- Email: [email protected]
Natural radiation and geochemical data for rocks and soils, in the North International Douro Cliffs (NE Portugal)
Journal of Geochemical Exploration 130 (2013) 60–64
Contents lists available at SciVerse ScienceDirect
Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp
Natural radiation and geochemical data for rocks and soils, in the North International Douro Cliffs (NE Portugal) M.E.P. Gomes a,⁎, L.M.O. Martins a, L.J.P.F. Neves b, A.J.C.S. Pereira b a b
Center for Geosciences, Department of Geology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal IMAR-CMA, Department of Earth Sciences, University of Coimbra, 3000-272 Coimbra, Portugal
a r t i c l e
i n f o
Article history: Received 19 November 2012 Accepted 18 March 2013 Available online 26 March 2013 Keywords: Lithology Natural radiation Radon Thoron Health
a b s t r a c t A cross-border multidisciplinary project has been developed in the region of Bemposta (cliffs of the Douro River) for the definition of walking trails, contributing to the promotion of health. The proposed routes traverse different lithologies of the region, visiting the interior of some “bodegas” (wineries in caves). The geochemical and radiometric characterization of lithologies revealed values of K, U and Th higher in granites and migmatites than the average crustal rocks and lower in sediments, which is in good agreement with results from a few hundred absorbed dose measurements. Soil gas radon and thoron concentrations, measured at a depth of 80 cm, also correlate well with geochemical information and indicate a low to moderate risk in the region. A preliminary assessment of the maximum contribution of the external radiation dose to the annual effective dose for an exposure scenario of 4 h/day shows an estimate of 0.19 mSv/year in the interior of the “bodegas” and 0.04 up to 0.19 mSv/year in trails according to the different lithologies, these values being reduced to 0.006 and 0.01 up to 0.006 mSv/year for a more likely scenario of exposure of 4 h/month. Exposure to radon gas is estimated not to exceed inside the “bodegas” 0.36 mSv/year for the latter scenario. © 2013 Elsevier B.V. All rights reserved.
1. Introduction There exists an innate human tendency to be drawn to the natural world and, in this context, the physical and emotional health benefits of a connection to nature have been well documented (Maller et al., 2009; Thompson et al., 2011). The region of Bemposta is located in the Natural Park of Douro International, particularly in cliffs of Douro International, implanted in the Mirandês Plateaux. This is a unique region with a highly attractive landscape by the geological, faunistic, floristic and heritage features, which can sustain the provision of nature tourism and outdoor activities, namely trekking. The rocks have different concentrations of radioactive isotopes (K, U and Th), the most important from the point of view of exposure to ionizing radiation. The distribution of those isotopes thus determines the external gamma radiation produced by rocks and also soils. There is still a current perception that natural radioactivity only affects people who live in the vicinity of uranium mines. However, all rocks contain uranium, some more than others, and depending on the concentration of this element and the subsequent contribution of certain geological factors, they also influence the presence of radon in the air. The emanation of radon gas is specifically dependent on lithology and geological structures. Thus, basement geochemistry influences the ⁎ Corresponding author. E-mail address: [email protected] (M.E.P. Gomes). 0375-6742/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2013.03.001
spatial distribution of radon levels at the soil/atmosphere interface (Buttafuoco et al., 2010). In the general environment, the natural radon isotopes contribute with more than 50% to the radiation dose received by individuals from natural radiation sources, and they have been identified as the second leading cause of lung cancer after tobacco smoking (UNSCEAR, 2000; WHO, 2009). Comparing the results of gamma dose rates in air inferred from the concentrations of radionuclides in rocks and soils obtained in Portugal with values of other European countries, according to the reports of UNSCEAR (2000, 2008), we can conclude that Portugal has a higher average dose rate than all other European countries studied (86 nGy/h). Granitoids exist in a significant portion of the Portuguese territory and have a tendency to incorporate higher amounts of uranium and thorium, compared to the crustal average. The distribution of these elements determines the gamma dose rate in air produced by rocks and soils, which explains the high average values found in Portugal. Border multidisciplinary projects have been developed in the region of the Douro cliffs for the definition of tracks and take full advantage of the natural patrimony, undoubtedly contributing to health promotion. The walking trails studied cross several lithologies, visiting the interior of some “bodegas” located in Fermoselle (Spain) and Urrós (Portugal). The objective of this work is the geochemical and radiometric characterization of the outcropping rocks and carries out a preliminary assessment of environmental risk associated with natural radioactivity, showing which routes are subjected to less exposure.
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
61
2. Geological setting
3. Materials and methods
The Bemposta region (Northeast of Portugal) is a part of the northwestern edge of the Variscan dome of Tormes that extends to Spain (Fig. 1). The dome of Tormes is a granitic batholith located in the inner area of the Iberian Massif characterized by the development of a plutono-metamorphic dome of complex geometry and the most significant feature of it is the presence of migmatites associated with anatectic granites. Cambrian and Ordovician metasedimentary country rocks from the North Douro International cliffs are locally migmatized and intruded by diverse granitic rocks, and the pre-Mesozoic rocks are locally covered by Tertiary sedimentary deposits. The granites are syn-tectonic with an elongated massive shape and are consistent with the NW–SE Variscan structures, revealing geometry and kinematics constrained by the Bemposta-Carviçais sinistral shear zone that strike ENE–WSW. Tonalite, three muscovite–biotite granites and muscovite granite are the granitoid facies recognized in the area. The tonalite outcrop in lenticules and the rock is dark, with a fine- to medium-grain, and exhibits strong foliation. The Bemposta granite is a medium-grained, muscovite– biotite and porphyritic, with phenocrysts of potassium feldspar generally less than 2 × 0.7 cm and strongly deformed. The Peredo de Bemposta granite is a medium-grained, muscovite–biotite, slightly porphyritic with phenocrysts below 1.5 × 5 cm. The Assumada granite is a fine- to medium-grained muscovite rock. The Urrós granite is a medium- to coarse-grained muscovite–biotite, and slightly porphyritic with phenocrysts of K-feldspar. Part of the granites and metasediments is covered by Cenozoic sedimentary deposits that extend across the Mirandês Plateaux and are well expressed in the vicinity of Sendim. The walking trails at the Douro cliffs are excellent for the practice of outdoor activities and consequently for the development of geotourism, crossing different lithologies and a large diversity of landscapes and forms. Some walking trails include visits to caves (“bodegas”) in the Urrós and Fermoselle villages.
