Soil & Tillage Research 69 (2003) 3–13

The use of environmental radionuclides as tracers in soilerosion and sedimentation investigations: recent advances

and future developments

Felipe Zapata∗Soil and Water Management& Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture,

International Atomic Energy Agency, P.O. Box 100, Wagramer Strasse 5, A-1400 Vienna, Austria

Abstract

Although much of the recent attention on the environmental problems has focused on climatic change, there is alsoincreasing concern that accelerated soil erosion and associated land degradation represent a major problem for sustainabledevelopment and environmental protection. There is an urgent need to obtain reliable quantitative data on the extent and ratesof soil erosion worldwide to provide a more comprehensive assessment of the magnitude of the problems and to underpinthe selection of effective soil conservation measures. The use of environmental radionuclides, in particular137Cs, affords aneffective and valuable means for studying erosion and deposition within the landscape. The key advantage of this approach isthat it can provide retrospective information on medium-term (30–40 years) erosion/deposition rates and spatial patterns ofsoil redistribution, without the need for long-term monitoring programmes. Advantages and limitations of the technique arehighlighted. The launching of two closely linked International Atomic Energy Agency (IAEA) research networked projectsin 1996 involving some 25 research groups worldwide has made a major contribution to co-ordinating efforts to refine andto standardise the137Cs technique. The efficacy and value of the approach has been demonstrated by investigations in anumber of environments. Significant developments that have been made to exploit its application in a wide range of studiesare reported in this review paper. Other environmental radionuclides, such as unsupported210Pb and7Be offer considerablepotential for use in soil erosion investigations, both individually and complementary to137Cs. The IAEA through researchnetworks and other mechanisms is promoting further development and applications of these radionuclides in soil erosion andsedimentation studies for a sustainable resource use and environmental protection.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Soil erosion; Sedimentation; Environmental radionuclides;137Cs;210Pb;7Be

1. Introduction

There is an increasing concern that accelerated soilerosion and associated land degradation represent amajor problem for the sustainable intensification ofthe agricultural production. This is particularly acutein the developing world, which is characterised by

∗ Tel.: +43-1-2600-21693; fax:+43-1-26007.E-mail address:f.zapata@iaea.org (F. Zapata).

limited land and water resources and a rapidly growingpopulation (Lal, 2000; Walling, 2002).

Soil erosion and associated sediment deposition arenatural processes but they can be accelerated by hu-man intervention through deforestation, overgrazing,and non-sustainable farming practices. They cause notonly on-site degradation of a natural resource basebut also off-site problems such as downstream sedi-ment deposition in fields, floodplains and water bod-ies. For example, it has been estimated that the world’s

0167-1987/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0167-1987(02)00124-1

4 F. Zapata / Soil & Tillage Research 69 (2003) 3–13

croplands are currently losing 23 billion tonnes of soilin excess of new soil formation each year. This isequivalent to a depletion of the global soil resource by7% each decade (Brown, 1984). An ISRIC/UNEP sur-vey on global assessment of soil degradation indicatesthat more than one-fourth of the world’s soils have losta substantial amount of their natural fertility over thepast 45 years. Deforestation and poor farming prac-tices are responsible for about 30% of the soil degrada-tion worldwide (UNEP, 1992). Sediment accumulationin water bodies affects water quality and biological ac-tivity because of pollution from various agrochemicalsadsorbed on the sediments. The sediments also causeloss of storage capacity and eutrophication of the wa-ter bodies (Clark, 1985; Ritchie and Mc Henry, 1990).

