HYDROLOGICAL PROCESSES, VOL. 5,215-227 ( 1 9 9 1 )

THE USE OF I3’Cs AS A TRACER IN AN EROSION STUDY IN SOUTH LIMBURG (THE NETHERLANDS) AND THE

INFLUENCE OF CHERNOBYL FALLOUT

A. P. J. DE ROO Institute of Geographical Research, University of Utrecht, Heidelberglaan 2, 3584 CS Utrecht, The Netherlands

ABSTRACT The 137Cs radioactivity of soils was used as a tracer of soil erosion in a catchment in the Netherlands: 143 samples were analysed to map the I3’Cs redistribution using geostatistical interpolation methods. Caesium-137 activities on grassland are significantly higher than on arable land. Also, 13’Cs activities on waning slopes are higher and activities on steep slopes are lower. The soil erosion estimates, derived from the 137Cs data, are used to validate the USLE erosion model. The recent Chernobyl nuclear accident also contributed to the lJ7Cs activity. However, the Chernobyl input of I3’Cs, with a constant ratio of 1.765:l to IJ4Cs, cannot be used as a tracer of soil erosion. Because of the rapid decay of 134Cs, we will not be possible to separate the sources of IJ7Cs in the near future in areas significantly influenced by Chernobyl fallout and in these areas I3’Cs can no longer be used as a soil erosion tracer.

K E Y WORDS I3’Cs Soil erosion Chernobyl Geostatistics Digital Elevation Models Soil erosion models

SOIL EROSION IN SOUTH LIMBURG

Soil erosion and flooding have increased in the hill country of South Limburg (The Netherlands) during the last decades. Schouten et al. (1985) stated that the expansion of urban areas and the increased number of sealed roads, as well as the intensification of agriculture are the major causes. Due to recent reallotment, fields have become larger, which causes surface runoff to be more erosive. Changes in land use also contribute to increasing erosion. The area of grassland has decreased in favour of urban areas and, after 1975, in favour of arable land (Van der Helm et al., 1989). Moreover there has been a change in land use and in the kinds of crops grown in South Limburg. Between 1960 and 1986, the growing of crops causing a higher erosion risk, such as maize and sugarbeet, has increased in South Limburg, replacing cereals such as winter wheat.

MAPPING SOIL EROSION

There are several methods for mapping soil erosion. One method consists of monitoring height changes in the landscape due to soil erosion, through the use of erosion pins (Haigh, 1977). However, several problems with this method have been noted, such as soil disturbance on insertion, and the influence of flow in their vicinity, causing enhanced turbulence in runoff (Rogowski et al., 1985). Because erosion rates are generally of the order of a few millimetres per year, a long period of measurement is needed to obtain a reliable estimate. This difficulty is confirmed by field experience in the Netherlands. Furthermore, farmers may move or disturb the pins on arable land.

Another group of methods consists of the use of sediment traps to measure erosion. A box dug into the slope with its top flush with the ground surface, catches surface runoff and sediment from the slope above it. A major problem with this method is defining the contributing area to the troughs (Statham, 1981). Therefore, the plots are often bounded. Consequently, the influence of slope length is not well accounted for. These sediment traps are used to measure both short-term and long-term erosion rates.

0885 6087/9l/020215- 13SO6.50 CJ 1991 by John Wiley & Sons, Ltd.

Received 12 May 1990 Revised 18 July 1990

216 A. P. J . DE ROO

Both groups of methods require intensive labour, and extra difficulties arise on arable land when farmers need to cultivate their fields. Mostly, only a small part of a catchment, or a slope segment, can be monitored.

The use of tracers in soil erosion research may provide an alternative approach on a broad catchment-wide scale for identifying the source of the sediment. These tracers are potentially useful to estimate soil erosion rates because the sampling of the drainage basin can be undertaken within a few days, without inconvenience to farmers. Caesium-137, a semi-‘natural’ radionuclide, remains in the soil for a substantial time and is easily detectable. The 37Cs-labelled sediment behaves within the natural environment as natural materials would do. The use of such tracers can provide erosion estimates for much larger areas than erosion plots. Moreover, 137Cs used as a tracer can provide an estimate of soil erosion since 1958. The Chernobyl nuclear accident in 1986, which produced 134Cs and 137Cs, can provide an opportunity to map short term soil erosion.

The aim of this study is to investigate the value of the 137Cs method for mapping soil erosion and to investigate the possible use of the derived erosion map to validate erosion models.

THE 137CS METHOD

The level of 137Cs in the soil can be used to map soil erosion patterns in a landscape (e.g. Ritchie et a/., 1974; Loughran et a/., 1987). Caesium-137 (half-life 30.1 7 years) is a product of atmospheric nuclear weapon tests conducted mainly between 1958 and 1965. I t is assumed that within a climatologically uniform area, a spatially uniform amount of fallout will have been received. This assumption has been verified in several areas of the northern hemisphere (Campbell et al., 1982; Walling and Bradley, 1988). Because of its relatively long half-life, part of this 137Cs is still detectable in the soil.

