Application of Chernobyl-derived 137Cs for the assessment of soil redistribution within a cultivated field

Soil & Tillage Research 69 (2003) 85–98

Application of Chernobyl-derived 137Cs for the assessment of soil redistribution within a cultivated field Valentin Golosov∗ Laboratory for Soil Erosion and Fluvial Erosion, Faculty of Geography, Moscow State University, GSP-2, Leninskie Gory, 119992 Moscow, Russia

Abstract Vast areas of Europe were contaminated by the Chernobyl-derived 137 Cs in April–May 1986. This paper reports a detailed study of the post-fallout 137 Cs redistribution within a 1 ha field located in the Chasovenkov Verh catchment in the northern part of the Middle-Russian upland. Particular attention was paid to the study of reference inventories. It is shown that the random spatial variability of 137 Cs is similar within undisturbed and cultivated parts of a flat interfluve. Systematic spatial variability is not essential for a relatively short (200 m) topographical unit with simple relief. The analysis of a soil redistribution pattern within the study field using the Chernobyl 137 Cs technique demonstrates that it is possible to identify areas of soil loss/gain. This pattern does not reflect soil redistribution for the whole field, because these have been only 12 years since the Chernobyl accident. Net erosion rates based on 137 Cs method were comparable to soil losses directly measured at the study field. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Chernobyl; 137 Cs; Soil redistribution; Erosion; Method

1. Introduction Natural and artificial radionuclide tracers have been used to assess the intensity and pattern of soil and sediment redistribution at a range of locations and spatial scales (Lal et al., 1991; Foster and Lees, 2000). Bomb-derived 137 Cs has been used to document soil degradation in different environments of the northern and southern hemispheres (Ritchie and McHenry, 1990; Loughran, 1994; Walling and Quine, 1992). Vast areas of Europe were contaminated by radionuclides after the Chernobyl accident in 1986. As a result, additional inputs of Chernobyl-derived 137 Cs was superimposed on the bomb-derived 137 Cs pattern within eroded areas. It was initially possible to distinguish the bomb-derived and Chernobyl-derived components ∗ Tel./fax: +7-95-9395044. E-mail address: [email protected] (V. Golosov).

using the 134 Cs measurements (t0.5 = 2.2 years). The evaluation of soil erosion rates in areas affected by the Chernobyl contamination encounters some additional complications (Higgitt et al., 1992; Litvin et al., 1996), partially discussed herein after. The nature, behaviour and spatial distribution of the Chernobyl-derived 137 Cs have been studied in great details (Borzilov et al., 1993; Bakunov and Arkhipov, 1995; Alberts et al., 1998). About 99% of the Chernobyl fallout was distributed across Europe during a very short period from 26 April up to 15 May 1986 (Izrael, 1996). The rain-associated Chernobyl fallout was more than 10 times larger than the dry fallout (Izrael, 1996; De Cort et al., 1998). Field measurements show that the depth distribution of the Chernobyl-derived 137 Cs is similar to that of the bomb-derived 137 Cs for the main soil types, except acid soils that are dominated in swampy landscapes near Chernobyl and have lost a substantial amount of

0167-1987/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 2 ) 0 0 1 3 0 - 7

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V. Golosov / Soil & Tillage Research 69 (2003) 85–98

leached away in a soluble form (Bakunov and Arkhipov, 1995; Haak and Rydberg, 1998). Most of the 137 Cs is concentrated within the top 5 cm of the soil profile and there is an exponential decrease with increasing depth (Golosov et al., 1999b). The problem of “new” 137 Cs loss (Walling and Quine, 1993) may be solved for the Chernobyl-derived 137 Cs based on rainfall intensity data in conjunction with information about crop rotation and land management during the period from late April 1986 till the first tillage

operation. The assessment of the spatial distribution of the Chernobyl-derived 137 Cs was undertaken for the most of Europe (De Cort et al., 1998). Some detailed maps of the Chernobyl contamination are available for a number of countries that received a substantial portion of the fallout (Atlas, 1998). It is assumed that the bomb-derived 137 Cs has relatively uniform spatial distribution, because the long-period period over (Walling and Quine, 1993). The spatial variability of the Chernobyl-derived 137 Cs

Fig. 1. The location of the Lokna river basin within the Middle-Russian upland and in relation to the regional pattern of the Chernobyl fallout inventories.

