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Short-term seasonal variability in 7Be wet deposition in a semiarid ecosystem of central Argentina
Journal of Environmental Radioactivity 100 (2009) 977–981
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Short-term seasonal variability in 7Be wet deposition in a semiarid ecosystem of central Argentina J. Juri Ayub a, D.E. Di Gregorio b, c, H. Velasco a, *, H. Huck b, c, M. Rizzotto a, F. Lohaiza a a
´tica Aplicada San Luis, Universidad Nacional de San Luis – CONICET, Ejercito de los Andes 950, D5700HHW San Luis, Argentina Grupo de Estudios Ambientales, Instituto de Matema ´n Nacional de Energı´a Ato ´ mica, Av. Gral. Paz 1499, 1650 San Martı´n, Provincia de Buenos Aires, Argentina Departamento de Fı´sica, Comisio c Escuela de Ciencia y Tecnologı´a, Universidad Nacional de San Martı´n, Martı´n de Irigoyen 3100, 1650 San Martı´n, Provincia de Buenos Aires, Argentina b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2008 Received in revised form 8 July 2009 Accepted 4 August 2009 Available online 27 August 2009
The 7Be wet deposition has been intensively investigated in a semiarid region at San Luis Province, Argentina. From November 2006 to May 2008, the 7Be content in rainwater was determined in 58 individual rain events, randomly comprising more than 50% of all individual precipitations at the sampling period. 7Be activity concentration in rainwater ranged from 0.7 0.3 Bq l1 to 3.2 0.7 Bq l1, with a mean value of 1.7 Bq l1 (sd ¼ 0.53 Bq l1). No relationship was found between 7Be content in rainwater and (a) rainfall amount, (b) precipitation intensity and (c) elapsed time between events. 7Be ground deposition was found to be well correlated with rainfall amount (R ¼ 0.92). For the precipitation events considered, the 7Be depositional fluxes ranged from 1.1 to 120 Bq m2, with a mean value of 32.7 Bq m2 (sd ¼ 29.9 Bq m2). The annual depositional flux was estimated at 1140 120 Bq m2 y1. Assuming the same monthly deposition pattern and that the 7Be content in soil decreases only through radioactive decay, the seasonal variation of 7Be areal activity density in soil was estimated. Results of this investigation may contribute to a valuable characterization of 7Be input in the explored semiarid ecosystem and its potential use as tracer of environmental processes. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: 7 Be atmospheric deposition Rainwater 7 Be areal activity density Central Argentina
1. Introduction 7 Be is a natural radionuclide (Eg ¼ 477.6 keV, t1/2 ¼ 53.3 d) originating from the interaction of cosmic rays with terrestrial atmosphere. The amount of 7Be that reaches the surface soil depends on the production rate (cosmic-ray intensity) which varies with latitude, altitude and solar activity. Other factors include stratosphere–troposphere mixing, circulation and advection processes within the troposphere, and the efficiency with which it is removed from the troposphere by wet and dry depositions (Kaste et al., 2002). 7 Be constitutes one of the most important environmental radionuclides, and the number of investigations regarding the application of 7Be as a tracer of environmental processes has increased notably in recent years. This radionuclide has been used as a tracer for the transport and residence time of aerosols in the atmosphere (Kaste et al., 2002) in sedimentological studies, and to assess soil redistribution after rain events. Wallbrink and Murray (1996) investigated 7Be in soils to assess the movement of topsoil in bare soil, grassland, and eucalypt forest. Walling et al. (1999) and
* Corresponding author. Tel./fax: þ54 2652 422803. E-mail address: [email protected] (H. Velasco). 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.08.001
Blake et al. (1999) used 7Be to investigate erosion and sedimentation in cultivated field in Devon, England. Blake et al. (1999) successfully demonstrated the potential for using 7Be as a tracer in soil erosion research. Later, Wilson et al. (2003) used the inventory balance for 7Be to calculate sediment erosion during a series of runoff-producing thunderstorms occurring over a short time period in Treynor, USA. Recently, Schuller et al. (2006) and Sepulveda et al. (2008) used 7Be to document soil erosion associated with a short period of extreme rainfall in south-central Chile. Various studies revealed that 7Be enters continuously into the ecosystem, principally through wet deposition. Salisbury and Cartwright (2005) estimated that wet fallout accounts for about 97% of the total 7Be ground deposition. Ioannidou et al. (2005) and Wallbrink and Murray (1994) suggested that dry depositions contribute, at a maximum, with only 8% of the total deposition. Further studies have shown that for different regions and dissimilar environmental conditions, 7Be wet deposition flux was dependent on rain precipitation volume (Kaste et al., 2002). Consequently, the seasonal 7Be wet deposition pattern may be closely associated with rainfall variability in the studied sites (Caillet et al., 2001). Potential correlations among 7Be wet deposition, total precipitation and precipitation intensity have been extensively studied by different authors with divergent results. Wallbrink and Murray
978
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981 Table 1 Rainfall amount and 7Be content in rainwater.
180
Accumulated rainfall (mm)
160
Event number
Date
1 2 3 4 5 6 7 8 9 10 11 12 13
Nov 15, 2006 Nov 18 Nov 25 Nov 25 Nov 26 Nov 26 Nov 1 Dec 5 Dec 8 Dec 10 Dec 13 Dec 17 Dec 18
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Rainfall (mm)
100 80 60 40 20 0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Fig. 1. Mean monthly accumulated rainfall amount in the studied zone. Mean values were obtained averaging over the last four years. Bars indicate the standard error.