Globally, the current economic costs of the on-siteand off-site impacts of erosion of agricultural landhave been estimated to amount to some US$ 400 bil-lion per year (Bernard and Iiavri, 2000). There is anurgent need for obtaining reliable quantitative data onthe extent and actual rates of soil erosion worldwide(Ritchie and Mc Henry, 1990; Walling and Quine,1995). Such data are required for a more comprehen-sive assessment of the magnitude of the problems toobtain a better understanding of the main environ-mental factors involved, to validate existing and newprediction models, and to provide a basis for select-ing effective soil conservation measures and improvedland management strategies, including assessment oftheir economic and environmental impacts (Zapata andGarcia Agudo, 2000; Walling, 2002). The objectives ofthis review paper are (a) to describe briefly the use ofradionuclides, in particular the137Cs technique in soilerosion studies, (b) to review recent advances on theapplication of the137Cs technique achieved by two re-search networks promoted by the International AtomicEnergy Agency, and (c) to present future developmentson the extension of the approach to other potential ra-dionuclides in soil erosion and sedimentation research.

2. Nuclear methods in soil erosion andsedimentation research

Despite extensive literature on the global, regionaland national problems of soil erosion, quantitative andreliable data on the extent and rates of soil erosionare scarce for many regions of the world. Soil ero-

sion research is capital and labour intensive as wellas a time-consuming exercise. Well-designed exper-iments using standardised methodologies should beperformed, so that the data obtained are comparableand representative of the study areas (FAO, 1977; Lal,1988; Zapata et al., 1995).

The existing methods for soil erosion assessmentcan be grouped into two main categories: erosionmodelling and prediction methods and erosion mea-surement methods. In all cases, there is a need fordirect measurement of soil erosion, which can be doneusing erosion plots, surveying methods and nucleartechniques. The choice of one or another method willbasically depend on the objectives of the study andthe availability of resources (Elliot et al., 1991). Ex-isting classical techniques such as erosion plots andsurveying methods for monitoring soil erosion are ca-pable of meeting some of these requirements but theypossess a number of important limitations in termsof the representativeness of the data obtained, theirspatial resolution, and potential to provide informa-tion on long-term rates of soil erosion and associatedspatial patterns over extended areas, and the costsinvolved (Loughran, 1989; Higgitt, 1991). The questfor alternative techniques for assessing soil erosion tocomplement existing methods has directed attentionto the use of radionuclides, in particular fallout137Csas tracers to obtain estimates of soil erosion and de-position on agricultural land (Ritchie and Mc Henry,1990; Walling and Quine, 1995).

By artificially labelling soil particles with an ap-propriate radionuclide, both the extent and source ofsoil loss can be determined. Several radionuclides,mainly gamma ray emitters, have been applied forfield erosion studies (59Fe,46Sc,110Ag, 198Au, 137Cs,51Cr). Double labelling techniques allow detectionof two particle sizes or the contribution from twosources (Menzel and Smith, 1984; IAEA, 1974). An-other group of these techniques involve the use of anumber of environmental radionuclides such as fall-out 137Cs, natural210Pb, cosmogenic7Be, and others(239–240Pu, 14C, 32Si, 26Al, 36Cl, etc.), which can beused to assess soil erosion, deposition patterns in waterbodies and other related problems depending on thetime-scale involved (IAEA, 1983, 1995). The 137Csis, however, by far the most widely used radionuclidein soil erosion and sedimentation research by virtueof its high affinity for fine particles, its relatively long

F. Zapata / Soil & Tillage Research 69 (2003) 3–13 5

half-life, its relative ease of measurement and thewell-defined temporal pattern of fallout input (Ritchieand Mc Henry, 1990; Walling and Quine, 1993).These radionuclides have been used successfully inseveral developed countries to measure soil loss, soilredeposition, and to develop budgets for erosion andsedimentation (Ritchie et al., 1970, 1974; Wise, 1980;Loughran et al., 1981; Campbell et al., 1982; de Jonget al., 1983; Walling et al., 1986; Vanden Berghe andGulinck, 1987; Lowrance et al., 1988).

3. The 137Cs technique

Early work in the 1960s demonstrated the link be-tween fallout radionuclides such as90Sr and137Csand soil erosion processes (Menzel, 1960; Rogowskyand Tamura, 1965). Ritchie and Mc Henry in the late1960s and early 1970s (Ritchie et al., 1970, 1974;Ritchie and Mc Henry, 1973, 1975) have clearlydemonstrated the potential for using fallout137Cs as atracer in soil erosion and sedimentation studies. Sincethen, the technique has been used by several groupsmainly in advanced countries and recently also indeveloping countries. To date there are over 2500citations on the applications of the technique (Ritchieand Ritchie, 2001).