Representative samples of all granites and metasedimentary rocks were collected and analyzed at the University of Bristol (UK). Major elements were determined by X-ray fluorescence with a detection limit of 0.01% and an accuracy better than ± 1%. U and Th were determined by ICP-MS with detection limits of 0.01 and 10 mg/kg and an accuracy of 5%. Minerals were analyzed in polished thin sections using a Jeol JXA-8500F electron-microprobe at the National Laboratory of Energy and Geology (LNEG) (Porto, Portugal). A radiometric survey was conducted in several lithologies of the region using the portable gamma-ray spectrometer GF Instruments — Gamma Surveyor compact 2, equipped with a NaI detector, able to measure the gamma dose rate in air (nGy/h) and estimate in situ the concentrations of K, U and Th of the rocks. Several profiles were performed to characterize each lithology. In selected locations (n =284) the external radiation was measured with the spectrometer placed at 1 m above the rock or soil, and concentrations of radiogenic elements K, U and Th were estimated in outcrops with a large amount of rock exposed. After a preliminary evaluation of the regional radiometric background, the determination of the soil-gas concentrations of radon and thoron was carried out in 37 representative places of the dominant lithologies by alpha emanometry, using a Scintrex RD200 equipment, according to the protocol described in Pereira et al. (1998). Sampling depth was 0.8 m and radon/thoron was discriminated following three consecutive one minute measurements. 4. Results and discussion 4.1. Geology and geochemistry of rocks and minerals Rocks such as granites contain more uranium than other rock types and consequently should give rise to higher radon levels at
Fig. 1. Simplified geological map in the region of Bemposta and location on the maps of Iberian and Portugal. Legend: 1. Paleozoic metasediments, 2. Migmatites, 3. Tonalite, 4. Bemposta granite, 5. Peredo de Bemposta granite, 6. Assumada granite, 7. Urrós granite, 8. Aplite–pegmatite veins, 9. Quartz veins and 10. Sedimentary deposits of the Cenozoic age.
62
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
the surface, namely in dwellings. However some granites contain more uranium than others and this element may be present in different minerals, some of which may not release radon during decay (Ball et al., 1991; Martins et al., 2013). Sakoda et al. (2008) showed that the radon exhalation rates and emanation fractions of the Misasa granite were much higher than those of the Badgastein granite, regardless of the 226Ra activity concentrations. The geological units from the Douro cliffs have been evaluated for their radioactive background with gamma-ray portable spectrometers, which allowed to recognize important differences in their uranium contents. Some radiometric anomalies that occur are mainly associated with geological faults and also in contacts and veins; significant differences in the radionuclide contents of some of the granites were recognized, namely regarding U and Th (Table 1). Data from whole rock geochemistry and measurements performed with the gamma-ray spectrometer allowed to register very close values of potassium, uranium and thorium in granites and migmatites (Table 1), in almost all the cases higher than the crustal abundance (ca. K — 3%; U — 3 mg/kg; and Th — 11 mg/kg). The sediments, however, have lower values than the crustal average. The tonalite and the granite of Bemposta contain the highest levels of Th and the Assumada muscovite granite contains the highest levels of U. The uranium concentrations were similar to those found in similar lithologies elsewhere in the north and center of Portugal (Gomes et al., 2010; Martins et al., 2013; Pereira et al., 2003). The rate of release of radon from rocks and soils (exhalation) is controlled not only by the uranium concentration but also by the way that this element is distributed in minerals (Appleton, 2005). Its presence in the structure of heavy accessory minerals reduces radon exhalation, while the presence of uranium associated with secondary oxi-hydroxides, mineral borders or weathered surfaces potentiates radon release from the rock (Gomes et al., 2010; Martins et al., 2013; Pereira et al., 2003, 2012). The observation of the accessory minerals of the granites and migmatites under a petrographic microscope as well as the observation with backscattered electrons images, allowed to identify apatite, zircon, ilmenite and monazite as common accessory minerals in all lithologies (Fig. 2a, b). Bemposta granite has major amounts of monazite rich in Th (Figs. 2a, b; 3a) while xenotime (Fig. 2c), uraninite (Figs. 2d, 3b), thorianite (Fig. 3c) and uranophane (Fig. 3d) were only detected in the Assumada granite, that is the most evolved granite and also has sulfides (pyrite, arsenopyrite, and sphalerite). In general the zircons of the Assumada granite have high U metamitic zircons (cyrtolite, Fig. 2c). We can conclude that uranium and thorium are likely related with the referred accessory minerals, which would result in principle in a lower radon exhalation rate. Th contents are relatively high in the Bemposta granite. This is probably due to the presence of larger amounts of monazite in this granite, concentrating high quantities of Th. However, due to the short half life (55 s) of the radon isotope produced in the decay chain of 232Th, known as thoron ( 220Rn), it is not usually considered
