A SIMPLE NATIONAL INTERCOMPARISON OF RADON IN WATER

A SIMPLE NATIONAL INTERCOMPARISON OF RADON IN WATER Abstract Radon-222, a naturally occurring radioactive gas, responsible together with its progeny of around 50% of the average effective dose received by the population, has not been regulated by law until the recent Directive 2013/51 /Euratom. Its transposition into Spanish legislation was made in the recent RD 314/2016, which sets at limit value of 500 Bq l−1 for radon-222 in water for human consumption. Intercomparison exercises, such as those carried out by IPROMA SL and the Laboratory of Environmental Radioactivity of the Cantabria University (LARUC) in November 2015 and December 2016, represent the most useful tool available for detecting problems and taking corrective actions necessary for an efficient measurement by part of the laboratories. The participants in these exercises used three techniques: liquid scintillation counting, gamma spectrometry and desorption followed by ionisation chamber detection. INTRODUCTION Radon gas has essentially three isotopes: 222Rn (T1/2 = 3.82 d), 220Rn (T1/2 = 55.6 s) and 219Rn (T1/2 = 3.96 s)(1). 222Rn (hereinafter referred as radon in this article) is a natural radioisotope belonging to the series of 238U (an alpha particle emitter, 5590.3 keV, with a half-life of 3.82 days). Its two short-lived alpha emitting progeny are 218Po (6002.55 keV, 3.11 min) and 214Po (7686.90 keV, 163.69 μs)(2). Exposure to radon and its progeny has an estimated average effective dose of around 50% (1.3 mSv) of the total effective dose received by the general population(3), based on results of the numerous studies conducted in dwellings(4–6). However, little attention has been paid to the radon that is ingested in drinking water, and to the additional risk that arises due to the low transferability of radon from water to air, with an estimated transfer coefficient for dwellings of 10−4(7). Although the dose received by ingestion of water with radon is significantly less than by inhalation of its progeny(8), the measurement of radon concentration in water has additional interest in other respects. The radon coming from water contributes very little to the concentration of radon inside dwellings, but it can be significant in certain workplaces such as some thermal spas. In addition, radon gas dissolved in water has proved to be a useful tracer of hydrodynamic processes in aquifers and underground currents(9). Being a noble gas, it is not assimilated by any chemical compounds in the environment, but due to its moderate solubility, (0.225 cm3 g−1 at 20°C), it can be detected in water, especially groundwater(2). Its concentration in groundwater will depend mainly on the radium content of the substrate, the specific surface area of the aquifer, the permeability of the soil and the characteristics of the water itself. When these groundwaters discharge at the surface, the concentration of dissolved radon decreases abruptly due to water movement and purification processes. However, where these waters are consumed directly at the point of upwelling, the risk of ingesting radon and its progeny may be significant. There was no limit for radon in the recently repealed RD 140/2003(10), which concerned drinking water quality. Thus, the range 100–1000 Bq l−1 in the EC Directive 2013/51/Euratom of 22 October 2013 represents the first time that a radon limit for drinking water has been set(11). Transposition of this Directive into Spanish legislation was by means of the recent RD 314/2016(12) which sets a limit value of 500 Bq l−1 for radon in water for human consumption. The intercomparison exercise reported in this paper, organised by the Radon Group of Cantabria University in collaboration with IPROMA S.L., arose from the need for a quality control for all the national laboratories that measure radon in water. Overall, 11 laboratories participated in November 2015, and 17 in December 2016 (Table 1). Table 1. Name and location of the 17 participating laboratories in the intercomparison of December 2016. 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid Table 1. Name and location of the 17 participating laboratories in the intercomparison of December 2016. 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid MATERIALS AND METHODS Sample collection The chosen sampling location was the spa of Las Caldas de Besaya, which has been studied by the Radon Group Cantabria University since the 1980s(9, 13). The geographical location of the site can be seen in Figure 1. Figure 1. View largeDownload slide Location of Caldas Besaya Spa. Figure 1. View largeDownload slide Location of Caldas Besaya Spa. The spa is located on the banks of the river Besaya in the town of Corrales de Buelna, ~30 km from Santander. These hot springs are characterised by temperatures of 34–37°C, and are rich in sodium chloride, bicarbonates and nitrates(9). A single homogenous water sample was collected from the spa on 23 November 2016 in a 25-l container, and taken to the LARUC laboratory where, on the same day, aliquots were transferred to 1-l low density plastic bottles for shipment to the participating laboratories. These bottle have double stoppers, in order to minimise leaks and the formation of bubbles, which could introduce uncertainty into the participants’ measurements. A ‘protocol’ was drawn up by the organisers of the exercise, and sent to each participant. This protocol explained everything necessary