Once in contact with the soil, 137Cs becomes strongly adsorbed, especially by temperate region soils containing illite. This adsorption is very rapid, so that the caesium is concentrated within the top few centimetres of the soil. Further movement by infiltration and root penetration is limited (Rogowski and Tamura, 1970). I t can be assumed that all the 137Cs fallout will still be present in the upper layer of a non-eroded soil, apart from that lost through natural decay.

Ploughing and biological activity cause redistribution within the profile producing a uniform mixing in the cultivation layer and some movement downwards due to soil fauna.

Removal rates of 137Cs from soil by most crop types are limited and average approximately 0.2 per cent yr- I of the total 137Cs added to the soil (Coughtrey and Thorne, 1983). Vanden Berghe and Gulink (1987) proposed a total removal over 30 years of 150 Bq m-’ through crops. Martz and De Jong (1987) assumed a 5 per cent loss of IJ7Cs due to drifting snow and harvested grain.

As a result of soil erosion, soil particles and the attached caesium are redistributed in the landscape. Several studies have shown that 137Cs movement can be associated with soil transport (e.g. Ritchie et a/., 1974; McCallan et al., 1980; Brown et al., 1981a, 1981b; Campbell et al., 1982; Loughran et a/., 1987; Martz and De Jong, 1987). In non-eroded sites a baseline level of 13’Cs will be present. In the Netherlands this represents an average 137Cs activity of 1950 Bq m-’ in the upper 25 cm (Koster et al., 1987). Activities of 137Cs in the soil, greater than in the (non-eroded) reference sites, indicate accumulation, and lesser activities indicate soil erosion.

The recent Chernobyl nuclear accident on April 26, 1986 also contributed to the 137Cs activity. However, the fallout associated with the Chernobyl accident is spatially variable, in contrast with the 137Cs activity of the nuclear tests, which provided spatially uniform inputs over local areas in the northern hemisphere (Vanden Berghe and Gulink, 1987; Walling and Bradley, 1988). Nuclear test fallout can also vary substantially across a much larger area in response to variation in annual rainfall. There is a general trend of decreasing Chernobyl fallout with increasing distance from the reactor site. Furthermore, local patterns reflect air mass trajectories and the incidence of precipitation at the time that the Chernobyl cloud passed overhead (Walling el al., 1989). The lJ7Cs from Chernobyl can be separated from the 137Cs from nuclear tests by simultaneously measuring the 34Cs activity (half-life 2.06 years), which originates exclusively from the Chernobyl accident. In Western Europe, the Chernobyl 137Cs deposition was on the average doublc the 134Cs deposition (Frissel et al., 1987). In the Netherlands the average deposition of l j7Cs and 134Cs in May 1986 was 1800 Bq m-’ and 920 Bq m-’(Koster et al., 1987), a ratio of 1:1.96. This ratio was also reported by

USE OF 37Cs AS A TRACER 217

Table 1. I 37Cs/’34Cs ratios in Chernobyl fallout, reported in the literature

Source

Koster et al. (1987)

Beentjes et a/. (1987) Cambray er a/. (1987) Bonnett et al. (1989) Persson er a/. (1987)

Denschlag et a/ . (1987) Hotzl er 01. (1987) Hoffmann et a/. (1987)

Description

Average for The Netherlands Deposition Beek (Limburg, NL) Rainwater Nijmegen (NL) Average for U.K. Rainwater Severn (Wales) Deposition Sweden Deposition U.S.S.R. Total emission (U.S.S.R. report) ORIGEN (U.S.A.) inventory Derived inventory Rainwater Mainz (FRG) Average Munich/Neuherberg FRG Chattia (FRG)

Ratio

1.956 1.889 1.944 1.667 1.714 1.754 1.47 1 1.947 0.794 1.474 1.826 1.775 2.0

I3711 34cs

1800/ 920 Bq m-’ 1700/ 900 Bq m-’ 2430/1230 Bq m-’

7201 420 Bq m - ’ 110.68

-

1/0.57

0.037/0.019 EBq 0.2710.34 EBq 0,2810.19 EBq 84/46 Bq I

19.7/11.1 kBq m-’ ~

Beentjes and Duysings (1987) for rainfall on May 3 near Nijmegen, the Netherlands. However, a different ratio (1:1.765) was derived from the field measurements reported below. Also, different ratios have been reported in Germany, Sweden and in the U.K. (Table I). These differences are important for the determination of the Chernobyl component of the total 137Cs inventory of soil samples, and therefore affect the results.

AIM OF THE STUDY

This study investigates the extent to which the 137Cs content of soils can be used as a tracer to estimate soil erosion rates over the past 30 years in the Etzenrade drainage basin in South Limburg (The Netherlands) (Figure 1). In this catchment surface runoff and soil erosion have also been quantified using field

Roetmond

Sitlard

Geleen

f Heerlen

Kerkrade OValkenburg

Maastricht

0 lOkm I - Figure I . Location of the Etzenrade catchment

218 A. P. J. DE ROO

measurements and simulation models. Vanden Berghe and Gulink (1987) have proposed that the extra fallout input of 137Cs from the Chernobyl nuclear accident in 1986 provides new possibilities to map short-term soil erosion rates. This paper evaluates these new possibilities. The 137Cs data are also used to evaluate the use of the Universal Soil Loss Equation (USLE) (Wischmeier and Smith, 1978) at a catchment scale. In addition, other methods to predict '"Cs patterns, such as geomorphological methods and the use of Digital Elevation Models, are evaluated.

