Journal of Environmental Radioactivity xxx (2014) 1e12
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
210
Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment Gerald Matisoff* Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, Cleveland, OH 44106-7216, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2012 Received in revised form 18 November 2013 Accepted 9 March 2014 Available online xxx
Although 137Cs has been used extensively to study soil erosion and particle transport in the terrestrial environment, there has been much less work using excess or unsupported 210Pb (210Pbxs) to study the same processes. Furthermore, since 137Cs activities in soils are decreasing because of radioactive decay, some locations have an added complication due to the addition of Chernobyl-derived 137Cs, and the activities of 137Cs in the southern hemisphere are low, there is a need to develop techniques that use 210 Pbxs to provide estimates of rates of soil erosion and particle transport. This paper reviews the current status of 210Pbxs methods to quantify soil erosion rates, to identify and partition suspended sediment source areas, and to determine the transport rates of particles in the terrestrial landscape. Soil erosion rates determined using 210Pbxs are based on the unsupported 210Pb (210Pbxs) inventory in the soil, the depth distribution of 210Pbxs, and a mass balance calibration (‘conversion model’) that relates the soil inventory to the erosion rate using a ‘reference site’ at which neither soil erosion nor soil deposition has occurred. In this paper several different models are presented to illustrate the effects of different model assumptions such as the timing, depth and rates of the surface soil mixing on the calculated erosion rates. The suitability of model assumptions, including estimates of the depositional flux of 210Pbxs to the soil surface and the post-depositional mobility of 210Pb are also discussed. 210Pb can be used as one tracer to permit sediment source area identification. This sediment ‘fingerprinting’ has been extended far beyond using 210Pb as a single radioisotope to include numerous radioactive and stable tracers and has been applied to identifying the source areas of suspended sediment based on underlying rock type, land use (roads, stream banks, channel beds, cultivated or uncultivated lands, pasture lands, forested lands, construction sites, undisturbed lands) or style of erosion (sheet wash, rills, bank). The transport time of particles in the terrestrial system can be estimated using 7Be/210Pbxs radionuclide ratios and from mass balance models of 210Pbxs and/or 7Be in streams. Watershed residence times can be calculated from the radionuclide inventory and the erosional loss rate. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: 210 Pb Soil erosion Sediment fingerprinting Particle transit time
1. Introduction Understanding the movement of particles on the landscape is fundamental to devising erosion control methodologies and establishing sustainable agricultural techniques, protecting aquatic habitats from flooding and siltation, and predicting future landscape changes. The best known of the tracers for quantifying soil erosion and tracing the movement of particles across the landscape and through the drainage network are natural and anthropogenic radionuclides. This paper reviews the state of knowledge of the application of one tracer, excess or unsupported 210Pb (210Pbxs), on
* Tel.: þ1 216 368 3677; fax: þ1 216 368 3691. E-mail address: [email protected].
determining soil erosion rates, sediment source identification and particle transport. The majority of anthropogenic radionuclides found on the landscape were produced largely by atmospheric nuclear bomb testing in the 1960s and the fallout was distributed globally. The list of fission products is extensive, although many of these radionuclides are too short-lived to be useful tracers of landscape processes. Of the longer-lived fission products, the best known is 137 Cs, but the list of other useful tracers includes 134Cs, 238,239,240 Pu, and 241Am as minute solid particles or sorbed to soil particles; and 3H and 90Sr as soluble tracers. There are also naturally-occurring radionuclides produced by various nuclear or cosmogenic reactions, or are intermediates in uranium or thorium decay chains and include 7Be, 210Pb, 210Po and a few others.