for the proper development of the exercise, including implementation a data protection policy that assigned a code to each laboratory to maintain anonymity of results. Participants received the samples 24–36 h after sampling, except for the University of Palmas, where customs clearance took 6 days, though no incident occurred during shipment that would have delayed the exercise. Measurement techniques The measurement techniques used by the participating laboratories in the intercomparison exercise can be seen in Table 2, which also shows the number of participants using each. Three participants sent results using two different techniques, which is why there were twenty results from only 17 laboratories. Table 2. Results sent by participants (‘GS’ = gamma spectrometry HPGe detector, ‘LSC’ = liquid scintillation counting and ‘D’ = desorption with ionisation chamber). Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Table 2. Results sent by participants (‘GS’ = gamma spectrometry HPGe detector, ‘LSC’ = liquid scintillation counting and ‘D’ = desorption with ionisation chamber). Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Desorption technique 222Rn in water with ionisation chamber The equipment used is an AlphaGuard PQ2000-PRO which uses a specific attachment for measuring 222Rn in water. By means of a pump, the water is bubbled continuously, which causes desorption of radon from the water and directs it to the detector via a desiccator column. Once inside the detector, the radon enters an ionisation chamber (where a potential of 750 V is maintained) flowing over a large-surface fibreglass filter that prevents entry of radon progeny and aerosols. Alpha particles emitted by the radon ionise the air, the cathode attracts the positively charged particles, while the anode attracts the negatively charged ones(14). To calculate the concentration of 222Rn in water, the following equation is used: Cwater=Cair⋅(Vsystem−VsampleVsample+k)−C01000 (1) where Cwater is the concentration of 222Rn in the water sample (Bq l−1), Cair is the concentration of 222Rn in air (Bq m−3) on the AlphaGuard screen, C0 is the background (Bq m−3) that can be considered 0, Vsystem is the inside volume of the equipment (1117.58 ml), Vsample is the volume of the sample (100 ml) and ‘k’ is a factor for the transfer of radon from water to the air, which is a function of temperature(15). Gamma spectrometry This technique is for detecting gamma emissions from soil, sludge, ash, environmental filters and, ultimately, from any sample whose gamma emission falls between 30 and 3000 keV. The equipment used is a HPGe detector. The photons resulting from gamma emissions from the sample enter the active volume of the detector and interact with its atoms. These interactions are converted to electrical pulses that are proportional to the energy of the photons emitted, and which are stored in finite energy increments equivalent over the range of the spectrum(16). 222Rn activity is determined three hours after preparing the bottle with the water sample with the count made in the area of the spectrum for 214Pb (351.93 keV). This elapsed time that is necessary to achieve secular equilibrium between radon and its short-lived progeny (218Po, 214Pb, 214Bi and 214Po). To calculate the activity due to 222Rn in the water, the following equation is used for peak 214Pb: A=(N−f⋅t)v⋅t⋅PE⋅Ef (2) where - A is the radon concentration (Bq l−1). - N are the counts. - f is the background in counts per second (cps). - t is the count time (s). - v is the volume of the sample (l). - PE is the emission intensity (%). - Ef is the efficiency (cps/Bq). Liquid scintillation counting with alpha/beta separation The water sample containing 222Rn is mixed with a liquid scintillation solution in a transparent vial. The alpha/beta emissions of 222Rn and its progeny transfer energy to the scintillator, which releases this energy as photons (measured as light pulses). A distinction between alpha and beta emissions is possible since alpha particles lead to slightly longer light pulses than beta particles. The electrical pulses derived from the photon release is proportional to the radioactive energy emitted. The continuous emission of alpha/beta particles from the radioactive material causes a continuous generation of pulses, so that the counts accumulates progressively(17). Equation (3) is used to calculate activity from the 222Rn in water (alpha particles) after 3 h elapsed time explained above: A=G−BEf⋅60⋅V (3) where A is the activity in Bq l−1; G are the counts per minute (cpm); B is the background in cpm what the equipment counts for a sample prepared with distilled water; Ef is the equipment’s efficiency for 222Rn, 218Po and 214Po; V is the sample volume in litres; the inclusion of 60 in the denominator is to transform counts per minutes to counts per second. RESULTS AND DISCUSSION Measurements were returned to the organiser by 14 December. By 22 December, each participant received the report of the exercise showing the results and techniques used by each one (Table 2). Statistical treatment of the measurements by the participants began by discarding data outside of the range median ± 50% as being incorrect data. By this means, the result from laboratory R04 was eliminated. The most relevant statistics were then calculated, as seen in Table 3. Table 3. Statistics of the results, with values expressed in Bq l−1. Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 Table 3. Statistics of the results, with values expressed in Bq l−1. Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 A quantile–quantile plot (q–q plot) was applied to test whether results fit a normal distribution, whereby any deviation from linearity as in Figure 2 implies a non-normal distribution. Figure 2. View largeDownload slide Graph q–q plot of the results of the exercise. Dashed line represents the 95% confidence interval. Figure 2. View largeDownload slide Graph q–q plot of the results of the exercise. Dashed line represents the 95% confidence interval. To establish the consensus statistic for the exercise, an iterative algorithm was applied, according to ISO 13528:2015, whereby extreme values are given less weight than in a classical treatment of statistical data. This algorithm considers the measurements, of all participants and repositions the extreme values within the interval of acceptable deviation, thus obtaining robust estimators of the consensus value X and the standard deviation σexercise. As for the objective sigma, σp, was established as 20%, while for uncertainty μx, the following equation is applied: μx=1.25⋅σexerciseN (4) where - σexercise is the standard deviation. - N is the number of results sent by laboratories. The statistics of the exercise are presented in Table 4. Table 4. Parameters of the exercise. Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 Table 4. Parameters of the exercise. Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 For the determination of the Zscore of each participant, the following equation is applied: Zscore=x−Xσp (5) where - x is the measurement provided by each participant. - X is the consensus value calculated according to ISO 13 528:2015. - σp is the target standard deviation, set at 20%. The Z score values are interpreted as follows: |Zscore| ≤ 2 indicates satisfactory performance. 2 < |Zscore| ≤ 3 indicates dubious performance and generates an alert. |Zscore| > 3 indicates unsatisfactory performance and generates a signal for action. A graphical presentation of the Z scores obtained by each laboratory is given in Figure 3, where the Z scores are ranked low to high. Figure 3. View largeDownload slide Z score of laboratories participating in the exercise. Figure 3. View largeDownload slide Z score of laboratories participating in the exercise. Figure 4 shows the concentrations reported by each laboratory with their respective uncertainties. The red horizontal lines represent the consensus value (112.1 Bq l−1) and the target standard deviation of ±20% (134.5 and 89.7 Bq l−1). Figure 4. View largeDownload slide Activities measured by each laboratory, showing the consensus value and the target standard deviation of ±20%. Figure 4. View largeDownload slide Activities measured by each laboratory, showing the consensus value and the target standard deviation of ±20%. As a result of the 6-day delay in the delivery of the sample to one of the participants, the organisers decided to do a radon-leak test of the containers used to send samples to the participants. The bottles used in both exercises were made of low density polyethylene (LDPE). The importance of the bottle material is not significant, other than to know its permeability to radon for future exercises with a reference value (in these, exercises were used consensus value). In order to quantify the leakage λleaks of the material, a sample of water containing 222Rn was divided and stored in two bottles made of different materials, one LDPE plastic and another glass. Samples were taken every 3–4 days and analysed in a liquid scintillation alpha spectrometer (Triathler 425-034); the results were plotted to observe the decay in the sample over several days. Each graph was fitted to a function of the type: A=Ao⋅e(λradon+λleaks) (6) where: - A is the final activity of the sample (Bq l−1). - Ao is the initial activity of the sample (Bq l−1). - λradon is the decay constant for radon (h−1). - λleaks is the constant of the permeability of the material to radon (h−1). This graph (Figure 5) indicates a value of λleaks of 3.3×10−3 h−1 for LDPE, compared to 3.5×10−4 h−1 for glass. Figure 5. View largeDownload slide Exponential adjustment of the radon decay of a water sample in LDPE plastic and glass containers. Figure 5. View largeDownload slide Exponential adjustment of the radon decay of a water sample in LDPE plastic and glass containers. CONCLUSIONS More laboratories participated in the performance testing in 2016 than 2015, demonstrating a growing interest in measuring 222Rn in drinking water. The reason is probably the recent transposition of Directive 2013/51/EURATOM into Spanish law (as RD 314/2016 of 29 July), establishing for the first time the legally permissible levels of 222Rn in drinking water. This intercomparison exercise by IPROMA and LaRUC, included 17 national laboratories in 14 provinces, representing 8 of the 17 Autonomous Communities in Spain. The liquid scintillation counting (LSC) technique, is the most widely used technique by Spanish laboratories specialising in the measurement of 222Rn in water (Figure 6). One of the great advantages of this technique is the small amount of sample required for measurements, with the majority of participants using between 6 and 10 ml. Figure 6. View largeDownload slide Distribution of results by measurements techniques employed. Figure 6. View largeDownload slide Distribution of results by measurements techniques employed. In terms of Zscore, the results of the intercomparison exercise (Figure 3), indicate that all the participants produced a satisfactory measurement of 222Rn in water, even though (Figure 4), laboratories R04 and R17 presented out of range results with respect to the consensus value ± σp. In general, the results of both the 2015 and 2016 exercises, demonstrate the good preparedness of national laboratories for measuring 222Rn in water. As for the suitability of bottle material used in terms of radon leaks, the results clearly show that the most suitable material is glass, which gives a λleaks 10 times less than plastic. REFERENCES 1 Chu , S. Y. F. , Ekström , L. P. and Firestone , R. B. ( 1999 , February). Table of Radioactive Isotopes. 