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Journal of Environmental Radioactivity 136 (2014) 112e120

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate / jenvrad

Uncertainty related to input parameters of 137Cs soil redistributionmodel for undisturbed fields

Andra-Rada Iurian a, *, Lionel Mabit b, Constantin Cosma a

a Babes-Bolyai University, Faculty of Environmental Science and Engineering, 30 Fantanele, 400294 Cluj-Napoca, Romaniab Soil and Water Management and Crop Nutrition Laboratory, Joint FAO/IAEA Programme Nuclear Techniques in Food and Agriculture,Department of Nuclear Sciences and Applications, Austria

a r t i c l e i n f o

Article history:Received 6 January 2014Received in revised form14 May 2014Accepted 17 May 2014Available online

Keywords:137Cserosion rateconvectionediffusion equationdiffusion and migration modelradionuclide migrationundisturbed soil

* Corresponding author. Present address: TerrestrIAEA Environment Laboratories, Department of NucleInternational Atomic Energy Agency, Vienna InternatioVienna, Austria. Tel.: þ43 1 2600 28371; fax: þ43 1 2

E-mail addresses: iurian.andra@ubbcluj.ro, A.Iuria

http://dx.doi.org/10.1016/j.jenvrad.2014.05.0130265-931X/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This study presents an alternativemethod to empiricallyestablish the effective diffusion coefficient and theconvective velocity of 137Cs in undisturbed soils. This approach offers the possibility to improve theparameterisation and the accuracy of the 137Cs Diffusion and Migration Model (DMM) used to assess soilerosionmagnitudes. The impact of the different input parameters of this radiometricmodel on the derived-soil redistribution rates has been determined for a Romanian pastureland located in the northwest ex-tremity of the Transylvanian Plain. By fitting the convectionediffusion equation to the available experi-mental data, the diffusion coefficient and convection velocity of 137Cs in soil could be determined; 72% ofthe 137Cs soil content could be attributed to the 137Cs fallout originating fromChernobyl. Themedium-termnet erosion rate obtained with the calculated input parameters reached �6.6 t ha�1 yr�1. The modelhighlights great sensitivity to parameter estimations and the calculated erosion rates for undisturbedlandscapes can be highly impacted if the input parameters are not accurately determined from theexperimental data set. Upper and lower bounds shouldbe establishedbased on the determined uncertaintybudget for the reliable estimates of the derived redistribution rates.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Soil degradation processes are currently affectingmore than 33%of the Romanian territory and among them water erosion is pre-dominant. The Transylvanian Tableland is one of the identifiedareas with very high erosion risk (Motoc et al., 2010).

137Cs is an artificial radionuclide generated in the atmosphereafter the nuclear weapon tests (the maximum activity has beenreached in 1963's according to Poręba and Bluszcz (2007)) and alsoafter the Chernobyl nuclear power plant (NPP) release inAprileMay 1986. In the late 1960's, due to the conservative physi-cochemical characteristics and behaviours of the bomb-derived137Cs (e.g. uniform spatial distribution within the landscape,strong binding to fine soil particles) this anthropogenic falloutradionuclide (FRN) has been suggested to the research communityas new soil and sediment tracer. Since the 1960s, the 137Cs method

ial Environment Laboratory,ar Sciences and Applications,nal Centre, PO Box 100, 1400600 28222.n@iaea.org (A.-R. Iurian).

represents aworldwide-applied tracing technique for assessing soilerosion and sedimentation rates which has been validated invarious natural and agricultural landscapes (Ritchie and McHenry,1990; Bernard et al., 1998a; Matisoff and Whiting, 2011; Mabitet al., 2008a, 2013). Particular attention must be paid to the Cher-nobyl contaminated areas since it is supposed that the additionalinputs of the Chernobyl-derived 137Cs induced higher spatial vari-ability than the bomb-derived fallout, especially in regions with alevel of contamination between 5 and 40 k Bq m�2 (Golosov, 2002).

Several radiometric models to investigate soil redistribution oncultivated and uncultivated fields have been developed during thelast 20 years (Walling andHe,1999;Wallinget al., 2002, 2011). Theseconversionmodels consider the radionuclide inventories across thefield, the 137Cs fallout pattern and the processes associated with thesoil particle movement (Walling and He,1999; Walling et al., 2002).

The standardization of models for deriving estimates of erosionand deposition rates from 137Cs and the development of a user-friendly software for the model implementations (Walling et al.,2011) have allowed researchers to test and use the same modelsin different countries under various environmental conditionsproducing worldwide comparable results. However, there is still alack of information and guidance for an accurate determination of

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120 113

some of the parameters (i.e. Chernobyl contribution) involved inthesemodels. This is especially the casewhen an accidental nuclearrelease has a major contribution to the overall 137Cs inventory insoil, which could be a significant source of uncertainty in erosionand sedimentation radiometric-derived evaluation. Moreover, inrecent review and discussion papers, Guzm�an et al. (2013) high-lighted that reducing the uncertainty associated with the conver-sion of radiotracer concentrations into erosion rates should be a keygoal for future applications of FRNs in soil science research. Mabitet al. (2013) suggested that conversion models and their associ-ated parameters can still be refined to reach an improved accuracyas compared to the results provided by conventional erosion andsedimentation measurement.