a health hazard due to its limited mobility in geological systems. The presence of uraninite in the muscovite granite from Assumada, a mineral readily leached in contact with meteoric water, presents the potential to increase uranium mobility and its deposition as a secondary mineral such as uranophane (possibly in fracture fillings) subsequently raises radon exhalation. According to its higher content of radionuclides, the Assumada granite also shows the highest average gamma dose rate between all studied lithologies (Table 2), with the lowest value being observed, as expected, in sediments. The soil gas concentrations of radon and thoron show a very good correlation with the geochemistry of rocks (Table 3), confirming that radon and thoron are higher in the Bemposta granite and much lower in the sediments. It also confirms that the radiogenic potential of migmatites is higher relatively to thoron when compared to the Urrós granite. The values given in Table 3 can be considered low to normal, except for thoron in the Bemposta granite, which is relatively high. The rather low soil-gas radon concentration is in agreement with the low exhalation rate expected from the study of accessory minerals and this observation may have an impact on the evaluation of the geogenic indoor radon risk in these rocks. 4.2. Exposure to the ionizing radiation from rocks and soils (external radiation) The sediments, as expected, have much lower radiometric background (39 nGy/h) relatively to the migmatites and granitic rocks of the region (Table 2). The Bemposta granite and the muscovite granite have the highest dose rate values (156 and 174 nGy/h, respectively). The highest values were, however, recorded inside the “bodegas” (183 nGy/h), with an estimated contribution of external radiation to the annual effective dose of 0.19 mSv/year, on the basis of an exposure of 4 h/day. This scenario is unlikely to occur in practice and should be considered as an upper limit for the contribution of the external radiation to the global exposure to ionizing radiation in the region. A more realistic scenario can be taken into account, for example considering a 4 h/month exposure inside the “bodegas”, or a monthly jogging of 4 h on the trails. Under this new scenarios, the estimated contribution decreases down to a maximum of 0.06 mSv/year (Table 2), which is negligible from a radiological point of view. The main risk in the “bodegas” is likely related with exposure to radon. No direct measurements of indoor air were carried out; however the results obtained for soil-gas radon concentrations in the region suggest a low to moderate risk on the basis of international classification systems (Åkerblom, 1994; Barnet et al., 2008). Moreover, the uranium content of the rock is only twice the typical value for building materials, and the exhalation rate of radon, as previously discussed, might be low. Assuming a ventilation rate ten times less than the average, the radon concentration is likely not to exceed 1000 Bq/m 3, the maximum allowed for buildings with public access in the new European directive project. The dose for visitors would be no higher
Table 1 Contents of K2O, U and Th in different lithologies of the studied region obtained by gamma spectrometry in situ and (2) number of analyzed samples and its values obtained by XRF (K2O) and ICP-MS (U and Th) for the diverse lithologies of the Bemposta region. Lithologies
Migmatite Tonalite Bemposta granite Peredo de Bemposta granite Urrós granite Assumada granite Sediments Inside “bodegas”
n
27 (2) 4 (3) 31 (12) 27 (5) 38 (6) 12 (4) 7 15
K2O (wt.%)
U (mg/kg)
Th (mg/kg)
Min–max
M
Min–max
M
Min–max
M
2.3–5.7 (4.4–4.9) 4.4–5.1 (3.0–4.4) 3.5–8.3 (4.6–7.3) 3.6–7.3 (5.6–6.0) 2.1–7.8 (5.1–6.5) 3.8–5.9 (4.2–5.1) 0.7–2.8 3.3–7.8
4.4 (4.7) 4.7 (3.9) 5.5 (5.4) 5.8 (5.8) 5.5 (5.9) 5.1 (5.0) 2.2 5.4
1–8 (4) 3–7 (3–10) 3–15 (4–22) 3–9 (7–18) 1–12 (4–8) 5–32 (5–54) 1–3 4–14
5 (4) 4 (6) 8 (11) 5 (11) 5 (6) 17 (5) 2 8
17–44 (18–19) 30–35 (25–31) 23–70 (7–51) 6–52 (15–24) 2–47 (6–13) 6–17 (4–13) 2–15 17–36
24 (19) 33 (27) 40 (29) 19 (20) 17 (9) 11 (9) 10 25
n — number of measurements; M — mean; min — minimum; and max — maximum.
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
63
Fig. 2. Backscattered electron images of accessory minerals: a) biotite (Bt) with apatite inclusion (Ap), which has zircon (Zrn) and monazite (Mnz) inclusions in the Bemposta granite; b) ilmenite (Ilm) with monazite and zircon inclusions in the Bemposta granite; c) Rich U metamitic zircon and xenotime (Xtm) inclusions in apatite in the Assumada granite and c) uraninite (Urn) inclusions in apatite that is an inclusion of muscovite (Ms) in the Assumada granite.
than 0.36 mSv/year, assuming an exposure of 4 h/month, and this is well below the limit usually considered of 1 mSv/year. Taking into consideration that these spaces are not used on a permanent basis, a scenario of higher occupancy is not considered here. Thus, the dose
due to radon exposure in the “bodegas” is not likely to be significant for the global annual exposure of the members of the public. In spite of this conclusion, a suggestion was made to the owners to increase the natural ventilation of the “bodegas” as a strategy for
Fig. 3. a) X-ray spectrum of monazite from the Bemposta granite and b) X-ray spectrum uraninite from the Assumada granite, c) X-ray spectrum of thorianite from Assumada granite, and d) X-ray spectrum of uranophane from the Assumada granite.
64
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
Table 2 Measurements of external radiation dose (DRE) and this contribution to the annual effective dose (CAED) in different exposition scenarios. The contribution of cosmic radiation is included. Lithologies
Migmatite Tonalite Bemposta granite Peredo de Bemposta granite Urrós granite Assumada granite Sediments Inside “bodegas”
n
21 6 22 27 26 11 6 12
DRE (nGY/h)
CAED (mSv/a)
Min–max
Mean
4 h/day
4 h/month
68–151 121–139 105–315 81–192 58–174 92–220 36–41 143–226
125 130 156 117 120 174 39 183
0.127 0.132 0.159 0.119 0.122 0.177 0.039 0.187
0.004 0.004 0.005 0.004 0.004 0.006 0.001 0.006
n — number of measurements; min — minimum; max — maximum; DRE — external radiation dose; CAED — contribution to the annual effective dose; (nGy/h) — nanograys per hour; and (mSv/a) — milisieverts per year.