http://dx.doi.org/10.1016/j.jenvrad.2014.03.008 0265-931X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Matisoff, G., 210Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/j.jenvrad.2014.03.008
2
G. Matisoff / Journal of Environmental Radioactivity xxx (2014) 1e12
The natural and anthropogenic radionuclides that are deposited onto the land surface from the atmosphere can be called ‘fallout radionuclides’. The fallout radionuclides identified above have been used extensively to trace the movement of soil particles on the landscape because it is widely believed that they rapidly and strongly sorb to the particles, they do not have significant differential adsorption to various sizes of particles, mineralogic constituents or organic constituents that cannot be accounted for, they are conservative (except for their radioactive decay which can be accounted for), and they may be measured using relatively simple methods. Of these radionuclides 137Cs has been most widely studied, particularly because of potential health effects associated with its incorporation into the food chain following global stratospheric fallout in the 1960s and from Chernobyl fallout in 1986. Matisoff and Whiting (2011) updated the Ritchie and Ritchie (2007) count of papers that study the environmental fate of 137Cs and conclude that there are now a total of about 4500 137Cs references with the vast majority of the papers following the Chernobyl accident. In comparison, there are about 2700 references on the use of 210 Pb in studies of soils and sedimentation. The current use of 210Pb in such studies now exceeds the use of 137Cs. There have been a number of excellent reviews of the application of 210Pb to study both aquatic and terrestrial surficial processes. The application of 210Pb activity profiles in sediment cores to determine sedimentation rates and bioturbation rates and depths have been reviewed by Robbins (1978) and in a collection of papers (Appleby, 1993, 2008). However, this review covers soil erosion rates, sediment source identification and particle transport and will not address sedimentation nor bioturbation in aquatic sediments. Mabit et al. (2008) identified the advantages and limitations of techniques using 137Cs, 210Pb, and 7Be in soil erosion and sedimentation studies. They note that the reported atmospheric depositional fluxes of 210Pbxs are highly globally spatially variable since the source of 210Pbxs would be greater over continental rocks and the depositional flux depends on the direction of passage of air masses and the distribution of oceans and continents. They show that the vertical distributions of 210Pbxs in undisturbed and in cultivated soils are very similar to those documented for 137Cs except that the 210Pbxs activity maximum is at the soil surface rather than at a small depth below the surface. They conclude that the application of 210Pbxs to measuring soil redistribution rates has the same advantages as using 137Cs as well as enabling a longer time scale because of the continuous fallout of 210Pbxs compared to the brief interval of 137Cs fallout. Persson and Holm (2011) reviewed the activities of both 210Po and 210Pb in ground-level air, in groundwaters, in soils and in the natural food chain. Of particular interest is that they report that in soils 210Po is in equilibrium with 210 Pb which suggests that 210Pb is the main source of 210Po. Zapata (2003) suggests that the behavior of 210Pbxs in soils is similar to that of 137Cs so that it is a good alternative to 137Cs in areas where 137Cs cannot be used. He notes that Walling and Quine (1995) and He and Walling (1997) suggest that there is potential to use both 137Cs and 210 Pb as independent indicators in soil erosion studies to provide additional information on the erosional history of a site. Walling et al. (1995) obtained similar landscape patterns and rates of erosion using 137Cs and 210Pb. They note that since the 210Pb methodology evaluates a longer time interval than the 137Cs methodology that the differences between the erosion rates provides information about the relative timing of the erosion. Du et al. (2011) reviewed how several short-lived radionuclides have been used to study the sources, transport pathways, and deposition of particles/sediments in rivers, estuaries and coasts. They summarize several example studies in which 210Pb and 210Po have been used, often in conjunction with other radiotracers, to quantify particle scavenging and dynamics in rivers and shallow coastal waters.
2. Retention and migration of
210
Pb in soils