The following hypotheses were tested: I . 137Cs originating from atmospheric nuclear weapon tests can be used as a tracer of soil erosion at a

catchment scale. The spatial redistribution of lJ7Cs at a catchment scale can be mapped accurately using geostatistical interpolation methods and a reasonable number of soil samples.

2. Extra ' 37Cs and ' 34Cs originating from the Chernobyl accident in 1986 provides information for mapping very short term and detailed erosion patterns.

3. A significant relation exists between geomorphology (location on the slope, plan convexity and profile convexity) and ' 37Cs activity, but this geomorphological analysis alone does not yield enough informa- tion about soil erosion patterns.

4. The USLE estimates of soil erosion are correlated with the soil loss estimates derived from the 137Cs activity.

5. The soil erosion rates, calculated with the lJ7Cs-data, are comparable with other estimates of rates of soil loss for the Belgian loamy region.

FIELD AND LABORATORY METHODS

The Etzenrade catchment (225 ha, between Jabeek and Brunssum) in South Limburg, the Netherlands (Figure I ) , has a gently to moderately sloping topography. About 90 per cent of the slopes have a gradient of less than 10 per cent, and only 1 per cent of the slopes are steeper than 15 per cent. In most of the catchment, the parent material is loess. Three types of soil can be distinguished: luvisols (in Dutch: 'radebrik' soils), eroded luvisols (in Dutch: 'bergbrik' soils) and colluvial soils. The topsoil of the eroded luvisols have a higher clay content (15 per cent) than the luvisols and the colluvial soils (10 per cent). The silt content in the topsoil is very high (79 per cent) (De Roo and Riezebos, 1990).

The spatial variation of 137Cs over the catchment was investigated. To map this spatial variation in the Etzenrade catchment, the soil was sampled at 143 sites with an Edelmannprobe (Figure 2). At each site the whole plough layer (0-30 cm) and the layer below (30-50 cm) were sampled. At several points, samples up to 90cm were taken to define the maximum depth at which caesium occurs. Bulk density was measured simultaneously.

For geostatistical analysis of the data, the first 62 sites were sampled in June 1988 along four transects at varying distances. The profiles were aligned in two directions, perpendicular to each other. At 12 non-eroded sites (old orchards on flat land), reference samples were taken. From the first 62 sites a semivariogram was calculated (Journel and Huijbregts, 1978; Burrough, 1986). A spherical model was fitted through the data with a 'Weighted Least Squares Fit' program, developed at Utrecht University. This fit produces a 'nugget variance', a 'sill' and a 'range'. The 'nugget variance' is the semivariance at zero lag (distance close to zero) and can arise from measurement error, discrete random variation and spatially dependent variation occurring over distances much less than the sampling interval (Oliver and Webster, 1986). In most instances i t is found that the semivariance increases from the nugget variance to a maximum level, known as the sill, at which it levels out. The sill value is the a priori variance of the variable. The lag at which the sill is reached is known as the 'range'. Beyond this distance there is no longer any spatial dependence between points. The optimum sampling distance is a distance between 50 per cent and 100 per cent of the range. The geostatistical analysis of the transect samples provided a range of 415 m. To obtain enough point data to produce a map of 137Cs distribution in the catchment by 'kriging', in June 1989 another 69 sites on both arable land and grassland, were sampled approximately on a regular grid (200*200 m). At each of these sites, bulk samples consisting of three subsamples, were taken to obtain a representative sample of the Cs activity within a 20*20 m grid, the gridsize used in this study for soil erosion modelling.

USE OF ' 37Cs AS A TRACER 219

192500 193000 193500 194000 194500 195000 195500 332500

332000

33 1500

331000

330500

330000

332000

331 500

331000

330500

330000

329500 1 1 I I 1 I I ' * I " I ' 1 I I I I I I I ' I I I 329500 192500 193000 193500 194000 194500 195000 195500

Figure 2. Location of the 143 sample points in the Etzenrade catchment (coordinates are in metres)

The loess samples were air dried, ground, and passed through a 2 mm sieve, and positioned around a 90 cm3 Ge detector inside a 5 cm lead shield. The 134Cs and 137Cs activities are calculated from the observed E , = 605 and 662 keV lines, respectively. The resolution is sufficient to separate other (natural) background lines. Typical counting times were 24 hours. A comparison with a 134*1 37Cs-doped standard sample produced at the IRI (Interfaculty Reactor Institute, Delft, The Netherlands) reactor yields absolute activities. The spectra were analysed with a Nucleus Personal Computer Analyzer installed in a IBM personal computer.

THE DISTRIBUTION OF 137CS IN THE ETZENRADE CATCHMENT

A sampling depth of 50 cm proved sufficient to measure total activities in the soil. The amount of 137Cs below 50 cm is hardly detectable because of background radiation and measurement errors. This depth also defines the zone of mixing by ploughing and soil fauna.