10 August 2015. Available on http://nucleardata.nuclear.lu.se/toi/ 2 Galán López , M. , Martín Sánchez , A. and Gómez Escobar , V. Application of ultra-low level liquid scintillation to the determination of 222Rn in groundwater . J. Radioanal. Nucl. Chem. 261 ( 3 ), 631 – 636 ( 2004 ). Google Scholar CrossRef Search ADS 3 Belloni , P. , Cavaioli , M. , Ingrao , G. and Mancini , C. Optimization and comparison of three different methods for the determination of Rn-222 in water . Sci. Tot. Environ. 173/174 , 61 – 67 ( 1995 ). Google Scholar CrossRef Search ADS 4 Quindós , L. S. , Fernández , P. L. and Soto , J. National Survey on indoor radon in Spain . Environ. Int. 17 , 449 – 453 ( 1991 ). Google Scholar CrossRef Search ADS 5 Quindós , L. S. Un Gas Radiactivo de Origen Natural en su Casa ( Santander, Cantabria : Consejo de Seguridad Nuclear y Universidad de Cantabria ) ( 1995 ) ISBN: 84-87275-59-1. 6 Vázquez , B. F. , Consuegra Ávila , F. M. , Olaya , M. A. and Fernández , C. S. Técnica de ventilación como medida de rehabilitación frente a la inmisión de gas radón en edificios y su repercusión en la eficiencia energética. Comunicación Congreso Latinoamericano REHABEND 2014 del 1 al 4 abril del 2014. Santander, España ( 2014 ). 7 Hess , C. T. , Vietti , M. A. , Lachapelle , E. B. and Guillemette , J. F. Chapter 5—Radon transferred from drinking water into house air. In: Radon, Radium and Uranium in Drinking Water . Richard Cothern , C. and Rebers , P. A. Eds ( Chelsea, MI : Lewis Publishers, Inc ) ( 1990 ) ISBN 0-87371-207-2. 8 National Research Council . National Research Council (US) Committee on Risk Assessment of Exposure to Radon in Drinking Water ( Washington, DC : National Academies Press (US) ) ( 1999 ) 1999. Risk Assessment of Radon in Drinking Water. 9 Sainz , C. , Rábago , D. , Fuente , I. , Celaya , S. and Quindós , L. S. Description of the behavior of an aquifer by using continuous radon monitoring in a thermal spa . Sci. Tot. Environ. 543 , 460 – 466 ( 2016 ). Google Scholar CrossRef Search ADS 10 de España , G. REAL DECRETO 140/2003 del 7 de Febrero por el que se establecen los criterios sanitarios de la calidad del agua de consumo humano. Madrid: BOE 45. 7228–7245 ( 2003 ). 11 Council Directive 2013/51/EURATOM . Council of the European Union . Council Directive 2013/51/EURATOM of 22 October 2013 laying down requirements for the protection of the health of the general public with regard to radioactive substances in water intended for human consumption. Brussels: O.J. EU ( 2013 ). 12 de España , G. , REAL DECRETO 314/2016 del 29 de Julio por el que se establecen los criterios sanitarios de la calidad del agua de consumo humano. Madrid: BOE 183. 53106–53126 ( 2016 ). 13 Soto , J. , Delgado , M. T. , Fernández , P. L. , Gómez y , J. and Quindós , L. S. Niveles de 222Rn en el Balneario Las Caldas de Besaya . Rev. Sanidad Higiene Públ. 65 ( 1 ), 71 – 75 ( 1991 ). 14 Aquakit . Accessory for radon in water measurement in combination with the radon monitor. Alphaguard, User Manual, Genitron Instruments Germany ( 1997 ). 15 Clever , H. L. , u.a. Temperature dependency of the diffusion coefficient ‘k’. In: Solubility Data Series, Krypton, Xenon, Radon Gas Solubilities 2. Pergamon Press, Oxford, pp. 463–468 ( 1985 ). 16 Fuente Merino , I. Puesta a punto de un equipo de fluorescencia de rayos x portátil con fuentes radiactivas: aplicaciones medioambientales. Tesis Doctoral, University of Cantabria ( 2015 ). 17 Neame , K. D. and Homewood , C. A. Introduction to Liquid Scintillation Counting London, England: ( Butterworth & Co ) ( 1974 ) ISBN: 0-408-70637-6. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

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Abstract

Abstract Radon-222, a naturally occurring radioactive gas, responsible together with its progeny of around 50% of the average effective dose received by the population, has not been regulated by law until the recent Directive 2013/51 /Euratom. Its transposition into Spanish legislation was made in the recent RD 314/2016, which sets at limit value of 500 Bq l−1 for radon-222 in water for human consumption. Intercomparison exercises, such as those carried out by IPROMA SL and the Laboratory of Environmental Radioactivity of the Cantabria University (LARUC) in November 2015 and December 2016, represent the most useful tool available for detecting problems and taking corrective actions necessary for an efficient measurement by part of the laboratories. The participants in these exercises used three techniques: liquid scintillation counting, gamma spectrometry and desorption followed by ionisation chamber detection. INTRODUCTION Radon gas has essentially three isotopes: 222Rn (T1/2 = 3.82 d), 220Rn (T1/2 = 55.6 s) and 219Rn (T1/2 = 3.96 s)(1). 