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in most cases follows local variations in the structure of one or two rainfalls, which fell during late April–first half of May 1986. The variability of initial deposition of the Chernobyl fallout may vary widely depending on the type of rain (drizzle, moderate, heavy, etc.), wind speed and direction, and local topography. The Chernobyl-derived 137 Cs is a very good marker for the assessment of sedimentation rates at different locations (Higgitt et al., 1993; Walling et al., 1992; Golosov et al., 1999a). A detailed study of the Chernobyl-derived 137 Cs re-deposition along pathways from cultivated slopes to river channels helps to evaluate sediment delivery ratio coefficients for different size catchments (Fridman et al., 1997; Golosov et al., 1998; Panin et al., 2001). The usefulness of the Chernobyl-derived 137 Cs for the assessment of soil redistribution within a cultivated field is less clear. Areas with a high level of the Chernobyl contamination (>10 kBq m2 ) provide an excellent opportunity for a detailed study of uncertainties and limitations, involved in the assessment of soil redistribution based on the Chernobyl-derived 137 Cs, because the proportion of the bomb-derived 137 Cs is negligible. A detailed study of soil redistribution using 137 Cs technique was undertaken at an intensively cultivated field located 250 km south of Moscow in the Lokna river basin, north of Middle-Russian plain (Fig. 1).

2. Study area The Middle-Russian upland is located in the central part of the Russian plain and represents an important topographical barrier for predominant westerly winds. This is the major reason for the serious contamination of the northwestern part of the Middle-Russian upland after the Chernobyl accident (Fig. 1). The highest inventories of 137 Cs fallout have been identified just before the major water division line between the Volga river basin and the Don river basin within the upstream Oka river basin. The study area is located in the middle of the basin of the Chasovenkov Verh, that is a dry tributary of the Lokna river (Fig. 2). The level of the initial Chernobyl contamination exceeds 300 kBq m−2 and the highest contamination is identified along the major valley of the Lokna river.

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The Lokna river basin is up to 240–250 m a.s.l. and its relative relief is of about 60–90 m. The topography is dominated by relatively flat interfluve areas and mostly convex slopes with various gradients dissected by a balka (dry creek) valleys. The area is underlain by Carboniferous limestone and dolomite mantled by Holocene loess. Typical and leached chernozems (Haplic, CHh, according to the FAO classification) with a loamy texture, cover about 80% of the area. The permeability of the soils is very low. A soil crust appears very quickly after spring rains. Main crops are winter and spring cereals, maize, potato, buckwheat and perennial grasses. According to a local meteorological station located about 1 km east of the study area, the mean annual precipitation for the 1986–1997 period was 650 mm, and snow comprised about a half of it. The most intensive rain occur during the period from June to August. The mean water equivalent of snow before the spring melting is 111 mm, for the 1986–1997 period. Erosion events happen almost each year during the spring snow-melting and as a result of heavy rain-storms during the May–September period. Soil erosion during the snow-melting is mostly observed at “warmer” (southwest) slopes due to more irregular melting of snow on at their most steepest convex parts compared to relatively flat interfluve areas. According to 11-year field measurements of water and sediment discharges organised at the Kashira field station (Braude, 1976), the mean annual erosion rates during the snow-melting are 0.54 kg m−2 at “warm” slope catchments. This station is located in the northern part of the Middle-Russian upland that has similar relief and grey forest loamy soils (Haplic, GRh according to the FAO classification). Based on the long-term field measurements at runoff plots (length 100–150 m, area 3000–5000 m2 ), the mean annual erosion rates vary from 0.04 kg m−2 for the grey forest soil (Barabanov, 1993) to 0.12 kg m−2 for loamy chernozem (Chernyshev, 1976). The effect of irregular melting of snow at convex slopes and the concentration of water within slope hollows (in the case of slope catchments) are the main reason of for different erosion rates detected at slope catchments and runoff plots. Long-term observations of soil erosion during heavy rains have not been organised at the Middle-Russian upland. However, some erosion consequences of the