The radionuclides 137Cs, 7Be and 210Pbxs are associated with aerosols in the atmosphere, are highly particle active and are delivered to the soil surface primarily in precipitation. Once delivered to the soil surface it is believed that the radionuclides rapidly become associated with the mineral grains and organic materials of soils. The nature of the fixation to soil and suspended matter of these radionuclides was reviewed by Matisoff and Whiting (2011). It is commonly assumed that these radionuclides are strongly particle bound, with partition coefficients, Kd, for these radionuclides w105 (Olsen et al., 1986; You et al., 1989; Hawley et al., 1986; Steinmann et al., 1999) indicating that these radionuclides are sufficiently particle-bound that they are suitable for tracing erosion, transport, and deposition of soil. However, a recent evaluation of the literature suggests that many sorption studies conducted with lead exceeded the solubility of lead phases and that the low concentration of lead observed in solution was not because of a high partitioning onto the solid phases, but rather because of precipitation reactions instead of adsorption reactions (EPA, 1999). That EPA report also notes that lead adsorption increases with increasing organic matter content of the soils and with increasing pH according to the relationship Kd (mL/g) ¼ 1639902.4 pH þ 150.4 pH2. They report Kd values for lead in several soils that range from 20 to 80,000 mL/g. Clearly these values are less than the w105 reported above for suspended matter in the water column and may reflect a difference between the nature and value of the distribution coefficient of lead in soils versus the value in suspended sediments. It is also possible that the Kd values reported for soils have been derived under conditions very different from those in situ. The kinetics of sorption is less understood, but Baskaran and Santschi (1993) reported that approximately 80% of 7Be became associated with particulates within an hour of a rainfall event. In a study where 134Cs was applied in solution to the surface of soils, it was adsorbed in the upper 2.5 cm of the soil (Owens et al., 1996). These results largely mimic the findings of Rogowski and Tamura (1970) who applied 137Cs to the soil surface and also found penetration of only a few cm. This modest penetration during infiltration suggests a timeframe for adsorption of less than 10 min if typical rates of infiltration for these silty sand soils are used. The determination of 210Pbxs is subject to error because of the nature of the measurement techniques and because 210Pbxs in suspended samples and surface soils may not be in secular equilibrium with its parent 226Ra. Most analytical techniques measure total 210Pb in a sample. Supported 210Pb can either be measured directly by determining the activity of a parent isotope and assuming secular equilibrium between the parent and the supported 210Pb or it can be inferred by assuming that the supported 210 Pb is equal to the total 210Pb at depth in the soil profile (i.e., where 210Pbxs ¼ 0). This assumption is not possible for suspended sediment. 210Pbxs is then calculated from the difference between total 210Pb and the supported 210Pb. Thus, the measurement of both total and supported 210Pb introduces additional error compared to other radionuclides such as 137Cs and 7Be where only a single measurement is required. In addition, considerable error may be introduced in suspended sediment or surface soil samples because gaseous 222Rn may be lost from the sample following decay of the 226 Ra parent. It has been shown that in suspended particles and in surface soils there is a 222Rn deficiency and that the 210Pb/226Ra can be as low as w0.5 (Ravichandran et al., 1995). Some proportion of the radionuclide fallout is retained on or incorporated into vegetation and can move through the food chain and can exceed health standards (Davis, 1986; Revelle and Revelle, 1988). Persson and Holm (2011) reviewed the literature
Please cite this article in press as: Matisoff, G., 210Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/j.jenvrad.2014.03.008
G. Matisoff / Journal of Environmental Radioactivity xxx (2014) 1e12
and report that mosses, lichens and peat have a high efficiency for capturing 210Po and 210Pbxs from atmospheric fallout and that caribou and reindeer accumulate these radionuclides from grazing on the plants. Doering et al. (2006) observed that 18% of 7 Be was retained on vegetation whereas only about 1% of 210Pbxs was retained on vegetation. These differences likely reflect the much shorter half-life of 7Be than any differences in affinity for organic material. 7 Be, 137Cs and 210Pbxs in the surface soils move downward into the soil with time. Processes that move radionuclides downward in the soil include infiltration with adsorption, bioturbation, leaching, diffusion, colloidal transport, microbial activity, preferential flow paths, translocation, and plowing. The depth of penetration of a radionuclide into the soil is determined by the rate of this downward movement and the half-life of the radionuclide (Bossew and Kirchner, 2004). Because 7Be and 210Pbxs are delivered continuously to the soil surface their soil profiles usually exhibit a surface maximum and decrease downcore. It is difficult to ascribe the shapes of those profiles to specific transport processes. However, since the half-life of 7Be is only 53 days the fact that it is found to depths of w3 cm indicates that at least some of the transport is rapid or it would not be found below the surface. Each radionuclide is distributed differently in the soil because of differences in half-lives, delivery rates, delivery histories, and land use. An undisturbed soil will exhibit higher radionuclide activities near the soil surface, which