The objectives of our study were:

(i) to apply the convectionediffusion equation for the 137Cs soildepth profile in a Romanian reference site using the leastsquare fitting procedure to determine the effective diffusioncoefficient and the convective velocity of both Chernobyl andbomb-originated 137Cs;

(ii) to highlight the impact of the input parameters (i.e. thereference inventory, the Chernobyl contribution, the diffu-sion coefficient, and the convective velocity for 137Cs, theparticle size correction factor) used in the Diffusion andMigration Model (DMM) for uncultivated fields; and

(iii) to assess the soil erosion and deposition rates affectingRomanian pastureland using the 137Cs method.

2. Materials and methods

2.1. Study site and sampling design

Soil samples were collected in a pastureland from Cluj County,Somes watershed, located at the west extremity of the Transyl-vanian Plain (N46�520, E23�450). The study site belongs to the Jucu

Fig. 1. Location of the study s

Research Station, was and has been covered with perennial plantsfor more than two decades to protect the soil against soil degra-dation and erosion processes. This grassland has a complextopography with prominent acclivities and average slope of 10%.The climate in this area is temperate continental.

Two sampling campaigns were conducted in November 2010and July 2011. Twelve soil cores were taken to 40 cm along twoparallel transects using two manually-operated cylindrical steelcorers (with inner diameters of 5.5 cm and 7.5 cm, respectively). Onthe first transect, the soil profiles were collected in 2.5 or 5 cm soilincrements and on the second transect bulk soil cores (0e40 cm)were taken. The distance between the two transects was approxi-mately 100 m. Sampling points of each transect were selectedaccordingly to the field micro-topography and the transect distancebetween the collected samples varied from 20 to 50 m (see Fig. 1).No sampling was performed in the lowest part of the field becauseof the presence of buildings and known human activity, whichcould have disturbed the 137Cs depth distribution in soil.

To establish the mean reference value for the 137Cs inventory,five sampling points were randomly considered in a flat undis-turbed grassed terrace situated about 500 m west from our studysite, with a minimum of 10 m between each collection points. Anincremental soil profile was taken to 40 cm to establish the 137Csdepth distribution and four additional bulk soil profiles weresampled to a depth of 30 cm to include all detectable 137Cs. Eachbulk soil profile included composite soil from three nearby sam-pling points, summing a number of 12 soil cores. To avoid theradionuclide ‘dilution’ in an overly voluminous soil mass, thecomposite soil was divided in three subsamples per 10 cm layers.The adopted sampling strategy aimed to take account of the spatialmicro-variability of the 137Cs inventory in the reference area.

Additional samples from the soil subsurface (5e10 cm) werecollected in the reference site and also close to each sampling pointin the study field to perform complementary physicochemicalanalysis.

ite and sampling design.

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120114

2.2. Laboratory analysis

2.2.1. Gamma spectrometric measurementsSoil samples were air-dried at room temperature and then

oven-dried at 105 �C for 24 h. After being manually ground andsieved to pass 2 mm, the soils were enclosed in cylindricalcontainers and tightly sealed with electrical band to be preparedfor gamma measurements. Two HPGe detectors with relativeefficiencies of 34% and 30% respectively and IAEA referencematerials were employed in the gamma analysis. The Maestro-32gamma software was used to perform the spectra evaluation.Measurement time ranged from 60000 to 80000 s and thecombined standard uncertainties were below 14% for the bulksamples and below 44% for the incremental samples (at 95%confidence level). The 661.6 keV gamma emission of 137mBa wasanalysed for the 137Cs activity determination. All the activitieswere decay corrected according to our first sampling date (1stNovember 2010).

2.2.2. Physicochemical characteristics of the soil investigatedSeveral soil physicochemical properties were determined such

as texture, pH, oxidation-reduction potential (ORP), electrical con-ductivity (EC), total dissolved solids (TDS), water content (%), bulkdensity (g cm�3) and organic matter (OM).

The wet weight of soil mass was registered to evaluate themoisture content (%). Then, the samples were oven-dried for oneday and the dry weight was recorded. Soil granulometry wasdetermined using a combination of hydrometer and sievingmethods, according to Romanian standards (STAS 1913/5-85).Fifty grams of dry soil per sample has been weighed and sievedusing the Retsch system AS200 with meshes of different aper-tures. The sedimentation time of the finest soil particles wasthen registered for different time intervals using the hydrometermethod and the diameter and the size distribution of the parti-cles were recorded. The resulting grain size distribution of thesoil sample was considered representative for the whole soilprofile.

The OM content was qualitatively determined using the methoddescribed in the Romanian standard STAS 7107/1-76. The pH, ORP(mV), EC (mS cm�1), TDS (mg L�1) were determined by the poten-tiometric method using two digital multiparameters (WTW720and WTW350, respectively, [pH electrode SenTix 41-3, pH 0.14/0.80 �C]).

2.3. Diffusion and migration erosion model for uncultivated fields

The 137Cs methodology to evaluate soil redistribution is basedon the comparison between the 137Cs inventories of samplingpoints with the inventory of a close undisturbed reference site. Fora deposition area, the 137Cs inventory is greater than the back-ground inventory established at the reference site. In contrast, in aplace where net soil erosion occurs, the 137Cs inventory is reducedas compared to this reference value (Walling and He, 1999; Wallinget al., 2002; Mabit et al., 2008a; Walling et al., 2011). This 137Cs soilcontent comparison allows to distinguish erosion and depositionprocesses and therefore to assess soil redistribution pattern.