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Fig. 2. Map of the Chernobyl 137 Cs contamination of the Plava river basin showing the study area within the Lokna river basin and land management of the middle section of the Chasovenkov Verh basin.

heavy rain, of the 10 June 1997 were directly measured at different cultivated fields within the Chasovenkov Verh basin. Volumes of rills and rill fans were independently measured. Average losses from different fields during the rain vary between 2.8 and 3.6 kg m−2 . Maximum soil losses from the most eroded parts of slopes were less than 20 kg m−2 . Wind erosion has not been identified in this area. However, some movement of dust occurs near unsealed roads and may further

complicate the distribution of the Chernobyl-derived 137 Cs. A relatively short slope located in the middle of the Chasovenkov Verh balka was chosen for the detailed study of soil redistribution (Fig. 2). This eastern slope has a convex profile, its length is 200–250 m and the gradient is 6–15%. An unsealed road forms a local water divide at this slope (Fig. 3). However, some water running along the

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road may overflow during extreme runoff events, and cause a sediment re-deposition at the upper part of the slope. Loamy soils contain 2.4–3.8% of humus. They are classified as moderately to strongly eroded soils and, according to the Russian soil classification (Shishov et al., 1985), this means that they have lost 1/3 to 1/2 of horizon A compared to undisturbed soils.

3. Methods Three parallel transects were chosen for sampling and in situ measurements of 137 Cs, because the field has a quite simple topography (Fig. 3). Measurement points were about 20–25 m apart and the transects spacing was about 20 m. Particular attention was given to the identification of reference locations where 137 Cs inventories should have to be representative for the total fallout input. Four reference sites were chosen at different locations around the study field (Fig. 2). The first reference site was on a flat interfluve area upslope from the study area. The second and third reference sites were located on gentle balka sides nearby and upstream from the study field. The last reference site was on a terrace downstream from the study field. Bulk core samples were collected using a 36.2 cm2

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core tube inserted to the depth of 30 cm on the slopes and interfluve. Three cores were taken at each point to decrease the effect of spatial variability. In addition, incremental samples from layers 0–30 and 30–40 cm were taken on at some points within the interfluve area. It was done to evaluate possible vertical migration of the Chernobyl-derived 137 Cs. Simultaneously with the collection of the bulk samples, in situ measurements of 137 Cs activity were made adjacent to the majority of the sampling points and at some additional sites using a Corad portable collimated spectrum sensitive NaI detector (Govorun et al., 1994; Chesnokov et al., 1997). The viability of using a portable detector in areas with high levels of radionuclide contamination is discussed in details elsewhere (Golosov et al., 2000). A detailed topographic survey of the study area as well as the sampling and measurement points was made using a differential GPS system that provided height and position records with a maximum error of ±2 cm (Panin et al., 2001) (Fig. 3). All soil samples were dried and sieved to <2 mm prior to laboratory measurement of their 137 Cs content by gamma spectrometry using an HPGe coaxial detector calibrated with Standard Reference Materials and laboratory standards made using standard solutions. Count times were sufficient to provide

Fig. 3. The study field topography and sampling points locations.

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Fig. 4. Relationship between actual soil erosion rates and simulated erosion rates, identified using a modified version of the USLE (for rain erosion) and the SHI (for erosion during snow-melting) empirical models.

a typical analytical precision of ±4–5%. All activities were corrected for radioactive decay to 1 June 1998. Information about land management, crop rotation and precipitation for the May 1986–May 1998 period was collected from a local collective farm and the meteorological station. Rain-storm related erosion rates at the study field were calculated using the EPIC model (Williams et al., 1984) and a modified version of the USLE developed by Laroniov (1993). The EPIC-based calculations were done using the latest version of the MUSLE. Mean erosion rates calculated using the modified version of the USLE for rain events and the State Hydrological Institute’s (SHI) model (Laroniov, 1993) for snow-melting period were compared with actual erosion rates for some similar slopes at different locations at the Russian plain (Fig. 4). The actual erosion rates were determined using data on sediment storage within field ponds with known time of construction. According to information provided by a local agricultural engineer, the retention efficiency of these particular ponds is about 100%. Possible errors in calculations of soil losses may result from some year to year changes of pond catchment

