Radon Concentrations in Drinking Water Supplied from Groundwater

Transcription

1 Radon Concentrations in Drinking Water Supplied from Groundwater T. Ishikawa 1, Y. Narazaki 2, S. Tokonami 1 and Y. Yamada 1 1 National Institute of Radiological Sciences, Anagawa, Inage-ku, Chiba , Japan tetsuo_i@nirs.go.jp 2 Fukuoka Institute of Health and Environmental Sciences, 39 Mukaizano, Dazaifu , Japan Abstract. In Japan, radon concentrations in water supplies are generally low because they are supplied from surface water in most cases. However, high radon concentrations can occur in water systems supplied from groundwater. Water supplies in a part of Nijo town (Fukuoka prefecture, Japan) are such cases. In this area, groundwater is used as a source of the water supplies. Radon concentrations in public/personal water supplies (drinking water) were surveyed for 18 points in Nijo town. For 14 points out of 18, two water samples were taken for each point. These samples were taken in February and August so that a factor of seasonal variation in concentration could be reduced. For each of other four points, a single sample was taken. Two measurement devices were used: an electrostatic radon detector (for the measurements in February) and a pulse ionization chamber (for the measurements in August). Average concentrations were calculated for the points where two samples were taken. The radon concentrations for 15 points ranged from 2 to 394 Bq/l (concentrations for three points were not detectable). These concentrations could be categorized into four: below 10 Bq/l (5 points), Bq/l (5 points), Bq/l (7 points) and 394 Bq/l (1 point). A difference in concentration might be related with the source of water supply (surface water or groundwater). For most points, there was no significant difference in radon concentration between samples taken in February and August. 1. Introduction Radon in water supplies presents a possible risk to the population in two pathways: ingestion of water and inhalation of radon (and its progeny) released from water [1]. In the US, Environmental Protection Agency (EPA) proposed regulations on radon concentration in drinking water: a MCL (maximum contaminant level) of 11 Bq/l and an AMCL (alternative maximum contaminant level) of 148 Bq/l. Public exposure from radon in drinking water is also a health concern for other countries. Surveys of radon concentrations in drinking water have been reported from some countries [2-4]. Although surface water contains small amounts of radon (typically < 4 Bq/l), water from groundwater systems can have relatively high levels of radon. In Japan, radon concentrations in water supplies are generally 1

2 low because they are supplied from surface water in most cases. For water systems supplied from groundwater, however, high radon concentrations have also been found in Japan. Water supplies in a part of Nijo town (Fukuoka prefecture, Japan, FIG.1) are such cases. In this area, groundwater is used as a source of the water supplies. Because Nijo town has a population of more than 13,000, radon in drinking water could be a health concern from the viewpoint of public exposure. Thus, a preliminary survey of radon concentration in drinking water was performed in this area. FIG. 1. A map of sampling location. 2

3 2. Method The water sampling was carried out for 18 public/personal water supplies in Nijo town. For 14 sampling points out of 18, two samples were taken from each water tap; one sample was taken in February and the other was taken in August to reduce a factor of seasonal variation in concentration. For other four points, a single sample was taken from each tap. The water samples were collected in vials suitable for measurement devices. Two different devices were used for the measurements: an electrostatic radon detector (commercial name: RAD7, manufactured at Durridge company, USA) and a pulse ionization chamber (commercial name: AlphaGUARD, manufactured at Genitron Instruments, Germany) [5]. The former device was used for the measurements in February and the latter was used for the measurements in August. Although the two different devices were used, they had been compared using samples with the same radon concentrations. As a result of the comparison, a difference in measured concentrations was within 5% [6]. Thus, concentrations measured with the two detectors could be compared each other. Schematics of the two detectors are shown in FIGs.2 and 3. The operation of the electrostatic radon detector is based on the following principle: (1) radon is expelled from a water sample by using a bubbling kit, (2) expelled radon enters a hemisphere chamber by air circulation, (3) polonium decayed from radon is collected onto a silicon solid-state detector by an electric field and (4) radon concentration is estimated from the count rate of polonium. On the other hand, the operation of the pulse ionization chamber is based on the following principle: (1) radon is expelled from a water sample by using a bubbling kit, (2) expelled radon enters an ionization chamber by air circulation, (3) alpha particles from decaying radon ionize air and (4) radon concentration is estimated by counting pulses produced by ionization. The detection limit for each detector depends on detector background and measurement time. These conditions were different from measurement to measurement for some practical reasons. In order to estimate the precision in each measurement, the ratio of measured concentration to its standard deviation was used as an index; if a measured concentration is smaller than its standard deviation, the concentration was treated as not detectable (ND). For example, a measured value of 2.0±2.3 Bq/l was treated as ND, while 0.3±0.2 was adopted. Measured concentrations were corrected for half-life (3.824 d) to calculate radon concentrations at the sampling times. The average concentrations were calculated for the points where two samples were taken. 3

