Lead-210 in Air and in Surface Soil in NE Estonia

Transcription

1 Lead-210 in Air and in Surface Soil in NE Estonia E. Realo 1, K. Realo 1, M. Lust 2, R. Koch 1, A. Uljas 2 1 Institute of Physics, University of Tartu, 51014, Tartu, Estonia. realo@fi.tartu.ee 2 Estonian Radiation Protection Centre, 10416, Tallinn, Estonia Abstract. In NE Estonia, 210 Pb activity concentration in air and in natural surface soils are determined. The region is characterized by varying 226 Ra( 238 U) activity concentrations and is affected by a significant fly ash radionuclide deposition from two large oil-shale power plants. The activity concentrations of 210 Pb in surface air and its depth distributions in natural surface soil profiles have been determined using HPGe γ spectrometry. A high-volume air sampler has been used to collect air-borne 210 Pb in aerosol filter samples in Narva-Jõesuu. The observed air concentrations of 210 Pb vary in the range of µbq m -3 with the average of 450 µbq m -3. The observed inverse correlation between the 210 Pb activity concentration in air and the precipitation rate confirms the role of wet deposition in removing lead from air. Two groups of low-radium soil profiles from sites: (a) far from the power plants and (b) near the power plants have been analyzed. The 226 Ra activity concentrations in the range of Bq kg -1 and radon emanation coefficients in the range of were determined. The found depth-dependent concentrations of unsupported 210 Pb have been fitted using a 1D diffusion model and a compartmental transfer model to determine the transfer parameters in soil. About 2/3 of the unsupported 210 Pb inventory is bounded in the upper 0-10 cm soil horizon. The average annual atmospheric deposition fluxes of 210 Pb with a mean value of 164 Bq m -2 a -1 and the average deposition velocity of 12 mm s -1 is found for natural locations, group (a). The maximum fluxes with a mean value of 321 Bq m -2 a -1 are found near the power plants. The results support an assumption about the naturally and/or technologically enhanced 210 Pb concentration in air in a number of locations in NE Estonia. 1. Introduction Lead-210 is characterized by a complex behaviour in the environment. Radioactive 210 Pb with a relatively long lifetime (t 1/2 = 22.3 a) forms from its parent 222 Rn (t 1/2 = 3.82 d), which is a daughter product from the α-decay of 226 Ra. A minor radium impurity occurs in almost all soils, minerals and in water. In soil, α-recoil of radium releases some noble gas radon atoms from mineral grains into the pore space of soil, while the others remain fixed in soil particles. The released radon atoms are transported by advection and diffusion through the pores and a fraction of them is released into the atmosphere. The worldwide average flux of Rn to the atmosphere is evaluated mbq m -2 s -1 from continental soils and 0.2 mbq m -2 s from ocean waters ([1] and references therein). Both released and fixed radon nuclei undergo a number of decays to produce 210 Pb. Air concentrations of 210 Pb determined by different authors vary in a wide range from 100 to 2500 µbq m -3 with the worldwide average of 500 µbq m -3 [2]. Its residence time in the atmosphere is considered to be relatively short, from one to two weeks [1,3] and significant seasonal, temporal and geographical variations of air concentration are observed. Precipitation in the form of rain and snow predominantly scavenges 210 Pb from the atmosphere and governs a reasonably constant annual depositional flux to the soil. The long-term accumulation compensates losses caused by radioactive decay and by migration to deeper soil layers. As a result, higher concentration of 210 Pb in comparison with 226 Ra is usually observed in the surface horizons of soil. In these soil layers, the total concentration of 210 Pb consists of two components: the atmospherically derived unsupported fraction and the in-situ produced supported fraction. Inventories of the unsupported fraction in soil can be related to the average annual atmospheric 210 Pb deposition fluxes. In addition to natural processes, 210 Pb is released to the environment by multiple industrial activities, e.g. energy production using fossil fuels, fertiliser production, metal production, oil industry, etc. In comparison to natural emission, the industry-related emissions of 210 Pb are considered insignificant, except the immediate surroundings of the sources [1]. All the above features have made the study of 210 Pb attractive for applications in atmospheric transport and removal processes, oceanography and sedimentation, geophysical dating. Here we present results of our study on 210 Pb in air and in soil profiles collected from the coastal areas of the Gulf of Finland in the Counties of Lääne-Virumaa and Ida-Virumaa, NE Estonia. This is a radioecologically interesting region characterized by locally enhanced natural 226 Ra( 238 U) soil 1