3.1. The137Cs as a tracer

137Cs (Ey = 0.622 MeV) is an artificial radionu-clide with a half-life of 30.12 years produced by nu-clear fission. Widespread global distribution of137Csinto the environment began with high-yield atmo-spheric tests of atomic weapons in the 1950s and early1960s.137Cs and other radionuclides were releasedinto the stratosphere, distributed globally, moved backto the troposphere and from the troposphere to theearth’s surface as fallout, which was strongly relatedto local precipitation patterns and rates (Ritchie andMc Henry, 1990). Temporal and spatial distribution ofradioactive fallout on the earth’s surface has been ex-tensively documented (Cambray et al., 1989; Larsen,1985). 137Cs fallout inputs were much greater in thenorthern hemisphere than in the southern hemisphere,because more atmospheric nuclear tests took place inthe northern hemisphere. Based on the available90Srdata, an assessment of the potential global distribu-

tion of 137Cs inputs has been made (Garcia Agudo,1998). Local events, such as the Chernobyl accidenthave had strong impacts on the fallout patterns at theregional level but are of limited significance on globalfallout patterns and rates (Volchok and Chieco, 1986;IAEA, 1991).

The value of137Cs as tracer lies in its rapid andstrong adsorption by fine soil particles, particularlyclay minerals and humic materials (Tamura, 1964;Bachhuber et al., 1982). In agro-ecosystems its re-distribution is assumed to be a direct result of theerosion, transport and deposition of soil particlesoccurring during the period from the main phaseof atmospheric deposition to the time of sampling(Walling and Quine, 1995). Where cultivation occurs,the radiocaesium will be distributed throughout theplough layer and soil redistribution will generallyinvolve a relatively small proportion of the originaltotal fallout. Where, however, soils are essentiallyundisturbed, as in the case of permanent pasture andrangelands,137Cs will be preferentially concentratednear the surface and a given rate of soil redistributionwill involve a substantially greater proportion of theoriginal inventory (Walling and Quine, 1990, 1993).

3.2. Assumptions, requirements, advantages andlimitations of the technique

The key assumptions and requirements of the137Cstechnique have been fully described in many publi-cations (Walling and Quine, 1991, 1993; Ritchie andMc Henry, 1990; de Jong et al., 1983; Loughran et al.,1988). Although the basis for the use of the137Cstechnique to document rates and patterns of soil loss isattractive in its simplicity, it is founded on several keyassumptions and a number of potential limitations anduncertainties must be recognised and addressed in anyapplication (Ritchie and Mc Henry, 1990; Walling andQuine, 1991, 1995; Loughran et al., 1992; Walling,1998).

The assessment of137Cs redistribution is commonlybased upon comparison of measured inventories (totalactivity per unit area) at individual sampling pointswith an equivalent estimate of the inventory repre-senting the cumulative atmospheric fallout input atthe site, taking due account of the different behaviourof cultivated and non-cultivated soils. Because directlong-term measurements of atmospheric fallout are

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rarely available, the cumulative input or reference in-ventory is usually established by sampling adjacent,stable sites, where neither erosion nor deposition hasoccurred. Where sample inventories are lower thanthe local reference inventory, loss of caesium-labelledsoil and therefore erosion may be inferred. Similarly,sample inventories in excess of the reference levelare indicative of addition of caesium-labelled soilby deposition. The magnitude and direction of themeasured deviations from the local reference levelprovide a qualitative assessment of soil redistribution(Walling and Quine, 1993).

To derive quantitative estimates of the rates of soilerosion and deposition from137Cs measurements, it isnecessary to establish a relationship between the mag-nitude of the deviation from the reference inventoryand the extent of soil loss or gain (Ritchie et al., 1974;Mc Henry and Ritchie, 1977). Many workers havefavoured the use of calibration procedures or conver-sion models that relate the erosion or deposition rateto the magnitude of the reduction or increase in the137Cs inventory (Ritchie and Mc Henry, 1990; Quineand Walling, 1981; Walling and Quine, 1990, 1993).