The total 137Cs activity has been divided into a component originating from nuclear weapon testing fallout and another part from the Chernobyl accident (Table 11). It has been separated using the 134Cs activity, an exclusive product of Chernobyl with a constant ratio of 137Cs. Because 134Cs has a relatively short half-life of 2 years and thus is rapidly decreasing, accurate measurements of 134Cs activity will be difficult in the near future. The estimate of Chernobyl 137Cs, having already a substantial measurement uncertainty, has to be subtracted from the nuclear test 137Cs. This subtraction produces an even greater relative error. To obtain the same precision as obtained in measurements of the 137Cs activity from nuclear weapon tests, longer counting times are needed to measure the 134Cs activity. Therefore, in the near future, when the 134Cs activity can no longer be accurately measured due to the rapid decay, 137Cs cannot be used as a tracer of soil erosion in areas where Chernobyl 137Cs is present in significant amounts.

As expected, a close correlation was found between 134Cs activity and total 137Cs activity (Figure 3). Linear regression produced a 137Cs/134Cs ratio of 1.765. The regression line intercepts the 137Cs ordinate at 2280 Bq m-* which, corrected for decay to June 1 , 1988, yields a value of 2174 Bq m-2. Exactly the same value was found for the mean of all estimates of bomb 13'Cs. The reference date June 1 , 1988 was chosen because the first sampling period was in June 1988, so soil erosion patterns could be evaluated up to this date,

220 A. P. J. DE ROO

Table 11. Mean 137Cs activity and precision limits for the Etzenrade catchment on June 1, 1988

Origin I3'Cs activity (Bq m-') Mean Standard deviation

Total 4430 f 75 1327 f 78 Nuclear tests 2174 f 249 714 & 43 Chernobyl 2256 239 1119+41

1 ~~ -I--- 1 0 1 2 3 4 5

0

134Cs activity (kBq/m2)

Figure 3 The relationship between Chernobyl IJ4Cs and total "'Cs activity in all 143 samples from the Etzenrade catchment

both long term (137Cs variations) and short term (IJ4Cs variations). Points above the line indicate sedimentation, points below the line indicate soil erosion.

137Cs from the nuclear tests is normally distributed. 134Cs from Chernobyl, and conseguently also total i37Cs, has a positive skewness.

Cs acriviries in r t + w n w sites ul. (1987) reported that in 1986 non-eroded soils in the Netherlands had an average IJ7Cs

activity of 1950 Bq m-' in the upper 25 cm, originating from nuclear tests. At the reference sites sampled in Limburg a higher i37Cs activity was found: 2472 Bq m-' (Table 111). This difference of 26.8 per cent may be attributed to the fact that samples below 25 cm also contribute to the 13'Cs activity due to biological activity in the soil. In refercnce soil 27 for instance, i37Cs was detected down to 50 cm (Figure 4). Soil profile analysis showed that only fauna activity, and not ploughing, had caused this vertical distribution. If only the upper 25 cm had been measured, the Cs activity would have been underestimated by 22-5 per cent.

An average of 4700 Bq m-' arrived in Western Europe between 1956 and 1963 (RIVM, 1986), and some 2600 Bq m-' will remain at present. Although the total bomb fallout varied from place to place in responsc to mean annual precipitation, this value corresponds well with the mean reference bomb 13'Cs activity found

Kiister

USE OF 13’Cs AS A TRACER 22 I

Table I l l . I3’Cs and ’34Cs activity in reference sites, including measurement precision (95 per cent confidence level)

Sample Lccation I3’Cs activity I3’Cs activity 134Cs activity site total nuclear tests Chernobyl

(Bq m - 2 ) (Bq m - 2 , (Bq m-2)

1 2 5 6

27 70 71 72 73 74 98

119

Douvergenhout Douvergenhout Douvergenhout Douvergenhout Douvergenhout Quabeek Douvergenhout Douvergenhout Merkelbeek Raath Raath Raath

Mean Standard deviation

3962 f 116 1197 f 412 3426 f 117 1361 f 305 2240 f 137 1495 f 356 8687f 96 2815 f 277 3719f 46 2479 f 120 4474 f 80 1784 f 272 7365 f 86 3686 f 268 4398 f. 76 2671 f 199 6633 f 89 3239 f 280 3894 f 66 2948 f 233 5142_+ 74 2263 f 285 4967 f 56 3727 f 194

1643 f 235 1227 f 168 443 f 195

3489 f. 155 737 f 66

1598 f 154 2186 f 151 1026 f 109 2017 f 158 562 f 133

1711 f. 164 737 f 110

4909 f 87 2472 f. 266 1448 f. 150 1820 f 27 871 76 863 f 43

~~ ~

*I3’Cs on June 1 , 1988, ”‘Cs on May 3, 1986

I .

