Regional Calibration of Erosion Radiotracers - ACS Publications

Inventories of radionuclides commonly used to study environmental processes, especially in erosion research, were determined in soil cores from two di...
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Environ. Sci. Technol. 2007, 41, 1324-1330

Regional Calibration of Erosion Radiotracers (210Pb and 137Cs): Atmospheric Fluxes to Soils (Northern Spain) ,†,§

JOAN-ALBERT SANCHEZ-CABEZA,* M A R T A G A R C I A - T A L A V E R A , ‡,| E D U A R D C O S T A , †,⊥ V I C T O R P E N ˜ A,‡ JORDI GARCIA-ORELLANA,† P E R E M A S Q U EÄ , † A N D CONSTANTINO NALDA‡

Universitat Auto`noma de Barcelona, Spain, Universidad de Valladolid, Spain, Marine Environment Laboratories, IAEA, Consejo de Seguridad Nuclear, Madrid, Spain, and Stanford University, Stanford, California

Inventories of radionuclides commonly used to study environmental processes, especially in erosion research, were determined in soil cores from two distant river basins in northern Spain. Results showed that 210Pb atmospheric fluxes correlate very well with mean annual rainfall across the region, and this is also the case for 137Cs inventories but only on the basin scale. Therefore we suggest that 210Pb is a better candidate as a radiotracer for soil erosion studies. In this region, the equation 210Pb flux (Bq m-2 yr-1) ) (0.19 ( 0.02) × rainfall (mm yr-1) - (24 ( 17) can be used as a calibration to estimate input 210Pb fluxes, a key parameter in soil erosion studies and models, when mean annual rainfall is known.

Introduction The study of atmospheric fluxes of some radionuclides (atmospheric radiotracers) has proven to be very useful for a large variety of applications such as the estimation of aerosol residence times and reactivity in the atmosphere (1-3), the distribution and migration of pollution in the environment (4-6), and radionuclide inputs and budgets in the terrestrial (7-9), freshwater (10-12), and marine environments (1315). In particular, atmospheric radiotracers such as 137Cs have become common tools for the quantification of soil erosion processes (16-19). Most soil erosion research using radiotracers has been based on the analysis of 137Cs (18). This is a man-made radionuclide injected into the atmosphere due to nuclear weapons tests which occurred mainly in the late 1950s and early 1960s (20) and subsequently deposited on the Earth’s surface (weapons or global fallout). The 137Cs levels changed dramatically in Europe due to the Chernobyl accident, when large amounts of 137Cs were released, dispersed, and deposited in the environment following a very heterogeneous pattern, closely related to rainfall during the passage of the polluted * Corresponding author e-mail: [email protected]. † Universitat Auto ` noma de Barcelona. ‡ Universidad de Valladolid. § IAEA, Marine Environment Laboratories. | Consejo de Seguridad Nuclear. ⊥ Stanford University. 1324

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TABLE 1. Main Characteristics of Sampling Locations from the Carrion River Basin (Palencia Province, Code P) and the Noguera Pallaresa River Basin (Lleida Province, Code L) sample code

latitude (X_UTM)

longitude (Y_UTM)

altitude (m)

rainfall (mm yr-1)

P1 P2 P3 P4 P5 P6a P7 P8

Palencia: UTM zone 30 371458 4660700 766 371640 4664248 784 368147 4678770 804 357031 4710069 962 358356 4711613 980 356100 4742108 1121 349020 4749120 1090 368216 4757231 1046

399 400 439 758 769 935 1084 1144

L1b L2 L3 L4 L5 L6 L7c

302769 293896 315270 318786 331526 316829 317205

Lleida: UTM zone 31 4504373 0 4615934 323 4631531 316 4652363 460 4679188 627 4721399 1649 4723526 2240

558 395 450 525 875 1275 1450

a The parameters given for location P6 correspond to the geographical center of a 5 km2 area where eight soil cores were collected. b From ref 42. c From ref 43.