radon reduction in these spaces (Gunn et al., 1991). In the past all “bodegas” at Fermoselle were linked by open spaces, with increased natural ventilation, but at the moment they are closed. 5. Conclusions The potassium, uranium and thorium amounts of migmatitic and granitic rocks in the Bemposta region are generally higher than the average crustal abundance, while sediments are lower. Measurements performed in several lithologies showed that granites generally have higher K, U and Th and also soil-gas concentrations of radon and thoron than migmatites, and all of these are higher than sediments. The tonalite and the Bemposta granite contain high levels of Th (>30 mg/kg) and the Assumada muscovite granite contains high levels of U (17 mg/kg, in average). The contribution of gamma dose rate in air to the annual effective dose for the different lithologies of the Douro cliffs, on a basis of 4 h/day contact time, reaches inside the “bodegas” 0.19 mSv/year and outside, in trails, 0.04 mSv/year in sediments and up to 0.18 mSv/year in Assumada granite. An appropriated scenario for visiting members of the public, with 4 h/month of exposure time (inside the “bodegas” or jogging on the trails), results in a maximum contribution of 0.006 mSv/year, which can be considered negligible from the radiological point of view. The dose component due to exposure to radon was not directly assessed in the “bodegas”. However, taking into account the contents of U in the rocks and the mineralogical distribution of this element, mostly related with accessory minerals with reduced exhalation, as well as the soil-gas radon measurements carried out at a depth of 80 cm, with average concentrations under 50 kBq · m−3 (a limit usually considered by international classification schemes as presenting a high risk), we can conclude also that exposure to radon is not critical in these spaces. A contribution of 0.36 mSv/year for the global dose of ionizing radiation is likely not to be exceeded for members of the public on the basis of an exposure of 4 h/month, well under the limit usually considered (1 mSv/year). Acknowledgments We are grateful to Prof. M.R. Machado Leite and Eng. Fernanda Guimarães for the use of the electron-microprobe at LNEG (Porto, Portugal). Thanks are due to the project Petrochron - PTDC/CTE-GIX/
Table 3 Soil-gas concentrations of radon and thoron (kBq · m−3) in the dominant lithologies of the region of Bemposta. Lithologies
Urrós granite
Bemposta granite
Migmatites
Sediments
n
17 34 17
10 49 153
3 10 29
7 4 11
222
Rn (radon) 220 Rn (thoron)
n — number of measurements.
112561/2009 for geochemical data and Programme Interreg IIIA Portugal–Espanha that supported the project Fluvial–Nuevas Ciudades Fluviales del S. XXI and the publication of twenty-three walking trails in the Douro International cliffs. The anonymous reviewers are acknowledged for their critical reviews that benefited the text. This research work was carried out in the program of the Geosciences Centre and IMAR-CMA, Coimbra University. References Åkerblom, G., 1994. Ground radon monitoring procedures in Sweden. Geoscientist 4, 21–27. Appleton, J.D., 2005. Radon in air and water. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, R., Smedley, P. (Eds.), Essentials of Medical Geology Impacts of the Natural Environment on the Public Health. Elsevier Academic Press, Amsterdam, pp. 227–257. Ball, T.K., Cameron, D.G., Colman, T.B., Roberts, P.D., 1991. Behaviour of radon in the geological environment: a review. Quarterly Journal of Engineering Geology & Hydrogeology 24, 169–182. Barnet, I., Pacherová, P., Neznal, M., Neznal, M., 2008. Radon in Geological Environment — Czech Experience. Czech Geological Survey Special Papers 19 (Prague, 70 pp.). Buttafuoco, G., Tallarico, A., Falcone, G., Gagliardi, I., 2010. A geostatistical approach for mapping and uncertainty assessment of geogenic radon gas in soil in an area of southern Italy. Environmental Earth Sciences 61 (3), 491–505. Gomes, M.E.P., Neves, L.J.P.F., Coelho, F., Carvalho, A., Sousa, M., Pereira, A.J.S.C., 2010. Geochemistry of granites and metasediments of the urban area of Vila Real (northern Portugal) and correlative radon risk. Environmental Earth Sciences 64 (2), 497–502. Gunn, J., Fletcher, S., Prime, D., 1991. Research on radon in British limestone caves and mines, 1970–1990. Cave and Karst Science 18, 63–66. Maller, C., Townsend, M., St Leger, L., Henderson, Wilson, Pryor, A., Prosser, L., Moore, M., 2009. Healthy Parks, Healthy People: The Health Benefits of Contact With Nature in a Park Context. 26 (2), 51–88 (www.georgewright.org/262maller.pdf Access 02/ 04/2012). Martins, L., Gomes, M.E.P., Neves, L.J.P.F., Pereira, A., 2013. Geochemistry of granites and metasediments of the region of Amarante (northern Portugal) and associate radon risk. Environmental Earth Sciences 68 (3), 733–740. Pereira, A.J.S.C., Neves, L.J.P.F., Godinho, M.M., Soares, A.F., Marques, J.F., 1998. Distribution of radon in soil in the region of Coimbra. Comunicações Instituto Geológico e Mineiro 84 (2), E110–E113. Pereira, A.J.S.C., Neves, L.J.P.F., Godinho, M.M., Dias, J.M.M., 2003. Natural radioactivity in Portugal: influencing geological factors and implications for land use planning. Radioprotecção 2 (2–3), 109–120. Pereira, D., Neves, L., Pereira, A., Peinado, M., Blanco, J.A., Tejado, J.J., 2012. A radiological study of some ornamental stones: the bluish granites from Extremadura (Spain). Natural Hazards and Earth System Sciences 12, 395–401. Sakoda, A., Hanamoto, K., Ishimori, Y., Nagamatsu, T., Yamaokaa, K., 2008. Radioactivity and radon emanation fraction of the granites sampled at Misasa and Badgastein. Applied Radiation and Isotopes 66, 648–652. Thompson, C.J., Boddy, K., Stein, K., Whear, R., Depledge, M.H., 2011. Does participating in physical activity in outdoor natural environments have a greater effect on physical and mental wellbeing than physical activity indoors? A systematic review. Environmental Science & Technology 45 (5), 1761–1772. UNSCEAR, 2000. Sources and Effects of Ionizing Radiation. United Nations, New York. UNSCEAR, 2008. Sources and effects of ionizing radiation, United Nations, New York. http://www.unscear.org/unscear/en/publications.html (Access 02/04/2012). WHO, 2009. WHO Handbook on Indoor Radon: A Public Health Perspective. World Health Organization, WHO Press, Geneva, Switzerland.