(1994) have concluded that there is no significant correlation between concentration and rainfall for precipitation events less than 25 mm, with activity concentration values ranging from 0.02 to 5.9 Bq l1. They reported that the total annual activity depositional flux delivered was 1030 100 Bq m2 y1, and found a correlation between rainfall events and the deposited areal activity of 7Be (R2 ¼ 0.6). Similar results were obtained by Caillet et al. (2001) after analyzing 46 rain events, at Versoix, Switzerland. These authors found that 7Be rainwater activity concentration of individual events ranged from 0.93 to 10.45 Bq l1, and calculated that the annual depositional flux for this radionuclide was 2087 23 Bq m2 y1. A linear correlation was also found between 7 Be integrated activity (Bq m2) and precipitation over the sampling interval (R2 ¼ 0.66) (November 1997–November 1998). In addition, when rain seasonal variability and atmospheric reload processes were considered, a very good agreement between calculated and measured 7Be deposition activity was obtained (R2 ¼ 0.92). For a relatively dry climate at Thessaloniki, Northern Greece, Ioannidou and Papastefanou (2006) showed that precipitation intensity qualitatively characterizes the 7Be content in rainwater. In fact, over a six-year rain sampling period, they concluded that the rainwater 7Be content was larger for low precipitation rates. Furthermore, for short duration events (<5 h) and low rainfall rates, the rainwater activity concentration content was almost five times larger than for high rainfall rates. Recently, 7Be atmospheric depositional fluxes were determined for a seven-year period in Boston, Massachusetts (Zhu and Olsen, 2009), measuring 7Be in monthly precipitation samples from September 2000 to August 2007. The monthly 7Be atmospheric deposition flux ranged from 1 to 670 Bq m2, with an average of 216 Bq m2. These authors found that the atmospheric deposition fluxes of 7Be were positively correlated with the total amounts of precipitation (R2 ¼ 0.46). When seasonality in the deposition fluxes was studied, they concluded that in spring and summer months the deposition flux was near 1.5 times larger than during autumn and winter months. Using this flux, the authors calculated the 7Be soil inventory and its seasonal dependence. The aim of this research is to investigate the content of 7Be in rainwater and its variability in a semiarid ecosystem at San Luis Province, central Argentina. Precipitation samples were collected
Be content in rainwater
Activity concentration (Bq l1)
Uncertainty (Bq l1)
42 32 7 18 10 5.5 4 38 4 8 5 33 22
1.0 0.9 1.8 1.3 1.3 1.9 2.3 1.6 2.6 2.1 1.8 1.5 2.3
0.3 0.3 0.3 0.3 0.2 0.3 0.5 0.4 0.5 0.6 0.4 0.4 0.4
Jan 26, 2007 Jan 29 Jan 29 Jan 31 Feb 14 Feb 15 Feb 21 Feb 23 Feb 23 Feb 26 Mar 1 Mar 7 Mar 13 Mar 15 Mar 28 Mar 29 Mar 30 Apr 4 Oct 13 Oct 14 Oct 15 Oct 30 Oct 31 Nov 8
25 16 8 5 32 22 5 17 5 7.5 12 6 25 54 8 38 21 8.5 30 4 21 1.7 24 38
0.7 2.0 3.1 2.1 1.9 1.5 1.8 3.0 2.2 1.6 1.7 1.6 1.6 1.7 1.5 1.7 1.1 1.4 0.7 1.7 1.8 1.2 1.4 2.6
0.4 0.7 0.7 0.5 0.4 0.4 0.5 0.6 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.4
Jan 18, 2008 Jan 21 Jan 22 Jan 24 Jan 25 Jan 26 Feb 4 Feb 15 Feb 16 Feb 17 Feb 20 Feb 21 Feb 28 Feb 29 Mar 6 Mar 6 Mar 7 Mar 9 Mar 28 Apr 7 May 28
30 40 6 4 4 11 38 7 55 36 30 57 14 1.5 14 1 2.5 9 47.5 28 11
1.9 2.2 2.0 1.4 0.7 0.7 1.2 1.8 2.1 2.1 2.0 2.1 1.5 2.1 1.6 1.1 2.0 2.0 1.8 1.9 0.8
0.5 0.5 0.6 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.5 0.4 0.4 0.5 0.5 0.6 0.5 0.4 0.4 0.5 0.2
140 120
7
during an18-month period, from November 2006 to May 2008. Possible relationships among the content of 7Be in rainwater in the regional regime were explored. The investigations were conducted in order to assess the 7Be annual depositional flux for the region and to predict seasonal changes in 7Be areal activity density in soil along the entire annual cycle on the basis of this depositional flux.
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981
10
5
0
c
20
Number of events
b
15
Number of events
Number of events
a
15
10
5
979
15
10
0
5
0
0 5 10 15 20 25 30 35 40 45 50 55 60
0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
Rainfall (mm)
Rainwater activity concentration (Bq l-1)
0
20
40
60
80
100
120
Activity deposition (Bq m-2)
Fig. 2. Histogram of the number of rainfall events taking into account: (a) the rainfall amount; (b) 7Be activity concentration in rainwater; (c) the 7Be activity density deposition.
A rainfall collector was installed in central Argentina (S 33 90 ; W 66 180 ) 15 km north of San Luis City (Province of San Luis). The altitude of the sampling site is 709 m above sea level. The average annual temperature is 17 C, while in summer (December–March) the mean temperature is 23 C. Annual rainfall ranges from 600 mm to 800 mm. In this region rainfall varies seasonally, with a dry season (from May to October) and a rainy season (from November to April). Rain events in the dry season are scarce and sporadic, with occasional drizzles. Fig. 1 presents, for each month, the accumulated rainfall mean value in the studied region considering the last four-year period. 2.2. Sampling procedures Rainwater samples were obtained using a 10 l capacity plastic collector of 0.16 m2 surface area, placed 1 m above the ground to avoid soil derived contributions. The rainwater collector was deployed when precipitation began and removed when it ended. After each rain event the collector was washed with distilled water. From the total amount of water collected for each rain event, a subsample of 500 ml was taken, filtered and transferred to plastic bottles without further treatment. The amount of precipitation was measured with a standard pluviometer. The associated error due to the water collection procedure was estimated to be less than 3%. This error is substantially lower than the activity measurement error. 2.3.
