Abstract: Radon and its radioactive progenies in indoor places are recognized as the main sources of public exposure from the natural radioactive sources. The tap water used for drinking and other household uses can increase the indoor radon level. In the present research drinking water samples were collected from various places and supplies of public water used in Mashhad city which has about 4 millions population. Then radon concentration has been measured by PRASSI system three times for each sample. Results show that about 75% of water samples have radon concentration >10 Bq L^-1 which advised EPA as a normal level. According to measurements data, the arithmetic mean of radon concentration for all samples was 16.238±9.322 Bq L^-1. As well as the annual effective dose in stomach and long per person has been evaluated in this research. According to the advised of WHO and the EU Council, just 2 samples induced the total annual effective dose greater than 0.1 mSv year^-1.

INTRODUCTION

Radon (²²²Rn) is a naturally occurring radioactive noble gas with a half-life of 3.82 days which is a member of the ²³⁸U decay series (Somlai et al., 2007). Radon and its short-lived decay products such as ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi and ²¹⁴Po at indoor places are recognized as the main sources of public exposure from the natural radioactivity contributing to nearly 50% of the global mean effective dose to the public (Somlai et al., 2007; UNSCEAR, 2000). The type of soil, building materials and water used for drinking and other household uses can make variable contributions to the indoor radon level (Sohrabi, 1998). The available data indicate that the main source of the indoor radon is the soil underlying a building. However, certain building materials with high concentrations of radium and even domestic water with high concentrations of radon can make major contributions to indoor radon exposure (Kearfott, 1989; Li et al., 2006). The most important aspect of radon in high concentrations can be health hazard for humans mainly a cause of lung cancer (Folger et al., 1994; Khan, 2000). However, a very high level of radon in drinking water can also lead to a significant risk of stomach and gastrointestinal cancer (Zhuo et al., 2001; Kendal and Smith, 2002). Knowledge of the levels of radon in each source including household water, particularly water from groundwater sources is necessary to protect public from consequences of excessive exposure to radiation mainly from the risk of lung cancer.

In Iran, the household water is supplied from various sources. Due to the dry climate condition in the most parts of the country, drilled wells have provided the main section of drinking water used by public. In a few parts with high annual rain, surface water is the main source for public usages.

In a number of cities including Mashhad, both groundwater and surface water are the sources of household water. Domestic water of Mashhad the second big city of Iran after Tehran which has about 4 million fixed population and >12 million religion tourists and business persons is supplied from two Torogh and Kardeh dams and >84 deep wells drilled in and around the city. Depending on raining condition the contribution of groundwater to the supplied domestic water, particularly in summer season may increase.

Depending on geographical situation of a specified region in the city, domestic water may be supplied from groundwater, surface water or a mixture of them. In addition, there are a number of large reservoirs in various parts of Mashhad for the collection and distribution of treated surface water and groundwater in the city.

In the present research results of radon measurement in 50 water samples, sources and tap water actually used for drinking and other household uses in Mashhad. Radon of water samples that have been measured using PRASSI system include a ZnS(Ag) scintillation detector.

MATERIALS AND METHODS

Water sampling: The water samples were collected in various points distributed in and around the city of Mashhad. Figure 1 shows the sampling sites. Water sampling has been done from each water supply including wells and surface water as well as from household water. The samples were collected from the head ports of active wells selected for sampling, rivers and surface water reservoirs as well as from domestic water taps of high consumption rates, using the standard procedure proposed by the USA Environmental Protection Agency, EPA (USEPA, 1991). In this procedure a plastic funnel was connected via a short plastic hose to the water tap. After the water flowed for several minutes, the flow rate was slowed down and the water was allowed to be collected in the funnel. Then, three 150 mL water samples have been collected from each source or region. The collected samples were transferred to the laboratory of Payame Noor University for analysis.

Fig. 1:	(a) Mashhad location in Iran and (b) The map of Mashhad city and • shows the sampling sites

Radon measurement: The PRASSI (Portable Radon Gas Surveyor SILENA) Model 5S has been used for radon concentration measurement in the water samples which is particularly well suited for this type of measurement that must be performed in the closed loop circuit. Figure 2 shows the system set up of measurement including bubbler and drier column. PRASSI pumping circuit operates with constant flow rate at 3 L min^-1 in order to degassing the water sample properly. The sensitivity of this system in continuous mode is 4 Bq m^-3 during the integration time of 1 h.

