Enhanced indoor radon concentration by using radon-rich well water ...

Journal of Radioanalytical and Nuclear Chemistry, Vol. 266, No. 3 (2005) 389–396

Enhanced indoor radon concentration by using radon-rich well water in a Japanese wooden house in Fukuoka, Japan Y. Kobayashi,1 S. Tokonami,1* Y. Narazaki,2 W. Zhuo,1 M. Furukawa1 1 Radon

Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan 2 Fukuoka Institute of Health and Environmental Sciences, 39 Mukaizano, Dazaifu 818-0135, Japan (Received December 6, 2004)

Radon measurements were carried out in a Japanese wooden house built on granitic geology, where radon-rich well water is used. Atmospheric radon concentrations were measured over one year with passive integrated radon monitors. The monitors were distributed at several locations in the house and were replaced every two months. In order to confirm the diurnal variation and heterogeneous distribution of radon, short-term measurements were carried out accordingly. Radon, its decay products and terrestrial gamma-radiations were measured in this survey. From the long-term measurement, the radon concentration in the house ranged from 14 to 184 Bq.m–3 with an arithmetic mean of 45 Bq.m–3. A radon concentration of 184 Bq.m–3 was observed in the bathroom in spring (March–May) though the radon level was normal in the living room and bedroom. In order to characterize the house, similar measurements were conducted in several surrounding houses. There was a significant difference in radon concentration between the investigated houses. There was a spatial distribution of the radon concentration and the highest value was found in the bathroom. Radon and its decay products concentrations varied with time, which increased from midnight to morning whereas they decreased during daytime. Although the radon concentration in tap water was 1 Bq.l–1, a high level of 353 Bq.l–1 was found in the well water.While well water was being used, the indoor radon concentration near the bathroom increased rapidly with a maximum value of 964 Bq.m–3. It is clear that the use of well water enhanced the radon level around the bathroom.

Introduction Since the correlation between high radon concentration and lung cancer was pointed out,1 measurements of indoor radon were carried out all over the world. According to the United Nations Scientific Committee on the Effect of Atomic Radiation Report (UNSCEAR) in 2000,2 the public exposure from radon and its progeny contributes to more than 50% of the global mean effective dose. Because the exposure due to radon and its progeny occurs mainly in dwellings, their high concentrations are measured as one of indoor air pollutants. In many countries, regulation and mitigation measures of indoor radon concentration have been advanced according to the 1993 recommendation of the International Commission on Radiological Protection (ICRP).3 Also, a setup of regulations of indoor radon level is required and several regional surveys are now being carried out in Japan. Indoor radon occurs from the soil and rock below the foundation of dwelling and building materials such as concrete and mud wall. Its concentration is dominated by volume, airtightness and ventilation of the dwelling, and depends on even occupant’s lifestyle. The ventilation rate will change by opening and closing the windows or doors, and using radon-rich water or natural gas may enhance the supply of radon gas. They will cause short-term variations of indoor radon concentration. On the other hand, a long-term measurement with a passive radon detector is generally used for regional surveys, whereas the detector cannot record any information about radon sources and short-

term variation of radon concentration during the measurement period. In order to indicate a possibility of high indoor radon concentration enhanced by use radonrich well water in this survey, both long-term and shortterm measurements were carried out in a Japanese wooden house, which is generally characterized by low radon level due to high natural ventilation rate. Investigated area Geological setting Futsukaichi the investigated area, is located in the Fukuoka prefecture (Fig. 1), in the northern part of Kyushu island, which is widely covered with the Late Cretaceous Granitoid with pegmatite of the inner zone of Southwest Japan. K-feldspar-porphyritic mediumgrained muscovite-bearing biotite granite, the Sawara Granite4 is found in Fukuoka area and the Kego Fault System5 that consists of several parallel NW-SE strike faults (Fig. 2). Figure 3 shows the location of the Futsukaichi hot spring, which is located a few hundred meters east of the Kego fault and is known as high radon content spring water. Investigated house The investigated house is located at 1 km northeast of the Futsukaichi hot spring. The house is a wooden mortar one-storied house, where both radon-rich well water and tap water are used, the well water mainly in bathroom. Figure 4 shows a room layout of the investigated house.

* E-mail: [email protected] 0236–5731/USD 20.00 © 2005 Akadémiai Kiadó, Budapest

Akadémiai Kiadó, Budapest Springer, Dordrecht

Y. KOBAYASHI et al.: ENHANCED INDOOR RADON CONCENTRATION BY USING RADON-RICH WELL WATER

Fig. 1. Location of the investigated area

Fig. 2. Geological map of Fukuoka area, Kyushu, Japan. The star indicates the location of the investigated house. The Sawara Granite is contained in Cretaceous Younger Granitoid (after the Geological Survey of Japan, 1993)

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Fig. 3. Location of the investigated house, its control houses and the Futsukaichi hot spring

Fig. 4. Room layout of the investigated house. The values in the rooms show the radon concentrations (in Bq.m–3) measured with Pico-Rad

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Control houses

Environmental radioactivity measurements

In order to characterize the investigated house, several houses were chosen in Futsukaichi as control houses (Fig. 3). The control house-1 and 2 are located about 0.7 km east, and the control house-3 at 2 km northwest of the investigated house. The control house-1 is a wooden one-storied house with mud wall, and control house 2 and 3 are two-story buildings of wooden mortar. In all control houses, only tap water is used for daily life.