7
Be activity analysis
The 7Be content in rainwater was determined by measuring its gamma emission at 477.6 keV. Gamma-ray measurements were performed using a special 40% efficiency high-purity germanium detector with an electroformed high-purity copper cryostat built by Princeton Gamma-Tech. The ultralow background technology of this detector was developed by the Pacific Northwest Laboratory/ University of South Carolina collaboration (Brodzinski et al., 1990). The detector was surrounded by approximately 50 cm of lead bricks to provide shielding against radioactive background. For gamma counting, 400 ml of each rainwater sample were placed in plastic Marinelli beakers. Counting periods were typically one day. For this geometry, an absolute photo-peak detection efficiency was performed following the method described by Di Gregorio et al. (2004). A mixture of the natural occurring long-lived isotopes, 176Lu, 138La, and 40K was used. These isotopes emit gamma-rays with energies ranging from 88 to 1460 keV and have simple and well understood decay schemes. The calibration source was prepared by diluting known amounts of the chemical compounds Lu2O3, La2O3, and KCl in 400 ml of deionized water. Data were acquired by using conventional modular electronic instruments and were accumulated in an 8K ADC (Analog to Digital Converter) multichannel analyzer connected to a personal computer. The activity concentrations of 7Be were determined by measuring the 477.6-keV gamma-ray and were corrected for decay to the time of deposition. The errors on the activity concentrations arise from the statistical uncertainty in the peak areas and the uncertainty in the absolute efficiency of the gamma detector. By summing in quadrature the different sources of error, the overall uncertainties range from 20% to almost 50% for activities in the order of 0.3 Bq l1. This value represents the detection limit of the present measurements. Two distinctive features should be pointed out regarding the present activity measurements: 1) the rainwater samples were just filtered after their collection; they did not receive any chemical treatment. For each rain event, a 400 ml
2.4. Data analysis and statistical procedures In this work, the descriptive statistical analysis, the statistical tests (ANOVA) and linear fits were carried out using OriginPro 7.0 (OriginLab, 2002). When the linear fit between the 7Be activity concentrations in rainwater and the precipitation records was performed, uncertainties in the measured activity values were taken into account. For the linear regression, error bars were used as statistical weights of experimental data (1/(error)2) (Bevington, 1969). With the aim of exploring a possible influence of rainfall intensity on 7Be rainwater content, data were clustered in three categories according to precipitation intensity and ANOVA test was performed to determine probable differences between the groups.
3. Results and discussion Table 1 presents the date, the rainfall amount, the 7Be activity concentration in rainwater and its corresponding uncertainty, for the 58 rainfall sampled events. The 7Be activity concentrations in rainwater ranged from 0.7 0.3 Bq l1 to 3.2 0.7 Bq l1, with a mean value of 1.7 Bq l1 (sd ¼ 0.53 Bq l1). Fig. 2(a) shows the frequency histogram for the sampled rainfall. The total precipitation was less than 10 mm in almost 40% of the sampled rainfall events. Precipitation was larger than 50 mm for only 3 rain events. Be activity concentration in rainwater (Bq l-1)
2.1. Study area
subsample was measured from the total rainwater that was collected; and 2) the counting rate of our ultralow environmental background detector is only approximately 20 counts keV1 d1 at the region of interest (477 keV). Both characteristics combined make the present activity measurements a particular case in comparison with previous experiments in which either rainwater samples were chemically treated or detectors did not have such high sensitivity (Caillet et al., 2001; Ioannidou and Papastefanou, 2006; Wallbrink and Murray, 1994; Zhu and Olsen, 2009).
7
2. Materials and methods
2007
2006
4,0
2008
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0
5
10
15
20
25
30
35
40
45
50
55
60
Rainfall event number Fig. 3. 7Be activity concentration in rainwater for each sampled rainfall event. Date of each event can be obtained from Table 1.
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0
10
20
30
40
50
60
Rainfall (mm)
7
7
4,0
Be activity concentration in rainwater (Bq l-1)
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981
Be activity concentration in rainwater (Bq l-1)
980
3,0
2,5
2,0
1,5
1,0
0,5
7
Fig. 4. Be activity concentration in rainwater versus rainfall amount.
0.05; 0.1
0.05
0.1
Precipitation intensity (mm min-1) Fig. 5. Box chart representation of 7Be activity concentration in rainwater for the rainfall events grouped according to precipitation intensity (lesser than 0.05 mm min1, between 0.05 and 0.1 mm min1 and greater than 0.1 mm min1). Boxes are determined by the 25th and 75th percentiles. Whiskers are determined by the 5th and 95th percentiles. The arithmetic mean 7Be activity concentration values are also represented.
The 7Be areal activity density in soil through the entire annual cycle (Fig. 7) was estimated using the parameters obtained from the linear fit and their respective errors, and considering the precipitation regime in the studied area. In order to perform this calculation a simple iteration model was applied. In this approach, for each time step of the iteration (ten days), the rainfall used is the mean rainfall value for this period, calculated on the basis of the last four years (see Fig. 1). The iteration stops when the difference in two successive areal activity density values is less than 0.02%, when comparing the same month in two successive years. The convergence to the steady value for each month was reached for an iteration period equivalent to three years. This procedure assumes that a) 7Be wet deposition is retained in soil, i.e. there are no superficial nor in-depth losses (only radioactive decay was considered), and b) the values of the 7Be activity concentration in rainwater remains constant.