reflects their surficial input and slow downward transport. For total 210Pb the surface maximum in activity decreases exponentially downward to the value of the supported 210Pb maintained by continuous in situ decay of 226Ra in soil and rock (Goldberg and Koide, 1962) (Fig. 1) or to 0 in the case of 7 Be. Most 210Pbxs is found in the upper 10 cm of soil whereas 7Be is found only in the top 2e3 cm (Bonniwell et al., 1999; Doering et al., 2006). The greater penetration of 210Pbxs than 7Be is due to its greater half-life of 22.6 years versus 53 days for 7Be. With more time to operate, downward migration extends further. Because 137 Cs had its peak delivery in the early 1960s and almost no delivery before 1951 or since 1975 (or had its peak delivery in 1986 in areas affected by Chernobyl fallout) its activities have a distinct peak at some depth (usually
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
210
Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment Gerald Matisoff* Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, Cleveland, OH 44106-7216, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2012 Received in revised form 18 November 2013 Accepted 9 March 2014 Available online xxx
Although 137Cs has been used extensively to study soil erosion and particle transport in the terrestrial environment, there has been much less work using excess or unsupported 210Pb (210Pbxs) to study the same processes. Furthermore, since 137Cs activities in soils are decreasing because of radioactive decay, some locations have an added complication due to the addition of Chernobyl-derived 137Cs, and the activities of 137Cs in the southern hemisphere are low, there is a need to develop techniques that use 210 Pbxs to provide estimates of rates of soil erosion and particle transport. This paper reviews the current status of 210Pbxs methods to quantify soil erosion rates, to identify and partition suspended sediment source areas, and to determine the transport rates of particles in the terrestrial landscape. Soil erosion rates determined using 210Pbxs are based on the unsupported 210Pb (210Pbxs) inventory in the soil, the depth distribution of 210Pbxs, and a mass balance calibration (‘conversion model’) that relates the soil inventory to the erosion rate using a ‘reference site’ at which neither soil erosion nor soil deposition has occurred. In this paper several different models are presented to illustrate the effects of different model assumptions such as the timing, depth and rates of the surface soil mixing on the calculated erosion rates. The suitability of model assumptions, including estimates of the depositional flux of 210Pbxs to the soil surface and the post-depositional mobility of 210Pb are also discussed. 210Pb can be used as one tracer to permit sediment source area identification. This sediment ‘fingerprinting’ has been extended far beyond using 210Pb as a single radioisotope to include numerous radioactive and stable tracers and has been applied to identifying the source areas of suspended sediment based on underlying rock type, land use (roads, stream banks, channel beds, cultivated or uncultivated lands, pasture lands, forested lands, construction sites, undisturbed lands) or style of erosion (sheet wash, rills, bank). The transport time of particles in the terrestrial system can be estimated using 7Be/210Pbxs radionuclide ratios and from mass balance models of 210Pbxs and/or 7Be in streams. Watershed residence times can be calculated from the radionuclide inventory and the erosional loss rate. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: 210 Pb Soil erosion Sediment fingerprinting Particle transit time
1. Introduction Understanding the movement of particles on the landscape is fundamental to devising erosion control methodologies and establishing sustainable agricultural techniques, protecting aquatic habitats from flooding and siltation, and predicting future landscape changes. The best known of the tracers for quantifying soil erosion and tracing the movement of particles across the landscape and through the drainage network are natural and anthropogenic radionuclides. This paper reviews the state of knowledge of the application of one tracer, excess or unsupported 210Pb (210Pbxs), on
* Tel.: þ1 216 368 3677; fax: þ1 216 368 3691. E-mail address: [email protected].
determining soil erosion rates, sediment source identification and particle transport. The majority of anthropogenic radionuclides found on the landscape were produced largely by atmospheric nuclear bomb testing in the 1960s and the fallout was distributed globally. The list of fission products is extensive, although many of these radionuclides are too short-lived to be useful tracers of landscape processes. Of the longer-lived fission products, the best known is 137 Cs, but the list of other useful tracers includes 134Cs, 238,239,240 Pu, and 241Am as minute solid particles or sorbed to soil particles; and 3H and 90Sr as soluble tracers. There are also naturally-occurring radionuclides produced by various nuclear or cosmogenic reactions, or are intermediates in uranium or thorium decay chains and include 7Be, 210Pb, 210Po and a few others.
