NUCLEAR TECHNIQUES SUPPORT TO ASSESS EROSION AND SEDIMENTATION PROCESSES : PRELIMINARY RESULTS OF THE USE OF 137 Cs AS SOIL TRACER IN SLOVENIA

Most studies of erosion and sedimentation evaluation in Slovenia have focused on the use of conventional approaches. This paper highlights potential advantages of nuclear techniques to assess soil redistribution magnitude and presents assessment of the initial 137Cs fallout in undisturbed site, located in Šalamenci, Eastern Slovenia. The 137Cs background activity in the selected forested site was evaluated at 7316 ± 2525 Bq m–2 with a coefficient of variation of 34 % (n = 20). This information will be used in future investigations to assess erosion and sedimentation processes of adjacent agricultural fields with 137Cs method.


INTRODUCTION
Conservation of soil and water resources has become a major agronomic and environmental concern.Degradation phenomena, such as erosion, desertification and salinization affect 65 % of the worldwide soil (Oldeman et al. 1990).Soil degradation is currently affecting 1.9 billion hectares and is increasing at a rate of 5 to 7 million hectares each year (Lal 2006).The degradation of arable lands affects especially arid areas with poor vegetation cover and tropical areas with high intensity rainfall, but it occurs under temperate and continental climate as well (Mabit et al. 2002a).Water erosion is by far the most common type of land degradation in both developed and developing countries.Accelerated erosion decreases soil productivity, increases sedimentation and is related to environmental pollution problems in agro-ecosystems (Pimentel et al. 1995;Boardman 2006).
In Europe, erosion by wind and water is a major threat to the soil resource and represents the main mechanism of landscape degradation.Around 12 % of the total area of Europe are highly affected by erosion processes (Commission of the European Communities 2006; Boardman and Poesen 2006) and, as an effect of climate change and global warming, water erosion risk is expected to increase in the European Union by the year 2050 for about 80 % of the agricultural areas (European Environmental Agency 1999;2000).
Slovenia is not spared from this phenomenon, with more than 80 % of its territory affected by major soil degradation problems mainly linked to water erosion process (Komac and Zorn 2005;Zorn and Mikoš 2010).Arable land represents around 8.8 % of Slovenia's territory (Ministry for Agriculture, Food and Forestry 2010) and is subjected to heavy urbanization (Vrščaj 2008), therefore it is imperative that remaining arable land and cultural landscape are protected from land degradation, i.e. water erosion.Associated environmental impacts such as surface water eutrophication and groundwater pollution by nitrate are serious problems in the most intensive agricultural Slovenian regions (Matičič 1999;Drolc et al. 2007).
To control soil erosion there is a need to assess the impact of major land use and the effectiveness of specific soil conservation technologies using various approaches (Pimentel et al. 1995;Stroosnijder 2005).Effective erosion control starts with the knowledge of soil erosion rates and mechanisms.
Soil erosion processes and impacts on soil/water resources in Slovenia have been comprehensively addressed by Mikoš (1996) and Hrvatin et al. (2006a).Various research projects on water erosion processes and erosion risks assessment have been presented, i.e. involving various conventional techniques such as remote sensing, morphometric investigation combining GIS, DEM and radar photos, sediment transport models and sediment loading measurements, runoff plots and rainfall erosivity measurements (e.g.Ceglar et al. 2008;Hrvatin et al. 2006b;Komac and Zorn 2005;2007;Mikoš et al. 2006;Petkovšek et al. 2003;Petkovšek and Mikoš 2004;Zorn and Petan 2007).However, only a few quantitative data on erosion and sedimentation magnitude under Slovenian agro-environmental condition are available (Zorn 2009).
Traditional monitoring and modelling techniques for soil erosion/sedimentation require many parameters and years of measurements of inter-annual and mid-term climatic variability and cropping practices evolution (Mabit et al. 2002a).Conventional erosion and sedimentation methods are limited to provide mid-term trends in soil erosion, however fallout radionuclides (FRN) have proven to be very powerful tools to trace soil erosion and sedimentation within the landscape from plot to basin scale and can complement the information provided by conventional erosion measurements and modelling (Mabit et al. 2008a).
Despite several environmental records and surveys (e.g.Brajnik et al. 1993;Jeršič 1972;1974;1975;Official Gazette of Republic of Slovenia 2007;Slovenian Nuclear Safety Administration 2007), FRN have never been used as soil redistribution tracer for assessment of erosion processes in physical geography and soil sciences researches under Slovenian agro-environment.
The purposes of this contribution are: • To present a synthetic overview in using nuclear techniques, i.e. 137 Cs to assess erosion and sedimentation processes; • To present a first investigation to assess the initial 137 Cs fallout to prepare future agricultural field investigations to quantify the magnitude of erosion processes in Slovenia.