Be areal activity density (Bq m -2 )
160 140 120 100
Linear fit (y = a + bx) a = 0.16 ± 0.34 b = 1.52 ± 0.07 R = 0.92 90% Confidence bands amin= -0.51; amax= 0.85 bmin= 1.39; bmax= 1.66
80 60 40 20
7
Fig. 2(b) shows the frequency histogram for the 7Be activity concentration in rainwater with intervals of 0.4 Bq l1. 7Be activity density deposition was assessed for each rainfall event by using the activity concentration and the rainfall amount. Fig. 2(c) shows the frequency histogram for 7Be activity density deposition. Values of 7 Be ground deposition ranged from 1.1 to 120 Bq m2 with a mean value of 32.7 Bq m2 (sd ¼ 29.9 Bq m2). Almost 50% of the individual rainfall events contributed, individually, less than 20 Bq m2. Fig. 3 presents the 7Be content in rainwater for each sampled rainfall event following a chronological order. The date corresponding to each event number can be obtained from Table 1. The larger values for rainwater activity (>2.5 Bq l1) correspond to the events identified as: 9, 16, 21, and 37, with dates: Dec 8, 2006; Jan 26, 2007; Feb 23, 2007 and Nov 8, 2007, respectively. When ANOVA test was applied no correlation was found between the 7Be content in rainwater and the precipitation date clustered by year, season, month, rainfall amount or elapsed time between events. Fig. 4 displays the 7Be activity concentration in rainwater as a function of rainfall amount. On the same lines as discussed above, no correlation was observed between the 7Be content in rainwater and the rainfall amount (R ¼ 0.025). So as to explore a possible dependence between the 7Be content in rainwater and the precipitation intensity, rainfall values were grouped in three categories according to the respective precipitation intensity values. The selected categories were a) lower than 0.05 mm min1 (drizzle), b) from 0.05 to 0.1 mm min1 (middle rainfall), and c) greater than 0.1 mm min1 (downpour). The box chart representation in Fig. 5 shows the distribution of the 7Be activity concentration in rainwater for each one of the three selected grouping values. As usual, boxes are determined by the 25th and 75th percentiles. The whiskers are determined by the 5th and 95th percentiles (OriginLab, 2002). Arithmetic mean values for each category are also represented. ANOVA test, at the 95.0% confidence level, shows no statistically significant difference among the values of the 7Be activity concentration in rainwater of these three data groups (F ¼ 0.51, p ¼ 0.61) (OriginLab, 2002). Taking into consideration the 7Be content in rainwater and the regional precipitation regime, the 7Be atmospheric depositional flux was estimated at 1140 120 Bq m2 y1. Since the 7Be activity concentration in rainwater appears to be independent of precipitation amount, a linear relationship is expected between the 7Be areal activity density deposition and rainfall amount. This relationship is shown in Fig. 6. The error bars were determined from the uncertainties in the 7Be activity concentration values. In this graph, the linear fit was performed considering error bars as weights (1/(error)2) (Bevington, 1969). The errors associated to each fit parameter are given in the same figure.
0 0
10
20
30
40
50
60
Rainfall ( mm) Fig. 6. 7Be activity density deposition versus rainfall amount. Linear fit was obtained with error as weight. Coefficient of the straight line, maximum and minimum coefficient values for the 90% confident band is indicated.
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981
700 7
Be soil content Be wet deposition
7
-2
Areal activity density (Bq m )
600
981
and decreases to values close to 100 Bq m2 during the dry period. Further research would be necessary, particularly regarding direct measure of 7Be soil activity density in the region, to confirm the theoretical prediction and the compliance of the assumptions of the study.
500
Acknowledgments 400
This research was supported by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), the Universidad Nacional de San Luis, and the Universidad Nacional de San Martı´n, Argentina. H. Velasco is grateful for the helpful support of the Mgter. Mariana Pascual.
300 200 100
References 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 7. Lower plot: 7Be wet activity density deposition for ten-day periods during the entire year (with bars). Upper plot: calculated areal activity density in soil running a three-year iteration process based on the 7Be wet deposition flux, and radioactive decay (circle). Error bars were obtained from the uncertainty in the determination of parameters a and b in the linear fit (Fig. 6).
4. Conclusions A semiarid region situated in central Argentina was selected in order to: a) explore the 7Be activity deposition through rainfall events and b) characterize this input along the annual cycle. In this research, 7Be activity concentrations were measured in 58 individual rain events during a period of 18 months. Results suggest that, in this region, 7Be concentration in rainwater is independent of individual rainfall events. Additionally, the elapsed time between rainfalls and precipitation intensity do not have a decisive influence on the 7Be content in rainwater. As a consequence, a very good relationship was found when the 7Be activity density deposition was represented as a linear function of precipitation amount. From this finding, a simple depositional model has been applied. Using the parameters obtained from the linear fit of the 7Be activity density deposition versus the precipitation amount and considering the precipitation regime in the region under study, the 7Be areal activity density through the entire annual cycle was modelled. The model presupposes that the total fallen 7Be is retained in soil, which constitutes one of the principal assumptions when this radionuclide is used as tracer of environmental processes, such as soil redistribution. The theoretical analysis allows a good characterization of 7Be inputs into the terrestrial ecosystem and their seasonal dependence. In keeping with this claim, the 7Be content in soil may be expected to reach values close to 600 Bq m2 during the wet period