There is a clear distinction to be made between the proceduresand models employed for cultivated and for uncultivated land toconvert 137Cs measurements to quantitative estimates of soilredistribution. Each model has its own specific set of parameters,although some parameters are common between models. The dif-ference is mostly due to the processes involved in the 137Cs redis-tribution within the soil profile, which in the case of pasturelandare only natural. The DMM represents one dimensional transportmodel characterized by an effective diffusion coefficient D

(kg2 m�4 yr�1) and a migration rate v (kg m�2 yr�1) for 137Cs withinthe soil profile. It also takes into account the temporal variation of137Cs in soil depth. This conversion model developed by Wallinget al. (2002) is further used to convert FRNs data set into soilerosion and deposition rates.

The model is based on the equation describing the variation ofthe 137Cs concentration C (t) (Bq kg�1) in soil with time t (yr)(Walling et al., 2002, 2011):

CðtÞzIðtÞH

þZ t�1

0

Iðt0Þe�RHffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

Dpðt � t0Þp e�v2ðt�t0Þ

4D �lðt�t0Þdt0 (1)

where I (t) is the annual 137Cs deposition flux at time t(Bq m�2 yr�1), H reflects the depth to which the fresh fallout inputof 137Cs penetrates the soil (kg m�2), R is the erosion rate(kg m�2 yr�1) and l is the 137Cs decay constant (yr).

3. Results and discussions

3.1. Application of the convectionediffusion equation for the 137Csdepth profile in the reference site

The Romanian territory was highly affected by the Chernobylcontamination, especially in areas where the radioactive cloud wasfollowed by heavy rainfalls. 137Cs is the main artificial radionuclideoriginating from this nuclear release in the atmosphere and is stillpresent in environmental media due to its ~30 year half-life(Cosma, 2002). Therefore, the 137Cs content in soil samples fromJucu area has two different sources: the global nuclear testingfallout (with a maximum in 1963) and the Chernobyl releases from1986. These two different sources will be found overlapped withinthe total 137Cs depth profile, and in some cases the peak corre-sponding to the bomb-derived 137Cs can be distinguisheddepending on the extension of the Chernobyl contribution in thetotal 137Cs inventory. According to Cosma et al. (2012), theFukushima NPP accident that took place in March 2011 had anegligible contribution to the total 137Cs inventory in Transylvania,and thereby will not influence the use of 137Cs as soil and sedimentredistribution tracer in the area of our study site.

The 137Cs content of the incremental soil profile in our referencesite demonstrated that the migration of this radionuclide is a slowprocess with most of its activity still found in the top 10 cm. Theconvectionediffusion equation was fitted to our available experi-mental data set to describe the 137Cs behaviour and penetration insoil after both contaminations occurred (firstly in 1963, and sub-sequently in 1986). Themodel assumes that the 137Cs soil migrationis controlled by physical and chemical processes, which take intoaccount contamination from the atmospheric nuclear weapon testsand the Chernobyl releases. Previous studies demonstrated that themobility of 137Cs in soil is determined by the soil physicochemicalproperties, as well as by the physicochemical characteristics of theradionuclide itself (Kirchner et al., 2009; Szerbin et al., 1999). Themodel has beenwidely applied, mostly in Chernobyl affected areas,and is based on a diffusive-convective transport equation, a con-servation equation and a linear equilibrium sorption equation.

The 137Cs effective migration velocity v (cm yr�1) and theeffective diffusion coefficient D (cm2 yr�1) values were determinedfor each 137Cs fallout input (from 1963, and 1986 respectively) usingthe least squares method to fit the function C (x,t) to the verticaldistribution data of 137Cs from the reference site. The 137Cs con-centration in soil (Bq cm�3), C (x,t) given by a single source at depthx and t years after that the initial deposition occurred, with l the137Cs decay constant (yr), is defined by the following equation(Bossew and Kirchner, 2004):

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120 115

�lt

8>< 1 �ðx�vtÞ2 v vx

v

ffiffiffiffit

rx!9>=

Fig. 2. Experimental depth distribution of 137Cs in the reference site, Chernobyl andthe nuclear tests fitting curves obtained by applying the convectionediffusion equa-tion (Error bars represent the uncertainty of the gamma-ray spectrometric measure-ments at 2s confidence limit).

Cðx; tÞ ¼ J0e >: ffiffiffiffiffiffiffiffiffipDt

p e 4Dt

2DeDerfc

2 Dþ2ffiffiffiffiffiffiDt

p >; (2)

Considering the two different 137Cs origins in Romanian soils,the total radionuclide concentration can be expressed as:

Cðx; tÞ ¼ CCh�x; t1; J0Ch;D1;v1

�þ CNp�x; t2; J0Np;D2;v2

�(3)

where CCh(x,t1) is the Chernobyl 137Cs concentration and CNp(x,t2) isthe 137Cs content in soil originating from nuclear tests. Therefore,considering both equations (2) and (3), the deriving terms used forthis application are: J0Ch the initial Chernobyl deposition (Bq cm�2)for AprileMay 1986, J0Np the initial nuclear tests deposition(Bq cm�2) considered for the time of the maximum release, middle1963, t1 ¼ 24.5 yr and t2 ¼ 47.5 yr the time elapsed between the137Cs deposition and the first sampling date (November 2010), themigration velocities v1 and v2, and the diffusion coefficients D1 andD2, respectively, for both 137Cs origins.