4 FIG. 2. A schematic of the electrostatic radon monitor with its bubbling kit. FIG. 3. A schematic of the ionization chamber with its bubbling kit. 4

5 3. Results The measurement results for the water samples are shown in Table I. The radon concentrations for 15 points ranged from 2 to 394 Bq/l (concentrations for three points were ND). These concentrations could be categorized into four: below 10 Bq/l (5 points), Bq/l (5 points), Bq/l (7 points) and 394 Bq/l (1 point). A difference in concentration might be related with the source of water supply (surface water or groundwater). For most points, there was no significant difference in radon concentration between samples taken in February and August. As a result of statistical test (t-test), a significant difference (p<0.05) was found for only two points out of 15. The two points are marked (*) in the last column in Table I. Table I. Measurement results for the water samples. Site No. Radon concentration (Bq/l) February August Average 1 36 ± 9 18 ± 1 27* 2 94 ± ± ± ± ± ± ± ± ± 9 55 ± ± ± ± 8 13 ± 2 20* 9 ND ND ± ± ± ± ± ± ND ND ND 14 ND ND ND ± ± ± ± ± Discussion As a result of the present survey, concentrations over the MCL were found for 13 points out of 18. However, the dose due to ingestion of drinking water is relatively small. When using a dose coefficient (3.5E 9 Sv/Bq for effective dose) proposed by UNSCEAR 2000 report [7], a daily ingestion of 0.6 l 5

6 water [1] with a radon concentration of 100 Bq/l causes an annual effective dose of 0.08 msv. This is much smaller than the total effective dose from natural radiation. With respect to the dose due to radon in drinking water, contribution from inhalation of radon (and its progeny) released from water could be greater than that from ingestion. In many situations such as showering, washing clothes, and flushing toilets, radon is released from water and mixes with radon in indoor air. Thus, radon in water contributes to the increase of indoor radon concentration. It is necessary to investigate indoor radon concentrations for houses where drinking water with high radon concentration is supplied. 5. Conclusion Radon concentrations in public/personal water supplies (drinking water) were surveyed for 18 points in Nijo town (Fukuoka prefecture, Japan). The radon concentrations for 15 points ranged from 2 to 394 Bq/l (concentrations for three points were ND). These concentrations could be categorized into four: below 10 Bq/l (5 points), Bq/l (5 points), Bq/l (7 points) and 394 Bq/l (1 point). A difference in concentration might be related with the source of water supply (surface water or groundwater). For most points, there was no significant difference in radon concentration between samples taken in February and August. References 1. National Research Council, Risk assessment of radon in drinking water. National Academy Press, Washington, D.C. (1999). 2. Otwoma, D. and Mustapha, A.O., Measurement of 222 Rn concentration in Kenyan groundwater. Health Phys., 74:91-95, (1998). 3. Sohrabi, M., Alirezazadeh, N. and Tajik Ahmadi, H., A survey of 222 Rn concentration in domestic water supplies of Iran. Health Phys., 75: , (1998). 4. Savidou, A., Sideris, G. and Zouridakis, N., Radon in public water supplies in Migdonia basin, Central Macedonia, Nothern Greece. Health Phys., 80: , (2001). 5. George, A.C., State-of-the-art instruments for measuring radon/thoron and their progeny in dwellings- a review. Health Phys., 70: , (1996). 6. Ishikawa, T., Yasuoka, Y., Narazaki, Y., Tokonami, S., Ishii, T., Suda, H. and Yamada, Y., Comparison of instruments for measuring radon in groundwater. Radioisotopes, in press (in Japanese) 7. UNSCEAR, Sources and effects of ionizing radiation. United Nations, New York (2000). 6