| BackgroundWith the increasing development and utilization of groundwater resources in our country past thirty years, groundwater is an important water source of drinking water in China. In north area of China, about 65% of water consumption comes from groundwater. Groundwater is the main drinking water source for more than 400 cities in 657 cities in the country. Drinking water in rural areas of China is mainly groundwater, and the people who drink groundwater account for 74.87% of the rural population. Radon content in water is closely related to the type of water. Radon concentration in surface water is usually low, and changes of radon concentration in groundwater is complex, and the level of enrichment depends mainly on U, Ra content and radon emanation coefficient of rocks and sediments flowed through in the process of the water cycle. The radon dissolved in water is a potentially important source of radiation exposure. On the one hand, radon in water could be released into the air due to domestic use of water, and radon and its decay products could be breathed in human body through respiratory, and cause radiation damage. On the other hand, directly ingestion of radon in drinking water can result to internal radiation hazard for digestive organs and other organs. Groundwater is an important source of indoor radon. The radon in groundwater has become a potential health risk of residents in China.ObjectiveThe purpose of this study is to investigate radon concentrations in drinking groundwater in typical city of China, to study radon transfer coefficient of groundwater, to explore the change of indoor radon and its progeny during and after domestic use of groundwater, to simulate the radon diffusion from water to the indoor air using Mckone’s model, and to estimate the public dose due to radon released from groundwater according to traditional house size and water use features, to further complement and improve radon in water measurement device with continuous radon monitor, which was developed by our laboratory previously.Methods(1) The main technical parameters, including detection efficiency, detection limit, equilibrium time and its influential factor of radon in water measurement device were studied. The combined uncertainty of radon in water measurement results was estimated. The device was calibrated with radium standard solution, and intercomparison excise was carried out. (2) A total of 12 cities from Beijing city, Inner Mongolia, Ningxia, Shaanxi, Henan, Liaoning and Heilongjiang province were selected. Underground drinking water samples were collected, and radon in water measurement device with the continuous radon monitor was used to determine the level and distribution of radon in groundwater of typical cities in China. (3) Hot spring water was chosen to explore radon transfer coefficient of groundwater, and changes of indoor radon and its progeny, including the equilibrium factor and the unattached fraction of radon decay product before and after shower in hotel room. Mckone’s model was used, combined with house size and water use, to simulate diffusion of radon released from domestic water use using Matlab7.1 software. (4) Annual effective dose due to radon released from groundwater due to domestic use was estimated by parameters including water radon transfer coefficient, radon concentration in water, indoor radon equilibrium factor, residence time, indoor radon concentration equilibrium equivalent dose conversion coefficients.Results(1) The detection efficiency of the developed radon in water measurement device was 2.14 cpm/(Bq/L) for 250 ml water samples, and the detection limit is 0.268 Bq/L. The equilibrium time is 15 min at 0.25 L/min flow rate of pump air, and the measurement device recycle 6 times at a 5 min counting cycle, and finish measurement in 30 min for one sample. The calibration factor is 0.90±0.01. The combined uncertainty of radon in water measurement results was estimated from counting statistic error and calibration error. The intercomparison results showed that the relative percentage deviation from RAD7H2O measurement device was in the range of 0.55% to 5.7%, and was in good agreement. (2) The average of radon content in underground drinking water was 11.8 Bq/L in 12 cities from 7 provinces, and ranged from 0.96 to 63.8 Bq/L. The radon concentration in 60.3% of water samples was less than 10 Bq/L, and 37% of water sample exceed 11 Bq/L the safe limit recommended by the United States Environmental Protection Agency (EPA). (3) The average of water radon transfer coefficient due to shower is 5.51×10-3 in shower room, in range of 1.78×10-3 to 1.29×10-2, and that of bedroom is 1.13×10-3, ranged from 8.57×10-4 to 1.69×10-3. At the end of 20 min of shower, the radon concentration in bathroom is 1.3 to 13.7 times higher than radon concentration before taking a shower. The equilibrium equivalent radon concentration is 1.4 to 11.3 times higher than that of before shower in bathroom. After a shower, the average of equilibrium factor F value was 0.15±0.05 in 30 min after shower turned off, and was 0.51±0.17 after 90 min the shower turned off. The equilibrium factor in bathroom showed a decreased at first and then increased trend. The unattached fraction of radon decay product was 0.101 after 5 min shower turned off, and was 0.028 after 2 h shower turned off. The unattached fraction of radon decay product showed a increased first and then decreased trend. The simulation of water radon diffusion in different regions of indoor air in 24 h showed that the indoor radon concentration is mainly affected by the impact of the shower in domestic use of groundwater. The residence time is an important factor affecting radon exposure due to use of water. (4) The average annual effective dose to inhaled radon released from groundwater was 0.312 mSv, in the range of 0.025 to 1.69 mSv. The average of the inhaled radon dose due to shower accounts for 42% of total inhaled dose to radon released from domestic use of groundwater.ConclusionThe radon concentrations in groundwater were investigated in this study, and baseline data of radon in drinking water in typical city of China were established. A preliminary result of indoor radon contribution due to the domestic use of the groundwater in China were obtained, the annual average effective dose due to inhaled radon released from long-term domestic use of groundwater accounts for 20% of natural radiation dose to radon and its progeny in China. The contribution of the groundwater supply to indoor radon is worthy of paying more attention. Data and results obtained in this study will provide important basic data for the study of natural radioactivity level in China. The radon in water measurement device which was previously developed by our laboratory was further improved, and can be applied for large scale radon in water survey. The results obtained in this work can also provide scientific information on setting control limit for radon in drinking water and risk assessment for radon radiation in future.
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