It should be pointed-out that this model oversimplifies a naturalsystem as it does not consider the horizontal component of thetransport (one-dimensional model) and the v and D are being heldconstant in the soil column length and over time for both 137Csorigins.

The initial 137Cs fallout was evaluated based on additional data.The 137Cs inventory of nuclear testing deposits was determined as766 Bq m�2 and was decay-corrected for 2010 for the investigatedarea, based on themodel of Sarmiento and Gwinn (1986) in relationto precipitation data. A mean value of 563 mmyr�1 was used in themodel and calculated over thewhole period of atmospheric nucleartesting releases (i.e. 1954e1974), taking into account the NCDC(2013) database for the Cluj-Napoca Meteorological Station,located 15 km south from the investigated area. The difference inthe inventory between the total experimental content of 137Cswithin the incremental soil profile and the nuclear tests representsthe expected Chernobyl derived 137Cs. A value of 1823 Bq m�2 wasdetermined in 2010. The variation of these parameters was limitedto a maximum of 5% for the least square fitting procedure.

The initial 137Cs fallout inputs produced by the fitting were0.2146 Bq cm�2 for the nuclear tests in 1963 and 0.3240 Bq cm�2 forChernobyl in 1986, translating into 737 Bq m�2 for the bomb-derived 137Cs and 1878 Bq m�2 for the nuclear accidental released137Cs at the time of sampling (2010). Comparing the total fittedinventory in the soil profile with our experimental value of2589 Bq m�2, the fitting uncertainty was around 1%.

Applying the least square fitting procedure, thev2 ¼ 0.350 cm yr�1 and D2 ¼ 0.321 cm2 yr�1 values were obtainedfor the depth migration of bomb-derived 137Cs, andv1 ¼ 0.097 cm yr�1 and D1 ¼ 0.15 cm2 yr�1 for the Chernobyl 137Csdistribution in soil. Themeasured and the fitted radionuclide valuesof each soil layer are plotted in Fig. 2 using the data analysis soft-ware Origin 8.0.

The depth migration of 137Cs with distinctive origin appeared tobe quite different. As highlighted by Fig. 2, the deeper 137Cs peakprofile which appeared around 15e20 cm indicates a broaderlongitudinal dispersion of nuclear test-derived 137Cs, while thepeak associated to the Chernobyl event was found close to the soilsurface at 2.5e5.0 cm. These findings are consistent with the 137Csdepth profiles found in the literature for both bomb-derived andChernobyl 137Cs (see Belyaev et al., 2005; Golosov et al., 2008;Montes et al., 2013). Sawhney (1970) stated that higher clay con-tent in soil enhances Cs fixation and therefore reduces its mobility.Considering that the convectionedispersion model has time-invariant parameters, which do not change with the residence

time of the radionuclide in the soil, higher migration rates can beexpected if the maximum deposition was accompanied by rainfallswhich permitted the infiltration of water at greater depths.

Applying this convectionediffusion equation for 528 Austriansoil profiles, Bossew and Kirchner (2004) reported that the typicalvalues of different artificial nuclides are 0.1e0.5 cm yr�1 for thevelocity, v, and 0.05e0.5 cm2 yr�1 for the diffusion constant, D,respectively. Our resulting values are in accordance with thoseabove and also with those obtained for both bomb-derived andChernobyl 137Cs in other studies performed in clay-loam soils(Kirchner et al., 2009; Montes et al., 2013; Schuller et al., 2004).

3.2. Assessment of erosion and deposition rates and uncertaintiesrelated to the input parameters of the radiometric model

The total 137Cs areal activities of the 12 sampling points in theinvestigated site ranged between 1157 Bq m�2 and 4629 Bq m�2

(Iurian et al., 2012). The inventories of 137Cs were higher than thereference value only for three soil cores of the first transect, high-lighting a predominant depositional process at these points. The137Cs data will be further used for the estimation of soil redistri-bution magnitude in Jucu area by applying the DMM for undis-turbed areas.

3.2.1. The assessment of soil redistribution rates in Jucu area usingthe DMM

The key parameters needed for running this radiometric modelare: (i) the reference inventory (Bq m�2), (ii) the migration velocity

Table 1List of parameters used in the radiometric conversion model.

Parameter Value

137Cs inventories (Bq m�2) 1157e4629Particle size correction factor P 1.01e1.67Particle size correction factor P0 0.33e0.76Reference inventory 137Cs (Bq m�2) 3160Relaxation depth (kg m�2) 5Sampling year (yr) 2010Chernobyl contribution (%) 72%Diffusion coefficient D (kg2 m�4 yr�1) 28.2Convection velocity v (kg m�2 yr�1) 2.012

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120116

v (kg m�2 yr�1) and the effective diffusion coefficient D(kg2 m�4 yr�1), (iii) the Chernobyl contribution (%) to the total 137Csinventory in the study area, (iv) the particle size correction factorsfor erosion (P) and deposition (P0), and (v) the relaxation depth (H),for which the input value of 5 kg m�2 is considered for undisturbedfields (Walling et al., 2002). These parameters have been deter-mined using our experimental data set (see Sections 3.2.2e3.2.5),apart from H value, which is difficult to obtain empirically as thecurrent 137Cs fallout flux is negligible, being impossible to replicatethe past situation during the main period of fallout. However, onepossibility to experimentally determine the relaxation depth wouldbe by simulating fallout input using a rainfall simulator or byestablishing the soil depth distribution of recent FRN fallout (e.g.7Be).