4 - l

0 2b

Total activity = 3719 B q h 2 (2479 Bq/m nuclear tests) (1240 Bq/m2 Chernobyl)

I I I I I I I I I I 5 10 15 20 25 30 35 40 45 50 55

Depth fcm) - 137Cs Chernobyl

1 3 7 0 nuclear tests Figure 4. Total I3’Cs in reference soil profile no. 27 (luvisol on plateau in orchard)

222 A. P. I . DE ROO

0.800 -

0.700 -

0.600 -

0.500 -

0.400 -

0.300 -

here (2472 Bq m-2). Furthermore, this reference input corresponds well to measurements in Canada and China. Martz and De Jong (1987) found reference activities of 2537 and 2593 Bq m-2, Zhang Xinbao er a/. (1990) found a reference value of 2529 Bq m-2. However, the reference value found here is higher than the reference value found in Belgium by Vanden Berghe and Gulink (1987). They measured a reference activity of 2000 Bq m-2, but limited their measurements to the upper 30 cm of the soil, which might be a serious underestimation of Cs, as mentioned above. However, in the same paper they also mention a value of 255 Bq m-2, which agrees with this study.

Additional 137Cs from Chernobyl at [he reference sires From Table 111 i t can be observed that the Chernobyl 134Cs activity is highly variable, even for the

reference sites. It is not possible to define a uniform input of Cs from Chernobyl, even though the investigated drainage basin is relatively small (225 ha). Possible explanations for this large spatial variation are the very local rainstorms in Limburg and interception/stemflow in orchards, causing high (sample no. 6) and low (sample no. 5, 74) activities. Although Vanden Berghe and Gulink (1987) stated that the extra Cs from Chernobyl is irregularly dispersed over the Belgian loamy region, due to irregular rainfall in May 1986, they proposed that this extra Cs provided the opportunity to study short-term erosion patterns and to shorten counting times. The underlying assumption was that there is little local spatial variation in Cs inputs from Chernobyl. However, because of the large variance in values from the reference sites (Table HI), the IJ4Cs and IJ7Cs from Chernobyl are of no value for erosion assessment in South Limburg. In general, in areas where the Cs failout from the Chernobyl accident is a substantial part of total Cs in the soil, the Chernobyl input of 137Cs cannot be used as a tracer for short-term soil erosion.

Mapping I-17Cs Geostatistical methods were used to map the spatial variation of bomb 137Cs (Journel and Huijbregts,

1978; Burrough, 1986). Using both the bomb lJ7Cs data from the transect sites and the data from the 200 m grid sites, a variogram was produced. A spherical model was fitted through the data with the ‘Weighted Least Squares Fit’ program (Figure 5). The nugget variance is 35.2 per cent of the sill and the range of the semivariance is 275 m. Based on these calculations, the bomb 137Cs point data were interpolated using ‘block kriging’ to obtain a map on a 20*20 m grid (Figure 6). However, kriging cross-validation was very poor and also mapping variances were relatively high.

* *

0 * *

* -

- -

-

* * t

0.100 0 0 75 150 225 300 ---A 375 450 525

O ’ * O O L - .

I I 1

lag,h (m) Figure 5. Semivariogram or IJ7Cs in the Etzenrade catchment calculated from all 143 samples

USE OF 37Cs AS A TRACER

1371:s i n s n L I . E t r e n r a d e catchment ( S o u t h Limhurg )

........................... .::::f ..................... .......... I ......... .....................

I . . . . . . . . . . - ... . . . . . . . . . . . . . . . . . . . . . . _. . - 0 500 meter

223

~ 1 not sampled 1 : < 1000 Bq/m2 [ 1 lo00 - 1250 Bq/m2 I 3 1250 - 1500 Bq/m2 ( .. 1 1500 - I 7 5 0 Bq/m2 [Wl 1750 - 2000 Bq/m2 m\ 2990 - 2500 Bq/m2 [m; 2500 - 3000 Bq/m2 1-1 3000 - 3500 Bq/m2 L-1 > 3500 Bq/m2

Figure 6 Bomb 13’Cs in the Etzenrade catchment, interpolated using block kriging

Rogowski et al. (1985) produced an experimental semivariogram of soil loss, estimated with the universal soil loss equation (USLE), based on a 100*100 m grid. They found a nugget variance of 64-7 per cent and a range of 400 m. This range and nugget variance were much higher than the values found here. However, Bachhuber et al. (1987) found no spatial correlation of 137Cs at all in a cultivated field of 150*100 m.

Calcularion of soil loss The mean 137Cs activity, originating from the nucler tests, is 2174 Bq m-’. A Mann-Whitney U-test

showed that the activities on arable land are significantly different from the reference activities. Subtraction of 2174 & 249 Bq m-’ from the mean 137Cs activity in the reference samples of 2472 f 266 Bq m-’ (Table 111) gives a mean loss of Cs of 298 f 399 Bq m-’ for the entire catchment. Following Martz and De Jong (1987), a mean loss of 137Cs of 5 per cent by harvesting crops is assumed. Tare losses in the sugarbeet harvest are an important factor here. Corrected for the area of grassland and forests in the Etzenrade catchment (ten per cent), which are not harvested, this represents a loss of 1 1 1 k 12 Bq m-*. Accounting for these losses, the mean loss of bomb l 37Cs by soil erosion is 187 f 399 Bq m-2. A mean 137Cs activity in the top soil of 4.47 Bq kg-’ was measured. Using this top soil activity, the average net erosion in the Etzenrade catchment is 418 tons ha-’ in 30 years (13.9 tons ha-’ yr-I). A maximum erosion rate of 154.9 tons ha-’ yr- I and a

224 A. P. J. DE ROO

maximum deposition rate of 144.5 tons ha- ’ yr- ’ were calculated. These soil erosion rates are comparable to the findings of Bolline (1975, 1977) in the loamy area of Belgium. However, one should take notice of the measurement precision of bomb 137Cs mentioned above, which has become relatively large ( 1 1.4 per cent) due to the Chernobyl IJ7Cs subtraction.