Besides the objectives of the study and the avail-ability of resources, the choice of a particular methodfor measuring soil erosion would require a thoroughknowledge of the advantages and limitations of thetechnique to be used. The following section sum-marises the main advantages and limitations of the137Cs technique to be considered in the context ofsoil erosion and sedimentation studies (Walling andQuine, 1995; IAEA, 1998; Walling, 2002).

3.2.1. Advantages

• Estimates are based on contemporary sampling andprovide a retrospective assessment of medium-term(30–40 years) rates of soil redistribution.

• Estimates can be obtained on the basis of a singlesite visit.

• Resulting estimates of soil redistribution rates areintegrated medium-term, average data and less in-fluenced by extreme events.

• Estimates are of individual points within the land-scape and information on rates and spatial patternscan be assembled.

• Sampling does not require significant disturbanceof the landscape or study area.

• The results are compatible with recent develop-ments in physically based distributed modellingand the application of GIS and geostatistics to soilerosion and sediment yield studies.

• Rates of soil redistribution represent integratedeffects of all landscape processes resulting inmovement of soil particles under defined landuse/management.

• The technique provides information on both erosionand deposition in the same watershed and thereforenet rates of sediment export.

• There are no major scale constraints, apart from thenumber of samples to be analysed.

• From the interpretation of the data, the techniquepermits quantification of processes such as soiltillage redistribution and soil loss and depositionassociated to sheet erosion.

• Application in fingerprinting suspended sedimentsources and estimating rates of over-bank floodplainaccretion.

3.2.2. Limitations

• Need for a multidisciplinary team for the successfulapplication of the technique. This is a particularlimitation in most developing countries with limitedscientific staff.

• Specialised laboratories with sample preparationand costly gamma counting equipment are required.

• Requirements for quality assurance/control of lowlevel gamma spectrometry measurements.

• The technique is effectively limited to documenta-tion of sheet and general surface lowering, but it canbe applied to study rill erosion in cultivated areas.

• It is an indirect approach that depends on the linkbetween measured soil redistribution and observed137Cs redistribution.

• There are uncertainties associated with the selectionof the conversion models used to estimate erosionand deposition rates from the137Cs measurements.

• There is inability to document short-term changesin erosion rates such as those related to changes inland use and management practices.

• The technique needs further standardisation of theprotocols for its worldwide application.

While some of the limitations are inherent to thetechnique, several others have been successfully ad-dressed in the frame of the research networks reported

F. Zapata / Soil & Tillage Research 69 (2003) 3–13 7

below. The significance of these limitations, in partic-ular the cost, must be evaluated in the context of theparticular advantages of the137Cs technique and thelimitations of other techniques.

4. The International Atomic Energy Agency(IAEA) research networks

Because of its mandate the IAEA has a long tra-dition of and wide experience in the use of nucleartechniques in sedimentation studies. Over the pasttwo decades, the Isotope Hydrology Section of theDivision of Physical and Chemical Sciences hasimplemented a number of projects in this field, in-cluding determination of the origin and measurementof suspended sediments in rivers, siltation of naviga-tion channels in estuarine areas, and investigation ofdumping sites for dredged sediments. In these studies,both artificial and naturally occurring radionuclideswere used; artificial tracers and sealed radioactivesources were mainly employed for sediment dynamicsstudies (IAEA, 1974, 1983).

As a result of the Chernobyl accident in April1986, the IAEA executed the “International Cher-nobyl Project”, in which the contamination of the soil(surface) by137Cs and other radionuclides was mon-itored among other activities. This project also con-tributed significantly to the development of gammaspectrometry instruments, training of local staff, stan-dardisation of field/laboratory protocols, and qualityassurance and control of the measurements (IAEA,1991). Furthermore, the Food and Agriculture Orga-nization of the United Nations and the IAEA throughtheir Joint Division of Nuclear Techniques in Foodand Agriculture joined efforts to conduct an extensiveliterature survey on the behaviour and significanceof the radioactive substances released into agricul-tural, forestry and fishery ecosystems, as well on soiland crop contamination by radioactive fallout (FAO,1993).