To measure the content of radon in water, researcher consider V_sample = 150 mL of the water sample in bubbler and the PRASSI will read a concentration of:

(1)

where, V_tot is the total volume of system equal 204x10^-3 m³ and A_Rn is the radon activity. It follows that the concentration of radon in water is:

(2)

The average value of three measurements was considered as the radon concentration in the water sample.

RESULTS AND DISCUSSION

In the present research, a total number of 50 water samples from groundwater of deep wells, surface water of rivers, tap water samples were collected and analyzed for radon concentrations.

Fig. 2:	The PRASSI system set up for radon measuring in the water sample

Table 1:	Average radon concentration data and annual effective dose of different water sources per adult person
SW = Surface Water; GW = Ground Water; MW = Mixes of surface and ground Water

The 3rd column of Table 1, shows the mean radon concentration in each water samples. According to the data the minimum and maximum radon concentrations in samples are 0.064 and 46.088 Bq L^-1, respectively. The arithmetic mean radon concentration of all samples was 16.238±9.322 Bq L^-1. As the data shown in Fig. 3, the radon concentrations is about 75% for samples used by people in Mashhad which are greater than the EPA advised level, 10 Bq L^-1. Researcher must mention that they sorted the experimental data in ascending order. The main reasons for large differences of radon concentration in sample seems to be due to mixing of surface water with groundwater in proportions mentioned earlier and storage of the mixed water in large reservoirs before distribution.

Unfortunately up to now, there is no specific national regulation for radioactivity concentrations in drinking water in Iran. Compared to maximum contaminant level of 10 Bq L^-1 for radon in public drinking water suggested by the EPA (Folger et al., 1994), the radon concentrations in most of the drinkable water samples in Mashhad, is significantly higher.

Fig. 3:	Radon concentration for various samples of drinking water

In addition, the EPA requires that action be taken to reduce radon levels above an alternative maximum contaminant level of 150 Bq L^-1 (Zhuo et al., 2001). A number of investigators have shown much higher radon concentrations in public drinking water (Savidou et al., 2001; Al-Kazwini and Hasan, 2003). Kusyk and Ciesla (2002) has been shown the mean value of 74 Bq L^-1 for tap water and mean value of 207 Bq L^-1 for wells in southern of Poland.

Evaluation of mean annual radon dose: The radon concentration of drinking water is an important issue from dosimetry aspect because more attention is paid to control of public natural radiation exposure. Regarding radiation dose to public due to waterborne radon, it is believed that waterborne radon may cause higher risk than all other contaminants in water (Vitz, 1991). Radon enters human body through ingestion and through inhalation as radon is released from water to indoor air. Therefore, radon in water is a source of radiation dose to stomach and lungs. The annual effective doses for ingestion and inhalation were calculated according to parameters introduced by UNSCEAR (2000). For ingestion the following parameters were used:

For inhalation the following parameters were used:

The annual effective dose due to inhalation corresponding to the concentration of 1 Bq L^-1 in tap water is 2.5 μSv year^-1. Therefore, waterborne radon concentration of 1 Bq L^-1 causes total effective dose of about 2.68 μSv year^-1 for adults. The mean annual effective dose per person for adults caused by different water samples are shown in Table 1.

The World Heath Organization and the EU Council recommend the determination of reference level of the annual effective dose received from drinking water consumption at 0.1 mSv year^-1 from these three radioisotopes: ²²²Rn ³H, ⁴⁰K (Somlai et al., 2007). So, 2 samples (No. 49 and 50) induced the total annual effective dose >0.1 mSv year^-1.

CONCLUSION

The results of this study well indicate that the radon concentrations in public drinking water samples of Mashhad are mostly low enough and below the proposed concentration limits. Measuring radon results show about 75% of samples actually used by people in Mashhad are greater than the EPA advised level 10 Bq L^-1. Although, according to the advised of WHO and the EU Council just 2 samples (No. 49 and 50) induced the total annual effective dose >0.1 mSv year^-1. Therefore, there is a radon problem for these two sources and requiring some action to reduce their radon level before public usage such as mixing with surface water in large reservoirs or aerate water in order to allowing some radon removal from the water. It is evident that if the wells are to be the only water supply for some parts of Mashhad, the required remedial action should be taken to reduce radon concentrations consumed by people.