Considering the calculated input parameters (given in Table 1)and the 137Cs inventories (Bq m�2) determined for the 12 samplingpoints in the investigated site, the soil erosion rates (t ha�1 yr�1)were estimated by means of the DMM.

Because of the preponderant Chernobyl 137Cs in the investigatedarea relative to the nuclear tests-derived 137Cs (ratio betweenChernobyl 137Cs and bomb-derived 137Cs about 4:1), it can beconsidered that the results derived from the 137Cs data modellinggive estimates of the yearly average soil redistribution rates overthe 1986e2010 period, 2010 being the sampling date (Table 2).

The resulting net soil erosion rate of �6.6 t ha�1 yr�1 is quitehigh compared to other values obtained for grassland areas (Katoet al., 2010). However, as there is no evidence of intensive grazingat the site, soil erosion processes were favoured by the fine soiltexture and also by the marked topography. Applying the SWATmodel to assess the spatial distribution of soil erosion over the EastRiver Basin, Wu and Chen (2012) concluded that soil erosion ishighly sensitive to soil properties and slope. Effectively, due to stepslopes and winter erosion processes, Konz et al. (2012), using the137Cs method, calculated a yearly erosion rate of 8 to 26 t�1 ha�1

affecting the Swiss alpine grasslands between 1986 and 2008. Inthis mountainous region, erosion magnitude can be higher than30 t ha�1 y�1 in pastures without dwarf shrubs (see Konz et al.,2009).

Further on, it will be demonstrated that neglecting the contri-bution of even one of these input parameters can significantly overor under estimate the final results.

Table 2Assessment of soil redistribution rates (summary for both transects).

Assessment of soil redistribution rates Value

Gross erosion rate (t ha�1 yr�1) �12.2Mean erosion rate in the eroding areas (t ha�1 yr�1) �16.2Mean deposition rate for the depositional areas (t ha�1 yr�1) 4.2Net erosion rate (t ha�1 yr�1) �6.6Sediment delivery ratio (%) 54%

3.2.2. Reference inventory valueA representative estimate of the local reference inventory also

termed ‘baseline’, is a key parameter in all soil erosion studiesconducted through radiometric modelling (Mabit et al., 2010). Theaveraged 137Cs control site inventory was measured by Iurian et al.(2012) as 3160 ± 867 Bq m�2 (areal inventory ± standard deviation,2s confidence interval), with a coefficient of variation (CV) of 27%.This fallout variability in the reference area is consistent with CVsfor grassland sites (5e41%) provided by the literature reviews ofSutherland (1991, 1996) and compare well with those measuredelsewhere in other undisturbed agrosystems (Rodway-Dyer andWalling, 2010; Mabit et al., 2002, 2012). Moreover, studies per-formed by Pennock (2000) and Mabit and Bernard (2010) on thespatial distribution of soil quality indicators (including 137Cs) inCanada and Ghana indicate that 137Cs typically has a coefficient ofvariation of about 20% in reference sites, a value comparable withother soil quality indicators such as organic carbon and nitrogen.

Iurian et al. (2013) established a mean 137Cs baseline level of5460 ± 880 Bq m�2 (n ¼ 10, CV ¼ 16%) in Mures County (Romania),located around 100 km east from our present study site. Values of6900 Bq m�2 and 4980 Bq m�2 were determined by Popa et al.(2011) for two reference sites from Barlad tableland located ineastern Romania. The different background level of the 137Cs in-ventories recorded in the Romanian territory is mostly due to thedifferent meteorological conditions in AprileMay 1986 associatedwith the NEeSW Chernobyl cloud passage over the country(Cosma, 2002). Other studies (Belyaev et al., 2005, 2009; Porębaand Bluszcz, 2007; Golosov et al., 2008) found a variation in 137Csinventories of less than 20% for spatially limited reference areashighly affected by Chernobyl release which meet the requirementsestablished for bomb-derived 137Cs fallout variations to be used insoil erosion radiometric modelling (Zapata, 2002).

The DMM appears very sensitive to the reference inventoryvalue; an increase of only 15% of this specific parameter (consideredas acceptable measurement uncertainty/coefficient of variation),with no change in the other parameters of the model, would resultin an increase of the derived net erosion rates by about 100% forinvestigated site (see Fig. 3). It must be pointed out that in otherspecific sites, the variation in the derived net erosion estimates canbe different when varying the mean reference inventory. However,special consideration must be made in the evaluation of the 137Csreference value in Chernobyl affected areas. Because of the falloutspatial variability concern, multiple sampling points must beconsidered in the reference area, and if possible, additional controlsites close to the study area should be investigated. Recently,Kirchner (2013) stressed the importance of the reference inventoryparameter and sampling design for soil redistribution studies. Theauthor argued that a very low number of samples collected in thereference area invalidates the 137Cs method even for sites wherelow spatial variability is found and a ‘tolerance’ interval should beestablish for the mean 137Cs reference value.

