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Procedia Earth and Planetary Science 17 ( 2017 ) 229 – 232

15th Water-Rock Interaction International Symposium, WRI-15 Radon in Icelandic Cold Groundwater and Low-Temperature Geothermal Water

Finnbogi Óskarssona,1, Ragnheiður St. Ásgeirsdóttira

aIceland GeoSurvey (ISOR), Grensasvegur 9, IS-108 Reykjavik, Iceland

Abstract

Samples of hot and cold water for radon analysis were collected from boreholes and springs in Iceland in 2014 and 2015. The majority of the samples was collected from municipal district heating services or potable water utilities. The total number of samples was 142, covering most towns and villages in Iceland. Radon activity is generally rather low, in most cases less than 5 Bq/L. Only 12 samples had a measured radon activity higher than 5 Bq/L, with a maximum activity of 10.8 Bq/L. The hot water samples generally have a higher radon activity than cold water samples, but samples from boiling borholes have a lower radon activity as radon fractionates into the vapour phase. The geographical distribution of the samples indicates that radon activity is generally lower within the active rift zones. This is most likely due to the very low uranium content of the tholeiites typically erupted within the rift zones. Higher radon values (> 5 Bq/L) are in most cases close to extinct central volcanoes and thus it seems plausible that the water sampled has been in contact with felsic rocks. © 20172017 The The Authors. Authors. Published Published by Elsevierby Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license Peer(http://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of the organizing). committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Radon; groundwater; low-temperature water; geothermal.

1. Introduction

Radon (Rn) is a radioactive gas that is formed by radioactive decay of radium (Ra). Both elements are parts of the natural decay chains of uranium (U) and thorium (Th), which ultimately lead to the formation of various isotopes of lead (Pb). Out of the 39 known isotopes of radon, only four have half-lives longer than 1 hour. The most stable radon isotope is 222Rn which has a half-life of 3.82 days. Radon is primarily formed in the earth’s crust, and is partly carried to the surface either as a gas or dissolved in water.

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1878-5220 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi: 10.1016/j.proeps.2016.12.078 230 Finnbogi Óskarsson and Ragnheiður St. Ásgeirsdóttir / Procedia Earth and Planetary Science 17 ( 2017 ) 229 – 232

As the concentrations of uranium and thorium are known to be low in the young primarily basaltic Icelandic bedrock, radon has not been considered a major health concern and therefore no systematic survey of radon in water has been performed in Iceland. However, in 2012 and 2013 the Icelandic Radiation Protection Agency conducted a survey of radon in residential air1. Their results showed low concentrations, with an annual average value of 13 Bq/m3 but slightly higher in Northern Iceland; 20 Bq/m3 on average. Other previous studies of radon in Icelandic water have mainly been focused on its use as an earthquake precursor. As reported by2 on the monitoring of radon concentrations in seven low-temperature wells in the South Iceland Seismic Zone and two low-temperature wells in the Tjörnes Fracture Zone, both of which are considered regions of high seismic risk. As reported by3 on the continued monitoring of the same seven wells in South Iceland, from 1977 to 1993. Several authors4 correlated changes in radon activity in some of the same wells in South Iceland with two large earthquakes in June 2000. In 2014 and 2015 Iceland GeoSurvey conducted a survey of radon in cold groundwater and low-temperature geothermal water in Iceland. This work was performed for the Centre of Public Health Sciences at the University of Iceland, where researchers were interested in estimating the radon exposure from potable water and district heating services. For this purpose, 142 samples of hot and cold water were collected and the activity of radon of the samples determined. The sampling campaign was focused on low-temperature water which is used by district heating services and potable water which is used by municipalities but to increase the coverage of the survey, samples were also collected from thermal and groundwater boreholes and springs which serve no or very few consumers.

2. Methods

Water samples were collected into clean, air-tight 250 mL screw-cap glass bottles which were rinsed three times with the sample before collection. The samples were run to the bottom of the bottle using a silicone rubber hose and the bottle filled from the bottom up to prevent aeration of the sample. Moreover, at least 250 mL of water were allowed to flow out of the bottle before the hose was removed and the cap tightened. Borehole samples were in most cases collected directly at the wellhead, but some samples were collected from the water pipeline from the well. Spring samples were collected from a depth of about 10 cm (wherever possible) using a peristaltic pump. Samples hotter than 30°C were collected through a stainless steel cooling coil and cooled that way to less than 30°C in order to prevent bubble formation upon cooling. Samples of hot water were generally collected in duplicate. The samples were analysed within 24 hours of collection using a Durridge RAD7 electronic radon detector with the H2O RAD accessory for analysis of radon in water. During the analysis, air was bubbled through the water sample in a closed loop in order to release the dissolved gas from the sample, followed by α-decay counting for 4 consecutive periods of 20 minutes each. Therefore, the values reported are the averages of 8 or 4 measurements, depending on whether samples were collected in duplicate or not. The relative standard deviation is on average about 30%, and obviously highest for the samples with lowest radon activity. Two RAD7 instruments were employed and duplicate samples were analysed using both instruments in order to detect any systematic error or bias. The values reported from the two instruments were usually within 5% of each other with no detectable bias.