The concept of a project on the use of environ-mental radionuclides in soil erosion and sedimenta-tion studies was first recommended at an AdvisoryGroup Meeting held in April 1993 in Vienna (IAEA,1995). Based on the recommendations of this meet-ing, proposals for the creation of two projects calledCo-ordinated Research Projects (CRP) were formu-

lated: a CRP on “Assessment of soil erosion throughthe use of137Cs and related techniques as a basis forsoil conservation, sustainable production and envi-ronmental protection” co-ordinated by the Soil andWater Management and Crop Nutrition Section ofthe Joint FAO/IAEA Division of Nuclear Techniquesin Food and Agriculture, and another CRP on “Sedi-mentation assessment by environmental radionuclidesand their application to soil conservation measures”organised by the Isotope Hydrology Section of theDivision of Physical and Chemical Sciences (Zapataet al., 1995). The IAEA provided the core fundingfor the projects, which were implemented during theperiod 1995–2000. The overall goal and objectives ofthese CRPs were consolidated at a consultants meet-ing held in Vienna in November 1995. Both meetingsemphasised the need to promote the standardisationof both field and laboratory protocols for the appli-cation of the137Cs technique (Pennock and Zapata,1995; IAEA, 1998). It was further recommended theco-ordinated implementation of both CRPs becauseof the inter-relationship between erosion and sedi-mentation processes at the watershed level and thesimilarities in the application of the137Cs technique.

The overall objective of both projects was to de-velop guidelines for estimating soil erosion and as-sociated sedimentation for sustainable agriculturalproduction and environmental protection. The spe-cific research objectives of the erosion project were(i) to refine (including validation and standardisation)relevant methodologies for documenting soil erosionusing the137Cs technique across a range of environ-ments which can then be used to test and calibrateexisting models of soil erosion, and (ii) to evaluatethe effect of specific land use management on soilerosion for providing data to underpin the selectionof soil conservation strategies.

The Soil Erosion CRP comprised 11 research con-tractors from Argentina, Brazil, Chile, China (2),Greece, Morocco, Romania, Russian Federation,Slovak Republic and Zimbabwe, and five researchagreement holders from Australia, Canada, Thailand(IBSRAM), UK and USA.

The Sedimentation CRP had 10 participants,of which four were contract holders from China,Morocco, Poland and Romania, and six agreementholders from Australia, Canada, France, New Zealand,Spain and UK.

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The purpose of these two closely linked IAEAnetworked research projects and the ResearchCo-ordination Meetings (RCM) held in Vienna, 1996;Bucharest, 1998 and Barcelona, 1999 was to bringtogether the 26 research groups and promote the ex-change of knowledge and sharing of experiences, thusmaking a major contribution to further developmentof the 137Cs technique. In addition, the IAEA sup-ported the exchange of scientists between the variousresearch groups and the training of young scientistsat laboratories with experience in the application ofthe137Cs technique (Zapata and Garcia Agudo, 1996,1998, 1999; Queralt et al., 2000).

5. Main outputs and achievements of the projects

During the final RCM of the Erosion CRP held inVienna in 2001, the participants evaluated achieve-ments/outputs of the project, drew main conclusionsand formulated recommendations for further research(Zapata, 2001a). The main outputs of the project canbe summarised as follows.

5.1. Refined and harmonised application of the137Cs technique

Standardised methods and protocols for the appli-cation of the technique were successfully developed,and will be disseminated through the publication ahandbook. The correct application of the techniqueprovides reliable erosion and sedimentation integrateddata over a medium-term (40 years) period. In this re-spect, the following conclusions were made:

1. The adoption of the137Cs technique requires asignificant commitment by a team of researchers,trained technicians and the availability of the ap-propriate analytical facilities.

2. The technique was shown to work in many differentenvironments in the course of this CRP, includingregions (e.g. tropical environments) where it hadnot been expected to perform well.

