Dr. Nitish Priyadarshi
Presence of high levels of Radon (222 Rn) has been reported from groundwater in Bangalore city, Keolari-Nainpur area, Seoni-Mandla district in Madhya Pradesh, Bathinda , Gurdaspur, Garhwal, Himachal Pradesh and Siwalik Himalayas and underground waters of the Doon valley in India.
To ascertain the ground reality and the nature of the hazards, if any, a study was conducted by the Central Ground Water Board, Bangalore in and around Bangalore city. The analytical results of all the groundwater samples collected from the gneissic and granitic rocks shows Radon concentration is above the permissible limit of 11.83 Bq/l and at places the concentration is as high as hundred times. The radon gas is occurring in the groundwater of the area ranging 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 bore wells. However it is observed that the formation waters from very shallow aquifers are having the least concentration of radon due to its easy loss to the atmosphere. Surface water samples are having negligible quantity of radon, which is well within the safe limits. It is observed that there is a good correlation between the presence of high radon content and the presence of granitic rocks.
Higher concentration of uranium and radon in groundwater of Keolari-Nainpur area have been observed during an exploration programmes for uranium. The average value of dissolved uranium in bore well waters 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 (b) 222Rn concentration on potable water exceeding 200 Bq/m3.
High radon concentration has been reported in river waters of Garhwal and Siwalik
Himalayas and underground waters of the Doon valley. Extremely high uranium content was reported in groundwater of Bathinda district in the Punjab state. The radon concentrations have been measured in all those areas where high uranium content was
reported in groundwater. The average radon concentration in hand-pump drawn water is 3.8 Bq /l and in tube-well drawn water, its value is 3.6 Bq /l. Radon values for Bathinda district 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 (25–92 Bq/l) were reported for groundwater from Quaternary alluvial gravels associated with uranium-rich sediments in the Doon Valley of the Outer Himalaya.
Radon activities in groundwater samples in different districts of Himachal Pradesh varies from 0.3 ± 0.2 to 792 ± 9 Bq /l. The maximum value of radon concentration is found in groundwater of thermal springs and the minimum value in a water tank. The highest value of radon concentration is recorded in thermal spring (no. III) at Kasol,
792 ± 9 Bq/ l which is in the Kullu 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 spring (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 222Rn activities in 16 groundwater samples of Varahi command area ranged between 0.2 ± 0.4 and 10.1 ± 1.7 Bq/ l with an average value of 2.07 ± 0.84 Bq /l. In contrast, the recorded 222Rn activities in 14 groundwater samples of Markandeya command area found to vary from 2.21 ± 1.66 to 27.3 ± 0.787 Bq/ l with an average value of 9.30 ± 1.45 Bq /l.
Despite its known health effects, no WHO guideline value exists for radon in drinking water because of the difficulties in defining a regionally-applicable value given the relative importance of inhalation compared to ingestion from drinking water. Radon concentrations in groundwater also change significantly on abstraction, aeration, storage and boiling.
Radon is essentially chemically inert, but radioactive (www. chemistrydaily.com). It is the heaviest noble gas at room temperature. At standard temperature and pressure radon is colorless. Natural radon concentrations in the Earth’s atmosphere are very low, the water in contact with the atmosphere will continually lose radon by volatilization, while groundwater has a higher concentration of 222 Rn than surface water. Likewise, the saturated zone of soil frequently has higher radon content than the unsaturated zone due to the diffusional losses to the atmosphere.
Radioactive substances in ground water, 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 ground water 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 into ground water from contact with sands and soils.
There are twenty known isotopes of radon. The most stable isotope is radon-222 which is decay product ( daughter product) of radium-226, has a half life of 3.823 days and emits radioactive alpha particles. Radon-220 is a natural decay product of thorium and is called thoron. It has a half life of 55.6 seconds and also emits alpha rays. Radon-219 as derived from actinium, is called action and is an alpha emitter having a half life of 3.96 seconds.
Radon being the daughter product of the uranium is expected in higher levels in rocks containing uranium. The studies indicate the granits, pegmatites and other acidic rocks are generally rich in uranium compared to other rocks types. When groundwater percolates through rocks rich in uranium, it is expected to contain high level of radon gas in groundwater.
Radon is a carcinogenic gas and is radioactive. It 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 respective daughter products, tend to form fine dust, which can easily enter the air passage and become permanently stuck to lung tissue, causing heavy localized exposure. Build-up of radon in homes has also been a more recent health concerns and many lung cancer cases are attributed to radon exposure each year. Radon escalates health hazard to smokers.
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Sinha, D.K., Shrivastava, P.K., Hansoti, S.K. and Sharma, P.K., 1997. Uranium and radon concentration in groundwater of Deccan Trap country and environmental hazard in Keolari-Nainpur area, Seoni-Mandla district, Madhya Pradesh. Geol. Surv. Ind. Spl. Pub. v.2, no.48, pp. 115-121.