THE USEFULNESS OF FRN APPROACHES TO INVESTIGATE SOIL REDISTRIBUTION IN AGRICULTURAL LANDSCAPE
Three FRN isotopes ( 137 Cs, 210 Pb and 7 Be) have been used worldwide to assess medium and short term soil erosion and deposition processes (Mabit et al. 2008a;Ritchie and Ritchie 2008): I) Caesium-137 ( 137 Cs) with a half-life of 30 years, an artificial radionuclide originating from thermonuclear weapon tests and nuclear reactors releases (Figures 1 and 2); The spatial distribution of the 137 Cs fallout was determined by the location of the weapons testing, the pattern of stratospheric circulation and transport, and the annual precipitation amount.It shows a clear latitudinal zoning, with total fallout in the northern hemisphere being substantially greater than in the southern hemisphere. 137Cs inventories in the southern hemisphere are, however, still measurable using appropriate detectors and counting times.However, at a small scale it can be assumed that the fallout was uniform.
The fallout of 137 Cs released into the troposphere by nuclear accidents -e.g.Chernobylhad a more heterogeneous distribution, reflecting the atmospheric circulation and precipitation distribution immediately after the release.II) Lead-210 ( 210 Pb) with a half-life of 22 years, is a geogenic radioisotope that originates from the decay of 226 Ra which produces short-lived gaseous 222 Rn (half-life = 3.8 days).Most of this 222 Rn decays to 210 Pb within the soil, producing supported 210 Pb, which is essentially in equilibrium with the parent 226 Ra.However, some of the 222 Rn diffuses upwards into the atmosphere, where it rapidly decays to 210 Pb.This 210 Pb is deposited as fallout, and, since it is not in equilibrium with the parent 226 Ra, it is commonly termed unsupported or excess 210 Pb ( 210 Pb ex ), to distinguish it from the supported 210 Pb in the soil which is in equilibrium with the parent 226 Ra (Figure 1); III) Beryllium-7 ( 7 Be) is a short-lived cosmogenic radioisotope with a half-life of 53 days produced in the upper atmosphere by cosmic ray spallation on nitrogen and oxygen nuclei (Figure 1).Due to their rapid and strong adsorption by fine soil particles, 137 Cs, 7 Be and 210 Pb ex are redistributed by mechanical processes of moving soil particles such as erosion (e.g. Figure 2 for 137 Cs).The use of these isotopes as well as their measurements is safe and they are already present in soil, one just measures the FRN soil activity background.
FRN techniques allow the estimation of short and medium-term rates of soil redistribution integrating land use and climatic variability.FRN can be used to obtain average soil redistribution figures for time scales ranging from single event to many years of erosion processes, while direct measurements on erosion plots are related to single rainfall event or rather short periods of time.FRN methodologies integrate all processes involving soil particle movements and allow quantification of soil loss and deposition associated with sheet erosion, which is difficult to assess using other conventional approaches.Sampling of individual points allows spatially distributed information on rates and patterns of soil redistribution.Also, one of the main advantages of the FRN is that time-consuming, costly maintenance, long-term monitoring programme and installations required by non-isotopic and conventional methods can be avoided.Soil sampling can be completed in a short time and the site disturbance during sampling is minimal and does not interfere with seeding and cultivation operations, which might occur with installation of bounded erosion plot.Since radionuclide-based measurements also provide information on the spatial distribution of erosion/sedimentation rates, they can be used to validate the results of distributed soil erosion models.The relative efficiency of soil conservation measures can also be assessed.Advantages and limitations in using the FRN as soil redistribution tracer compared to conventional methods have been summarized in Table 1.
The 137 Cs redistribution across the landscape is investigated to estimate soil redistribution rates and patterns based on the total 137 Cs areal activity in the eroding or depositing sites and comparing them with the 137 Cs initial fallout established at undisturbed area called also 'reference sites' where no soil erosion or deposition occurred (Mabit et al. 2008a;Walling and Quine 1993).
The 137 Cs method provides estimates of soil redistribution averaged over a period of several decades (i.e. from the beginning of global fallout in the mid 1950s until the time of sampling).It is therefore particularly useful for providing estimates of medium-term mean annual soil erosion and deposition rates.A full comparison of the advantages and limitations

REFERENCE SITE SELECTION
The measure of erosion assumes that the local fallouts of 137 Cs were originally uniform in the area under investigation.The commonly used approach involves comparison of measured inventories with a reference inventory, representing the inventory associated with a point experiencing neither erosion nor deposition.In view of its central importance to the reliability of the estimates of soil redistribution rates obtained, it is important to ensure that the sampling site, termed control site or reference site used to establish the reference inventory, provides a representative estimate of the local reference inventory (Sutherland 1996).These benchmark stations are localized near the studied sector, in areas without erosion or deposition since the first introduction of radiocaesium in the environment, e.g.old and stable pasture, permanent grassland, forested area (Mabit et al. 2008a).