3. Special attention should be paid when using theapproach in areas influenced by Chernobyl fallout.

4. A suite of approaches (e.g. grid versus transect ap-proaches, composite versus multiple samples) wasdeveloped for site selection and sampling by the

various research groups. No single approach is uni-versally applicable; instead the research team mustmake an informed selection from the available ap-proaches. The sampling strategy adopted must re-flect the processes believed to be operating at thesite and the site conditions.

5. There are environments that the technique may notwork in. Preliminary studies should be undertakenin regions where it has not been used previously.

6. Selection and adequate sampling of reference sitesis critical for the successful application of the tech-nique.

7. Standard volumetric soil sampling techniques areadequate for sampling soil for137Cs content.

8. The research team must make detailed site andsampling point observations of those factors of rel-evance for the processes operating in the studyarea/region.

9. Standard solid-state detectors proved to be appro-priate for measurement of137Cs concentrations.Low background shielding may be required in someenvironments.

10. The analytical equipment should be dedicated tothe technique, given the large number of samplesrequired.

11. Analytical quality control must be included in lab-oratory routine work.

12. Special care should be taken when analysing210Pbdue to its low gamma energy for instrumental mea-surement.

13. Gamma spectra should be analysed using appro-priate software. The choice of software should bemade by contacting the IAEA or a major analyticallaboratory.

14. The measured137Cs redistribution values should becalculated and analysed prior to soil redistributionconversion. The caesium residual values (especiallyoutlier values) should be checked against the fieldobservations and against the processes likely to beoperating in the area.

15. For calculating the soil redistribution values fromthe 137Cs measurements, selection of a conversion(calibration) model is required. The model selectedmust be appropriate for the site (e.g. uncultivatedversus cultivated sites) and the required parametersmust be available (Walling and He, 1997, 1999a).

16. The specific model and all parameters used in themodel should be clearly reported.

F. Zapata / Soil & Tillage Research 69 (2003) 3–13 9

17. The soil redistribution results should be comparedto the field observations and the literature availablefor the region.

18. The results should be used to evaluate the effectsof past land use practises and the possible futureimplications of the measured rates. The policy im-plications of these rates should be clearly stated.

5.2. Reliable data on soil erosion andsedimentation rates

The 137Cs technique was proven to work well ina range of environments and scales in the course ofthis CRP and a wealth of information on soil ero-sion/sedimentation rates was obtained. In this specialissue the majority of the contributing authors (Bac-chi, Bujan, Fulajtar, Yong Li, and Theorochapoulos)have reported on the testing of the available conver-sion models to obtain soil erosion/sedimentation ratesfrom the137Cs measurements. Some of them have de-veloped specific approaches, i.e. to take due account ofthe Chernobyl fallout (Golosov), to assess the relativecontribution of gully and inter-gully areas to sedimentproduction (Yong Li), and to refine existing models tobetter suit local conditions (Schuller, Zhang).

The soil erosion/sedimentation rates obtained withthe 137Cs technique have been validated with inde-pendent estimates provided by erosion plots, instru-mented catchments, and soil profile interpretation, aswell as local experience (cf. Yong Li, Zhang, Walling,Schuller, Ionita, Golosov, Theocharopoulos).

The data on soil redistribution rates assembled bythe contributors using standardised protocols for theapplication of the137Cs technique have provided di-rectly comparable and representative information onsoil erosion rates in a wide range of environmentsworldwide. Countries represented include Argentina,Australia, Brazil, Canada, Chile, China, Greece, Rus-sian Federation, Slovakia, UK, and USA. Valuesranged from≈0 to >100 t ha−1 per year and demon-strated that in general local land use exerts a more im-portant influence on the range of values encounteredthan climate or lithology and soil type. However, theinfluence of the latter can be readily distinguished.The contributions of Bacchi, Bujan, Fulajtar, Yong Li,Zhang, and Wallbrink in this issue provide valuableevidence of the influence of land use and managementpractices on rates of soil loss and sedimentation.