http://dx.doi.org/10.1016/j.jenvrad.2014.03.008 0265-931X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Matisoff, G., 210Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/j.jenvrad.2014.03.008
2
G. Matisoff / Journal of Environmental Radioactivity xxx (2014) 1e12
The natural and anthropogenic radionuclides that are deposited onto the land surface from the atmosphere can be called ‘fallout radionuclides’. The fallout radionuclides identified above have been used extensively to trace the movement of soil particles on the landscape because it is widely believed that they rapidly and strongly sorb to the particles, they do not have significant differential adsorption to various sizes of particles, mineralogic constituents or organic constituents that cannot be accounted for, they are conservative (except for their radioactive decay which can be accounted for), and they may be measured using relatively simple methods. Of these radionuclides 137Cs has been most widely studied, particularly because of potential health effects associated with its incorporation into the food chain following global stratospheric fallout in the 1960s and from Chernobyl fallout in 1986. Matisoff and Whiting (2011) updated the Ritchie and Ritchie (2007) count of papers that study the environmental fate of 137Cs and conclude that there are now a total of about 4500 137Cs references with the vast majority of the papers following the Chernobyl accident. In comparison, there are about 2700 references on the use of 210 Pb in studies of soils and sedimentation. The current use of 210Pb in such studies now exceeds the use of 137Cs. There have been a number of excellent reviews of the application of 210Pb to study both aquatic and terrestrial surficial processes. The application of 210Pb activity profiles in sediment cores to determine sedimentation rates and bioturbation rates and depths have been reviewed by Robbins (1978) and in a collection of papers (Appleby, 1993, 2008). However, this review covers soil erosion rates, sediment source identification and particle transport and will not address sedimentation nor bioturbation in aquatic sediments. Mabit et al. (2008) identified the advantages and limitations of techniques using 137Cs, 210Pb, and 7Be in soil erosion and sedimentation studies. They note that the reported atmospheric depositional fluxes of 210Pbxs are highly globally spatially variable since the source of 210Pbxs would be greater over continental rocks and the depositional flux depends on the direction of passage of air masses and the distribution of oceans and continents. They show that the vertical distributions of 210Pbxs in undisturbed and in cultivated soils are very similar to those documented for 137Cs except that the 210Pbxs activity maximum is at the soil surface rather than at a small depth below the surface. They conclude that the application of 210Pbxs to measuring soil redistribution rates has the same advantages as using 137Cs as well as enabling a longer time scale because of the continuous fallout of 210Pbxs compared to the brief interval of 137Cs fallout. Persson and Holm (2011) reviewed the activities of both 210Po and 210Pb in ground-level air, in groundwaters, in soils and in the natural food chain. Of particular interest is that they report that in soils 210Po is in equilibrium with 210 Pb which suggests that 210Pb is the main source of 210Po. Zapata (2003) suggests that the behavior of 210Pbxs in soils is similar to that of 137Cs so that it is a good alternative to 137Cs in areas where 137Cs cannot be used. He notes that Walling and Quine (1995) and He and Walling (1997) suggest that there is potential to use both 137Cs and 210 Pb as independent indicators in soil erosion studies to provide additional information on the erosional history of a site. Walling et al. (1995) obtained similar landscape patterns and rates of erosion using 137Cs and 210Pb. They note that since the 210Pb methodology evaluates a longer time interval than the 137Cs methodology that the differences between the erosion rates provides information about the relative timing of the erosion. Du et al. (2011) reviewed how several short-lived radionuclides have been used to study the sources, transport pathways, and deposition of particles/sediments in rivers, estuaries and coasts. They summarize several example studies in which 210Pb and 210Po have been used, often in conjunction with other radiotracers, to quantify particle scavenging and dynamics in rivers and shallow coastal waters.
2. Retention and migration of
210
Pb in soils
The radionuclides 137Cs, 7Be and 210Pbxs are associated with aerosols in the atmosphere, are highly particle active and are delivered to the soil surface primarily in precipitation. Once delivered to the soil surface it is believed that the radionuclides rapidly become associated with the mineral grains and organic materials of soils. The nature of the fixation to soil and suspended matter of these radionuclides was reviewed by Matisoff and Whiting (2011). It is commonly assumed that these radionuclides are strongly particle bound, with partition coefficients, Kd, for these radionuclides w105 (Olsen et al., 1986; You et al., 1989; Hawley et al., 1986; Steinmann et al., 1999) indicating that these radionuclides are sufficiently particle-bound that they are suitable for tracing erosion, transport, and deposition of soil. However, a recent evaluation of the literature suggests that many sorption studies conducted with lead exceeded the solubility of lead phases and that the low concentration of lead observed in solution was not because of a high partitioning onto the solid phases, but rather because of precipitation