Contents lists available at SciVerse ScienceDirect
Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp
Natural radiation and geochemical data for rocks and soils, in the North International Douro Cliffs (NE Portugal) M.E.P. Gomes a,⁎, L.M.O. Martins a, L.J.P.F. Neves b, A.J.C.S. Pereira b a b
Center for Geosciences, Department of Geology, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, Portugal IMAR-CMA, Department of Earth Sciences, University of Coimbra, 3000-272 Coimbra, Portugal
a r t i c l e
i n f o
Article history: Received 19 November 2012 Accepted 18 March 2013 Available online 26 March 2013 Keywords: Lithology Natural radiation Radon Thoron Health
a b s t r a c t A cross-border multidisciplinary project has been developed in the region of Bemposta (cliffs of the Douro River) for the definition of walking trails, contributing to the promotion of health. The proposed routes traverse different lithologies of the region, visiting the interior of some “bodegas” (wineries in caves). The geochemical and radiometric characterization of lithologies revealed values of K, U and Th higher in granites and migmatites than the average crustal rocks and lower in sediments, which is in good agreement with results from a few hundred absorbed dose measurements. Soil gas radon and thoron concentrations, measured at a depth of 80 cm, also correlate well with geochemical information and indicate a low to moderate risk in the region. A preliminary assessment of the maximum contribution of the external radiation dose to the annual effective dose for an exposure scenario of 4 h/day shows an estimate of 0.19 mSv/year in the interior of the “bodegas” and 0.04 up to 0.19 mSv/year in trails according to the different lithologies, these values being reduced to 0.006 and 0.01 up to 0.006 mSv/year for a more likely scenario of exposure of 4 h/month. Exposure to radon gas is estimated not to exceed inside the “bodegas” 0.36 mSv/year for the latter scenario. © 2013 Elsevier B.V. All rights reserved.
1. Introduction There exists an innate human tendency to be drawn to the natural world and, in this context, the physical and emotional health benefits of a connection to nature have been well documented (Maller et al., 2009; Thompson et al., 2011). The region of Bemposta is located in the Natural Park of Douro International, particularly in cliffs of Douro International, implanted in the Mirandês Plateaux. This is a unique region with a highly attractive landscape by the geological, faunistic, floristic and heritage features, which can sustain the provision of nature tourism and outdoor activities, namely trekking. The rocks have different concentrations of radioactive isotopes (K, U and Th), the most important from the point of view of exposure to ionizing radiation. The distribution of those isotopes thus determines the external gamma radiation produced by rocks and also soils. There is still a current perception that natural radioactivity only affects people who live in the vicinity of uranium mines. However, all rocks contain uranium, some more than others, and depending on the concentration of this element and the subsequent contribution of certain geological factors, they also influence the presence of radon in the air. The emanation of radon gas is specifically dependent on lithology and geological structures. Thus, basement geochemistry influences the ⁎ Corresponding author. E-mail address: [email protected] (M.E.P. Gomes). 0375-6742/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2013.03.001
spatial distribution of radon levels at the soil/atmosphere interface (Buttafuoco et al., 2010). In the general environment, the natural radon isotopes contribute with more than 50% to the radiation dose received by individuals from natural radiation sources, and they have been identified as the second leading cause of lung cancer after tobacco smoking (UNSCEAR, 2000; WHO, 2009). Comparing the results of gamma dose rates in air inferred from the concentrations of radionuclides in rocks and soils obtained in Portugal with values of other European countries, according to the reports of UNSCEAR (2000, 2008), we can conclude that Portugal has a higher average dose rate than all other European countries studied (86 nGy/h). Granitoids exist in a significant portion of the Portuguese territory and have a tendency to incorporate higher amounts of uranium and thorium, compared to the crustal average. The distribution of these elements determines the gamma dose rate in air produced by rocks and soils, which explains the high average values found in Portugal. Border multidisciplinary projects have been developed in the region of the Douro cliffs for the definition of tracks and take full advantage of the natural patrimony, undoubtedly contributing to health promotion. The walking trails studied cross several lithologies, visiting the interior of some “bodegas” located in Fermoselle (Spain) and Urrós (Portugal). The objective of this work is the geochemical and radiometric characterization of the outcropping rocks and carries out a preliminary assessment of environmental risk associated with natural radioactivity, showing which routes are subjected to less exposure.
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
61
2. Geological setting
3. Materials and methods
The Bemposta region (Northeast of Portugal) is a part of the northwestern edge of the Variscan dome of Tormes that extends to Spain (Fig. 1). The dome of Tormes is a granitic batholith located in the inner area of the Iberian Massif characterized by the development of a plutono-metamorphic dome of complex geometry and the most significant feature of it is the presence of migmatites associated with anatectic granites. Cambrian and Ordovician metasedimentary country rocks from the North Douro International cliffs are locally migmatized and intruded by diverse granitic rocks, and the pre-Mesozoic rocks are locally covered by Tertiary sedimentary deposits. The granites are syn-tectonic with an elongated massive shape and are consistent with the NW–SE Variscan structures, revealing geometry and kinematics constrained by the Bemposta-Carviçais sinistral shear zone that strike ENE–WSW. Tonalite, three muscovite–biotite granites and muscovite granite are the granitoid facies recognized in the area. The tonalite outcrop in lenticules and the rock is dark, with a fine- to medium-grain, and exhibits strong foliation. The Bemposta granite is a medium-grained, muscovite– biotite and porphyritic, with phenocrysts of potassium feldspar generally less than 2 × 0.7 cm and strongly deformed. The Peredo de Bemposta granite is a medium-grained, muscovite–biotite, slightly porphyritic with phenocrysts below 1.5 × 5 cm. The Assumada granite is a fine- to medium-grained muscovite rock. The Urrós granite is a medium- to coarse-grained muscovite–biotite, and slightly porphyritic with phenocrysts of K-feldspar. Part of the granites and metasediments is covered by Cenozoic sedimentary deposits that extend across the Mirandês Plateaux and are well expressed in the vicinity of Sendim. The walking trails at the Douro cliffs are excellent for the practice of outdoor activities and consequently for the development of geotourism, crossing different lithologies and a large diversity of landscapes and forms. Some walking trails include visits to caves (“bodegas”) in the Urrós and Fermoselle villages.