The potential use of137Cs measurements for vali-dating distributed soil erosion and sediment deliverymodels has been clearly demonstrated by several stud-ies undertaken within the CRP. Walling used137Csmeasurements to test the application of the AGNPSand ANSWERS models to two small catchments inDevon, UK; Golosov compared the soil redistributionrates within a small catchment with the output froma soil loss model developed for Russian conditionsand Bacchi compared the estimates provided by theWEPP and USLE models with the soil redistributionrates obtained using137Cs measurements in a smallcatchment in Piracicaba, SP, Brazil.

The information on spatially distributed estimatesof soil redistribution rates provided by the137Cs mea-surements offer a great potential for an improved un-derstanding of the relationships between soil loss/gainand soil quality parameters, soil carbon, nutrient re-distribution, and the fate of agrochemicals and othercontaminants in the landscape. This special issue in-cludes several investigations exploring the followingapplications:

• the relationship between soil redistribution and to-pographic controls (Pennock, Bacchi, Bujan, Fula-jtar, Yong Li);

• the relationship between soil quality parametersand rates of soil loss (Bacchi, Yong Li, Fulajtar,Theocharopoulos);

• the role of tillage in soil redistribution (Zhang, Li,Schuller);

• the relationship between137Cs-derived soil redistri-bution and soil organic carbon distribution patterns(Ritchie);

• the relationship between on-site soil erosion andnutrient export and off-site impacts (Wallbrink).

6. Future initiatives on the use of environmentalradionuclides in soil erosion and sedimentationstudies

The recent developments in the use of the137Cstechnique in soil erosion and sedimentation studieshave paved the way to both extending the approachto other environmental radionuclides and expandingapplications exploiting the essentially unique advan-tages provided by the technique to controlling soil

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erosion and sedimentation and protecting the environ-ment (Zapata and Garcia Agudo, 2000; Pennock andFrick, 2001; Walling, 2002).

6.1. Other potential environmental radionuclides

137Cs is one of only a number of environmental fall-out radionuclides for use in soil erosion investigations.The137Cs technique, however, has practically monop-olised most, if not nearly all, recent work in this fieldand there have been very few attempts to use otherfallout radionuclides.

Other environmental radionuclides, including un-supported210Pb and7Be, have attracted much less at-tention, but there is increasing evidence that they offerconsiderable potential for use in soil erosion investiga-tions, both individually and complementary to137Cs.

6.1.1. Unsupported or excess210Pb210Pb is a naturally occurring radionuclide, prod-

uct of the238U decay series with a half-life of 22.26years. It is derived from the decay of gaseous222Rn,the daughter of226Ra. 226Ra occurs naturally insoils and rocks and will generate210Pb that will bein equilibrium with its parent. Diffusion of a smallquantity of the222Rn from the soil introduces210Pbinto the atmosphere, and its subsequent fallout pro-vides an input of this radionuclide to surface soilsand sediments that is not in equilibrium with its par-ent 226Ra (Robbins, 1978). This fallout componentis termed “unsupported” or “excess”210Pb, since itcannot be accounted for (or supported) by decay ofthe in situ parent. The amount of unsupported oratmospherically derived210Pb in a sediment samplecan be calculated by measuring both210Pb and226Raand subtracting the supported or in situ component(Appleby and Oldfield, 1983; Joshi, 1987). Although210Pb has been widely used for dating lake sedimentcores, its potential for estimating soil erosion hasbeen essentially little exploited to date (Appleby andOldfield, 1978; Robbins, 1978). As a fallout radionu-clide, which is rapidly and strongly adsorbed by thesurface soil, unsupported210Pb will behave in a sim-ilar manner to137Cs, except that its fallout input isconstant though time and the supply to the soil surfaceis being continuously replenished. Slight differencesin the depth distributions of the two radionuclides areto be expected. As a result of the continuous surface

replenishment, unsupported210Pb concentrations aregreatest at the surface, whereas in the case of the137Cs, the lack of significant replenishment during thepast 20 years coupled with biological activity withinthe soil and some downward diffusion, have causedthe level of maximum concentration to be displaced afew centimetres below the surface. Also, soil invento-ries associated with unsupported210Pb are generallysomewhat greater than those of137Cs (Walling andQuine, 1995; Walling and He, 1999b).