2 concentrations varying in a wide range from 5 to 350 Bq kg -1 [4]. A relatively heavy atmospheric fly ash deposition load emitted by two large oil-shale-fired power plants also affects the region [5,6]. As a result, above-average 210 Pb deposition fluxes might be possible in this region. Our preliminary study [7] indicated the highest deposition fluxes near the power plants. Nevertheless, no clear evidence or evaluation supports the major contribution from 210 Pb deposition emitted with flue gases and fly ash. In a number of radium-rich soil sites intensive radon exhalation results in domination of the supported 210 Pb fraction over the unsupported fraction. Significant 210 Pb deficiencies relative to 226 Ra and aboveaverage depositions have been found. These results stimulated us to revisit 210 Pb studies in low-radium soils (< 35 Bq kg -1 ) both far from and near the power plants and to supplement them with air filter analysis data from the region. 2. Sampling, analysis and methods A high-volume continuous air sampler (SnowWhite, STUK, Finland) was used to collect air-borne 210 Pb in weekly aerosol filter samples in Narva-Jõesuu. The sampler operated by Estonian Radiation Protection Centre is located immediately on the coastline of the Gulf of Finland ( N, E), within ~ 20 km from both power plants and the high-radium areas. The sampler has a capacity of 900 m 3 h -1 and typically (5-6) 10 4 m 3 of air per week was sampled. The filter samples were pressed into pellets using a 10 t hydraulic press, put into plastic containers and analyzed for 210 Pb on γ spectrometer with a planar detector. In the present work weekly data over the time period are included. Additional analysis is in progress. Soil samples were collected from natural undisturbed grasslands in NE Estonia in (see below Table 1). Four soil profiles (group a) were sampled far from the power plants (> 10 km) and four profiles near them (group b). The soils differed in nature and in type. A MAGELLAN GPS receiver was used for the determination of geographical coordinates of all sampling sites. A special sharpened stainless steel corer of an 8.3-cm internal diameter served to take soil cores down to the depth of 20 cm. The corer design effectively minimizes soil compaction. From each location two soil cores at a distance of 1 m were collected. In laboratory, the cores were sliced into 2 or 3 cm sections: 0 2, 2 4, 4 6, 6-9 cm, etc., down to the core bottom. The corresponding sections of two cores were bulked, thoroughly mixed, sieved and stones (> 2 mm) were removed. All samples were dried at C for h, homogenized and stored in sealed 55-cm 3 metal containers. Both wet and dry weights of sections were determined and the corresponding depth-dependent densities of profiles were derived. The analysis of 210 Pb was performed on a low-background γ spectrometer with a planar 800-mm 2 HPGe detector by means of its 46.5 kev line. Spectra were measured typically during 3-7 d. A selfattenuation correction method basing on a direct 46.5 kev transmission measurement for each sample was applied [6]. For the 226 Ra analysis a low-background ORTEC γ spectrometer with a coaxial HPGe detector (42 % efficiency and 1.7 kev resolution at 1.3 MeV) was applied. The in-growth of radon progeny was recorded for two top sections of each profile. Time-dependent intensities of the 295 kev, 352 kev and 609 kev γ lines of 214 Pb/Bi determined from the successive measurements served to derive radon emanation coefficients for the samples. The calibration of spectrometers was carried out using the IAEA-RGU certified reference material measured in an identical geometrical arrangement. The IAEA GANAAS 3.1/ GAMANAL software served for γ spectrum analysis. From air filter samples activity concentrations of 210 Pb in air (µbq m -3 ) were determined. Depth-dependent activity concentrations of 210 Pb and 226 Ra, C Pb (x), and C Ra (x), respectively, were determined for soil samples. C Pb (x) denotes the total (sum of supported and unsupported fraction), activity concentration of 210 Pb (Bq kg -1 ) at depth x (m) in soil. Using the determined depth-dependent soil sections density, ρ(x) (kg m -3 ), the corresponding average activity concentrations were calculated: C(x)(Bq m -3 ) = ρ(x) C(x)(Bq kg -1 ). The concentration of the unsupported fraction was evaluated as a difference C uns (x)=c Pb (x) [1-κ(x)] C Ra (x), where κ(x) is the estimated depth-dependent radon emanation coefficient. The selection of low-radium soil samples results in relatively small uncertainties in the supported fraction evaluation. 2