areas due to different tillage directions. Erosion rates computed by empirical models overestimate actual erosion rate by 10–20% (Fig. 4). However, good correlation between the two data sets confirms the validity of these models for the evaluation of mean erosion rates for landscape conditions of the Russian plain. Data on actual water content in snow before snow-melting were not available for the study field. So it was not possible to use the SHI model to calculate erosion rate for the snow-melting period. It was assumed that soil losses during the snow-melting period do not exceed 0.1 kg m−2 per year, because of the “cold” orientation and small length of the study slope. This assumption was based on the results of long-term field observations during snow-melting within the northern and central regions of the Middle-Russian plain (Chernyshev, 1976; Barabanov, 1993). An extreme rain-storm (35 mm per 1.5 h, which followed 20 mm rain-storm per 2 h) on 10 June 1997 caused an intensive runoff and erosion in the study field. Immediately after the rain, the pattern and volumes of rills and rill fans were measured. According

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Fig. 5. Changes of crop coefficient and number of heavy rains for the study field during 1986–1997.

to the calculations of total soil losses, the mean erosion rate over the entire field was 3.6 kg m−2 (0.3 kg m−2 per year for the period 1986–1997) with a maximum loss of 20 kg m−2 identified within a strip of 50–100 m up from the cultivated slope bottom. The distribution of precipitation during strong rains for the May 1986–June 1997 period was compared with the dimensionless factor for cover and management (C-factor) for the study field (Fig. 5). It was determined that situations similar to that of 1997 (a high number of strong rainfalls and lower crop protection of soil surface) were observed at this field in 1989 and 1995 (Fig. 5). Also it is known from the meteorological data that >40 mm rains occurred in summer in 1989 and 1995. The actual erosion effects of these rains are not known. It was assumed that the soil losses were similar to those observed erosion after the rain of 10 June 1997, i.e. between about 0.5 kg m−2 (minimum) and 3.6 kg m−2 (maximum). This provides a rough indication of mean annual soil losses including erosion during snow-melting for June 1986–1997 period in the range of 0.4–0.9 kg m−2 .

4. Results 4.1. A comparison of in situ and laboratory measurements It was already shown (Golosov et al., 2000) that a good correlation between in situ and laboratory 137 Cs measurements exists for areas with a high level of the Chernobyl contamination. In this case, two data sets can be compared and one of them is associated to a flat ploughed interfluve. The very good correlation between these two data sets (Fig. 6A) reflects ploughing and cultivation mixing that reduced the microscale variability of 137 Cs inventories. However, mean values of the laboratory measurements exceed the in situ 137 Cs inventory. Probably, this is due to some differences in accuracy of applied methods. The second set of measurements is associated with the study slope. It also shows a quite good correlation (Fig. 6B). However, the correlation is lower due to the soil redistribution within the field and probably tillage at the bottom of the cultivated slope. This is an additional confirmation that a portable detector may be used for in situ

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Fig. 6. Correlation between in situ and laboratory measurements of the

137 Cs

inventory for: (A) flat interfluve; (B) study field.

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measurements of 137 Cs in areas with a high level of the Chernobyl contamination. 4.2. Reference inventory An appropriate reference inventory is the key issue in the application of the 137 Cs technique. A detailed analysis of random and systematic variability of initial 137 Cs inventories has been provided elsewhere (Golosov et al., 1999; Walling et al., 2000a). The Chernobyl contamination was associated with one or two precipitation events. The rainfall patterns mostly depend on topography. Therefore, the distribution of the Chernobyl fallout also depends on a relief (Walling and Quine, 1993; De Cort et al., 1998). A detailed study of the spatial variability of the Chernobyl fallout within the Lokna river basin shows a strong south–north trend in the southern part of the basin (Golosov et al., 1999a,b) (Fig. 2). The variability of the four reference inventories shown in Fig. 2 reflects a systematic increase in the fallout from south to north across the bottom of the main balka of the Chasovenkov Verh basin. There is no any substantial east–west trend in the 137 Cs inventory. A detailed study of the random variability was carried out at the flat interfluve. The 27 in situ 137 Cs measurements were made along five parallel transects lined up about 40 m apart within the cultivated area and at one site within an adjacent forest shelter belt. There is no substantial difference between mean 137 Cs inventories for the points at the cultivated area and at an open spot within the forest shelter belt. Therefore, the flat cultivated interfluve may be used as a reference site. Coefficients of variation do not vary substantially for the different sets (Table 1). The total length of the sampling area (about 1 ha) within the flat interfluve is 200 m. Consequently, it is possible to assume that the initial random and systematic variability was relatively uniform across the cultivated slope, because it has similar length and orientation (Fig. 3). 4.3. Estimation of soil redistribution from 137 Cs data Walling and He (1999) summarised the existing approaches transforming the 137 Cs distribution patterns into soil distribution patterns. They also suggested some improved models that are able to