MODELLING BOMB ‘j7CS USING LANDSCAPE FACTORS

A Mann - Whitney U-test showed that 1’7Cs activities of soils under grassland are statistically significantly higher than in soils with arable land use (Table IV), at a confidence level of 95 per cent. Other landscape factors, topography and geomorphology, were used to evaluate the bomb IJ7Cs data.

Bomb 137Cs activities in six geomorphological classes were evaluated (Tables V and VI). The geomorpho- logical classes consist of different slope segments (Young, 1972). A similar study was carried out by Loughran er al. (1989). Bomb 137Cs activities on the maximum slope angle locations are significantly lower and activities on the lower waning slopes are significantly higher. However, other differences between the slopes are not significant. Increased 137Cs loss due to slope length efiect, as introduced in the model of Loughran et al. (1989) is found in the constant slope class, but 137Cs activity is not lower than on the free faces. Analysis of slope form alone cannot explain the observed spatial pattern of I3’Cs.

Also, possible relations between bomb 137Cs activity and plan convexity (rate of change of slope aspect) and profile convexity (rate of change of slope gradient), calculated from a Digital Elevation Model (DEM) (Burrough, 1986), were evaluated. However, supporting the findings of Martz and De Jong (1987) and Pennock and De Jong (1987), no significant correlation exists (R2 0.0008 and 0.0020 respectively). Furthermore, the relation between 137Cs activity and the number of upslope elements, also calculated from a DEM using the WATERSHED toolbox (Van Deursen and Kwadijk, 1990) was examined. However, no significant correlation exists.

Table IV. Bomb 137Cs activity in the Etzenrade catchment on June I , 1988. Mean values for land use classes

Land use class mean st. dev. n

Arable land 2066 707 106 Grassland 2486 535 25 Orchard 2472 871 12 All 2174 714 143

I3’Cs activity (Bq m-’)

Table V. Mean bomb 137Cs/’34Cs activity and standard deviations for landform classes on arable land in the Etzenrade catchment

Landform 137Cs activity (Bq m-’) mean st. dev. n

~~

Plateau 2156 622 10 Waxing slope 1990 635 42 Maximum slope 1275 76 1 7 Constant slope 2020 625 24 Waning slope 1 2062 598 10 Waning slope 2 2760 690 13 All 2066 707 106

USE OF ' 37Cs AS A TRACER 225

Table VI. Results of the Mann-Whitney test

Landform Plateau Waxing Maximum Constant Waning 1

Plateau ~

Waxing slope 0.4575 ~

Maximum slope 0.220* 0.0217* -

Constant slope 0.5724 0.8279 0*0234* - Waning slope 0.345 1 0.7317 0.0354* 0-8344 ~

Waning slope 2 0.0372' 0.001 1* 0.0007* 0.0031* 0.0163'

*Significant at the 95 percent confidence level

Comparing bomb ' ~ ' C S with USLE estimates The bomb 137Cs data were compared with the estimates, simulated with the Universal Soil Loss Equation,

modified for catchment applications. There is a negative correlation between I3'Cs activity and USLE soil loss estimates. However, supporting the findings of Pennock and De Jong (1987), the correlation coefficient is very low (R2 0.03).

In a future paper, the soil erosion map calculated from the Cs data will be used in the validation of the 'ANSWERS erosion simulation model (Beasley et a/., 1980; De Roo et al., 1989). However, in view of the uncertainties associated with the bomb 137Cs map, this will be difficult. Probably only the measured point values of bomb 137Cs can be used for validating erosion models.

DISCUSSION OF ERRORS

The reference values measured here show a high variability. The mean is as expected but the reliability of several sites is questioned. The sampled reference sites are orchards on loess plateaus. In orchards sometimes old trees are removed, causing soil disturbance, which may be an explanation for the variability.

Furthermore, the 37Cs/134Cs ratio causes problems. Different ratios are reported in the literature. A different ratio (e.g. 1.95) results in a semivariance with a higher nugget. However, the range of the semivariance does not change. The larger nugget leads to a larger mapping variance. Soil loss estimates are only slightly influenced by using a different ratio.

Soil disturbance by ploughing may explain a large part of the nugget variance. However, it is expected that after many years of ploughing the soil is well mixed and the nugget variance is reduced. Another part of the nugget variance can be explained by the known large spatial variability of soil erosion processes, due to their complexity.