Bevington, P.H., 1969. Data Reduction and Error Analysis for the Physical Sciences. McGraw Hill. Blake, W.H., Walling, D.E., He, Q., 1999. Fallout beryllium-7 as a tracer in soil erosion investigations. Appl. Radiat. Isot. 51 (5), 599–605. Brodzinski, R.L., Miley, H.S., Reeves, J.H., Avignone III, F.T., 1990. Further reduction of radioactive backgrounds in ultrasensitive germanium spectrometers. Nucl. Instrum. Methods A292, 337–342. Caillet, S., Arpagaus, P., Monna, F., Dominik, J., 2001. Factors controlling 7Be and 210 Pb atmospheric deposition as revealed by sampling individual rain events in the region of Geneva, Switzerland. J. Environ. Radioact. 53, 241–256. Di Gregorio, D.E., Huck, H., Aristegui, R., De Lazzari, G., Jech, A., 2004. 137Cs contamination in tea and Yerba mate in South America. J. Environ. Radioact. 76, 273–281. Ioannidou, A., Manolopoulou, M., Papastefanou, C., 2005. Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (40 N). Appl. Radiat. Isot. 63, 277–284. Ioannidou, A., Papastefanou, C., 2006. Precipitation scavenging of 7Be and 137Cs radionuclides in air. J. Environ. Radioact. 85, 121–136. Kaste, J.M., Norton, S.A., Hess, C., 2002. Enviromental chemistry of 7Be. Rev. Mineral. Geochem. 50, 271–289. Origin, 2002. Scientific Graphing and Analysis Software. Version 7. OriginLab Corporation, Northampton, MA 01060. www.OriginLab.com. Salisbury, R.T., Cartwright, J., 2005. Cosmogenic 7Be deposition in North Wales: 7Be concentrations in sheep faeces in relation to altitude and precipitation. J. Environ. Radioact. 78, 353–361. Schuller, P., Iroume´, A., Walling, D.E., Mancilla, B., Castillo, A., Trumper, R.E., 2006. Use of beryllium-7 to document soil redistribution following forest harvest operation. J. Environ. Qual. 35, 1756–1763. Sepulveda, A., Schuller, P., Walling, D.E., Castillo, A., 2008. Use of 7Be to document soil erosion associated with a short period of extreme rainfall. J. Environ. Radioact. 99, 35–49. Wallbrink, P.J., Murray, A.S., 1994. Fallout of 7Be in South Eastern Australia. J. Environ. Radioact. 25, 213–228. Wallbrink, P.J., Murray, A.S., 1996. Distribution and variability of 7Be in soils under different surface cover conditions and its potential for describing soil redistribution processes. Water Resour. Res. 32, 467–476. Walling, D.E., He, Q., Blake, W., 1999. Use of 7Be and 137Cs measurements to document short and medium-term rates of water-induced soil erosion on agricultural land. Water Resour. Res. 35, 3865–3874. Wilson, C.G., Matisoff, G., Whiting, P.J., 2003. Short-term erosion rates from a 7Be inventory balance. Earth Surf. Process. Landforms 28, 967–977. Zhu, J., Olsen, C., 2009. Beryllium-7 atmospheric deposition and sediment inventories in the Neponset River estuary, Massachusetts, USA. J. Environ. Radioact. 100, 192–197.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Short-term seasonal variability in 7Be wet deposition in a semiarid ecosystem of central Argentina J. Juri Ayub a, D.E. Di Gregorio b, c, H. Velasco a, *, H. Huck b, c, M. Rizzotto a, F. Lohaiza a a
´tica Aplicada San Luis, Universidad Nacional de San Luis – CONICET, Ejercito de los Andes 950, D5700HHW San Luis, Argentina Grupo de Estudios Ambientales, Instituto de Matema ´n Nacional de Energı´a Ato ´ mica, Av. Gral. Paz 1499, 1650 San Martı´n, Provincia de Buenos Aires, Argentina Departamento de Fı´sica, Comisio c Escuela de Ciencia y Tecnologı´a, Universidad Nacional de San Martı´n, Martı´n de Irigoyen 3100, 1650 San Martı´n, Provincia de Buenos Aires, Argentina b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 October 2008 Received in revised form 8 July 2009 Accepted 4 August 2009 Available online 27 August 2009
The 7Be wet deposition has been intensively investigated in a semiarid region at San Luis Province, Argentina. From November 2006 to May 2008, the 7Be content in rainwater was determined in 58 individual rain events, randomly comprising more than 50% of all individual precipitations at the sampling period. 7Be activity concentration in rainwater ranged from 0.7 0.3 Bq l1 to 3.2 0.7 Bq l1, with a mean value of 1.7 Bq l1 (sd ¼ 0.53 Bq l1). No relationship was found between 7Be content in rainwater and (a) rainfall amount, (b) precipitation intensity and (c) elapsed time between events. 7Be ground deposition was found to be well correlated with rainfall amount (R ¼ 0.92). For the precipitation events considered, the 7Be depositional fluxes ranged from 1.1 to 120 Bq m2, with a mean value of 32.7 Bq m2 (sd ¼ 29.9 Bq m2). The annual depositional flux was estimated at 1140 120 Bq m2 y1. Assuming the same monthly deposition pattern and that the 7Be content in soil decreases only through radioactive decay, the seasonal variation of 7Be areal activity density in soil was estimated. Results of this investigation may contribute to a valuable characterization of 7Be input in the explored semiarid ecosystem and its potential use as tracer of environmental processes. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: 7 Be atmospheric deposition Rainwater 7 Be areal activity density Central Argentina
1. Introduction 7 Be is a natural radionuclide (Eg ¼ 477.6 keV, t1/2 ¼ 53.3 d) originating from the interaction of cosmic rays with terrestrial atmosphere. The amount of 7Be that reaches the surface soil depends on the production rate (cosmic-ray intensity) which varies with latitude, altitude and solar activity. Other factors include stratosphere–troposphere mixing, circulation and advection processes within the troposphere, and the efficiency with which it is removed from the troposphere by wet and dry depositions (Kaste et al., 2002). 7 Be constitutes one of the most important environmental radionuclides, and the number of investigations regarding the application of 7Be as a tracer of environmental processes has increased notably in recent years. This radionuclide has been used as a tracer for the transport and residence time of aerosols in the atmosphere (Kaste et al., 2002) in sedimentological studies, and to assess soil redistribution after rain events. Wallbrink and Murray (1996) investigated 7Be in soils to assess the movement of topsoil in bare soil, grassland, and eucalypt forest. Walling et al. (1999) and
* Corresponding author. Tel./fax: þ54 2652 422803. E-mail address: [email protected] (H. Velasco). 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.08.001
Blake et al. (1999) used 7Be to investigate erosion and sedimentation in cultivated field in Devon, England. Blake et al. (1999) successfully demonstrated the potential for using 7Be as a tracer in soil erosion research. Later, Wilson et al. (2003) used the inventory balance for 7Be to calculate sediment erosion during a series of runoff-producing thunderstorms occurring over a short time period in Treynor, USA. Recently, Schuller et al. (2006) and Sepulveda et al. (2008) used 7Be to document soil erosion associated with a short period of extreme rainfall in south-central Chile. Various studies revealed that 7Be enters continuously into the ecosystem, principally through wet deposition. Salisbury and Cartwright (2005) estimated that wet fallout accounts for about 97% of the total 7Be ground deposition. Ioannidou et al. (2005) and Wallbrink and Murray (1994) suggested that dry depositions contribute, at a maximum, with only 8% of the total deposition. Further studies have shown that for different regions and dissimilar environmental conditions, 7Be wet deposition flux was dependent on rain precipitation volume (Kaste et al., 2002). Consequently, the seasonal 7Be wet deposition pattern may be closely associated with rainfall variability in the studied sites (Caillet et al., 2001). Potential correlations among 7Be wet deposition, total precipitation and precipitation intensity have been extensively studied by different authors with divergent results. Wallbrink and Murray
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J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981 Table 1 Rainfall amount and 7Be content in rainwater.