3. Results

The total number of samples collected for this survey was 142; 75 samples of hot (>30°C) water and 67 samples of cold water. The sample temperatures ranged from 1.7 to 145°C. The radon activity results are reported as bequerel per litre of sample (Bq/L), i.e. dissociations per second and litre. The results are presented in Figure 1, where the blue dots denote cold water samples with radon activities ranging from <1 Bq/L (lightest blue) to >10 Bq/L (darkest blue), and the yellow and red dots denote hot water samples likewise ranging from <1 Bq/L (yellow) to >10 Bq/L (dark red). The highest radon activity measured was 10.8 Bq/L in a hot water well in Drangsnes, NW-Iceland, but two other samples had activities higher than 10 Bq/L; samples from a hot water well in Hrafnagil, N-Iceland (10.1 Bq/L), and a cold water spring in Svínadalur, W-Iceland (10.6 Bq/L).

Finnbogi Óskarsson and Ragnheiður St. Ásgeirsdóttir / Procedia Earth and Planetary Science 17 ( 2017 ) 229 – 232 231

Figure 1. Sample locations and measured radon activities. The colours of the dots indicate radon activity. The volcanic zones are shaded.

From Figure 1, it is apparent that most of the samples (~90%) have radon activity less than 5 Bq/L, and that the radon activity of the cold water samples is in general lower than that of the hot water samples. The cold water samples had a median radon activity of 0.71 Bq/L and an average activity of 1.58 Bq/L, whereas the median activity for the hot samples was 1.53 Bq/L and the average 2.39 Bq/L. Six hot water samples had radon activities between 5 and 10 Bq/L; one in N-Iceland (Barð í Fljótum), three in NW-Iceland (Húnavellir, Hrútafjörður and Borðeyri) and two within the South Iceland Seismic Zone (Kaldárholt and Flúðir). Only three cold water samples had radon activity between 5 and 10 Bq/L; in Eyrarsveit and Borgarfjörður in W-Iceland and in Þingeyri, NW-Iceland. Sixteen samples had radon activities lower than 0.1 Bq/L, in most cases either cold water or water from boiling wells (>100°C). As mentioned above, radon activity in water in Iceland has not been studied extensively, but the current results compare rather well with previous findings2-4; the radon values measured in this study are in most cases close to average values for the existing time series2-4 for the same wells, and in all cases within their ranges.

4. Discussion

The geographic distribution of the results indicates that the radon activity is generally lower in the youngest parts of the country; all the elevated radon activities (> 5 Bq/L) are found outside the active rift zones, mainly in the western and northwestern parts of the country. This is further illustrated in Figure 2A which shows the number of samples in each radon activity class relative to the age of the local bedrock, as presented by5. More than half of the samples collected in the active rift zones (age < 0.8 Ma) have radon activity < 1 Bq/L wheras the majority of samples collected outside the active rift zones have radon activity > 1 Bq/L. The geographic distribution of radon activity is most convincingly explained by the fact that the tholeitic basalts of the rift zones generally have very low uranium concentrations (0.01-0.54 mg/kg)6, whereas more alkaline basalts outside the rift zones have somewhat higher uranium concentrations (0.23-1.2 mg/kg)7 and felsic rocks e.g. in active or extinct central volcanoes have yet higher uranium concentrations [(1.4-4.0 mg/kg)8, (2.1-3.0 mg/kg)9]. And indeed the locations of the elevated radon activities are in general close to the mapped calderas of extinct central volcanoes5.

232 Finnbogi Óskarsson and Ragnheiður St. Ásgeirsdóttir / Procedia Earth and Planetary Science 17 ( 2017 ) 229 – 232

Figure 2. (A) Distribution of radon activity relative to the age of the local bedrock. (B) Measured radon activity against sample temperature.

The relationship between sample temperature and radon activity is shown in Figure 2B. It is apparent from the figure that the very highest radon values are found at temperatures ranging from 2.6 to 97°C, but there appears to be a slight trend towards lower radon activity with increasing temperature. Most samples with temperatures higher than 100°C have radon activity lower than 2 Bq/L and the radon activity of the hottest sample (145°C) is 0.03±0.03 Bq/L. This is due to the favoured fractionation of radon and other gases into the vapour phase upon boiling. The Icelandic regulation on potable water does not set a maximum admissible content for radon, but Recommendations No. 2001/928 of the Commission of the European Communities suggest setting an action limit of 100 Bq/L for commercial or public water services – an order of magnitude higher than the highest values detected in this study.

Acknowledgements

The authors wish to thank Professor Vilhjálmur Rafnsson at the University of Iceland Centre of Public Health Sciences for suggesting that this survey be undertaken and lending of the instruments, as well as the Icelandic Centre for Research for providing funding for RSÁ in Summer 2015. We also gratefully acknowledge the district heating services, community water services, power companies and other owners of boreholes and sample locations. Finally, we thank our colleagues Ester Eyjólfsdóttir, Vaiva Čypaite and Magnús Ólafsson, all at Iceland GeoSurvey, for assistance with sample collection and analysis.

References

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