Measurement of radon concentration in dwellings from the affected landslide area of Mamit town, Mizoram, India

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1 parent nucleus present in the bedrock and soil materials. Radon being a gas can migrate by mechanism of diffusion and convection through pore spaces in the soil, fractures in the rocks and along with weak zones 2 such as shear, faults, thrust, etc. Radon is continually formed in soil and is released to air. Subsequent decay products are formed in the air. The amount of radon exhaled from the soil to the surrounding atmosphere is influenced by radon emanating power in the materials, perwww.sciencevision.org Original Research Sci Vis 12 (3), July-September 2012 ISSN (print) ISSN (online) Measurement of radon concentration in dwellings from the affected landslide area of Mamit town, Mizoram, India P.C. Rohmingliana 1, L. Vanchhawng 1, R. K. Thapa 1, M. Lalthansangi 2, Lalrintluangi 2, Laltlanchhungi 2, Lalremruati Hmar 2, Lalnunthara 2, B. K. Sahoo 3, Y. S. Mayya 3 and B. Zoliana 4* 1 Department of Physics, Mizoram University, Aizawl , India 2 Department of Botany, Govt. Zirtiri Residential Science College, Aizawl , India 3 RPAD, Bhabha Atomic Research Centre (BARC), Mumbai , India 4 Department of Physics, Govt. Zirtiri Residential Science College, Aizawl , India Received 13 August 2012 Accepted 25 September 2012 ABSTRACT Radon concentration in dwellings in Mamit town area has been observed for about two years from May 2009 to February Seasonal variations of these radon concentration levels were observed for a year and after which landslide took place in a part of Mamit town and indoor radon concentration levels were also measured in landslide affected area. Pre- and post-landslide radon data are compiled and compared. For this, indoor radon concentration measurement, a time integrated method was employed by using a solid state nuclear track detector (SSNTD) type (LR-115films) kept in a twin cup dosimeter. It was found that the radon concentration level during pre-landslide was more than that of the post-landslide level. This behaviour is similar to the changes of concentrations in soil-gas radon which is associated with the earthquake activities as observed in Garhwal Himalayas. The radon content in the soil gas measured using RAD7 even after the landslide phenomena was found to be not so high. Key words: Indoor radon concentration; landslide; RAD7; soil gas; SSNTD. INTRODUCTION Radon is a radioactive gas which is produced from Ra 224 and more than 50% of the natural radiation is contributed by radon and thoron (collectively called radon), and their progenies. 1 The amount of radon concentration in the soil is determined by the amount of Corresponding author: Zoliana Phone: bzoliana@rediffmail.com 92

2 Measurement of radon concentration in dwellings from the affected landslide area of Mamit town, Mizoram, India meability of the soil and underlying rocks, moisture content of the soil. 3 The movement of soil due to earthquake or landslide is suppose to enhance radon exhalation from the soil to surrounding atmosphere by providing more gaps for exhalation to surrounding atmosphere. The mechanism for entry of radon from the soil gas into a house is convectional flow through cracks at soil-foundation interface, often driven by a pressure differential between the structure and the surrounding soil. 4 The indoor radon concentration level is also influenced by its exhalation from building materials, ventilation pattern, architectural style of building, water, burning gas, etc. 5 Measurement of indoor radon concentration was carried out in 22 different locations within Mamit town by recording seasonal variation of the gas from June 2009 to April Mamit is located in the western part of Mizoram which is a part of northeastern India located at N and E. During rainy season, in the month of August 2010, landslide took place in some area within this Mamit town. Around 63 building and cottages fall under landslide effected area and about 42 houses were dismantled or abandoned. As a result of this landslide, cracks were formed on the floor and walls of the dwellings near the landslide area. In order to study radon exhalation from the soil to the surrounding atmosphere after the landslide took place, measurements were taken for indoor radon concentration in 10 dwellings located in and around landslide area. These data were compared with the radon concentration obtained prior to this landslide incidence. MATERIALS AND METHODS A passive time integrated solid state nuclear track detector (SSNTD) made of cellulose nitrate film (LR-115, Type II) was used for detection of this radioactive radon gas. This detector is kept in a twin cup dosimeter, 6 a device developed at BARC, Mumbai. This dosimeter is a cylindrical plastic chamber divided into two equal compartments, each having an inner volume of 135 cm 3 and height 4.5 cm. The detector film cut into appropriate size is affixed at the bottom of each cup. One of the cups called filter cup allows the entry of both radon and thoron inside by covering the cup with a glass fiber filter and hence tracks formed on the film in the filter compartment are due to both radon and thoron gases. On the other side of the compartment only radon gas was allowed to enter which has a modification from previous measurement 7 by using a cap with a pin hole in it. This pin hole is designed in thickness and size so as to block thoron from entry inside by considering its diffusion length and half life. So tracks formed on the film in the pinhole compartment are due to radon gas only while tracks formed in the other compartment is due to radon and thoron. As a result the concentration of thoron can be deducted by subtracting the radon concentration from that of radon + thoron values after substitution of required parameters. In this report, we shall consider only the concentration level of radon even though the contribution of thoron in population dosimetry is important. The concentration level of these gases including thoron and hence the necessary radon anomalies are reported elsewhere. 8 The dosimeter is hung over on the ceiling of the selected houses at a height of minimum 1.5 m from floor and at least 10 cm away from any surface in order to prevent the direct thoron influence for a period not less than 90 days. When the period of exposure is over the detector is retrieved and chemically etched using 2.5 N NaOH solution at constant temperature of 60 C for 90 minutes. The track formed due to these gases in an etched SSNTD films are then counted by using a spark counter. 9 Concentrations of the gases are then obtained from the track density by multiplying with an appropriate calibration factor. 93