180
Accumulated rainfall (mm)
160
Event number
Date
1 2 3 4 5 6 7 8 9 10 11 12 13
Nov 15, 2006 Nov 18 Nov 25 Nov 25 Nov 26 Nov 26 Nov 1 Dec 5 Dec 8 Dec 10 Dec 13 Dec 17 Dec 18
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Rainfall (mm)
100 80 60 40 20 0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Fig. 1. Mean monthly accumulated rainfall amount in the studied zone. Mean values were obtained averaging over the last four years. Bars indicate the standard error.
(1994) have concluded that there is no significant correlation between concentration and rainfall for precipitation events less than 25 mm, with activity concentration values ranging from 0.02 to 5.9 Bq l1. They reported that the total annual activity depositional flux delivered was 1030 100 Bq m2 y1, and found a correlation between rainfall events and the deposited areal activity of 7Be (R2 ¼ 0.6). Similar results were obtained by Caillet et al. (2001) after analyzing 46 rain events, at Versoix, Switzerland. These authors found that 7Be rainwater activity concentration of individual events ranged from 0.93 to 10.45 Bq l1, and calculated that the annual depositional flux for this radionuclide was 2087 23 Bq m2 y1. A linear correlation was also found between 7 Be integrated activity (Bq m2) and precipitation over the sampling interval (R2 ¼ 0.66) (November 1997–November 1998). In addition, when rain seasonal variability and atmospheric reload processes were considered, a very good agreement between calculated and measured 7Be deposition activity was obtained (R2 ¼ 0.92). For a relatively dry climate at Thessaloniki, Northern Greece, Ioannidou and Papastefanou (2006) showed that precipitation intensity qualitatively characterizes the 7Be content in rainwater. In fact, over a six-year rain sampling period, they concluded that the rainwater 7Be content was larger for low precipitation rates. Furthermore, for short duration events (<5 h) and low rainfall rates, the rainwater activity concentration content was almost five times larger than for high rainfall rates. Recently, 7Be atmospheric depositional fluxes were determined for a seven-year period in Boston, Massachusetts (Zhu and Olsen, 2009), measuring 7Be in monthly precipitation samples from September 2000 to August 2007. The monthly 7Be atmospheric deposition flux ranged from 1 to 670 Bq m2, with an average of 216 Bq m2. These authors found that the atmospheric deposition fluxes of 7Be were positively correlated with the total amounts of precipitation (R2 ¼ 0.46). When seasonality in the deposition fluxes was studied, they concluded that in spring and summer months the deposition flux was near 1.5 times larger than during autumn and winter months. Using this flux, the authors calculated the 7Be soil inventory and its seasonal dependence. The aim of this research is to investigate the content of 7Be in rainwater and its variability in a semiarid ecosystem at San Luis Province, central Argentina. Precipitation samples were collected
Be content in rainwater
Activity concentration (Bq l1)
Uncertainty (Bq l1)
42 32 7 18 10 5.5 4 38 4 8 5 33 22
1.0 0.9 1.8 1.3 1.3 1.9 2.3 1.6 2.6 2.1 1.8 1.5 2.3
0.3 0.3 0.3 0.3 0.2 0.3 0.5 0.4 0.5 0.6 0.4 0.4 0.4
Jan 26, 2007 Jan 29 Jan 29 Jan 31 Feb 14 Feb 15 Feb 21 Feb 23 Feb 23 Feb 26 Mar 1 Mar 7 Mar 13 Mar 15 Mar 28 Mar 29 Mar 30 Apr 4 Oct 13 Oct 14 Oct 15 Oct 30 Oct 31 Nov 8
25 16 8 5 32 22 5 17 5 7.5 12 6 25 54 8 38 21 8.5 30 4 21 1.7 24 38
0.7 2.0 3.1 2.1 1.9 1.5 1.8 3.0 2.2 1.6 1.7 1.6 1.6 1.7 1.5 1.7 1.1 1.4 0.7 1.7 1.8 1.2 1.4 2.6
0.4 0.7 0.7 0.5 0.4 0.4 0.5 0.6 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.4
Jan 18, 2008 Jan 21 Jan 22 Jan 24 Jan 25 Jan 26 Feb 4 Feb 15 Feb 16 Feb 17 Feb 20 Feb 21 Feb 28 Feb 29 Mar 6 Mar 6 Mar 7 Mar 9 Mar 28 Apr 7 May 28
30 40 6 4 4 11 38 7 55 36 30 57 14 1.5 14 1 2.5 9 47.5 28 11
1.9 2.2 2.0 1.4 0.7 0.7 1.2 1.8 2.1 2.1 2.0 2.1 1.5 2.1 1.6 1.1 2.0 2.0 1.8 1.9 0.8
0.5 0.5 0.6 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.5 0.4 0.4 0.5 0.5 0.6 0.5 0.4 0.4 0.5 0.2
140 120
7
during an18-month period, from November 2006 to May 2008. Possible relationships among the content of 7Be in rainwater in the regional regime were explored. The investigations were conducted in order to assess the 7Be annual depositional flux for the region and to predict seasonal changes in 7Be areal activity density in soil along the entire annual cycle on the basis of this depositional flux.
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981
10
5
0
c
20
Number of events
b
15
Number of events
Number of events
a
15
10
5
979
15
10
0
5
0
0 5 10 15 20 25 30 35 40 45 50 55 60
0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2
Rainfall (mm)
Rainwater activity concentration (Bq l-1)
0
20
40
60
80
100
120
Activity deposition (Bq m-2)
Fig. 2. Histogram of the number of rainfall events taking into account: (a) the rainfall amount; (b) 7Be activity concentration in rainwater; (c) the 7Be activity density deposition.