3.2.3. Chernobyl contributionSeveral approaches can be used to derive the Chernobyl 137Cs

contribution relative to the total radionuclide inventory in undis-turbed soil. Therefore it is not always easy to make a decisionconcerning the most appropriate approach considering the avail-able data. Using the model of Sarmiento and Gwinn (1986) for thebomb-derived 137Cs in our site and the convectionediffusionequation (see Section 3.1), approximately 72% of the measuredmean reference inventory had a Chernobyl origin. This Chernobyl137Cs contribution is a realistic value and is in agreement with thedata for the total 137Cs deposition on Romanian territory fromaccidental atmospheric release (~71%) derived from Table III.1 of

Fig. 3. Sensitivity analysis of the input parameters in DMM radiometric derived-soil redistribution rates model (negative values of the soil redistribution rates represent net erosionand positive values net sedimentation).

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120 117

the Atlas of Caesium Deposition on Europe after the Chernobylaccident (see De Cort et al., 1998).

The estimation of the global 137Cs fallout based on the precipi-tation record was also performed by Poręba and Bluszcz (2007) forthe Proboszczowicki tableland, a Chernobyl contaminated regionlocated in south-west Poland. These authors estimated the contri-bution of Chernobyl fallout to the total 137Cs soil content at 69%.

Another possibility to determine the Chernobyl contribution isto use records of the 137Cs cumulative atmospheric fallout comingfrom the nuclear tests before Chernobyl event, if these data areavailable. However, to our knowledge, no measurements wereperformed to evaluate the 137Cs concentrations in soils of theinvestigated area before 1986.

Data for the estimated bomb-derived 137Cs contamination level,derived from maps published by De Cort et al. (1998) in theCaesium Atlas of Europe (Residual levels of 137Cs deposition fromatmospheric weapon tests, effective on 1st May, 1986), can be alsoused to distinguish between the 137Cs originating from Chernobyland fallout associated with nuclear tests (Szab�o et al., 2012). Thisapproach was also recommended by Golosov (2002) for the esti-mation of the Chernobyl contribution parameter. The 137Cs patternfor these maps was derived based on data for 90Sr deposition acrossthe Northern Hemisphere, considering the ratio of the totalamounts of these two nuclides in the atmosphere. According to themap by De Cort et al. (1998), the average level of 137Cs depositionfor the latitude of our study site (46e47�N), derived from the at-mospheric testing of nuclear weapons determined prior to theChernobyl accident (May 1986) was approximately 2500 Bq m�2.After correcting this value for decay with sampling time (2010), thederived Chernobyl contribution was determined as 69%.

Schimmack et al. (2001) determined the Chernobyl contributionin the 137Cs inventory using the Chernobyl ratio established be-tween 137Cs and 134Cs. Because of the short half-life of 134Cs (2.06years), it is no longer possible to use this isotopic ratio (i.e.137Cs/134Cs) for estimating the Chernobyl origin of the 137Cs contentin the total inventory in areas with significant Chernobyl impact.

Different approaches presented above and used for deriving theChernobyl contribution parameter would give comparable results.However, if no data are available for 137Cs activity in soils beforeChernobyl, or data regarding the Chernobyl 134Cs contamination in

the area, the model of Sarmiento and Gwinn (1986) appears to bemore site-specific than the low-resolution map of De Cort et al.(1998).

This radiometric model (DMM) is expected to be particularlysensitive to the contribution of this parameter, especially in areahighly affected by Chernobyl nuclear release. If the parameter is setat 80% (the maximum value allowed by the model computation),the net erosion rate will be 125% higher than the scenario when thereal value of 72% is used, since a 25% Chernobyl contribution willgive a net erosion rate reduced by 74% (see Fig. 3).

3.2.4. Diffusion coefficient D and convection velocity vThe diffusion coefficient (cm2 yr�1) and the migration velocity

(cm yr�1) parameters were determined in Section 3.1 for theChernobyl source 137Cs and for nuclear testing fallout, based on thediffusive-convective transport equation. Furthermore, using themean bulk density of the incremental soil core (1194 kg m�3), weobtained D and v values for the nuclear accidental release of21.4 kg2 m�4 yr�1 and 1.16 kg m�2 yr�1, and for the bomb-derived137Cs of 45.6 kg2 m�4 yr�1 and 4.18 kg m�2 yr�1 respectively. Theweighted mean of the two parameters was then calculatedconsidering the contribution of each 137Cs input in the total arealactivity of the radionuclide. The derived mean values of D(28.2 kg2 m�4 yr�1) and v (2.012 kg m�2 yr�1) were used for theapplication of the radiometric model of soil erosion.

Estimates of v and D of the reference site can be also empiricallydetermined from the distribution of 137Cs mass activity (Bq kg�1)versus the cumulative mass depth (kg m�2) using the followingequations of Walling et al. (2002):

vzWp

t � 1963¼ 1:043 kg m�2yr�1 (4)

Dz

�N �Wp

�22ðt � 1963Þ ¼ 29:88 kg2m�4yr�1 (5)

where t is the year when the soil profile was collected, Wp is themass depth of the maximum 137Cs activity (Bq kg�1) and Np char-acterises the mass depth where the 137Cs concentration decreasesto 1/e of the maximum mass activity (Bq kg�1).