222Rn (hereinafter referred as radon in this article) is a natural radioisotope belonging to the series of 238U (an alpha particle emitter, 5590.3 keV, with a half-life of 3.82 days). Its two short-lived alpha emitting progeny are 218Po (6002.55 keV, 3.11 min) and 214Po (7686.90 keV, 163.69 μs)(2). Exposure to radon and its progeny has an estimated average effective dose of around 50% (1.3 mSv) of the total effective dose received by the general population(3), based on results of the numerous studies conducted in dwellings(4–6). However, little attention has been paid to the radon that is ingested in drinking water, and to the additional risk that arises due to the low transferability of radon from water to air, with an estimated transfer coefficient for dwellings of 10−4(7). Although the dose received by ingestion of water with radon is significantly less than by inhalation of its progeny(8), the measurement of radon concentration in water has additional interest in other respects. The radon coming from water contributes very little to the concentration of radon inside dwellings, but it can be significant in certain workplaces such as some thermal spas. In addition, radon gas dissolved in water has proved to be a useful tracer of hydrodynamic processes in aquifers and underground currents(9). Being a noble gas, it is not assimilated by any chemical compounds in the environment, but due to its moderate solubility, (0.225 cm3 g−1 at 20°C), it can be detected in water, especially groundwater(2). Its concentration in groundwater will depend mainly on the radium content of the substrate, the specific surface area of the aquifer, the permeability of the soil and the characteristics of the water itself. When these groundwaters discharge at the surface, the concentration of dissolved radon decreases abruptly due to water movement and purification processes. However, where these waters are consumed directly at the point of upwelling, the risk of ingesting radon and its progeny may be significant. There was no limit for radon in the recently repealed RD 140/2003(10), which concerned drinking water quality. Thus, the range 100–1000 Bq l−1 in the EC Directive 2013/51/Euratom of 22 October 2013 represents the first time that a radon limit for drinking water has been set(11). Transposition of this Directive into Spanish legislation was by means of the recent RD 314/2016(12) which sets a limit value of 500 Bq l−1 for radon in water for human consumption. The intercomparison exercise reported in this paper, organised by the Radon Group of Cantabria University in collaboration with IPROMA S.L., arose from the need for a quality control for all the national laboratories that measure radon in water. Overall, 11 laboratories participated in November 2015, and 17 in December 2016 (Table 1). Table 1. Name and location of the 17 participating laboratories in the intercomparison of December 2016. 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid Table 1. Name and location of the 17 participating laboratories in the intercomparison of December 2016. 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid 1 Iproma Physic-Chemical Laboratory, Castellón 2 University of Extremadura Dept. of Atomic Physics, Badajoz 3 University of Las Palmas de Gran Canaria Dept. of Physics, Las Palmas 4 Laboratory Labaqua Alicante 5 University Politécnica de Valencia Lab. of Environmental Radioactivity, Valencia 6 University Politécnica de Cataluña Lab. of Radioactivity analysis, Barcelona 7 University of Barcelona Environmental Radiology Lab, Barcelona 8 University of Valencia Institute of Corpuscular Physics, Burjassot 9 Environmental and Sanitary Radiochemistry Unit Emergency laboratory and water quality, Tarragona 10 University of Cáceres Lab. of Environmental Radioactivity, Cáceres 11 University of Cantabria Radon Group, Santander 12 University of Bilbao Dept. of Nuclear Engineering and Fluid Mechanics, Bilbao 13 University of Málaga Radioactive Installation, University of Málaga 14 Canal de Isabel II Management Area of Instrumental Analysis, Madrid 15 University of Granada Dept. of Inorganic Chemistry, Radiochemical Laboratory and Environmental Radiology, Granada 16 AGQ Labs & Technological Services Lab. of Environmental Radioactivity, Sevilla 17 Health Institute Carlos III Radioprotection Service, Madrid MATERIALS AND METHODS Sample collection The chosen sampling location was the spa of Las Caldas de Besaya, which has been studied by the Radon Group Cantabria University since the 1980s(9, 13). The geographical location of the site can be seen in Figure 1. Figure 1. View largeDownload slide Location of Caldas Besaya Spa. Figure 1. View largeDownload slide Location of Caldas Besaya Spa. The spa is located on the banks of the river Besaya in the town of Corrales de Buelna, ~30 km from Santander. These hot springs are characterised by temperatures of 34–37°C, and are rich in sodium chloride, bicarbonates and nitrates(9). A single homogenous water sample was collected from the spa on 23 November 2016 in a 25-l container, and taken to the LARUC laboratory where, on the same day, aliquots were transferred to 1-l low density plastic bottles for shipment to the participating laboratories. These bottle have double stoppers, in order to minimise leaks and the formation of bubbles, which could introduce uncertainty into the participants’ measurements. A ‘protocol’ was drawn up by the organisers of the