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take into consideration some relationships between a fresh bomb-derived 137 Cs fallout, soil and sediment grain size and tillage. Proportional models (Mitchell et al., 1980; Walling and Quine, 1990) and standard mass-balance models (Kachanoski and de Jong, 1984; Quine, 1989; Ostrova et al., 1990; Walling and Quine, 1990) are applicable to the Middle-Russian upland area contaminated by the Chernobyl-derived 137 Cs because the deposition of the Chernobyl radionuclides took place over a very short period (27 April–15 May 1986) and all bare slopes in central Russia were cultivated shortly after the fallout. Some uncertainties associated with the behaviour of freshly deposited 137 Cs prior to its incorporation into a plough layer by cultivation have been substantially clarified. This is also true for fields under winter corn and perennial grass because erosion rates are relatively low under these crops during summer (Edwards and Owens, 1991). There is a similar grain size distribution for suspended sediments, from a flow near the lower end of the cultivated field and for surface soil samples of from the cultivated field in the Chasovenkov Verh basin (Walling et al., 2000b). The effect of tillage is not significant for the study field, it has a very simple morphology and only twice per year there is a diagonal tillage parallel to the dirt road (Fig. 2). Besides there was no enough time since 1986. However, some increase in 137 Cs may occur due to tillage operations at the bottom of the slope. This should be taken into account in the future. Proportional and standard mass-balance models were applied for the calculation of erosion and deposition rates at the study slope using software developed by Walling and He. The proportional model is as follows: Y = 10 ×

BdX 100TP

(1)

where Y is the mean annual soil loss (t ha−1 per year), d the depth of plough or cultivation layer (m), B the bulk density of soil (kg m−3 ), T the time elapsed since initiation of 137 Cs accumulation (year), P the particle size correction factor, and X the percentage reduction in total 137 Cs inventory. Reduction in total 137 Cs inventory defined as X=

Aref − A × 100 Aref

(2)

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Table 1 Summary statistics for

137 Cs

inventory from reference area at flat interfluve (in situ measurements)

Set of measurements

Number of measurements

Mean (kBq m−2 )

Standard deviation (kBq m−2 )

Coefficient of variation (%)

Range (kBq m−2 )

Entire set of measurements Cultivated part Forest shelter belt Southern half of flat interfluve Northern half of flat interfluve

27 20 7 14 13

347 344 358 354 340

38 37 41 34 42

11 11 11 10 12

297–440 297–440 313–421 312–440 297–421

where Aref is the 137 Cs reference inventory (Bq m−2 ), and A the measured total 137 Cs inventory at the sampling point (Bq m−2 ). The standard mass-balance model (MBM-1) was also used to calculate erosion–deposition rates within the sites. For calculation of erosion rates the following equation was applied:
   10dB X 1/(t−1986) Y = 1− 1− (3) P 100 where Y is the mean annual soil loss (t ha−1 per year), d the depth of plough or cultivation layer (m), B the bulk density of soil (kg m−3 ), X the percentage reduction in total 137 Cs inventory (defined as [(Aref − A)/Aref ] × 100), t the time (year), and P the particle size correction factor. Deposition rate was estimated from the 137 Cs concentration in deposited sediments according to R =  t

Aex (t)

1986 Cd (t

= t

 ) e−λ(t−t  ) dt 

A(t) − Aref 

 −λ(t−t ) dt  1986 Cd (t ) e

(4)

where Aex (t) is the excess 137 Cs inventory of the sampling point over reference inventory at year t (Bq m−2 ), Cd (t ) the 137 Cs concentration of deposited sediment at year t (Bq kg−1 ), and λ the decay constant for 137 Cs (per year). Gross erosion rate (Yg ) was defined according to Yg =

Wt TSt

(5)

where Wt is the total volume of eroded sediment for T (kg), T the time elapsed from May 1986 till June 1998 (year), and St the total area of study field (m2 ).