Representative samples of all granites and metasedimentary rocks were collected and analyzed at the University of Bristol (UK). Major elements were determined by X-ray fluorescence with a detection limit of 0.01% and an accuracy better than ± 1%. U and Th were determined by ICP-MS with detection limits of 0.01 and 10 mg/kg and an accuracy of 5%. Minerals were analyzed in polished thin sections using a Jeol JXA-8500F electron-microprobe at the National Laboratory of Energy and Geology (LNEG) (Porto, Portugal). A radiometric survey was conducted in several lithologies of the region using the portable gamma-ray spectrometer GF Instruments — Gamma Surveyor compact 2, equipped with a NaI detector, able to measure the gamma dose rate in air (nGy/h) and estimate in situ the concentrations of K, U and Th of the rocks. Several profiles were performed to characterize each lithology. In selected locations (n =284) the external radiation was measured with the spectrometer placed at 1 m above the rock or soil, and concentrations of radiogenic elements K, U and Th were estimated in outcrops with a large amount of rock exposed. After a preliminary evaluation of the regional radiometric background, the determination of the soil-gas concentrations of radon and thoron was carried out in 37 representative places of the dominant lithologies by alpha emanometry, using a Scintrex RD200 equipment, according to the protocol described in Pereira et al. (1998). Sampling depth was 0.8 m and radon/thoron was discriminated following three consecutive one minute measurements. 4. Results and discussion 4.1. Geology and geochemistry of rocks and minerals Rocks such as granites contain more uranium than other rock types and consequently should give rise to higher radon levels at
Fig. 1. Simplified geological map in the region of Bemposta and location on the maps of Iberian and Portugal. Legend: 1. Paleozoic metasediments, 2. Migmatites, 3. Tonalite, 4. Bemposta granite, 5. Peredo de Bemposta granite, 6. Assumada granite, 7. Urrós granite, 8. Aplite–pegmatite veins, 9. Quartz veins and 10. Sedimentary deposits of the Cenozoic age.
62
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
the surface, namely in dwellings. However some granites contain more uranium than others and this element may be present in different minerals, some of which may not release radon during decay (Ball et al., 1991; Martins et al., 2013). Sakoda et al. (2008) showed that the radon exhalation rates and emanation fractions of the Misasa granite were much higher than those of the Badgastein granite, regardless of the 226Ra activity concentrations. The geological units from the Douro cliffs have been evaluated for their radioactive background with gamma-ray portable spectrometers, which allowed to recognize important differences in their uranium contents. Some radiometric anomalies that occur are mainly associated with geological faults and also in contacts and veins; significant differences in the radionuclide contents of some of the granites were recognized, namely regarding U and Th (Table 1). Data from whole rock geochemistry and measurements performed with the gamma-ray spectrometer allowed to register very close values of potassium, uranium and thorium in granites and migmatites (Table 1), in almost all the cases higher than the crustal abundance (ca. K — 3%; U — 3 mg/kg; and Th — 11 mg/kg). The sediments, however, have lower values than the crustal average. The tonalite and the granite of Bemposta contain the highest levels of Th and the Assumada muscovite granite contains the highest levels of U. The uranium concentrations were similar to those found in similar lithologies elsewhere in the north and center of Portugal (Gomes et al., 2010; Martins et al., 2013; Pereira et al., 2003). The rate of release of radon from rocks and soils (exhalation) is controlled not only by the uranium concentration but also by the way that this element is distributed in minerals (Appleton, 2005). Its presence in the structure of heavy accessory minerals reduces radon exhalation, while the presence of uranium associated with secondary oxi-hydroxides, mineral borders or weathered surfaces potentiates radon release from the rock (Gomes et al., 2010; Martins et al., 2013; Pereira et al., 2003, 2012). The observation of the accessory minerals of the granites and migmatites under a petrographic microscope as well as the observation with backscattered electrons images, allowed to identify apatite, zircon, ilmenite and monazite as common accessory minerals in all lithologies (Fig. 2a, b). Bemposta granite has major amounts of monazite rich in Th (Figs. 2a, b; 3a) while xenotime (Fig. 2c), uraninite (Figs. 2d, 3b), thorianite (Fig. 3c) and uranophane (Fig. 3d) were only detected in the Assumada granite, that is the most evolved granite and also has sulfides (pyrite, arsenopyrite, and sphalerite). In general the zircons of the Assumada granite have high U metamitic zircons (cyrtolite, Fig. 2c). We can conclude that uranium and thorium are likely related with the referred accessory minerals, which would result in principle in a lower radon exhalation rate. Th contents are relatively high in the Bemposta granite. This is probably due to the presence of larger amounts of monazite in this granite, concentrating high quantities of Th. However, due to the short half life (55 s) of the radon isotope produced in the decay chain of 232Th, known as thoron ( 220Rn), it is not usually considered