3 3. Depth distribution models of unsupported 210 Pb in soil Assuming a steady-state long-term one-dimensional migration of 210 Pb from surface to deeper soil horizons, an exponential distribution model for depth-dependent concentration of unsupported fraction, C uns (x), can easily be derived: C uns ( x) = Cuns, 0 exp( x l uns ) (1) where C uns,0 surface concentration of unsupported 210 Pb (C uns (0)); l uns the relaxation length, (m), which can be used for calculation of diffusion and/or advection parameters (see, e.g., [8]). The least-square fitting procedure of the Eq. 1 to the analyzed depth-dependent concentrations of unsupported fraction was performed using the ORIGIN5 software. The best-fit parameters were used to derive the annual atmospheric deposition flux of 210 Pb, F (Bq m -2 a -1 ): F = 0 λ C uns, l uns (2) where λ= a -1 is the radioactive decay constant of 210 Pb. A compartmental model referred in [9] was used to describe the 210 Pb activity per unit area (Bq m -2 ) in soil sections of different depth. In this simplified model, the time evolution of depth-dependent activity in a soil profile is modelled, assuming only downward migration and radioactive decay of the radionuclide. The immediate mixing and a uniform concentration in a compartment is assumed. A soil profile is assumed to consist of four layers (compartments) with unit-area activities (Bq m -2 ) of A, B, C and D and with the thicknesses of m, m, m and m, respectively. The model is given in the following form: da = k dt db = ( k dt dc = ( k dt dd = ( k dt BC CD DO + λ) A+ A ( AB 0 + λ) B + k + λ) C + k + λ) D + k AB BC CD A B C (3) where k ij are time constants (a -1 ) describing the transfer rate from compartment i to compartment j, e.g., A B, etc. The system was programmed and solved as a SCIENTIST 2.01 model. The unit area activities of unsupported 226 Pb in the corresponding compartments were calculated using an interpolation of the experimental depth-dependent activity concentration of unsupported 226 Pb, C uns (x), in the soil profiles. The obtained activities were fitted to the solutions of the above system of equations to find the corresponding transfer rates. 3

4 4. Results and discussion 4.1. Air Weekly averaged activity concentrations of 210 Pb in air at Narva-Jõesuu in are summarized in Fig. 1. FIG. 1. Concentrations of 210 Pb in air at Narva-Jõesuu, , weekly collected samples. Lead-210 concentrations in the surface air range between µbq m -3, with the arithmetic and the geometric means of the weekly concentrations of 453 µbq m -3 and 387 µbq m -3, respectively. During the study period, the concentrations display a lognormal distribution with the width parameter of The mean concentration is comparable to the worldwide mean of 500 µbq m -3 [2]. There is two other air samplers in Estonia: in Harku ( N, E, NW Estonia) and in Tõravere ( N, E, S Estonia). A comparison reveals that the arithmetic mean of 2l0 Pb concentration in the Narva-Jõesuu is higher than in Harku (333 µbq m -3 ) and ~15 % less than in the inland Tõravere (512 µbq m -3 ). Correlation analysis shows that the monthly averaged 210 Pb air concentrations are moderately inversely correlated with the corresponding monthly precipitation amounts, the correlation coefficient is ( 0.40). This result seems to confirm the general conclusions found by J.Paatero et al. for Finland [10]: (a) high pressure situations are often connected with continental air masses rich in 210 Pb, (b) the maritime air masses associated with cyclones from the North Atlantic Ocean contain very low 2l0 Pb concentrations. Further analysis on longer time-scales should clarify to what extent local sources might influence the measured air concentrations Soil The concentration of 226 Ra is generally fairly constant along the profile showing only about ±15% difference in the top-most layer. The profiles with 226 Ra concentration smaller than 30 Bq kg -1 are selected for the present analysis. All profiles indicate a considerable surplus of 210 Pb (relative to 226 Ra). 4