plume (7, 21, 22). This makes difficult the use of 137Cs as radiotracer of soil erosion in zones where the Chernobyl input was significant in relation to global fallout and/or presents marked inhomogeneous distribution. Yet, Golosov (23) showed that when the Chernobyl input dominates the total 137Cs inventory (or is very small) this is still a valuable tracer. is a member of the 238U decay series. 222Rn (T1/2 ) 3.8 d) exhales from the continental crust to the lower troposphere and is dispersed until it decays into 210Pb, which attaches to aerosols and is deposited on the Earth’s surface (excess 210Pb). On the other hand, 210Pb formed in situ in soils and in equilibrium with its parent radionuclide 226Ra (T1/2 ) 1601 yr) is referred to as base (or supported) 210Pb. It is surprising to note that comparatively few studies have used excess 210Pb to study erosion processes (18, 24, 25) although it should be increasingly applied in the future, since 137 Cs is a transient tracer that will disappear from the environment unless other significant inputs occur, whereas 210Pb, of natural origin, will always be present. 210Pb

The local atmospheric flux of the radiotracer should be determined from reference (unaltered) soils in the area of interest, but these are often not available and the flux must be estimated from locations far away from study area. In general, the governing mechanism in the deposition of atmospheric substances to soils is wet deposition (2, 26). Many studies have shown a good correlation of radiotracer atmospheric fluxes to soils (such as 90Sr, 137Cs, 210Pb, and plutonium isotopes) with rainfall (26-28). In this work we determine the atmospheric fluxes of the radiotracers most commonly used for research on soil erosion (137Cs and 210Pb) in reference soils from two well-defined and distant (ca. 400 km) river basins (Carrion and Noguera Pallaresa, see Supporting Information) in order to study their correlation with mean annual rainfall. This correlation can be used to estimate atmospheric fluxes to reference soils and be valuable for future erosion studies in these regions, an important socio-economic problem in all Mediterranean countries. 10.1021/es061356z CCC: $37.00

 2007 American Chemical Society Published on Web 01/19/2007

FIGURE 1. Sampling locations and mean annual rainfall in (a) the Carrion Basin (Palencia Province) and (b) in the Noguera Pallaresa Basins (Lleida Province). Note that in the Noguera Pallaresa Basin the mean annual rainfall data used was obtained from nearby meteorological stations.

Materials and Methods Soil cores were collected by using a stainless steel cylinder sampler 4.5 cm in diameter and 50 cm long. Five cores were sampled in February 2001 in the Noguera Pallaresa Basin and 15 cores were sampled in January 2003 in the Carrion Basin (Table 1), 8 of which were collected within a small area of 5 km2 and were used to provide an average result for that zone. Sampling locations were undisturbed flat grasslands, free of trees or other obstacles. The length of the collected soil cores ranged from 22 to 39 cm. After recovery, the core was visually described and sliced in situ at 5 cm intervals. Samples were placed in plastic bags and kept at 4 °C until subsequent analysis. A description of the study areas, sample pretreatment, and analysis is included in the Supporting Information.

Results Mean Annual Rainfall. The mean annual rainfall distribution in the Carrion Basin was determined using data from 30 meteorological stations provided by the Instituto Nacional de Meteorologı´a (INM). In order to have the best mean annual rainfall estimate at each station, the yearly data from the oldest available record (ranging from the 1930s to the 1980s) up to 2003 were averaged. A contour map showing lines of equal mean annual rainfall was created by using the kriging algorithm in Surfer 8 software (see Figure 1) and the rainfall value at each sampling site was then determined by interpolation (Table 1). The mean annual rainfall at each sampling site of the Noguera Pallaresa Basin was determined using historical data records from INM stations, consisting of daily precipitation measurements, which in some cases span from the 1920s to present. Soil coring locations were selected as close as possible to the INM network stations to directly assign the instrumental rainfall record to the sampling location. In order to have the best possible mean annual rainfall estimate at each station, the annual average precipitation was determined by averaging the daily precipitation values (ranging from 18 to 80 years; Table 1). Radionuclide Distributions. Figures 2 and 3 show the depth distributions of 137Cs and excess 210Pb (210Pbex) in soil cores from the two catchments. For 137Cs, the depth profiles