exercise, and sent to each participant. This protocol explained everything necessary for the proper development of the exercise, including implementation a data protection policy that assigned a code to each laboratory to maintain anonymity of results. Participants received the samples 24–36 h after sampling, except for the University of Palmas, where customs clearance took 6 days, though no incident occurred during shipment that would have delayed the exercise. Measurement techniques The measurement techniques used by the participating laboratories in the intercomparison exercise can be seen in Table 2, which also shows the number of participants using each. Three participants sent results using two different techniques, which is why there were twenty results from only 17 laboratories. Table 2. Results sent by participants (‘GS’ = gamma spectrometry HPGe detector, ‘LSC’ = liquid scintillation counting and ‘D’ = desorption with ionisation chamber). Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Table 2. Results sent by participants (‘GS’ = gamma spectrometry HPGe detector, ‘LSC’ = liquid scintillation counting and ‘D’ = desorption with ionisation chamber). Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Laboratory Result (Bq l−1) Uncertainty (Bq l−1) Technique Volume (ml) Time (min) R01 106 8 LSC 10 60 R02 100 14 LSC 10 100 R03 120 20 LSC 10 400 R04 49.7 3.6 LSC 10 200 R05 93 10 LSC 10 30 R06 114.6 5.9 LSC 10 30 R07 110 16 LSC 10 30 R08-1 104 17 GS 1000 3583 R08-2 75.1 17 D 490 30 R09 82.4 8.5 LSC 10 10 R10-1 127.5 8.9 GS 270 120 R10-2 121 18 LSC 6 10 R11-1 125 4 LSC 10 30 R11-2 127 17 GS 100 16.6 R12 125 19 LSC 6 10 R13 117.6 6.6 LSC 5 30 R14 136.63 1.75 LSC 200 100 R15 115.98 5.97 LSC 10 10 R16 113 14 LSC 10 200 R17 76.6 12.2 D 100 100 Desorption technique 222Rn in water with ionisation chamber The equipment used is an AlphaGuard PQ2000-PRO which uses a specific attachment for measuring 222Rn in water. By means of a pump, the water is bubbled continuously, which causes desorption of radon from the water and directs it to the detector via a desiccator column. Once inside the detector, the radon enters an ionisation chamber (where a potential of 750 V is maintained) flowing over a large-surface fibreglass filter that prevents entry of radon progeny and aerosols. Alpha particles emitted by the radon ionise the air, the cathode attracts the positively charged particles, while the anode attracts the negatively charged ones(14). To calculate the concentration of 222Rn in water, the following equation is used: Cwater=Cair⋅(Vsystem−VsampleVsample+k)−C01000 (1) where Cwater is the concentration of 222Rn in the water sample (Bq l−1), Cair is the concentration of 222Rn in air (Bq m−3) on the AlphaGuard screen, C0 is the background (Bq m−3) that can be considered 0, Vsystem is the inside volume of the equipment (1117.58 ml), Vsample is the volume of the sample (100 ml) and ‘k’ is a factor for the transfer of radon from water to the air, which is a function of temperature(15). Gamma spectrometry This technique is for detecting gamma emissions from soil, sludge, ash, environmental filters and, ultimately, from any sample whose gamma emission falls between 30 and 3000 keV. The equipment used is a HPGe detector. The photons resulting from gamma emissions from the sample enter the active volume of the detector and interact with its atoms. These interactions are converted to electrical pulses that are proportional to the energy of the photons emitted, and which are stored in finite energy increments equivalent over the range of the spectrum(16). 222Rn activity is determined three hours after preparing the bottle with the water sample with the count made in the area of the spectrum for 214Pb (351.93 keV). This elapsed time that is necessary to achieve secular equilibrium between radon and its short-lived progeny (218Po, 214Pb, 214Bi and 214Po). To calculate the activity due to 222Rn in the water, the following equation is used for peak 214Pb: A=(N−f⋅t)v⋅t⋅PE⋅Ef (2) where - A is the radon concentration (Bq l−1). - N are the counts. - f is the background in counts per second (cps). - t is the count time (s). - v is the volume of the sample (l). - PE is the emission intensity (%). - Ef is the efficiency (cps/Bq). Liquid scintillation counting with alpha/beta separation The water sample containing 222Rn is mixed with a liquid scintillation solution in a transparent vial. The alpha/beta emissions of 222Rn and its progeny transfer energy to the scintillator, which releases this energy as photons (measured as light pulses). A distinction between alpha and beta emissions is possible since alpha particles lead to slightly longer light pulses than beta particles. The electrical pulses derived from the photon release is proportional to the radioactive energy emitted. The continuous emission of alpha/beta particles from the radioactive material causes a continuous generation of pulses, so that the counts accumulates progressively(17). Equation (3) is used to calculate activity from the 222Rn in water (alpha particles) after 3 h elapsed time explained above: A=G−BEf⋅60⋅V (3) where A is the activity in Bq l−1; G are the counts per minute (cpm); B is the background in cpm what the equipment counts for a sample prepared with distilled water; Ef is the equipment’s efficiency for 222Rn, 218Po and 214Po; V is the sample volume in litres; the inclusion of 60 in the denominator is to transform counts per minutes to counts per second. RESULTS AND DISCUSSION Measurements were returned to the organiser by 14 December. By 22 December, each participant received the report of the exercise showing the results and techniques used by each one (Table 2). Statistical treatment of the measurements by the participants began by discarding data outside of the range median ± 50% as being incorrect data. By this means, the result from laboratory R04 was eliminated. The most relevant statistics were then calculated, as seen in Table 3. Table 3. Statistics of the results, with values expressed in Bq l−1. Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 Table 3. Statistics of the results, with values expressed in Bq l−1. Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 Parameter Value Number of participants (dimensionless) 16 Number of measurements (dimensionless) 19 Average 110 Median 115 Geometric average 109 Minimum 75 Maximum 137 Standard deviation 18 Standard deviation geometric 1.2 A quantile–quantile plot (q–q plot) was applied to test whether results fit a normal distribution, whereby any deviation from linearity as in Figure 2 implies a non-normal distribution. Figure 2. View largeDownload slide Graph q–q plot of the results of the exercise. Dashed line represents the 95% confidence interval. Figure 2. View largeDownload slide Graph q–q plot of the results of the exercise. Dashed line represents the 95% confidence interval. To establish the consensus statistic for the exercise, an iterative algorithm was applied, according to ISO 13528:2015, whereby extreme values are given less weight than in a classical treatment of statistical data. This algorithm considers the measurements, of all participants and repositions the extreme values within the interval of acceptable deviation, thus obtaining robust estimators of the consensus value X and the standard deviation σexercise. As for the objective sigma, σp, was established as 20%, while for uncertainty μx, the following equation is applied: μx=1.25⋅σexerciseN (4) where - σexercise is the standard deviation. - N is the number of results sent by laboratories. The statistics of the exercise are presented in Table 4. Table 4. Parameters of the exercise. Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 Table 4. Parameters of the exercise. Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 Parameter Consensus value, X Rob. standard deviation, σexercise Objective sigma, σp Uncertainty, μx No. of results Radon (Bq l−1) 112.1 15.2 22.4 4.4 19 For the determination of the Zscore of each participant, the following equation is applied: Zscore=x−Xσp (5) where - x is the measurement provided by each participant. - X is the consensus value calculated according to ISO 13 528:2015. - σp is the target standard deviation, set at 20%. The Z score values are interpreted as follows: |Zscore| ≤ 2 indicates satisfactory performance. 2 < |Zscore| ≤ 3 indicates dubious performance and generates an alert. |Zscore| > 3 indicates unsatisfactory performance and generates a signal for action. A graphical presentation of the Z scores obtained by each laboratory is given in Figure 3, where the Z scores are ranked low to high. Figure 3. View largeDownload slide Z score of laboratories participating in the exercise. Figure 3. View largeDownload slide Z score of laboratories participating in the exercise. Figure 4 shows the concentrations reported by each laboratory with their respective uncertainties. The red horizontal lines represent the consensus value (112.1 Bq l−1) and the target standard deviation of ±20% (134.5 and 89.7 Bq l−1). Figure 4. View largeDownload slide Activities measured by each laboratory, showing the consensus value and the target standard deviation of ±20%. Figure 4. View largeDownload slide Activities measured by each laboratory, showing the consensus value and the target standard deviation of ±20%. As a result of the 6-day delay in the delivery of the sample to one of the participants, the organisers decided to do a radon-leak test of the containers used to send samples to the participants. The bottles used in both exercises were made of low density polyethylene (LDPE). The importance of the bottle material is not significant, other than to know its permeability to radon for future exercises with a reference value (in these, exercises were used consensus value). In order to quantify the leakage λleaks of the material, a sample of water containing 222Rn was divided and stored in two bottles made of different materials, one LDPE plastic and another glass. Samples were taken every 3–4 days and analysed in a liquid scintillation alpha spectrometer (Triathler 425-034); the results were plotted to observe the decay in the sample over several days. Each graph was fitted to a function of the type: A=Ao⋅e(λradon+λleaks) (6) where: - A is the final activity of the sample (Bq l−1). - Ao is the initial activity of the sample (Bq l−1). - λradon is the decay constant for radon (h−1). - λleaks is the constant of the permeability of the material to radon (h−1). This graph (Figure 5) indicates a value of λleaks of 3.3×10−3 h−1 for LDPE, compared to 3.5×10−4 h−1 for glass. Figure 5. View largeDownload slide Exponential adjustment of the radon decay of a water sample in LDPE plastic and glass containers. Figure 5. View largeDownload slide Exponential adjustment of the radon decay of a water sample in LDPE plastic and glass containers. CONCLUSIONS More laboratories participated in the performance