Radon is a carcinogenic gas and therefore is hazardous to inhale this element since it emits alpha particles. Radon in water may therefore present dual pathways of exposure for individuals -- through drinking water and inhalation of air containing radon released from groundwater, says Dr Nitish Priyadarshi.
There have been a number of reports of the presence of dissolved radon in groundwater from India. The reports say that people are being affected after consuming water contaminated with radon, and the most affected are the Malwa region of Punjab and Bengaluru city.
Presence of high levels of radon (222 Rn) has been reported from groundwater in Bengaluru city, apart from the Keolari-Nainpur area and the Seoni-Mandla district in Madhya Pradesh, and parts of Bhatinda, Gurdaspur, Garhwal, Himachal Pradesh, the Shivalik Himalayas and the underground waters of the Doon Valley in India.
To find out how hazardous radon was, the Central Ground Water Board in Bengaluru conducted a study in and around Bangalore city. The analytical results of all the groundwater samples collected from the gneissic and granitic rocks shows the concentration of radon is above the permissible limit of 11.83 Becquerel per litre and at places is hundred times higher. The radon gas occurs in the groundwater of the area and ranges from 55.96 Bq/l to 1189.30 Bq/l plus or minus error values.
There is no relation between the radon concentration and the depth of borewells. However, it is observed that waters from very shallow aquifers have the least concentration of radon because of the loss to the atmosphere. Surface water samples have negligible radon, well within safe limits. It is also observed that there is a good correlation between the presence of high radon content and the presence of granite rocks.
Higher concentration of uranium and radon in the groundwater of Keolari-Nainpur area has been observed during exploration programmes for uranium. The average value of dissolved uranium in a bore well is 13 ppb (parts per billion). Considering 200 ppb as a safe limit, it has been possible to delineate several pockets where groundwater is contaminated by very high uranium (217-4,500 ppb in 13 villages) and radon (34,151 Bq/m3 to 1,146,075 Bq/m3 in 6 villages).
These pockets, therefore, have been classified as high background radiation area on the basis of (a) long-lived alpha radioactivity through ingestion of more than 2 Bq/day and (b) 222Rn concentration on potable water exceeding 200 Bq/m3.
High radon concentration has been reported in river waters of Garhwal and Shivalik Himalayas and the groundwaters of the Doon valley. Extremely high uranium content was reported in the groundwater of Bhatinda district in Punjab. The radon concentrations have been measured in all those areas where high uranium content was reported in groundwater.
The average radon concentration in water drawn by a hand-pump is 3.8 Bq/l, while in water drawn by a tubewell, the value is 3.6 Bq /l. Radon values for Bhatinda are lower than the corresponding values for Gurdaspur district. The occurrence of radon in groundwater is reasonably related to the uranium content of the bedrocks and it can easily enter into the interacting groundwater by the effect of lithostatic pressure. Relatively high concentrations of radon were reported for groundwater from Quaternary (in geology, pertaining to the present period of earth history) alluvial gravels associated with uranium-rich sediments in the Doon Valley of the outer Himalayas.
Radon activities in groundwater samples in different districts of Himachal Pradesh vary and the maximum radon concentration is found in the groundwater of thermal springs and the minimum value in a water tank. The highest value of radon concentration is recorded in thermal spring at Kasol (792 9 Bq/l), which is in the Kulu district of Himachal Pradesh. The uranium content of water in the Kasol thermal springs was found to be 37.40 0.41 ppb. The radon anomalies are related to Shat-Chinnjra and Kasol mineralisation. The radon concentration at Chinnjra also shows a high value of 144 4 Bq/l in natural springs (bauli) as compared to other natural springs at Takrer and Bradha in the same area.
The study was also carried out in Varahi and Markandeya river basins, Karnataka State, India. The measured levels in 16 groundwater samples of the Varahi command area ranged between 0.20.4 and 10.11.7Bq/l with an average value of 2.070.84Bq/l.
In contrast, the recorded activities in 14 groundwater samples in the Markandeya command area were found to vary from 2.211.66 to 27.30.787 Bq/l with an average value of 9.301.45 Bq/l.
Radon concentrations in groundwater changes significantly on abstraction, aeration, storage and boiling. Radon is essentially chemically inert, but radioactive. It is the heaviest noble gas at room temperature.
At standard temperature and pressure radon is colourless. Natural radon concentrations in the Earth's atmosphere are very low; the water in contact with the atmosphere will continually lose radon by volatilisation, while groundwater has a higher concentration of 222 Rn than surface water.
Radioactive substances in groundwater such as radium, uranium and thorium, occur naturally. They are present at least to some extent in almost all rocks and radium, in particular, dissolves more readily into groundwater in contact with sands or soils. The acidity of the water, which may be increased by the presence of elevated levels of nitrates associated with agricultural land use, is believed to increase the amount of radium that dissolves in groundwater from contact with sands and soils.
The most stable isotope is radon-222 which is decay product of radium-226, has a half-life of 3.823 days and emits radioactive alpha particles.
Radon being the daughter product of uranium is expected in higher levels in rocks containing uranium. The studies indicate that granite, pegmatite and other acidic rocks are generally rich in uranium compared to other rocks. When groundwater percolates through rocks rich in uranium, it is expected to contain a high level of radon gas in groundwater.
Radon is a carcinogenic gas and therefore is hazardous to inhale this element since it emits alpha particles. Radon in water may therefore present dual pathways of exposure for individuals -- through drinking water and inhalation of air containing radon released from groundwater.
Its solid decay products and their daughter products tend to form fine dust, which can easily enter the air passage and become permanently stuck to lung tissue, causing heavy localised exposure. Build-up of radon in homes has also been a recent health concerns and many lung cancer cases are attributed to radon exposure each year. Radon escalates health hazard to smokers.
The writer is a geologist and lecturer in the department of environment and water management, Ranchi University, Jharkhand