vary from nearly exponential (e.g., L4) to nearly uniform (e.g., P3). Some exhibit subsurface concentration maxima of varying widths (e.g., compare P2 with P8). The shapes of the 210Pb profiles are more homogeneous as they all present a ex surface maximum and decrease with depth either with an exponential or near-quadratic shape. For the Lleida soils, 90% of the activity of both radionuclides is always found in the top 15 cm. The same occurs for 210Pbex in soils from Palencia, while 137Cs often extends deeper. It is usual to find such diversity in 137Cs or 210Pbex profiles in undisturbed soils (29-31). The depth distribution of a fallout radionuclide is the result of many factors, such as soil properties (contents and depth distribution of clay minerals and organic matter, permeability, etc.), vegetation cover, hydrology or underlying geology (32). Processes such as sorption, leaching, mass transport through macropores (33) or bioturbation (34), can also drastically affect the radiotracer profile. The total inventory (I) of 137Cs or excess 210Pb in a sampling site was obtained by the addition of the total activity in each section of the core as values below the detection limits were observed in lower layers. The resulting values are shown, together with other published values, in Table 2 and they are considered to constitute records of atmospheric fallout alone since they have been obtained from flat areas of open grassland showing no tillage activity for the past few decades. Assuming that 210Pbex in soils is in steady state between input from the atmosphere and radioactive decay, the average atmospheric 210Pb flux can be calculated according to the following expression: Pb flux (Bq m-2 yr-1) ) λ (yr-1) × 210Pbex inventory (Bq m-2)

210

where λ is the 210Pb decay constant (0.0311 yr-1). The atmospheric 210Pb fluxes at all sites are presented in Table 2. The 137Cs inventories from the Carrion Basin ranged from 564 ( 14 to 3090 ( 263 Bq m-2 for an annual rainfall range of [399, 1144] mm yr-1. For the Noguera Pallaresa Basin, where average rainfall ranged from 395 to 1450 mm yr-1, 137Cs inventories ranged from 1448 ( 53 to 6911 ( 146 Bq m-2. Given that fallout of radionuclides is largely governed by rainfall, the large variability found in both regions can VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2.

210Pb

ex

and

137Cs

profiles of soil samples collected in the Carrion River Basin (Palencia Province).

possibly be ascribed to the wide range of annual precipitation values. In fact, the two catchments selected for this study were chosen for this specific characteristic. The 210Pb fluxes in Palencia ranged from 32 ( 3 to 219 ( 7 Bq m-2 yr-1, while for Lleida the values obtained were between 64 ( 5 and 255 ( 8 Bq m-2 yr-1.

Discussion 210Pb

fluxes to soils depend on regional 222Rn exhalation, which in turn depends on soil properties and, in general, on the geological setting of the area. In principle one would not expect 210Pb fluxes in the two areas of interest to be comparable, and, therefore, an independent regression analysis of 210Pb with mean annual rainfall was carried out for each area, showing excellent correlations in both cases (Carrion Basin: r 2 ) 0.88, p < 0.001; Noguera Pallaresa Basin: r 2 ) 0.94, p < 0.001). In order to compare the two regression lines, Snedecor’s F-test was used to compare the regression variances and it was concluded that they were equivalent. Then, the comparison of the regression parameters using the Student t-test showed that both regression lines were not statistically different. This interesting result indicates that 210Pb fluxes are similar in both regions and possibly, by extrapolation, in other regions in northern Spain, maybe with the exceptions of (i) coastal locations affected by oceanic low-radon air masses, and (ii) areas with substantially high radon exhalation rates. A map of theoretical 222Rn exhalation rates in Spain and the procedure followed to produce the map are included in the Supporting Information . Although, in general, the 210Pb flux to a region is largely dominated by long-range transport processes (35), for environments with significantly 1326

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enhanced radon exhalation rates, the local component could be also substantial. The combination of both datasets in a single regression analysis showed that excess 210Pb fluxes correlate very well with mean annual rainfall in both regions (Figure 4, r 2 ) 0.86, p < 0.001). Therefore, it is proposed that the following regression equation can be used to estimate 210Pb fluxes to reference soils in the north of Spain when mean annual rainfall is known: Pb flux (Bq m-2 yr-1) ) (0.19 ( 0.02) × rainfall (mm yr-1) - (24 ( 17)

210

The slope of the regression equation (0.19 ( 0.02 Bq m-2 mm-1) indicates the deposition of 210Pb per unit (mm) rainfall. This can be transformed into an average 210Pb concentration in rainfall of 190 ( 20 mBq L-1 in the region. Although the uncertainty of the deposition at null rainfall is large, negative 210Pb soil inventories have also been reported in dry regions, with the explanation being that 222Rn exhalation from dry soils is greater than 210Pb deposition (36, 37). However, a small positive deposition at null rainfall has been reported for dryer areas of the Mediterranean region, maybe due to the impact of Saharan dust events (38) thus showing that an extension to the database is needed to provide a more realistic value of this parameter. The 137Cs flux from global fallout is known to vary evenly with latitude (20). Although both regions of interest are at similar latitude (41° N) it must borne in mind that, though the impact of the Chernobyl accident was small, it may have been different in each area. For example, Ferrero and coworkers (39) showed a clear increase of 137Cs in air over

210Pb

FIGURE 3.

ex

and

137Cs

profiles in soil samples collected in the Noguera Pallaresa Basin (Lleida Province).