5 The determined radon emanation coefficients, κ, of the soil profiles differ significantly in the range from 0.14 to Their values demonstrate poor correlation with both the density of soil and the 226 Ra concentration values. The determined depth-dependent unsupported 210 Pb activity concentrations (Bq m -3 ) in soil are plotted in Fig. 2. Here the averaged curves for two groups of sampling sites are presented: (a) four sites, #136, #141, #150, #151, far (> 10 km) from and (b) four sites, #122, #124, #126, #131, near (< 2 km) the power plants. Error bars indicate the range of individual profile values. Significant differences exist between the depth-dependent concentration behaviour for two pairs of soil profiles in the group (b). These differences result in variation of the corresponding fitting parameters and are explained probably by different soil type and water level. (Table 1). Fig. 2. Activity concentration of unsupported fraction of 210 Pb (kbq m -3 ) vs depth (m) for group averages (a) of four sites, far (> 10 km) from the power plants and (b) of four sites near the power plants. It can be seen that a simple exponential depth-dependence of Eq.1 can satisfactorily be used for fitting. Table 1 summarizes the fitting parameters for the depth-dependencies of 210 Pb in soils for individual sites and for the corresponding group mean profiles: surface unsupported activity concentrations, C uns0, and relaxation lengths, l uns. Geographical coordinates of the sites, average 226 Ra activity concentrations, C Ra, radon emanation coefficients, κ, and calculated annual 210 Pb atmospherical deposition fluxes, F, are also presented. For different sites 226 Ra activity concentrations vary in a rather narrow range from 15 to 35 Bq kg -1. The measured radon emanation coefficients, κ, of soils range between (the mean values for group (a) and group (b) being 0.26 and 0.29, respectively). κ- values demonstrate poor correlation with both the density of soil and the 226 Ra concentration values [11]. Significant site-specific variations in parameters occur in the group (b), especially in the relaxation length of unsupported fraction, l uns. The latter ranges from 0.07 to 0.23 m. The mean values of l uns for both groups are equal, 0.10 m. It follows that about 2/3 of the atmospherically deposited (unsupported) 210 Pb is bound in the upper 0.10 m soil layer. This finding may confirm the attachment of unsupported 210 Pb to soil particles or to organic matter in the surface soil [12]. It should be noted 5

6 that at present the analysis of uncertainties of the fitting process is far from complete and further efforts are needed. Table 1. Fitting parameters for depth-dependent 210 Pb unsupported fractions for individual soil profiles and their group means, assuming Eq Pb Site Geographical coordinates C Ra Rn eman. C uns0 Relaxation deposition no N E (Bq kg -1 ) coeff., κ± 15 % (Bq m -3 ) length, l uns (m) flux, F (Bq m -2 a -1 ) Group (a) # '48" '06" # '04" '05" # '58" '54" # '11" '00" Mean Group (b) # '29" '51" # '55" '46" # '15" '16" # '40" '23" Mean The compartmental model fitting of the mean depth-dependent activities to Eq. 3 results in annual deposition flux, F, and transfer rate, k ij, parameter values for the soil groups (a) and (b). This analysis presents information on the migration velocities of the radionuclide into deeper soil layers. The fitting data and interpolated unsupported 210 Pb activity concentration values (Bq m -2 ) for the selected soil compartments are presented in Table 2. Table 2. Interpolated unsupported 210 Pb activity values for the selected soil compartments A, B, C and D in mean profiles of both (a) and (b) soil groups and the fitting parameters. Site Unsupp. 