3 Rohmingliana et al. Radon concentration in soil gas has been also measured by using RAD7 after the landslide took place. This device using soil probe measures radon concentration in the soil gas, for 5 minutes in 4 cycles in sniff mode, which will be able to determine the radon content in the soil at a depth of 1.2 m. RESULTS AND DISCUSSIONS Figure 1. Average annual concentration of Radon in Mamit town during , before the landslide occurred in which case, the dosimeters were deployed in 22 houses for different seasons of a year and the average concentration for these seasons was recorded. After landslide took place a total of 10 houses were selected in the affected area in which dosimeters were deployed for about 4 months starting from November 2010 to February Figure 2 shows the result of the concentration of radon soon after the landslide took place. From the above two figures it is clear that radon concentration in dwellings has not been increased as expected due to the landslide formation in Mamit town. The period of deployment after landslide took place is nearly 4 months during winter season in which there is suppose to be less ventilation rate due to cold weather. However, the poor ventilation system, the factor which is most responsible for the increased radon concentration in dwellings in Mizoram 10, has not contributed much to the effect. This is clearly proved by the seasonal variation of radon concentration in Mamit districts (as shown in Fig. 3) surveyed before the landslide took place. Survey of radon concentration was done in Mamit districts as a whole in which 33 houses were selected, out of which 11 were outside Mamit town from nearby villages. The seasonal Figure 2. Radon concentration in dwellings in affected landslide area of Mamit. Figure 1 gives the annual average values of radon concentration in different locations within Mamit town which was determined in Figure 3. Seasonal variation of radon concentration level in Mamit district before landslide took place (2009 May-2010 April). 94

4 Measurement of radon concentration in dwellings from the affected landslide area of Mamit town, Mizoram, India maximum concentration was Bq/m 3 during rainy season (June-September 2009) and Bq/m 3 during winter season (October 2009-January 2010) respectively before landslide occurred. These maxima values are higher than the maximum value obtained after the landslide occurrence which is only Bq/m 3 during November 2010-January As it comes to summer (February-April 2010) the radon concentration values are getting lower having a maximum of Bq/m 3 which is lower due to well ventilation system. Similar observations are made by Ramola 11 while studying the changes of concentration in soil-gas radon with earthquake activity in Garhwal Himalaya in In his observation the radon concentration level before the earthquake occurred, higher than the post earthquake phenomenon. The radon increases were more likely to be caused by crustal disturbance than any atmospheric disturbance. Using RAD7, radon concentration had been observed in seven locations, five of which were in landslide area and the other two in unaffected areas. The radon content in soil gas in four locations, except one place of landslide areas were lower than the average of the two in the unaffected areas which is shown by Figure 4. This measurement shows that movement of soil due to landslide did not increase the radon content in soil gas. L- landslide area O - unaffected area CONCLUSION Radon exhalation from the landslide area of Mamit town was studied by measuring the radon concentration in dwellings in and around the landslide area. The level of concentration was compared with the prelandslide phenomena data of radon concentration level for Mamit town and Mamit district. The following conclusions are made from the observations: 1. Radon concentration in and around the landslide area has not been increased due to the formation of landslide in Mamit town. In fact, the concentration level before the landslide was higher than those measured after landslide took place. 2. The level of concentration of radon was found to be much below the ICRP limit and below the action limit of the recent data of WHO. 13 Drawbacks Measurements of radon concentrations were carried out using a time integrated method over a period of more than three months for each season by employing twin cup dosimeter. This method would not be able to determine the exact time at which radon exhalation from the soil was highest in a day, a week or a month. A continuous on-line monitoring device would be helpful to provide accurate and satisfying data for the study of radon exhalation from the soil gas. However, the present measurement would be able to provide a part of the necessary data in determination of pre and post landslide radon concentration in dwellings. ACKNOWLEDGEMENT Figure 4. Radon in soil gas measurement at Mamit town after landslide took place. The authors would like to acknowledge the helpful and supportive nature of residents where observations were made within and outside Mamit town. 95

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