TABLE 2. 137Cs Inventories (Decay Corrected to February 2001), 210Pbex Inventories and Fluxes, and Inventory Ratios in Soils from Carrion (Palencia) and Noguera Pallaresa (Lleida) Basins (Including Published Values from refs 42 and 43) sample code

137

Cs inventory (Bq m-2)

210

Pbex inventory 210Pbex flux (Bq m-2) (Bq m-2 yr-1)

137

Cs/210Pbex

P1 P2 P3 P4 P5 P6a P7 P8

589 ( 15 1071 ( 20 1613 ( 27 1781 ( 38 2336 ( 29 3228 ( 275 2214 ( 48 3006 ( 26

Palencia 1044 ( 111 1134 ( 140 1863 ( 148 2453 ( 277 2922 ( 255 3674 ( 230 6593 ( 356 7044 ( 210

32 ( 3 35 ( 4 58 ( 5 76 ( 9 91 ( 8 114 ( 7 205 ( 11 219 ( 7

0.56 ( 0.06 0.95 ( 0.12 0.87 ( 0.07 0.73 ( 0.05 0.80 ( 0.08 0.88 ( 0.09 0.34 ( 0.02 0.43 ( 0.01

L1b L2 L3 L4 L5 L6 L7

d 1749 ( 44 1448 ( 53 2414 ( 75 2943 ( 107 5142 ( 95 6378 ( 135c

Lleida 2612 ( 45 2048 ( 178 3155 ( 85 2988 ( 161 5033 ( 275 5984 ( 322c 8204 ( 257c

81.2 ( 1.4 64 ( 5 98 ( 3 93 ( 5 156 ( 8 186 ( 10 255 ( 8

0.85 ( 0.08 0.46 ( 0.02 0.81 ( 0.05 0.58 ( 0.04 0.86 ( 0.05 0.78 ( 0.03

a The inventories at P6 were obtained as an average of the values measured from eight soil cores collected within a 5 km2 area. The values ranged from 2042 to 4050 Bq m-2 for 137Cs and from 2428 to 4484 Bq m-2 for excess 210Pb. b From ref 42. c From ref 43. d Not available.

Valencia (eastern Spain) and Chernobyl radiocesium was also observed on the Spanish coast of the Mediterranean Sea (40). An independent regression analysis of 137Cs vs rainfall was carried out for each area and very good correlations

were found (Carrion Basin: r 2 ) 0.73, p < 0.001; Noguera Pallaresa Basin: r 2 ) 0.96, p < 0.001). The F-test was performed to compare the regression variances and it was concluded that they were equivalent. However, the comparison of the regression slopes using the Student t-test showed that they were different at a 0.06 significance level. The independent calibrations of 137Cs soil inventories with mean annual rainfall from these two areas are (Figure 5): Palencia: 137

Cs inventory (Bq m-2) ) (2.5 ( 0.6) × rainfall (mm yr-1) + (92 ( 503)

Lleida: 137

Cs inventory (Bq m-2) ) (4.3 ( 0.4) × rainfall (mm yr-1) - (227 ( 409)