a health hazard due to its limited mobility in geological systems. The presence of uraninite in the muscovite granite from Assumada, a mineral readily leached in contact with meteoric water, presents the potential to increase uranium mobility and its deposition as a secondary mineral such as uranophane (possibly in fracture fillings) subsequently raises radon exhalation. According to its higher content of radionuclides, the Assumada granite also shows the highest average gamma dose rate between all studied lithologies (Table 2), with the lowest value being observed, as expected, in sediments. The soil gas concentrations of radon and thoron show a very good correlation with the geochemistry of rocks (Table 3), confirming that radon and thoron are higher in the Bemposta granite and much lower in the sediments. It also confirms that the radiogenic potential of migmatites is higher relatively to thoron when compared to the Urrós granite. The values given in Table 3 can be considered low to normal, except for thoron in the Bemposta granite, which is relatively high. The rather low soil-gas radon concentration is in agreement with the low exhalation rate expected from the study of accessory minerals and this observation may have an impact on the evaluation of the geogenic indoor radon risk in these rocks. 4.2. Exposure to the ionizing radiation from rocks and soils (external radiation) The sediments, as expected, have much lower radiometric background (39 nGy/h) relatively to the migmatites and granitic rocks of the region (Table 2). The Bemposta granite and the muscovite granite have the highest dose rate values (156 and 174 nGy/h, respectively). The highest values were, however, recorded inside the “bodegas” (183 nGy/h), with an estimated contribution of external radiation to the annual effective dose of 0.19 mSv/year, on the basis of an exposure of 4 h/day. This scenario is unlikely to occur in practice and should be considered as an upper limit for the contribution of the external radiation to the global exposure to ionizing radiation in the region. A more realistic scenario can be taken into account, for example considering a 4 h/month exposure inside the “bodegas”, or a monthly jogging of 4 h on the trails. Under this new scenarios, the estimated contribution decreases down to a maximum of 0.06 mSv/year (Table 2), which is negligible from a radiological point of view. The main risk in the “bodegas” is likely related with exposure to radon. No direct measurements of indoor air were carried out; however the results obtained for soil-gas radon concentrations in the region suggest a low to moderate risk on the basis of international classification systems (Åkerblom, 1994; Barnet et al., 2008). Moreover, the uranium content of the rock is only twice the typical value for building materials, and the exhalation rate of radon, as previously discussed, might be low. Assuming a ventilation rate ten times less than the average, the radon concentration is likely not to exceed 1000 Bq/m 3, the maximum allowed for buildings with public access in the new European directive project. The dose for visitors would be no higher
Table 1 Contents of K2O, U and Th in different lithologies of the studied region obtained by gamma spectrometry in situ and (2) number of analyzed samples and its values obtained by XRF (K2O) and ICP-MS (U and Th) for the diverse lithologies of the Bemposta region. Lithologies
Migmatite Tonalite Bemposta granite Peredo de Bemposta granite Urrós granite Assumada granite Sediments Inside “bodegas”
n
27 (2) 4 (3) 31 (12) 27 (5) 38 (6) 12 (4) 7 15
K2O (wt.%)
U (mg/kg)
Th (mg/kg)
Min–max
M
Min–max
M
Min–max
M
2.3–5.7 (4.4–4.9) 4.4–5.1 (3.0–4.4) 3.5–8.3 (4.6–7.3) 3.6–7.3 (5.6–6.0) 2.1–7.8 (5.1–6.5) 3.8–5.9 (4.2–5.1) 0.7–2.8 3.3–7.8
4.4 (4.7) 4.7 (3.9) 5.5 (5.4) 5.8 (5.8) 5.5 (5.9) 5.1 (5.0) 2.2 5.4
1–8 (4) 3–7 (3–10) 3–15 (4–22) 3–9 (7–18) 1–12 (4–8) 5–32 (5–54) 1–3 4–14
5 (4) 4 (6) 8 (11) 5 (11) 5 (6) 17 (5) 2 8
17–44 (18–19) 30–35 (25–31) 23–70 (7–51) 6–52 (15–24) 2–47 (6–13) 6–17 (4–13) 2–15 17–36
24 (19) 33 (27) 40 (29) 19 (20) 17 (9) 11 (9) 10 25
n — number of measurements; M — mean; min — minimum; and max — maximum.
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
63
Fig. 2. Backscattered electron images of accessory minerals: a) biotite (Bt) with apatite inclusion (Ap), which has zircon (Zrn) and monazite (Mnz) inclusions in the Bemposta granite; b) ilmenite (Ilm) with monazite and zircon inclusions in the Bemposta granite; c) Rich U metamitic zircon and xenotime (Xtm) inclusions in apatite in the Assumada granite and c) uraninite (Urn) inclusions in apatite that is an inclusion of muscovite (Ms) in the Assumada granite.
than 0.36 mSv/year, assuming an exposure of 4 h/month, and this is well below the limit usually considered of 1 mSv/year. Taking into consideration that these spaces are not used on a permanent basis, a scenario of higher occupancy is not considered here. Thus, the dose
due to radon exposure in the “bodegas” is not likely to be significant for the global annual exposure of the members of the public. In spite of this conclusion, a suggestion was made to the owners to increase the natural ventilation of the “bodegas” as a strategy for
Fig. 3. a) X-ray spectrum of monazite from the Bemposta granite and b) X-ray spectrum uraninite from the Assumada granite, c) X-ray spectrum of thorianite from Assumada granite, and d) X-ray spectrum of uranophane from the Assumada granite.
64
M.E.P. Gomes et al. / Journal of Geochemical Exploration 130 (2013) 60–64
Table 2 Measurements of external radiation dose (DRE) and this contribution to the annual effective dose (CAED) in different exposition scenarios. The contribution of cosmic radiation is included. Lithologies
Migmatite Tonalite Bemposta granite Peredo de Bemposta granite Urrós granite Assumada granite Sediments Inside “bodegas”
n
21 6 22 27 26 11 6 12
DRE (nGY/h)
CAED (mSv/a)
Min–max
Mean
4 h/day
4 h/month
68–151 121–139 105–315 81–192 58–174 92–220 36–41 143–226
125 130 156 117 120 174 39 183
0.127 0.132 0.159 0.119 0.122 0.177 0.039 0.187
0.004 0.004 0.005 0.004 0.004 0.006 0.001 0.006
n — number of measurements; min — minimum; max — maximum; DRE — external radiation dose; CAED — contribution to the annual effective dose; (nGy/h) — nanograys per hour; and (mSv/a) — milisieverts per year.