In accordance with “The guideline for mineral spring analyzing method” (Ministry of Environment, Japan),6 the radon concentrations in well water and tap water were measured with a TRI-CARB 2260XL (Packard) liquid scintillation counter. The radionuclide contents in topsoil in the garden of the investigated house were measured with a gammaspectrometer with a IGC1619E (Toshiba Co.) Ge(Li) semiconductor detector. After removing foreign substances such as vegetable roots or gravels, and drying at 105 °C, a soil sample has been sieved through a 2 mm-∅ sieve. Then a U-8 container made of styrene resin was filled up with the sample, and sealed with an adhesive material. The container was measured one month after reaching radioactive equilibrium. In addition, terrestrial gamma-radiation in the garden of the investigated house and its surrounding area was also measured with a JSM-102 (Aloka Co., Ltd.) NaI(Tl) scintillation spectrometer.

Experimental Long-term measurement For the long-term measurement, a passive radon detector, commercially called Radopot (Radosys, Co., Ltd.), with CR-39 was used so as to obtain an annual mean radon concentration and its seasonal variation in the investigated house and its control houses. The detector was placed on a shelf or furniture at each place, and it was replaced with a new one every two months. The measurement period ranged from November 2000 to January 2002. Short-term measurement Continuous measurement in the bathroom: In order to account for diurnal change of indoor radon concentration in the investigated house, the radon concentration and the equilibrium equivalent radon concentration were continuously measured with a passive pulse-type ionization chamber, AlphaGuard (Genitron Instruments GmbH), and a WL meter (Pylon Model AEP-47 Alpha Detection Assembly Model AB-5), respectively. The measurements were carried out in July 1998 (1 day) and in October 2000 (3 days). Indoor radon measurements in the investigated house and surrounding area: The indoor radon concentration in the investigated house and its surrounding houses was measured in April 1998 with another passive radon detector, PICO-RAD (Packard), based on activated charcoal adsorption. The detectors were distributed in the living room or the office of the investigated house and its surrounding houses so that the mean radon concentrations are checked at as many measuring points as possible. The indoor radon concentration in each room of the investigated house was also measured in July 1998. The exposure periods were 24 hours.

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Results Long-term measurements From the long-term measurement, the radon concentration in the bathroom of the investigated house ranged from 36 to 184 Bq.m–3 with an arithmetic mean of 77 Bq.m–3, and radon concentration of 184 Bq.m–3 was observed in the bathroom in spring (March 2001– May 2001). A significant seasonal variation of radon concentration was found in the bathroom (Fig. 5). The mean radon concentration in the bathroom of the investigated house was five times higher than that of the control houses. There were no large differences of indoor radon concentration in the living room and bedroom of the investigated house and its control houses, and no seasonal variation has been found. Short-term measurements Diurnal variation of indoor radon concentration in the bathroom: In July (Fig. 6a), the bathtub (0.3 m3) was filled with well water 5 hours after starting the measurement, and kept filled during the measurement period. As soon as water was running into the bathtub, radon concentration increased rapidly and decreased 2 hours later. It temporarily concentration exceeded 300 Bq.m–3. Then, the radon concentration increased up to around 100 Bq.m–3 from 22:00 again, and the mean radon concentration was 62±17 Bq.m–3 from 22:00 to 9:00. Meanwhile, the window of the bathroom was opened.


In the October experiment (Fig. 6b), the bathtub was filled with well water 20 hours after starting the measurement, and was left filled throughout the measurement period. Although between 12:00 and 20:00 the mean radon concentration was 19 Bq.m–3 before running the well water into the bathtub, between 21:00 and 11:00 the mean radon concentration reached 112±32 Bq.m–3 afterwards. The radon concentration increased rapidly up to 964 Bq.m–3 right after running the well water into bathtub and decreased 2 hours later as shown in July. Then, the radon concentration repeated the diurnal variation of translating high level of around 150 Bq.m–3 from 21:00 to 11:00 and low level of around 50 Bq.m–3 from 12:00 to 20:00. The window of the bathroom was closed during that period. Using well water, the equilibrium factors varied from 0.04 to 0.35 (average: 0.23) in July, and from 0.29 to 0.78 (average: 0.55) in October, respectively.