Higher fluxes were found in the Noguera Pallaresa Basin as the impact of the Chernobyl accident was greater in northeast Spain. However, the excellent correlation found is puzzling, as a more heterogeneous distribution of Chernobyl fallout would be expected because of the patchy distribution recorded over Europe. In any case, this study shows that the calibration of 137Cs inventories using mean annual rainfall is only possible on the scale of the river basin. Also, it is clear that this problem will be more important in countries heavily affected by the impact of the Chernobyl accident. The 137Cs/210Pb inventory ratio was determined for all locations and is shown in Table 2. The Shapiro-Wilks test of the inventory ratios at Palencia showed that the data are normally distributed while for Lleida the distribution is not normal; therefore robust estimators were used to characterize VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Relationship between annual rainfall and unsupported 210Pb fluxes measured at the Carrion River Basin (b) and the Noguera Pallaresa River Basin (9). The solid line is a linear fit to the data (210Pb flux (Bq m-2 yr-1) ) (0.19 ( 0.02) × rainfall (mm yr-1) - (24 ( 17), r 2 ) 0.86, p < 0.001) and the dashed lines indicate the 95% confidence interval for the mean. both distributions. For the Carrion Basin the median is 0.76, and its 95% confidence interval estimated by bootstrapping is [0.65, 1.09]. For the Noguera Pallaresa Basin the median is 0.79 and its 95% confidence interval, calculated also by bootstrapping, is [0.73, 1.07]. These findings are consistent with the fact that Noguera Pallaresa soils were more affected by the Chernobyl accident. In erosion research, the value ascribed to the reference inventory is a key parameter to convert field measurements into estimates of soil erosion or sediment deposition rates. Such reference value is usually characterized by the average and the standard deviation of one or a few measurements of undisturbed stable soils found in the study area. In an extensive literature review of 137Cs variability for reference sites in over 70 published studies, Sutherland (41) found a range of coefficients of variation (CV) from 1.5 to 86%, with a median of 19%. The estimated 137Cs inventory from the proposed calibrations show CVs of 21% and 14% at the Carrion and Noguera Pallaresa Basins at mean rainfall rates, respectively, which is comparable to, if not better than, the estimation from the mean of reference soils. In the higher rainfall regime, the CVs are also good (25% and 15% for Carrion and Noguera Pallaresa Basins, respectively), but this is not the case for the lowest rainfall stations, where the estimated CVs are high (62% and 50% for Carrion and Noguera Pallaresa Basins, respectively). Further research is needed to provide a more useful calibration in low rainfall regions. The estimated 210Pb flux from the calibrations shows a CV of 12% at mean rainfall rates, which is better than the CV determined by Sutherland (41) for 137Cs inventories. For maximum and minimum rainfall rates in the regions, these CVs are 13% and 40%, respectively. These comparisons show that literature values on inventories’ local variability and the confidence intervals for the mean provided by the calibrations are of the same order. Therefore, other effects not included in the model are of minor importance. This emphasizes the usefulness of the calibration approach since it provides results comparable in precision to those that one would expect from local replicate sampling at every site. 1328

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FIGURE 5. Relationship between annual rainfall and 137Cs inventories measured in soils (a) from the Carrion Basin (137Cs inventory (Bq m-2) ) (2.5 ( 0.6) × rainfall (mm yr-1) + (92 ( 503), r 2 ) 0.73, p < 0.001) and (b) from the Noguera Pallaresa Basin (137Cs inventory (Bq m-2) ) (4.3 ( 0.4) × rainfall (mm yr-1) - (227 ( 409), r 2 ) 0.96, p < 0.001). The linear fits to the data (solid lines) have been extrapolated to allow a better comparison between the two lines. This study shows that 137Cs inventory calibrations with mean annual rainfall are valid only at the river basin scale in northern Spain. Although it is feasible to perform specific basin calibrations, this would not be justified in small-scale studies (e.g., erosion in a plot) or if a clear reference soil (contrasted with existing calibrations) is found, which is not always possible. In these cases it is possibly better to use 210Pb as an erosion radiotracer, as regional calibration curves with mean annual rainfall can be produced with relatively little effort, although other factors such as changing erosion rates and derivation of appropriate models must also be taken into account when choosing a radiotracer. Also, to use 210Pb as an erosion radiotracer is possibly the best option in areas where the impact of the Chernobyl accident cannot be clearly distinguished from global fallout. It would be desirable to have additional calibration curves of 210Pb with mean annual rainfall from as many world regions as possible. A synthesis of existing basin/regional correlations would be of great help for soil erosion research and the use of these environmental radiotracers in other disciplines.

Acknowledgments We acknowledge financial support received from the Junta de Castilla y Leo´n, the Generalitat de Catalunya (2001 FPI 00471), and the Comisio´n Interministerial de Ciencia y Tecnologı´a (Proyecto MORE, CICYT 2FD97-1133). We thank the Consejo de Seguridad Nuclear for providing the data of γ radiation values in Spain (Proyecto MARNA). We are grateful to Manel Garcı´a for the construction of the soil sampling device, Joan-Manel Bruach for help in sample collection and analysis, and to Anna Jimenez for elaboration of the GIS cartography of the Lleida Basin. The IAEA is grateful for the support provided to its Marine Environment Laboratories by the Government of the Principality of Monaco.

Supporting Information Available Description of the study areas, sample pretreatment and analysis, the results of an interlaboratory study, and a map of theoretical 222Rn exhalation rates in Spain. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review June 7, 2006. Revised manuscript received October 27, 2006. Accepted December 4, 2006. ES061356Z