Measurement precision of nuclear test 137Cs is & 11-4 per cent (95 per cent confidence limits) on the average. A large part of this uncertainty results from the rapid decay of Chernobyl 134Cs. The mean measurement precision of total 137Cs is much better: f 1.7 per cent (95 per cent confidence limits). The Chernobyl input of both 134Cs and 137Cs requires longer counting times to obtain the same accuracy in measurements of the 137Cs activity from nuclear weapon tests.

CONCLUSIONS

The Chernobyl input of 134Cs cannot be used as a tracer for short-term soil erosion. This is due to the large spatial variation of 134Cs between the reference sites, even though the distances between them are small. Caesium-134 is an exclusive product of Chernobyl with a constant ratio of 1:1.765 to lJ7Cs. Chernobyl Cs can only be used to examine river sediment sources (top soil us. subsoil), as demonstrated by Walling et al. (1989) and Bonnett et al. (1989).

In the near future, it will no longer be possible to separate Chernobyl 137Cs from nuclear weapon tests 137Cs, because the 134Cs will decay rapidly due to its short half-life. The variability of reported 137Cs/134Cs

226 A. P. J. D E R O O

ratios causes minor additional problems in separating the Chernobyl input from the older caesium input. Due to the rapid decay of 134Cs, in the near future it will be impossible to use 137Cs as a tracer of soil erosion in areas where the 137Cs fallout from the Chernobyl accident represents a substantial part of total 137Cs in the soil. In this study, however, 137Cs originating from nuclear tests could be used as a tracer of soil erosion. Using block kriging, the lJ7Cs distribution in the catchment was mapped using 143 samples. From geostatistical analysis it was found that 13'Cs, originating from nuclear tests, no longer shows any spatial dependence beyond a distance of 275 m. The 'nugget' variance is 35 per cent of the sill. Due to the relatively high nugget and the relatively large sampling distance (200 m), the mapping variance is high. More samples seem necessary to map Cs on a catchment scale.

Modelling bomb 137Cs patterns and thus soil erosion is more difficult. Caesium-137 activities on grassland are significantly higher than on arable land. Also, 137Cs activities on waning slopes are significantly higher, and activities on maximum slope locations are lower. However, no clear relation exists between geomorpho- logical units and IJ7Cs. Furthermore, plan and profile convexity and the number of upslope elements, calculated from a DEM, are not correlated with 137Cs. Also, the USLE estimates and the bomb 137Cs data are not significantly correlated. Therefore, more advanced models are needed to predict soil erosion patterns. The catchment models ANSWERS (Beasley et al., 1980) and SWAM (Alonso and DeCoursey, 1985) may yield better estimates. A forthcoming paper will evaluate the ANSWERS model results.

From the bomb 137Cs data, an average net soil loss of 13.9 tons ha-' yr-l from the drainage basin is estimated. Locally within the catchment, gross erosion rates are as high as 155 tons ha- yr- and deposition rates ranges up to 145 tons ha-' yr-'. These values are comparable to data reported for the Belgian loamy region and other areas affected by soil erosion.

ACKNOWLEDGEMENTS

The author wishes to thank Dr. C. Alderliesten, Dr. K. v.d. Borg, Mr. B. R. Hoogervorst, and Mr. H. D. Kaegi from the Robert J. Van De Graaff Laboratory, Nuclear Physics Department, University of Utrecht, for their great cooperation and enthusiasm in the setup of the Cs experiment. Without their help these experiments could not have taken place. Dr. C. J. Schouten is thanked for his effort to initialize this research. The author thanks Prof. Dr. P. A. Burrough and Drs. S. M. De Jong for their constructive comments on the manuscript.

R E F E R E N C E S

Alonso, C. V. and DeCoursey. D. G . 1985. 'Small watershed model', Proceedings of the Nurural Resources Modelling Symposium. LISDA.

Bachhuber. H., Bunzl, K., and Schimmack, W. 1987. 'Spatial variability or fallout I3'Cs in the soil o f a cultivated field', Enuironmenral

Beasley, D. B.. Huggins, L. F., and Monke, E. J. 1980. 'ANSWERS: a model for watershed planning'. Transarrions o f f h e ASAE, 23, 3,

Beentjes, L. B. and Duijsings, J. H. 1987. 'Radioactive contamination in Nijmegen rainwater after the Chernobyl accident', The Science

Bolline. A. 1975. 'La mesure de I'intensite du splash sur sol limoneux. Mise au point d'une technique de terrain et premiers resultats'.

Bolline, A. 1977. 'La vitesse de I'erosion sous culture en region limoneuse', PI.duloyir, 27. 191 206. Bonnett, P. J. P., Leeks, G . J. L.. and Cambray, R. S. 1989. 'Transport processes for Chernohyl-labelled sediements: Preliminary evidence

Brown, R. B.. Cutshall. N. H., and Kling, G . F. 1981a. 'Agricultural erosion indicated by '"Cs redistribution: 1. Levels and distribution

Brown, R. B.. Kling, G. F., and Cutshall, N. H. 1981b. 'Agricultural erosion indicated by '"Cs redistribution: 11. Estimates of erosion

Burrough. P. A. 1986. Principles qf Geogruphicul Infornturiun Sj's!evi.rfor L a r i d Resources Assewm-nr, Oxford. Oxford University Press.