A rainfall collector was installed in central Argentina (S 33 90 ; W 66 180 ) 15 km north of San Luis City (Province of San Luis). The altitude of the sampling site is 709 m above sea level. The average annual temperature is 17 C, while in summer (December–March) the mean temperature is 23 C. Annual rainfall ranges from 600 mm to 800 mm. In this region rainfall varies seasonally, with a dry season (from May to October) and a rainy season (from November to April). Rain events in the dry season are scarce and sporadic, with occasional drizzles. Fig. 1 presents, for each month, the accumulated rainfall mean value in the studied region considering the last four-year period. 2.2. Sampling procedures Rainwater samples were obtained using a 10 l capacity plastic collector of 0.16 m2 surface area, placed 1 m above the ground to avoid soil derived contributions. The rainwater collector was deployed when precipitation began and removed when it ended. After each rain event the collector was washed with distilled water. From the total amount of water collected for each rain event, a subsample of 500 ml was taken, filtered and transferred to plastic bottles without further treatment. The amount of precipitation was measured with a standard pluviometer. The associated error due to the water collection procedure was estimated to be less than 3%. This error is substantially lower than the activity measurement error. 2.3.
7
Be activity analysis
The 7Be content in rainwater was determined by measuring its gamma emission at 477.6 keV. Gamma-ray measurements were performed using a special 40% efficiency high-purity germanium detector with an electroformed high-purity copper cryostat built by Princeton Gamma-Tech. The ultralow background technology of this detector was developed by the Pacific Northwest Laboratory/ University of South Carolina collaboration (Brodzinski et al., 1990). The detector was surrounded by approximately 50 cm of lead bricks to provide shielding against radioactive background. For gamma counting, 400 ml of each rainwater sample were placed in plastic Marinelli beakers. Counting periods were typically one day. For this geometry, an absolute photo-peak detection efficiency was performed following the method described by Di Gregorio et al. (2004). A mixture of the natural occurring long-lived isotopes, 176Lu, 138La, and 40K was used. These isotopes emit gamma-rays with energies ranging from 88 to 1460 keV and have simple and well understood decay schemes. The calibration source was prepared by diluting known amounts of the chemical compounds Lu2O3, La2O3, and KCl in 400 ml of deionized water. Data were acquired by using conventional modular electronic instruments and were accumulated in an 8K ADC (Analog to Digital Converter) multichannel analyzer connected to a personal computer. The activity concentrations of 7Be were determined by measuring the 477.6-keV gamma-ray and were corrected for decay to the time of deposition. The errors on the activity concentrations arise from the statistical uncertainty in the peak areas and the uncertainty in the absolute efficiency of the gamma detector. By summing in quadrature the different sources of error, the overall uncertainties range from 20% to almost 50% for activities in the order of 0.3 Bq l1. This value represents the detection limit of the present measurements. Two distinctive features should be pointed out regarding the present activity measurements: 1) the rainwater samples were just filtered after their collection; they did not receive any chemical treatment. For each rain event, a 400 ml
2.4. Data analysis and statistical procedures In this work, the descriptive statistical analysis, the statistical tests (ANOVA) and linear fits were carried out using OriginPro 7.0 (OriginLab, 2002). When the linear fit between the 7Be activity concentrations in rainwater and the precipitation records was performed, uncertainties in the measured activity values were taken into account. For the linear regression, error bars were used as statistical weights of experimental data (1/(error)2) (Bevington, 1969). With the aim of exploring a possible influence of rainfall intensity on 7Be rainwater content, data were clustered in three categories according to precipitation intensity and ANOVA test was performed to determine probable differences between the groups.
3. Results and discussion Table 1 presents the date, the rainfall amount, the 7Be activity concentration in rainwater and its corresponding uncertainty, for the 58 rainfall sampled events. The 7Be activity concentrations in rainwater ranged from 0.7 0.3 Bq l1 to 3.2 0.7 Bq l1, with a mean value of 1.7 Bq l1 (sd ¼ 0.53 Bq l1). Fig. 2(a) shows the frequency histogram for the sampled rainfall. The total precipitation was less than 10 mm in almost 40% of the sampled rainfall events. Precipitation was larger than 50 mm for only 3 rain events. Be activity concentration in rainwater (Bq l-1)
2.1. Study area
subsample was measured from the total rainwater that was collected; and 2) the counting rate of our ultralow environmental background detector is only approximately 20 counts keV1 d1 at the region of interest (477 keV). Both characteristics combined make the present activity measurements a particular case in comparison with previous experiments in which either rainwater samples were chemically treated or detectors did not have such high sensitivity (Caillet et al., 2001; Ioannidou and Papastefanou, 2006; Wallbrink and Murray, 1994; Zhu and Olsen, 2009).
7
2. Materials and methods
2007
2006
4,0
2008
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0
5
10
15
20
25
30
35
40
45
50
55
60
Rainfall event number Fig. 3. 7Be activity concentration in rainwater for each sampled rainfall event. Date of each event can be obtained from Table 1.
3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0
10
20
30
40
50
60
Rainfall (mm)
7
7
4,0
Be activity concentration in rainwater (Bq l-1)
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981
Be activity concentration in rainwater (Bq l-1)
980
3,0
2,5
2,0
1,5
1,0
0,5
7
Fig. 4. Be activity concentration in rainwater versus rainfall amount.
0.05; 0.1
0.05
0.1
Precipitation intensity (mm min-1) Fig. 5. Box chart representation of 7Be activity concentration in rainwater for the rainfall events grouped according to precipitation intensity (lesser than 0.05 mm min1, between 0.05 and 0.1 mm min1 and greater than 0.1 mm min1). Boxes are determined by the 25th and 75th percentiles. Whiskers are determined by the 5th and 95th percentiles. The arithmetic mean 7Be activity concentration values are also represented.