210 Pb activity in compartment (Bq m -2 ) Annual flux, F (Bq m -2 a -1 ) Transfer rate, k ij (a -1 ) A B C D k A0 k AB k BC k CD k DO mean (a) mean (b) Cs [9] In general, the found transfer rates for two soil groups are approximately equal. The average downward migration velocity, mm a -1, calculated as x i k ij, where x i is the thickness of the compartment i (m), is also equal for both mean groups. Transfer rates for 137 Cs in soil [9] are presented in Table 2 for a comparison. For 210 Pb, the transfer rates k AB and k BC are slightly larger and k CD is significantly smaller than the respective 137 Cs values. The atmospheric deposition flux values, F, range from 124 Bq m -2 a -1 to 207 Bq m -2 a -1 and from 243 Bq m -2 a -1 to 460 Bq m -2 a -1 for groups (a) and (b), respectively. At the same time, the mean value, 321 Bq m -2 a -1, for the sites near the power plants (group b) is about two times larger than the value for the group (a), 164 Bq m -2 a -1. Our previous study [11] revealed larger deposition fluxes at sites characterized with higher 226 Ra concentration in soil. Although all the sampling sites are located in the coastal region of the Gulf of Finland, i.e. within km from the sea, the calculated deposition flux values are typical for those found for the inland sites (see, e.g., [1,13]). The calculated F-values can be compared with those found by other authors [1,13-15] (Table 3). It follows from the comparison that in NE Estonia mean 210 Pb atmospheric deposition flux in the sites far from the power plants are within the range or almost equal to the values reported for other countries. The mean flux near the power plants is significantly higher than the fluxes reported in references. In 6

7 this region, the contributions to the 210 Pb inventory from emissions of flue gases and fly ash by two large oil-shale-fired power plants are a possible explanation. Table 3. Comparison of 210 Pb deposition flux, F, and precipitation, p, data Location Precipitation, p 210 Pb flux, F Reference (m a -1 ) (Bq m -2 a -1 ) Munich, Germany [13] Geneva, Switzerland [14] Devoke Water, UK [15] Harwell, UK [15] Groningen, Netherlands [1] NE Estonia, mean (a) mean (b) present work This conclusion seems to support an assumption about locally enhanced 210 Pb (and possibly 222 Rn) concentration in air in NE Estonia. The most probable intensive radon and 210 Pb sources are the areas with a high 226 Ra( 238 U) soil concentration and the low-lying uranium-rich alum shale (dictyonema argillite) layers overlain by limestone in the background of this region. Indirect evidence follows also from the results of an atmospheric transport modelling study for the air-borne 210 Pb in Finland [16]: areas in E Estonia were identified among the relatively intensive 210 Pb source regions. The role of snow and ice, which cover soil during ~ 100 d per year, in the increase of 210 Pb in the surface soil inventory needs a further study. Using the data on mean concentration in air for NE Estonia, C air,pb = 450 µbq m -3, and the derived atmospheric deposition flux values, F, the deposition velocity of 210 Pb, v d, can be calculated: v d = F C air, Pb (4) For natural sites far from power plants (a), the deposition velocity, v d, is m s -1, while for the sites near the power plants (b) the value is m s -1. The latter is formal, because no air concentration measurements are made near the power plants and the data on historical annual fly ash releases to the atmosphere are highly unreliable. At the same time, deposition velocity values in the range of m s -1 are typically found for other natural areas [1]. The observed high deposition fluxes and deposition velocities support an assumption about industrially enhanced 210 Pb inventory in soil at sites near the power plants. 5. Conclusions Lead-210 air concentration, its atmospheric deposition flux, and its concentrations in soil are studied by applying γ spectrometric analysis and modelling of natural soil cores collected in areas with low 226 Ra content. In , 210 Pb concentrations in surface air at the seaside Narva-Jõesuu indicate considerable variations with time in the range from 100 µbq m -3 to 1900 µbq m -3 with a mean value of 450 µbq m -3. The latter is comparable to the 210 Pb air concentration, 540 µbq m -3, in Tõravere, the Estonian inland. The observed moderate inverse correlation between the 210 Pb activity concentration in air and the precipitation rate confirms the important role of the wet deposition in removing 210 Pb from air. Soil sampling sites cover low-radium natural soils near the two large oil-shale-fired power plants and areas far from them. A significant excess of 210 Pb, in comparison with 226 Ra characterizes these surface soils. For each site, radon emanation coefficients were determined using the γ spectrometric method. Depth-dependent activity concentrations of unsupported 210 Pb fractions were least-square fitted using a 1D diffusion model and a compartmental model. Near the power plants, inventories of the 210 Pb unsupported fraction are approximately two times larger those for other locations. About 2/3 of the 210 Pb unsupported fraction inventory was found bounded in the upper 0-10 cm soil horizon. 210 Pb atmospheric deposition fluxes revealed practically no correlation with the soil density values or with its radon emanation coefficients. 7