radon reduction in these spaces (Gunn et al., 1991). In the past all “bodegas” at Fermoselle were linked by open spaces, with increased natural ventilation, but at the moment they are closed. 5. Conclusions The potassium, uranium and thorium amounts of migmatitic and granitic rocks in the Bemposta region are generally higher than the average crustal abundance, while sediments are lower. Measurements performed in several lithologies showed that granites generally have higher K, U and Th and also soil-gas concentrations of radon and thoron than migmatites, and all of these are higher than sediments. The tonalite and the Bemposta granite contain high levels of Th (>30 mg/kg) and the Assumada muscovite granite contains high levels of U (17 mg/kg, in average). The contribution of gamma dose rate in air to the annual effective dose for the different lithologies of the Douro cliffs, on a basis of 4 h/day contact time, reaches inside the “bodegas” 0.19 mSv/year and outside, in trails, 0.04 mSv/year in sediments and up to 0.18 mSv/year in Assumada granite. An appropriated scenario for visiting members of the public, with 4 h/month of exposure time (inside the “bodegas” or jogging on the trails), results in a maximum contribution of 0.006 mSv/year, which can be considered negligible from the radiological point of view. The dose component due to exposure to radon was not directly assessed in the “bodegas”. However, taking into account the contents of U in the rocks and the mineralogical distribution of this element, mostly related with accessory minerals with reduced exhalation, as well as the soil-gas radon measurements carried out at a depth of 80 cm, with average concentrations under 50 kBq · m−3 (a limit usually considered by international classification schemes as presenting a high risk), we can conclude also that exposure to radon is not critical in these spaces. A contribution of 0.36 mSv/year for the global dose of ionizing radiation is likely not to be exceeded for members of the public on the basis of an exposure of 4 h/month, well under the limit usually considered (1 mSv/year). Acknowledgments We are grateful to Prof. M.R. Machado Leite and Eng. Fernanda Guimarães for the use of the electron-microprobe at LNEG (Porto, Portugal). Thanks are due to the project Petrochron - PTDC/CTE-GIX/
Table 3 Soil-gas concentrations of radon and thoron (kBq · m−3) in the dominant lithologies of the region of Bemposta. Lithologies
Urrós granite
Bemposta granite
Migmatites
Sediments
n
17 34 17
10 49 153
3 10 29
7 4 11
222
Rn (radon) 220 Rn (thoron)
n — number of measurements.
112561/2009 for geochemical data and Programme Interreg IIIA Portugal–Espanha that supported the project Fluvial–Nuevas Ciudades Fluviales del S. XXI and the publication of twenty-three walking trails in the Douro International cliffs. The anonymous reviewers are acknowledged for their critical reviews that benefited the text. This research work was carried out in the program of the Geosciences Centre and IMAR-CMA, Coimbra University. References Åkerblom, G., 1994. Ground radon monitoring procedures in Sweden. Geoscientist 4, 21–27. Appleton, J.D., 2005. Radon in air and water. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, R., Smedley, P. (Eds.), Essentials of Medical Geology Impacts of the Natural Environment on the Public Health. Elsevier Academic Press, Amsterdam, pp. 227–257. Ball, T.K., Cameron, D.G., Colman, T.B., Roberts, P.D., 1991. Behaviour of radon in the geological environment: a review. Quarterly Journal of Engineering Geology & Hydrogeology 24, 169–182. Barnet, I., Pacherová, P., Neznal, M., Neznal, M., 2008. Radon in Geological Environment — Czech Experience. Czech Geological Survey Special Papers 19 (Prague, 70 pp.). Buttafuoco, G., Tallarico, A., Falcone, G., Gagliardi, I., 2010. A geostatistical approach for mapping and uncertainty assessment of geogenic radon gas in soil in an area of southern Italy. Environmental Earth Sciences 61 (3), 491–505. Gomes, M.E.P., Neves, L.J.P.F., Coelho, F., Carvalho, A., Sousa, M., Pereira, A.J.S.C., 2010. Geochemistry of granites and metasediments of the urban area of Vila Real (northern Portugal) and correlative radon risk. Environmental Earth Sciences 64 (2), 497–502. Gunn, J., Fletcher, S., Prime, D., 1991. Research on radon in British limestone caves and mines, 1970–1990. Cave and Karst Science 18, 63–66. Maller, C., Townsend, M., St Leger, L., Henderson, Wilson, Pryor, A., Prosser, L., Moore, M., 2009. Healthy Parks, Healthy People: The Health Benefits of Contact With Nature in a Park Context. 26 (2), 51–88 (www.georgewright.org/262maller.pdf Access 02/ 04/2012). Martins, L., Gomes, M.E.P., Neves, L.J.P.F., Pereira, A., 2013. Geochemistry of granites and metasediments of the region of Amarante (northern Portugal) and associate radon risk. Environmental Earth Sciences 68 (3), 733–740. Pereira, A.J.S.C., Neves, L.J.P.F., Godinho, M.M., Soares, A.F., Marques, J.F., 1998. Distribution of radon in soil in the region of Coimbra. Comunicações Instituto Geológico e Mineiro 84 (2), E110–E113. Pereira, A.J.S.C., Neves, L.J.P.F., Godinho, M.M., Dias, J.M.M., 2003. Natural radioactivity in Portugal: influencing geological factors and implications for land use planning. Radioprotecção 2 (2–3), 109–120. Pereira, D., Neves, L., Pereira, A., Peinado, M., Blanco, J.A., Tejado, J.J., 2012. A radiological study of some ornamental stones: the bluish granites from Extremadura (Spain). Natural Hazards and Earth System Sciences 12, 395–401. Sakoda, A., Hanamoto, K., Ishimori, Y., Nagamatsu, T., Yamaokaa, K., 2008. Radioactivity and radon emanation fraction of the granites sampled at Misasa and Badgastein. Applied Radiation and Isotopes 66, 648–652. Thompson, C.J., Boddy, K., Stein, K., Whear, R., Depledge, M.H., 2011. Does participating in physical activity in outdoor natural environments have a greater effect on physical and mental wellbeing than physical activity indoors? A systematic review. Environmental Science & Technology 45 (5), 1761–1772. UNSCEAR, 2000. Sources and Effects of Ionizing Radiation. United Nations, New York. UNSCEAR, 2008. Sources and effects of ionizing radiation, United Nations, New York. http://www.unscear.org/unscear/en/publications.html (Access 02/04/2012). WHO, 2009. WHO Handbook on Indoor Radon: A Public Health Perspective. World Health Organization, WHO Press, Geneva, Switzerland.