Because the deposition of fallout137Cs and210Pbexhibits different temporal patterns, potential shouldalso exist to use the two radionuclides as indepen-dent indicators in soil erosion studies to provide ad-ditional information on the erosional history of a site(Walling and Quine, 1995; He and Waling, 1997). Twoapproaches have been postulated for the conjunctiveuse of210Pb and137Cs in soil erosion investigations(Wallbrink and Murray, 1996a; He and Waling, 1997).

The similar behaviour of fallout210Pb in soils to137Cs makes it an alternative to137Cs in soil ero-sion investigations in areas where137Cs measurementsprove to be inapplicable. For instance, in areas wheresignificant amount of Chernobyl-derived fallout wasreceived or where the levels of bomb-derived137Csfallout were too low or heterogeneous. Further studiesare required to explore the potential for using unsup-ported210Pb measurements in these environments.

6.1.2. 7Be7Be is also a naturally occurring radionuclide pro-

duced by the bombardment of the atmosphere by cos-mic rays causing spallation of nitrogen and oxygenatoms in the troposphere and stratosphere. In this case,the radionuclide is extremely short-lived (half-life of53.3 days) relative to137Cs and210Pb, and it offerspotential for investigating soil erosion dynamics overmuch shorter time scales.7Be is typically concen-trated in the upper 5 mm of the soil profile and itis therefore capable of providing good discriminationbetween sediment derived from the uppermost soilsurface and that derived from depths >10 mm, whereconcentrations will be effectively zero (Walling andQuine, 1995).

This radionuclide has been used in erosion plots ex-periments to distinguish sediments mobilised by sheeterosion and rill erosion, and to demonstrate the pro-gressive development of rills during the course of

F. Zapata / Soil & Tillage Research 69 (2003) 3–13 11

a single simulated runoff event (Burch et al., 1988;Wallbrink and Murray, 1994, 1996b).

Monitoring of the spatial distribution of the7Beactivity across small plots, immediately after stormevents could also provide a basis for investigating localerosion patterns in response to micro-topographic con-trols (Walling and Quine, 1995). There is also scopeto couple measurements of7Be activity with equiv-alent measurements of137Cs and unsupported210Pbto provide increased capability for sediment sourcediscrimination. Combined measurements of137Cs and7Be have been used to derive estimates of medium-and short-term rates of soil redistribution within thestudy field (Walling et al., 2000).

6.2. Further applications of environmentalradionuclides

The IAEA through research networks and othermechanisms is promoting further development andapplications of the137Cs and other environmentalradionuclides in soil erosion and sedimentation re-search. Thus, the Isotope Hydrology Section is cur-rently implementing a new Co-ordinated ResearchProject entitled “Isotope techniques for sedimentsource fingerprinting” (2001–2005) with the overallobjective to apply nuclear and selected allied tech-niques to the planning and design of erosion and sed-imentation remediation strategies and assessing theireffectiveness, with emphasis on sustainable water-shed management (Turner, 2000). A new FAO/IAEACo-ordinated Research Project will be launched in2003 to further develop these methodologies, in par-ticular the combined use of137Cs,210Pb and7Be formeasuring rates and patterns of soil redistribution atseveral time and spatial scales, and to pilot test soilconservation measures tailored to local conditions andresources (Zapata, 2001b). Both new IAEA projectshave the ultimate goal of sustainable resource use andenvironmental protection.

Acknowledgements

The International Atomic Energy Agency (IAEA)provided core funding for implementing bothCo-ordinated Research Projects through its IAEAResearch Contract Programme. The close interaction

and sharing of experience and knowledge among theparticipating groups in the research networks havegreatly contributed to the developments on the appli-cation of the137Cs technique reported in this issue.The author is grateful to Profs. M. Kutilek and D.J.Pennock for their editorial comments to improve themanuscript.

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