Indoor radon concentration in the surrounding area: Figure 7 shows the distribution of indoor radon concentration and gamma dose rate in the surrounding area. The radon level in the investigated house was 22 Bq.m–3, which is slightly high as compared with that in surrounding houses of 3–17 Bq.m–3. There were significant differences of indoor radon concentration and gamma dose rate between the investigated house and its control houses, and they are slightly correlated with each other. However, a definite evidence of the correlation requires further investigations in this area. Figure 4 shows the distribution of indoor radon concentration in the investigated house. A heterogeneous distribution of indoor radon concentrations was observed in the investigated house. The highest value of 33 Bq.m–3 was found in the bathroom, and the average of the other rooms was 19 Bq.m–3. The bathtub was filled with well water during the measurement period.

Fig. 5. Radon concentrations in the investigated house and three control houses measured with the passive radon detector

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Fig. 6. Diurnal variation of radon concentration, equilibrium equivalent radon concentration (EERC), equilibrium factor, temperature and relative humidity in the investigated houses in July 1998 (a) and in October 2000 (b)

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Table 1. Radioactivity of radon sources in the investigated house Radon source

Discussion

Radioactivity

Soil

19 Bq.kg–1 45 Bq.kg–1 353 Bq.l–1 1 Bq.l–1

238U

series 232 Th series Well water Tap water

Environmental radioactivity The radon concentration in the well water used in the investigated house was 353 Bq.l–1. 238U, 232Th and 40K contents in dry soil were 17, 37 and 964 Bq.kg–1, respectively (Table 1). The gamma dose rate of 60 nSv.h–1 was observed in the garden of the investigated house. Those in the surrounding area were ranged 30–50 nSv.h–1 with an arithmetic mean of 46 nSv.h–1.

From the continuous short-term measurement in the bathroom, a rapid increase of both radon concentration and EERC were temporarily observed while running the well water. It is considered that the enhanced radon concentration and EERC were caused by exhalation of radon from well water. The differences of maximum values of radon concentration and equilibrium factor between July of 316 Bq.m–3 and October of 964 Bq.m–3 might be caused by different ventilation rates depending on whether the window was opened or not. On the other hand, the result of long-term measurements show a seasonal variation of radon concentration in the bathroom. It might depend on whether not only the window was opened but also the bathroom was frequently used.

Fig. 7. Natural radiation levels in the surrounding area of the investigated house

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There are no anomalous values of radon concentrations in the investigated house measured in the short-term measurements with the PICO-RAD except for the case of the bathroom. From the long-term results, the mean radon concentration in the bathroom was of about five times higher than the mean value of the control houses. However, a significant difference of radon concentrations in the living room and bedroom between the investigated house and its control houses was not found. These results suggest that the indoor radon concentrations are locally affected by use of radon-rich well water despite the high ventilation rate of the Japanese wooden house. Actually, with 22 Bq.m–3 the indoor radon concentration in the living room of the investigated house is higher than the annual mean value in the nationwide survey in Japan of 15.5 Bq.m–3 and that in the surrounding houses of 9 Bq.m–3,7 but they were not remarkably different. In addition, the gamma dose rate measured in the investigated house was also higher than that of the surrounding houses. This fact suggests that the exhalation rate of radon from soil must be high. Because there is little information to relate directly the source of indoor radon in the living room to the use of well water, further investigations will be needed. Conclusions The annual mean radon concentration in the living room of the investigated house was 22 Bq.m–3, which was slightly higher than the annual mean indoor radon concentrations of 12.9 Bq.m–3 in wooden house and 15.5 Bq.m–3 in whole house measured in a nationwide survey in Japan, and 9 Bq.m–3 higher than that of the surrounding houses.

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From the long-term measurements, the annual mean radon concentration in the bathroom was found to be 77 Bq.m–3 though the radon levels were normal in the living room and bedroom. The radon concentration measured in short-term measurements temporarily exceeded several hundreds Bq.m–3 when well water was used. This exemplifies a possibility of high indoor radon concentration caused by use of radon-rich well water, which is hardly detected in long-term integrating measurements because of its temporal and heterogeneous distribution.

References 1. W. JACOBI, Expected lung cancer risk from radon daughter exposure in dwellings, Proc. Intern. Conf. on Indoor Air Quality, Stockholm, 1984. 2. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), Source and Effects of Ionizing Radiation, United Nations, New York, 2000. 3. International Commission on Radiological Protection (ICRP), Protection Against Radon-222 at Home and at Work, ICRP Publication 65, Annuals of the ICRP, 23 (2), Pergamon, Oxford, 1993. 4. Geological Survey of Japan, 1 : 200,000 Geological Map, Fukuoka, 1993. 5. Research Group for Active Fault of Japan, Active Fault in Japan: Sheet Maps and Inventories (Revised ed.), University of Tokyo Press, Tokyo, 1991. 6. Nature Conservation Bureau, Ministry of Environment, Japan. The Guideline of Mineral Spring Analyzing Method, Tokyo, 2002. 7. T. SANADA, K. FUJIMOTO, K. MIYANO, M. DOI, S. TOKONAMI, M. UESUGI, Y. YAMADA, J. Environ. Radioact., 45 (1999) 129.