MATERIAL AND METHODS
An undisturbed flat forest of 2 ha situated in Šalamenci (46°44'N, 16°7'E) located in Pomurje in Goričko in Eastern Slovenia was selected as reference site to assess the initial 137 Cs fallout for future investigations on erosion and sedimentation process affecting the neighborhood agricultural fields and areas (Figure 3).The climate is moderate inland continental with mean annual precipitation reaching 870 mm (Murska Sobota, 46°39'N, 16°11'E; Bat et al. 2008).Developed on non-carbonate rocks, the soil of the Šalamenci forest was described as a strongly acidic Haplic Stagnosol with a silt loam texture.
In the area under investigation, 20 soil profiles of 40 cm, divided into four increments of 10 cm were collected according to a 40 x 30 m grid, using bulk density cylinders based on the protocol proposed by Mabit et al. (2008b).The soil samples (n = 80) were pre-treated (oven-dried 48 hours at 70 o C and sieved at 2 mm) prior to γ-spectrometry measurements using a high resolution HPGe coaxial detector (relative efficiency at 1.33 MeV, 60 Co = 115.6 %).Calibration and quality control were performed using sealed radioactive source FG 607 from Amersham and IAEA reference materials according to the protocol proposed by Shakhashiro and Mabit (2009).The Minimum Detectable Activity (MDA) for 137 Cs was 0.2 Bq kg -1 and the average error at 2 σ precision for 50,000 seconds counting times was 2.6 % ± 1.2 %, 6.5 % ± 3.3 %, 26 % ± 14 % and 46 % ± 24 % for the soil layers 0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm, respectively.
After gamma measurement, the 137 Cs mass activity (Bq kg -1 ) was converted into areal activity (Bq m -2 ) using the bulk density of the different incremental samples.The total inventory at each sampling point was calculated as the sum of the depth interval areal activity.The 137 Cs areal activity data set was tested for normality by using Kolmogorov-Smirnov test at a confidence level of 95 %.All descriptive statistics calculus and statistical tests in this study were carried out by SPSS programme version 11.5 for Windows.

PRELIMINARY RESULTS AND FUTURE PERSPECTIVES IN THE APPLICATION OF THE 137 CS METHOD AS SOIL TRACER IN THE STUDY AREA
As expected in a forested soil with a fibric and organic matter enriched top soil, the bulk density increased with incremental depth reaching 1.5 t m -3 at 40 cm (Table 3).The activity of the 137 Cs measured in the layer 20-30 cm was close or even under the detection limit of our gamma detector (0.2 Bq kg -1 ).Below the first 30 cm, 137 Cs soil content can be neglected (Table 4).The Kolmogorov-Smirnov test at a confidence level of 95 % confirmed that the 137 Cs content (total inventory 0-40 cm) is distributed along a normal distribution.The maximum 137 Cs mass activity was found in the first 10 cm with an average of 70 ± 33 Bq kg -1 (Table 4).In undisturbed and uneroded mineral or organic soils, the profile shape distribution of 137 Cs typically decreases with depth (Mabit et al. 2008a).In this forest also a clear exponential decrease in 137 Cs content was observed with 98 % of the 137 Cs in the first 20 cm.
Based on the 20 profiles collected (0-40 cm), the initial 137 Cs fallout was evaluated at 7316 ± 2525 Bq m -2 with a coefficient of variation (CV) of 34 %.The CVs of the 137 Cs initial inventory in undisturbed forested areas -if chosen as reference sites -are generally relatively high, from 5 % in the best case to 41 % (Sutherland 1996;Owens and Walling 1996).An acceptable CV should be around 25 %.
Under similar climatic conditions, e.g. in France (Bernard et al. 1998) and Austria (Mabit et al. 2008b) without a significant contribution of Chernobyl the base level in 2009 could be expected to be around 1800-2000 Bq/m -2 .A value three times higher in this study area in Slovenia suggests a major contribution of the Chernobyl fallout (April-May 1986).
Based on these preliminary results additional investigations and actions are needed: • Firstly, the baseline 137 Cs evaluation should be improved in reducing the coefficient of variation, then an additional sampling in the reference site should be planned to collect 10 additional soil profiles to refine the value obtained; • Secondly, the contribution of the Chernobyl accident to 137 Cs soil inventories should be estimated for future assessment of its contribution comparing to the nuclear test deposits occurring in the 1950 and 60's.This value will be included in the conversion model to convert the areal activity measured in the agricultural field to assess more accurately the soil redistribution magnitude (Walling et al. 2002); • Thirdly, a sampling along a multi-transect design will be organized in an adjacent agricultural field to assess quantitatively the soil erosion and sedimentation magnitudes.

CONCLUSIONS
To control water erosion processes and subsequent land degradation, impact assessment of major land uses is needed.FRN and especially 137 Cs provide very effective means of quantifying erosion and sedimentation rates and represent a valuable complement to validate conventional measurement techniques, especially computer models.
The methodology of 137 Cs can also be a useful and valuable technique to complement conventional runoff and sediment measurements when assessing soil redistribution in agricultural landscape.
A typical exponential distribution of 137 Cs in the soil profile was found and the baseline level in this undisturbed Slovenian forest was established at 7316 ± 2525 Bq m -2 with a coefficient of variation of 34 %.The variability of the fallout is rather high and before any additional agro-environmental investigation using 137 Cs as soil tracer in this area, the reference site should be refined to obtain an acceptable level of the initial baseline level.
A new sampling to assess a more reliable value for the initial 137 Cs fallout is planned and in the mean time a small agricultural field to evaluate the magnitude of soil redistribution will be investigated.

Table 3 :
Bulk density summary statistics per soil depth increments of the forested site(Šalamenci,