps that can help in this direction,” she added.

“We will need to think how we proceed in India with uranium monitoring because our district level water testing labs are inadequate for much simpler testing too,” said Krishnan. He pointed out that “the response needs to be strategic and selective to begin with based on studies such as the current one.”

CITATION:
Coyte, R.M., Jain, R.C., Srivastava, S.K., Sharma, K.C., Khalil, A., Ma., L., Vengosh, A. (2018). Large-scale uranium contamination of groundwater resources in India. Environmental Science & Technology Letters, 5 (6), 341-347. DOI: 10.1021/acs.estlett.8b00215