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120118

In both cases, the calculated values are in agreement with therange of value specified by Walling et al. (2002), precisely20e65kg2m�4 yr�1 and0.2e2 kgm�2 yr�1, apart from the velocity ofthebomb-derived 137Cs,whichwas found tobegreater for thepresentinvestigated soil. The DMMis less sensitive to the variation of v andD.Testing the model with the two distinctive D and v mean valuesdetermined above, a decrease of 32% was recorded when usingparameter values derived from Equations (4) and (5) (see Fig. 3).

3.2.5. Particle size correction factorsTextural analysis was carried out for representative samples of

the 12 sampling points from the study site and for the referencearea. The soil specific surface area was calculated considering thespherical approximation of the particles and amedium particle sizediameter for each textural class i.e.: 0.0025 mm for clay, 0.026 mmfor silt, and 1.025 mm for sand. The particle-size correction factorfor eroded areas (e.g. P) was obtained from the ratio of the surfacearea of mobilized soil to that of the original soil. The particle-sizecorrection factor at deposition sites (e.g. P0) was evaluated by us-ing the ratio of the surface area of deposited soil to that of themobilized soil, by a power of 0.75 (see Walling et al., 2002).

In fact, as reported by Bernard et al. (1998b) during the erosiveprocesses, the fine and 137Cs-rich particles are more easily trans-ported, while a larger proportion of coarse and 137Cs-poor particleswill settle when the carrying capacity of runoff is reduced orexceeded. This can be translated in P values (i) greater than 1 ineroded areas, considering that particles originating from an erodedsoil are finer than the source soil, and (ii) lower than 1 in depositionareas, where generally particles are expected to be coarser than themobilized soil (e.g. the soil of the reference site). In full agreementwith the statement presented above, our resulting P values rangedfrom 1.01 to 1.67 and the P0 values fluctuated from 0.33 to 0.76.

The particle size correction factor was added in the conversionmodel to include the selectivity of soil redistribution processes.Ignoring this parameter or using its default value (e.g. 1) whenapplying the DMM for the Jucu area, will have increased by 80% thesoil net erosion rate assessed with our experimental data (Fig. 3).Scott Van Pelt et al. (2007) also concluded that the soil loss ratesbased on the 137Cs radiometric models may be over-estimated if theparticle size selectivity is neglected. Moreover, following a statis-tical analysis on the variability of the model settings, Walling et al.(2002) reported that the DMM is very sensitive to all the parame-ters and particular care is required in their estimation.

Table 3Pearson's correlation coefficients between 137Cs inventories and measured physicochem

137Cs(Bq m�2)

Clay(%)

Silt(%)

Sand(%)

Gr.(%)

pH

137Cs (Bq m�2) 1.00Clay (%) �0.30 1.00Silt (%) 0.55 �0.81** 1.00Sand (%) �0.43 �0.24 �0.37 1.00Gr. (%) �0.03 �0.04 0.15 �0.25 1.00pH �0.60* 0.79** �0.93*** 0.30 �0.26 1.0ORP (mV) 0.59* �0.79** 0.93*** �0.29 0.24 �1.0EC (mS cm�1) �0.09 0.19 0.00 �0.31 0.20 0.1TDS (mg L�1) �0.09 0.19 0.00 �0.31 0.20 0.1OM (%) 0.63* �0.40 0.27 0.20 �0.12 �0.4Water (%) �0.24 0.40 �0.34 �0.08 0.12 0.3BD (g cm�3) 0.33 �0.70** 0.65* 0.05 �0.27 �0.6

137Cs (137Cs areal activity for the total soil profile 0e40 cm), Clay (d < 0.005 mm), Silt (0.0reduction potential), EC (electrical conductivity), TDS (total dissolved solids), OM (organ***Correlation significant at the 0.001 level.**Correlation significant at the 0.01 level.*Correlation significant at the 0.05 level.

3.3. Correlation between 137Cs and the physicochemical soilproperties

Soils of the investigated area are all fine textured (clay-loam),however a differentiation could be made between the two tran-sects. Whereas the second transect was mostly dominated by veryfine clay (d < 0.002 mm) content, the first had balanced percent-ages of silt, very fine and fine clay grain size. The variability in theOM content of the collected soil samples from Jucu area range fromlow (about 2%) to high (about 5%), with lower values recorded forthe second transect, where also the erosion processes appeared tobe marked. For the subsurface layers, the pH values ranged from 7.7to 10.2, showing high alkaline soil characteristic of the secondtransect and slightly alkaline for the first transect. This differencebetween the two transects was also revealed by the differentoxidation-reduction potential values, which were three timessmaller for the first transect compared with the second one(averaged �182.6 mV). The electrical conductivity and the totaldissolved solids exhibited high variation for the soils on the firsttransect, whereas the different values for the first transect werevery similar. The bulk density ranged between 0.88 g cm�3 and1.52 g cm�3, the lowest values being measured at the secondtransect.

Using the statistical software GraphPad Prism 6, the Pearson'scorrelation coefficients between 137Cs total inventories (0e40 cm)and the different soil parameters (i.e. the soil texture parameters(clay, silt, sand and gravel), pH, ORP, EC, TDS, water content, bulkdensity and OM) measured in the 12 soil samples, showed that the137Cs inventory was significantly positively correlated (0.05 level)with the OM content (r ¼ 0.63) and the ORP (r ¼ 0.59) and nega-tively correlated with the pH (r ¼ �0.60) (see Table 3).