The 7Be areal activity density in soil through the entire annual cycle (Fig. 7) was estimated using the parameters obtained from the linear fit and their respective errors, and considering the precipitation regime in the studied area. In order to perform this calculation a simple iteration model was applied. In this approach, for each time step of the iteration (ten days), the rainfall used is the mean rainfall value for this period, calculated on the basis of the last four years (see Fig. 1). The iteration stops when the difference in two successive areal activity density values is less than 0.02%, when comparing the same month in two successive years. The convergence to the steady value for each month was reached for an iteration period equivalent to three years. This procedure assumes that a) 7Be wet deposition is retained in soil, i.e. there are no superficial nor in-depth losses (only radioactive decay was considered), and b) the values of the 7Be activity concentration in rainwater remains constant.
Be areal activity density (Bq m -2 )
160 140 120 100
Linear fit (y = a + bx) a = 0.16 ± 0.34 b = 1.52 ± 0.07 R = 0.92 90% Confidence bands amin= -0.51; amax= 0.85 bmin= 1.39; bmax= 1.66
80 60 40 20
7
Fig. 2(b) shows the frequency histogram for the 7Be activity concentration in rainwater with intervals of 0.4 Bq l1. 7Be activity density deposition was assessed for each rainfall event by using the activity concentration and the rainfall amount. Fig. 2(c) shows the frequency histogram for 7Be activity density deposition. Values of 7 Be ground deposition ranged from 1.1 to 120 Bq m2 with a mean value of 32.7 Bq m2 (sd ¼ 29.9 Bq m2). Almost 50% of the individual rainfall events contributed, individually, less than 20 Bq m2. Fig. 3 presents the 7Be content in rainwater for each sampled rainfall event following a chronological order. The date corresponding to each event number can be obtained from Table 1. The larger values for rainwater activity (>2.5 Bq l1) correspond to the events identified as: 9, 16, 21, and 37, with dates: Dec 8, 2006; Jan 26, 2007; Feb 23, 2007 and Nov 8, 2007, respectively. When ANOVA test was applied no correlation was found between the 7Be content in rainwater and the precipitation date clustered by year, season, month, rainfall amount or elapsed time between events. Fig. 4 displays the 7Be activity concentration in rainwater as a function of rainfall amount. On the same lines as discussed above, no correlation was observed between the 7Be content in rainwater and the rainfall amount (R ¼ 0.025). So as to explore a possible dependence between the 7Be content in rainwater and the precipitation intensity, rainfall values were grouped in three categories according to the respective precipitation intensity values. The selected categories were a) lower than 0.05 mm min1 (drizzle), b) from 0.05 to 0.1 mm min1 (middle rainfall), and c) greater than 0.1 mm min1 (downpour). The box chart representation in Fig. 5 shows the distribution of the 7Be activity concentration in rainwater for each one of the three selected grouping values. As usual, boxes are determined by the 25th and 75th percentiles. The whiskers are determined by the 5th and 95th percentiles (OriginLab, 2002). Arithmetic mean values for each category are also represented. ANOVA test, at the 95.0% confidence level, shows no statistically significant difference among the values of the 7Be activity concentration in rainwater of these three data groups (F ¼ 0.51, p ¼ 0.61) (OriginLab, 2002). Taking into consideration the 7Be content in rainwater and the regional precipitation regime, the 7Be atmospheric depositional flux was estimated at 1140 120 Bq m2 y1. Since the 7Be activity concentration in rainwater appears to be independent of precipitation amount, a linear relationship is expected between the 7Be areal activity density deposition and rainfall amount. This relationship is shown in Fig. 6. The error bars were determined from the uncertainties in the 7Be activity concentration values. In this graph, the linear fit was performed considering error bars as weights (1/(error)2) (Bevington, 1969). The errors associated to each fit parameter are given in the same figure.
0 0
10
20
30
40
50
60
Rainfall ( mm) Fig. 6. 7Be activity density deposition versus rainfall amount. Linear fit was obtained with error as weight. Coefficient of the straight line, maximum and minimum coefficient values for the 90% confident band is indicated.
J. Juri Ayub et al. / Journal of Environmental Radioactivity 100 (2009) 977–981
700 7
Be soil content Be wet deposition
7
-2
Areal activity density (Bq m )
600
981
and decreases to values close to 100 Bq m2 during the dry period. Further research would be necessary, particularly regarding direct measure of 7Be soil activity density in the region, to confirm the theoretical prediction and the compliance of the assumptions of the study.
500
Acknowledgments 400
This research was supported by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), the Universidad Nacional de San Luis, and the Universidad Nacional de San Martı´n, Argentina. H. Velasco is grateful for the helpful support of the Mgter. Mariana Pascual.
300 200 100
References 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 7. Lower plot: 7Be wet activity density deposition for ten-day periods during the entire year (with bars). Upper plot: calculated areal activity density in soil running a three-year iteration process based on the 7Be wet deposition flux, and radioactive decay (circle). Error bars were obtained from the uncertainty in the determination of parameters a and b in the linear fit (Fig. 6).
4. Conclusions A semiarid region situated in central Argentina was selected in order to: a) explore the 7Be activity deposition through rainfall events and b) characterize this input along the annual cycle. In this research, 7Be activity concentrations were measured in 58 individual rain events during a period of 18 months. Results suggest that, in this region, 7Be concentration in rainwater is independent of individual rainfall events. Additionally, the elapsed time between rainfalls and precipitation intensity do not have a decisive influence on the 7Be content in rainwater. As a consequence, a very good relationship was found when the 7Be activity density deposition was represented as a linear function of precipitation amount. From this finding, a simple depositional model has been applied. Using the parameters obtained from the linear fit of the 7Be activity density deposition versus the precipitation amount and considering the precipitation regime in the region under study, the 7Be areal activity density through the entire annual cycle was modelled. The model presupposes that the total fallen 7Be is retained in soil, which constitutes one of the principal assumptions when this radionuclide is used as tracer of environmental processes, such as soil redistribution. The theoretical analysis allows a good characterization of 7Be inputs into the terrestrial ecosystem and their seasonal dependence. In keeping with this claim, the 7Be content in soil may be expected to reach values close to 600 Bq m2 during the wet period
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