Table 2 Integrated data of soil loss/gain for the study field based on different calibration modelsa Measure

Gross erosion rate (kg m−2 per year) Eroding sites Mean erosion rate (kg m−2 per year) Percentage of total area Aggrading sites Mean deposition rate (kg m−2 per year) Percentage of total area Net erosion rate (kg m−2 per year) Sediment delivery ratio (%)

Proportional model

Standard mass-balance model

1.1/1.0

1.31/1.1

1.80/1.77

2.16/2.04

62/56 1.33/1.76 38/44 0.61/0.24 55/24

61/55 1.61/1.98 39/45 0.68/0.25 52/22

a

The values before slash “/” indicate results based on laboratory analysis of 137 Cs and after slash indicate the results based on the in situ measurements of 137 Cs.

Net erosion rate (Yn ) was defined according to Yn =

Wt − Wd TSt

(6)

where Wd is the total volume of sediment deposited within study field (kg). The results for both calibration models are presented in Table 2 and Fig. 7.

5. Discussion It is likely that the current discrimination of eroded and deposition areas in terms of reduced 137 Cs inventories is insufficient, an objective soil redistribution

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Fig. 7. Soil redistribution for the May 1986–June 1998 period within the study field determined using the measurements): (A) proportional model; (B) standard mass-balance model.

pattern even within slopes with relatively high erosion rates because of the short period of time since the Chernobyl-derived 137 Cs deposition. For an essential part of the study field soil losses/gains for the study

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137 Cs

technique (in situ

period are in the range of ±2.0 kg m−2 and this is less than possible measurement errors as well as the initial variability of the Chernobyl 137 Cs fallout. The most eroded strip is located 50–100 m up from the slope

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(Fig. 7). Two areas with high deposition rates are at the bottom and in the middle of the slope. The deposition at the bottom is likely to be associated with the cumulative effect of sediment re-deposition in front of the field edge as well as with the tillage erosion effect. The deposition area in the middle of the slope may be explained by possible re-deposition of sediments originated from the overflow at the top of the field during extreme runoff events. We observed such an overflow at the Chasovenkov Verh catchment during the extreme rain on 10 June 1997. A comparison between the erosion and deposition rates identified in the field (in situ measurements) and the laboratory-based measurements shows very similar results except for the net erosion rates (Table 2). This confirms the possibility to use in the future field 137 Cs measurements for the assessment of soil redistribution in areas with a high level of the Chernobyl contamination. Some differences between the net erosion rates are due to a different number of the in situ measurements (36) and sampling points for laboratory analysis (31). Table 3 shows mean soil loss rates based on different approaches. The results indicate that the Chernobyl-derived 137 Cs may already be used for the calculation of mean erosion rates for cultivated fields with intensive soil redistribution. In the near future it will be possible to apply the Chernobyl-derived 137 Cs to study of soil redistribution at any cultivated slope in areas with high levels of the Chernobyl contamination. The results of the modified USLE version of Table 3 Mean annual soil losses identified by different methods for the 1986–1997 period Method of assessment

Mean annual rates (kg m−2 per year)

Measurements of rill volume combine and analysis of rain distribution Modified version of USLE EPIC

0.4–0.9

137 Cs

technique (in situ measurement of Proportional model Net erosion Gross erosion

Standard mass-balance model Net erosion Gross erosion

1.15 1.25 137 Cs

inventory)