reactions instead of adsorption reactions (EPA, 1999). That EPA report also notes that lead adsorption increases with increasing organic matter content of the soils and with increasing pH according to the relationship Kd (mL/g) ¼ 1639902.4 pH þ 150.4 pH2. They report Kd values for lead in several soils that range from 20 to 80,000 mL/g. Clearly these values are less than the w105 reported above for suspended matter in the water column and may reflect a difference between the nature and value of the distribution coefficient of lead in soils versus the value in suspended sediments. It is also possible that the Kd values reported for soils have been derived under conditions very different from those in situ. The kinetics of sorption is less understood, but Baskaran and Santschi (1993) reported that approximately 80% of 7Be became associated with particulates within an hour of a rainfall event. In a study where 134Cs was applied in solution to the surface of soils, it was adsorbed in the upper 2.5 cm of the soil (Owens et al., 1996). These results largely mimic the findings of Rogowski and Tamura (1970) who applied 137Cs to the soil surface and also found penetration of only a few cm. This modest penetration during infiltration suggests a timeframe for adsorption of less than 10 min if typical rates of infiltration for these silty sand soils are used. The determination of 210Pbxs is subject to error because of the nature of the measurement techniques and because 210Pbxs in suspended samples and surface soils may not be in secular equilibrium with its parent 226Ra. Most analytical techniques measure total 210Pb in a sample. Supported 210Pb can either be measured directly by determining the activity of a parent isotope and assuming secular equilibrium between the parent and the supported 210Pb or it can be inferred by assuming that the supported 210 Pb is equal to the total 210Pb at depth in the soil profile (i.e., where 210Pbxs ¼ 0). This assumption is not possible for suspended sediment. 210Pbxs is then calculated from the difference between total 210Pb and the supported 210Pb. Thus, the measurement of both total and supported 210Pb introduces additional error compared to other radionuclides such as 137Cs and 7Be where only a single measurement is required. In addition, considerable error may be introduced in suspended sediment or surface soil samples because gaseous 222Rn may be lost from the sample following decay of the 226 Ra parent. It has been shown that in suspended particles and in surface soils there is a 222Rn deficiency and that the 210Pb/226Ra can be as low as w0.5 (Ravichandran et al., 1995). Some proportion of the radionuclide fallout is retained on or incorporated into vegetation and can move through the food chain and can exceed health standards (Davis, 1986; Revelle and Revelle, 1988). Persson and Holm (2011) reviewed the literature
Please cite this article in press as: Matisoff, G., 210Pb as a tracer of soil erosion, sediment source area identification and particle transport in the terrestrial environment, Journal of Environmental Radioactivity (2014), http://dx.doi.org/10.1016/j.jenvrad.2014.03.008
G. Matisoff / Journal of Environmental Radioactivity xxx (2014) 1e12
and report that mosses, lichens and peat have a high efficiency for capturing 210Po and 210Pbxs from atmospheric fallout and that caribou and reindeer accumulate these radionuclides from grazing on the plants. Doering et al. (2006) observed that 18% of 7 Be was retained on vegetation whereas only about 1% of 210Pbxs was retained on vegetation. These differences likely reflect the much shorter half-life of 7Be than any differences in affinity for organic material. 7 Be, 137Cs and 210Pbxs in the surface soils move downward into the soil with time. Processes that move radionuclides downward in the soil include infiltration with adsorption, bioturbation, leaching, diffusion, colloidal transport, microbial activity, preferential flow paths, translocation, and plowing. The depth of penetration of a radionuclide into the soil is determined by the rate of this downward movement and the half-life of the radionuclide (Bossew and Kirchner, 2004). Because 7Be and 210Pbxs are delivered continuously to the soil surface their soil profiles usually exhibit a surface maximum and decrease downcore. It is difficult to ascribe the shapes of those profiles to specific transport processes. However, since the half-life of 7Be is only 53 days the fact that it is found to depths of w3 cm indicates that at least some of the transport is rapid or it would not be found below the surface. Each radionuclide is distributed differently in the soil because of differences in half-lives, delivery rates, delivery histories, and land use. An undisturbed soil will exhibit higher radionuclide activities near the soil surface, which reflects their surficial input and slow downward transport. For total 210Pb the surface maximum in activity decreases exponentially downward to the value of the supported 210Pb maintained by continuous in situ decay of 226Ra in soil and rock (Goldberg and Koide, 1962) (Fig. 1) or to 0 in the case of 7 Be. Most 210Pbxs is found in the upper 10 cm of soil whereas 7Be is found only in the top 2e3 cm (Bonniwell et al., 1999; Doering et al., 2006). The greater penetration of 210Pbxs than 7Be is due to its greater half-life of 22.6 years versus 53 days for 7Be. With more time to operate, downward migration extends further. Because 137 Cs had its peak delivery in the early 1960s and almost no delivery before 1951 or since 1975 (or had its peak delivery in 1986 in areas affected by Chernobyl fallout) its activities have a distinct peak at some depth (usually