testing in 2016 than 2015, demonstrating a growing interest in measuring 222Rn in drinking water. The reason is probably the recent transposition of Directive 2013/51/EURATOM into Spanish law (as RD 314/2016 of 29 July), establishing for the first time the legally permissible levels of 222Rn in drinking water. This intercomparison exercise by IPROMA and LaRUC, included 17 national laboratories in 14 provinces, representing 8 of the 17 Autonomous Communities in Spain. The liquid scintillation counting (LSC) technique, is the most widely used technique by Spanish laboratories specialising in the measurement of 222Rn in water (Figure 6). One of the great advantages of this technique is the small amount of sample required for measurements, with the majority of participants using between 6 and 10 ml. Figure 6. View largeDownload slide Distribution of results by measurements techniques employed. Figure 6. View largeDownload slide Distribution of results by measurements techniques employed. In terms of Zscore, the results of the intercomparison exercise (Figure 3), indicate that all the participants produced a satisfactory measurement of 222Rn in water, even though (Figure 4), laboratories R04 and R17 presented out of range results with respect to the consensus value ± σp. In general, the results of both the 2015 and 2016 exercises, demonstrate the good preparedness of national laboratories for measuring 222Rn in water. As for the suitability of bottle material used in terms of radon leaks, the results clearly show that the most suitable material is glass, which gives a λleaks 10 times less than plastic. REFERENCES 1 Chu , S. Y. F. , Ekström , L. P. and Firestone , R. B. ( 1999 , February). Table of Radioactive Isotopes. 10 August 2015. Available on http://nucleardata.nuclear.lu.se/toi/ 2 Galán López , M. , Martín Sánchez , A. and Gómez Escobar , V. Application of ultra-low level liquid scintillation to the determination of 222Rn in groundwater . J. Radioanal. Nucl. Chem. 261 ( 3 ), 631 – 636 ( 2004 ). Google Scholar CrossRef Search ADS 3 Belloni , P. , Cavaioli , M. , Ingrao , G. and Mancini , C. Optimization and comparison of three different methods for the determination of Rn-222 in water . Sci. Tot. Environ. 173/174 , 61 – 67 ( 1995 ). Google Scholar CrossRef Search ADS 4 Quindós , L. S. , Fernández , P. L. and Soto , J. National Survey on indoor radon in Spain . Environ. Int. 17 , 449 – 453 ( 1991 ). Google Scholar CrossRef Search ADS 5 Quindós , L. S. Un Gas Radiactivo de Origen Natural en su Casa ( Santander, Cantabria : Consejo de Seguridad Nuclear y Universidad de Cantabria ) ( 1995 ) ISBN: 84-87275-59-1. 6 Vázquez , B. F. , Consuegra Ávila , F. M. , Olaya , M. A. and Fernández , C. S. Técnica de ventilación como medida de rehabilitación frente a la inmisión de gas radón en edificios y su repercusión en la eficiencia energética. Comunicación Congreso Latinoamericano REHABEND 2014 del 1 al 4 abril del 2014. Santander, España ( 2014 ). 7 Hess , C. T. , Vietti , M. A. , Lachapelle , E. B. and Guillemette , J. F. Chapter 5—Radon transferred from drinking water into house air. In: Radon, Radium and Uranium in Drinking Water . Richard Cothern , C. and Rebers , P. A. Eds ( Chelsea, MI : Lewis Publishers, Inc ) ( 1990 ) ISBN 0-87371-207-2. 8 National Research Council . National Research Council (US) Committee on Risk Assessment of Exposure to Radon in Drinking Water ( Washington, DC : National Academies Press (US) ) ( 1999 ) 1999. Risk Assessment of Radon in Drinking Water. 9 Sainz , C. , Rábago , D. , Fuente , I. , Celaya , S. and Quindós , L. S. Description of the behavior of an aquifer by using continuous radon monitoring in a thermal spa . Sci. Tot. Environ. 543 , 460 – 466 ( 2016 ). Google Scholar CrossRef Search ADS 10 de España , G. REAL DECRETO 140/2003 del 7 de Febrero por el que se establecen los criterios sanitarios de la calidad del agua de consumo humano. Madrid: BOE 45. 7228–7245 ( 2003 ). 11 Council Directive 2013/51/EURATOM . Council of the European Union . Council Directive 2013/51/EURATOM of 22 October 2013 laying down requirements for the protection of the health of the general public with regard to radioactive substances in water intended for human consumption. Brussels: O.J. EU ( 2013 ). 12 de España , G. , REAL DECRETO 314/2016 del 29 de Julio por el que se establecen los criterios sanitarios de la calidad del agua de consumo humano. Madrid: BOE 183. 53106–53126 ( 2016 ). 13 Soto , J. , Delgado , M. T. , Fernández , P. L. , Gómez y , J. and Quindós , L. S. Niveles de 222Rn en el Balneario Las Caldas de Besaya . Rev. Sanidad Higiene Públ. 65 ( 1 ), 71 – 75 ( 1991 ). 14 Aquakit . Accessory for radon in water measurement in combination with the radon monitor. Alphaguard, User Manual, Genitron Instruments Germany ( 1997 ). 15 Clever , H. L. , u.a. Temperature dependency of the diffusion coefficient ‘k’. In: Solubility Data Series, Krypton, Xenon, Radon Gas Solubilities 2. Pergamon Press, Oxford, pp. 463–468 ( 1985 ). 16 Fuente Merino , I. Puesta a punto de un equipo de fluorescencia de rayos x portátil con fuentes radiactivas: aplicaciones medioambientales. Tesis Doctoral, University of Cantabria ( 2015 ). 17 Neame , K. D. and Homewood , C. A. Introduction to Liquid Scintillation Counting London, England: ( Butterworth & Co ) ( 1974 ) ISBN: 0-408-70637-6. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Published: Mar 9, 2018

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