8 The average annual atmospheric deposition fluxes of 210 Pb range from 124 to 207 Bq m -2 a -1 (mean value of 164 Bq m -2 a -1 ) and from 243 to 460 Bq m -2 a -1 (mean value of 321 Bq m -2 a -1 ) for groups (a) and (b), respectively. The latter values are among the highest concentrations reported for other locations. The areas with high 226 Ra( 238 U) concentration in soil and the low-lying uranium-rich alum shale layers in the background are assumed as the most probable sources of 222 Rn in air of the region. The role of two large oil-shale-fired power plants in the 210 Pb-inventory increase in near-by soils seems to be proved. Additional analysis of the present results may be useful to clarify the contribution of snow cover to 210 Pb inventory in soil profile. Acknowledgements Partial financial support provided by the Estonian Science Foundation grant no 4691 is acknowledged. References 1. Beks, J.P., Eisma, U. D., van der Plicht, J., A record of atmospheric 210 Pb deposition in The Netherlands. Sci. Tot. Environ., 222:35-44, (1998). 2. United Nations, UNSCEAR. Sources and Effects of Ionizing Radiation. Volume I: Sources, UN, New York, (2000). 3. Tokieda, T., Yamanaka, K., Harada, K., Tsunogai, S., Seasonal variations of residence time and upper atmospheric contribution of aerosols studied with Pb-210, Bi-210, Po-210 and Be-7. Tellus, 48B: , (1996). 4. Realo, E., in Radiation Protection Issues in the Baltic Region. Proceedings of the Regional IRPA Congress, Stockholm, 1998, edited by J. Søgaard-Hansen and A. Damkjær (Riso National Laboratory, Roskilde, 1998), p Realo, E., Realo, K., Jõgi, J., Releases of Natural Radionuclides from Oil-shale-fired Power Plants in Estonia. J. Environ. Radioactivity, 33:77-89, (1996). 6. Realo, K., Realo, E., in Proceedings of the 12 th NSFS/ IRPA ordinary meeting, Skagen, 1999, edited by J. Søgaard-Hansen and A. Damkjær (Riso National Laboratory, Roskilde, 1999), p Realo, K., Realo, E., in Radiation Protection in Central Europe: Radiation Protection and Health. Proceedings of the IRPA Regional Congress, Dubrovnik, 2001, edited by B.Obelić, M.Ranogajec- Komor, et al. (IRPA/ CRPA, Zagreb, 2002, CD-ROM, No 5p-23), p He, Q., Walling, D.E., The Distribution of Fallout 137 Cs and 210 Pb in Undisturbed and Cultivated Soils. Appl. Radiat. Isot., 48: , (1997). 9. European Commission, Methodology for assessing the radiological consequences of routine releases of radionuclides to the environment. Report EUR 15760, Radiation Protection series no 72, Office for Official Publications of EC, Luxembourg, (1995). 10. Paatero, J., Hatakka, J., Mattsson, R., Aaltonen, V., Viisanen,Y., in Transport and Chemical Transformation in the Troposphere: Proceedings of the EUROTRAC-2 Symposium 2000, edited by P. M. Midgley, M. Reuther, M. Williams, (CD-ROM, Springer-Verlag, Berlin, 2000), p Realo, E., Realo, K., in Book Series " Radioactivity in the Environment", Symposium Proceedings Volume, VII International Symposium NATURAL RADIATION ENVIRONMENT (NRE-VII), Rhodes, 2002, (Elsevier, 2004) (accepted for publication). 12. Moore, H.E., Poet, S.E., 210 Pb fluxes determined from 210 Pb and 226 Ra in soil profiles. J. Geophys. Res., 81: , (1976). 13. Winkler, R., Rosner, G., Seasonal and long-term variation of 210 Pb concentration in air, atmospheric deposition rate and total deposition velocity in south Germany. Sci.Tot. Environ.,263:57-68, (2000). 14. Caillet, S., Arpagaus, P., Monna, F., Domonik J., Factors controlling 7 Be and 210 Pb atmospheric deposition as revealed by sampling individual rain events in the region of Geneva, Switzerland. J. Environ. Radioactivity, 53: , (2001). 15. Smith, J.T., Appleby, P.G., Hilton, J., Richardson, N., Inventories and Fluxes of 210 Pb, 137 Cs and 241 Am Determined from the Soils of Three Small Catchments in Cumbria, UK. J. Environ. Radioactivity, 37: , (1997). 16. Paatero, J., Hatakka, J., Source Areas of Airborne 7 Be and 210 Pb Measured in Northern Finland. Health Physics, 79: , (2000). 8