0.61 1.10 0.68 1.31

EPIC models are in agreement with the gross erosion rates estimated from the 137 Cs measurements and very close to the upper limit of actual soil losses (Table 3). Three independent methods (model calculations, Chernobyl 137 Cs and the combination of rill measurements) and the analysis of the variability of main erosion factors during 1986–1997 have identified the mean annual erosion rates in the range of 0.4–1.1 kg m−2 for the study field. These results correspond to erosion rates available for the central and northern regions of the Russian plain. The evaluation of sediment storage in field ponds for slopes with similar topography provides erosion rates from 0.2 to 0.57 kg m−2 (Golosov, 1998). Soil morphological methods show erosion rates in the range of 0.2–0.65 kg m−2 (Rozhkov, 1977). If soil losses have been in the range of 0.4–1.1 kg m−2 , cultivated chernozem soils should have lost about 8–25 cm since the beginning of cultivation 200–300 years ago. Therefore the local soils are mostly low eroded soils according to the Russian classification (Shishov et al., 1985). However, soils from the most eroded part of the study slope (erosion rates >2.0 kg m−2 ) are in fact moderately eroded soils.

6. Conclusion The results of this and previous investigations in areas with high levels of the Chernobyl contamination demonstrate that the Chernobyl-derived 137 Cs can be used for the evaluation of soil and sediment redistribution within cultivated slopes. The identification of initial fallout patterns for study areas is the key issue that should be addressed before designing sampling programs for areas affected by erosion and deposition processes. If there is a very high spatial variability of the initial fallout without any obvious trends, the 137 Cs technique is not suitable for the assessment of soil losses at discrete points. Nevertheless, it may be applied for monitoring 137 Cs dynamics using DGPS-supported repetitive sampling and field measurements by a portable detector. This would provide an opportunity to evaluate the dynamics of soil degradation at various morphological units. Similar approach should be applied in areas where levels of the Chernobyl contamination are comparable with those of the bomb-derived 137 Cs. However, in such

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environments each repeated point measurement of 137 Cs inventory may be used for the evaluation of soil redistribution based on calibrating models.

Acknowledgements The author gratefully acknowledges A. Panin, N. Ivanova, M. Markelov and V. Belyaev for assistance in the field; E. Stukin and E. Kvasnikova, Institute of Global Climate and Ecology for radionuclide analysis. This research was funded by INTAS-RFBR (grant no. 95-0734) and by the IAEA Co-ordinated Research Project on the Assessment of Soil Erosion through the use of 137 Cs and related techniques (IAEA contract no. 9044). The critical and constructive comments on the original manuscript made by two anonymous reviewers are also acknowledged. References Alberts, B.P., Rackwitz, R., Schimmack, W., Bunzl, K., 1998. Transect survey of radiocaesium in soils and plants of two alpine pastures. Sci. Total Environ. 216, 159–172. Atlas, 1998. In: Izrael, Yu.A., Soudakova, E.A. (Eds.), Atlas of radioactive contamination of European, Russia, Belarus and Ukraine. Federal Service of Geodesy and Cartography, Moscow. Bakunov, N.A., Arkhipov, N.R., 1995. Behaviour of 90 Sr and 137 Cs of weapons and reactor origin in the soil–plant system. Eurasian Soil Sci. 28, 40–52. Barabanov, A.T., 1993. Agro- and Forest Melioration in Soil Conservation Farming. VNIALMI Press, Volgograd (in Russian). Borzilov, V.A., Konoplev, A.V., Bulgakov, A.A., 1993. Application of the Chernobyl experience in developing methodology for assessing and predicting the consequences of radioactive contamination of the hydrosphere. In: Proceedings of the International Workshop on Hydrological Considerations in Relation to Nuclear Power Plants, UNESCO, Paris, pp. 246–263. Braude, I.D., 1976. Rational Use of Eroded Grey Forest Soils in the Non-Chernozem Zone of Russia. Lesnaya Promyshlennost Press, Moscow (in Russian). Chernyshev, E.P., 1976. Tendency of erosion changes on southern part of Russian plain. In: Timofeev, D.A. (Ed.), Voprocy Antropogennyh Izmenenii Vodnyh Resursov. AN SSSR Press, Moscow, pp. 47–63 (in Russian). Chesnokov, A.V., Fedin, V.I., Govorun, A.P., Ivanov, O.P., Liksonov, V.I., Potapov, V.N., Smirnov, S.V., Scherbak, S.B., Urutskoev, V.I., 1997. Collimated detector technique for measuring a 137 Cs deposit in soil under a clean protected layer. Appl. Radiat. Isot. 48, 1265–1272.

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