Our significant correlation between the 137Cs inventories andthe OM content in top soils conforms to similar relationships, be-tween 137Cs and soil organic matter and/or soil organic carbon e

which have been already established in various soil types forexample in Australia (Martinez et al., 2010), Canada (Mabit et al.,2008b), China (Li et al., 2006; Zhang et al., 2006), France (Mabitand Bernard, 1998), Japan (Takenaka et al., 1998), Spain (Gasparand Navas, 2013; Navas et al., 2011) and in the USA (Ritchie et al.,2007). The relationship between 137Cs and OM content makesobvious that both are affected by similar physical processes of soilredistribution. The negative correlation between 137Cs inventoriesand pH conforms to a relative higher mobility of 137Cs in soils with

ical soil properties.

ORP(mV)

EC(mS cm�1)

TDS(mg L�1)

OM(%)

Water(%)

BD(g cm�3)

00*** 1.004 �0.15 1.004 �0.15 1.00*** 1.008 0.48 �0.51 �0.51 1.004 �0.35 �0.06 �0.06 �0.34 1.002* 0.63* 0.01 0.01 0.39 �0.58* 1.00

05 < d < 0.05 mm), Sand (0.05 < d < 2 mm), Gr. (gravel, d > 2 mm), ORP (oxidation-ic matter), BD (bulk density), Water (water content).

A.-R. Iurian et al. / Journal of Environmental Radioactivity 136 (2014) 112e120 119

pH 10, compared to the soils with pH 8, which are known toimmobilize 137Cs effectively (Giannakopoulou et al., 2007).

No significant correlation was found between 137Cs values andthe fine soil particles, as reported by the findings of several otherauthors (Erlinger et al., 2009; Gaspar et al., 2013; Mabit et al.,2008b). However, the correlation coefficient between the radio-nuclide inventory and silt was 0.55. These results concur withWischmeier's statements regarding the relationship between soiltexture and its erodibility (Wischmeier et al., 1971).

4. Conclusions

The convectionediffusion equation applied to the 137Cs depthprofile in our reference area fit well the experimental results. Anestimation of the bomb-derived 137Cs was made according to theprecipitation regime and the geographical location of our investi-gated area making it possible to determine the Chernobyl inputversus the nuclear weapon testing deposition. Thus, the Chernobylcontribution in the selected reference sitewas determined at 72% ofthe total 137Cs inventory. The effective diffusion coefficient and theconvective velocity of 137Cs for undisturbed soil were empiricallyderived for both Chernobyl and nuclear tests using the con-vectionediffusion equation. The resulting values are consistentwith data reported in the available literature. This alternativeapproach offered the possibility to: (i) improve the accuracy of theDMM parameter determination and therefore the precision of themodel application, and (ii) to get a better insight into the 137Csdepth characteristics in undisturbed soil. The assessment of net soilerosion rate on Romanian pastureland using the DMM for 137Cswas �6.6 t ha�1 yr�1.

Conversion models, which differ in their underlying assump-tions, are a key requirement in the use of 137Cs to evaluate soilredistribution rates. A sound understanding of the models and thederivation of their input parameters is an essential precursor totheir accurate use. As highlighted in this contribution, the DMM isvery sensitive to the different input parameters and particular careis required in their estimation, especially in areas affected byChernobyl fallout. Our data show predicted erosion rates arehighly sensitive to an increase of the Chernobyl contributionparameter and also to the variation in the reference inventoryvalue. All comparisons discussed within the sensitivity analysiswere performed according to the net erosion rate obtained usingthe experimental parameter values. Our study highlights the needfor developing an improved accuracy of the parameters incorpo-rated in the conversion models which are used to derive estimatesof soil redistribution rates from measurements of FRN inventories.Tolerance intervals should be established for the reliable estimatesof the derived redistribution rates and upper and lower boundsshould be considered and integrated in the conversion models. Ontheir review on the use of 137Cs in soil erosion research, Parsonsand Foster (2011) also mentioned that the variability of thereference-site values was not taken into account until recentlyand a greater statistical rigour in the use of measures of variabilityis required in order to provide reliable estimates of soilredistribution.

Moreover, other uncertainties which are not directly related tothe radiometric model andwhich were not discussed in the presentstudy (e.g. transfer to vegetation, preferential infiltration rate ofrainwater, sampling strategy, samples handling during soil samplespreparation) must be quantified and an uncertainty budgetmust berealised based upon all available data to finally assess the reliabilityof the estimated soil redistribution rates. As suggested by thefindings of our study, integrating away in the conversion models toassess the impact of the uncertainty around themodel's parameterswill be a big asset and would contribute to the transition of the use

of FRN tracers from research tool to a more widely accepted deci-sion support methods.

Acknowledgement

This study was supported by the POSDRU CUANTUMDOC“Doctoral studies for European performances in research andinnovation” ID79407 project funded by the European Social Fundand Romanian Government.

The authors wish to thank the local landowner who grantedaccess to the study site andwho provided as well information aboutthe field history. The help of Dr. Dan Constantin Nita and Dr. Robert-Csaba Begy for the field sampling is gratefully acknowledged. Theassistance of Dr. Ramona Balc and Ioana Catinas for the texturalanalysis are also much appreciated. The authors wish to thank alsothe Environmental Chemistry Laboratory technician for her help ingrinding the soil samples.

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