Cambray. R. S., Cawse. P. A.. Garland, J.A.. Gibson. J. A. B.. Johnson, P., Lewis, G. N. J.. Newton, D., Salmon, L., and Wade, B. 0. 1987.

Campbell. E. L.. Loughran, R. J.. and Elliot. G. L. 1982. 'Caesium-137 as an indicator of geomorphic processcs in a drainage has~n

ARS-30.40-46.

moniroriny und u.w.wnen/ , 8, 93- 101.

938-944.

u/ rhe Toral Enoironmen/, 64. 253-258.

Rdohgie, 25, 1 99-2 1 0.

from upland Mid-Wales', Lund deurudurioti & rehahilitorion, 1, I , 39-50.

of 13'Cs activity in soils', Soil Sci. Soc. Am. J., 45, 1184- 1190.

rates', Soil Sri. Soc. Am. J. , 45, I191 1197.

193 pp.

'Observations on radioactivity from the Chernohyl accident'. Nuclcur Energy, 26. 77- 101.

system'. Au.\/ruliuti Geoqruphicul Sludies. 20. 49-64.

USE OF I3’Cs AS A TRACER 221

Coughtrey, P. J. and Thorne, M. C. 1983. Radionuclide Distribution and Transporr in Terrestrial and Aquatic Ecosystems, Vol. 1.

Denschlag, H. O., Diel, A., Glasel, K.-H.. Heimann, R., Kafirell, N., Knitz, U., and Menke, H. 1987. ‘Fallout in the Maim area from the

De Roo, A. P. J., Hatelhoff, L., and Burrough, P. A. 1989. ‘Soil erosion modelling using “ANSWERS” and Geographical Information

De Roo, A. P. J. and Riezebos, H. Th. 1990. ‘Validating the “ANSWERS” erosion model: infiltration experiments and their implications

Frissel, M. J., Soutjesdijk. J. F., Hoolwijk. A. C., and Koster, H. W. 1987. ‘The Cs-137 contamination of soils in the Netherlands and its

Haigh, M. J. 1977. ‘The use of erosion pins in the study of slope evolution’, British Geomorphological Research Group Tech. Bull., 18,

Hoffmann, P., P i l l N., and Lieber, K. H. 1987. ‘Radionuclides from the Chernobyl accident in the environment of Chattia, a region of

Journel, A. G. and Huijbregts, Ch. J. 1978. Mining Geostarisrics. Academic Press, New York, N.Y.. 600 pp. Koster, H. W.. Mattern, F. C. M., and Pennders, R. M. J. 1987. The radioactive conramination of the Dutch Soil due to the accident of the

Hotzl, H., Rosner, G., and Winkler, R. 1987. ‘Ground depositions and air concentrations of Chernobyl fallout radionuclides at

Loughran, R. J., Campbell, B. L., and Walling, D. E. 1987. ‘Soil erosion and sedimentation indicated by caesium 137: Jackmoor Brook

Loughran, R. J., Campbell, B. L., Elliott, G. L., Cummings, D., and Shelly, D. J. 1989. ‘A caesium-137-sediment hillslope model with tests

Martz, L. W. and De Jong, E. 1987. ‘Using Caesium-137 to assess the variability of net soil erosion and its association with topography

McCallan, M. E., OLeary, B. M.. and Rose, C. W. 1980. ‘Redistribution of Caesium-I37 by erosion and deposition on an Australian

Oliver, M. A. and Webster, R. 1986. ‘Semi-variograms for modelling the spatial pattern of landform and soil properties’, Earth Surface

Pennock, D. J. and De Jong. E. 1987. ‘The influence ofslope curvature on soil erosion and deposition in hummock terrain’, SoilScience,

Persson, C.. Rodhe. H., and De Geer, L-E. 1987. ‘The Chernobyl accident - a meteorological analysis of how radionuclides reached and were deposited in Sweden’, AMBIO, 16, 1, 20-31.

Ritchie, J. C., Spraberry, J. A., and McHenry, J. R. 1974. ‘Estimating soil erosion from the redistribution of fallout I3’Cs’, Soil Science Society of America Proceedings, 38, 137- 139.

RIVM 1986. Samenvar rend voorrgangsrapport radioactiviteirsmetingen in verban met her nucleaire ongeval te Tsjernobyl over de periode 1-12 mei 1986, In opdracht van de hoofdinspectie voor milieuhygiene, Rapport nr. 248606001, Dutch Ministry of Public Health, 31 PP.

Balkema, Rotterdam, 5 11 pp.

Chernobyl Reactor accident’, Radiochimica Acra, 41, 163- 172.

Systems’, Earth Surface Processes and Landforms, 14, 5 17-532.

for modelling surface runoff and soil erosion’, CATENA, under review.

consequences for the contamination of crop products’, Nerh. J. of Agr. Sci., 35, 339-346.

31 -49.

the FRG’. Radiochimica Acra. 41, 173- 179.

nuclear power station at Chernobyl, Working document VI, RIVM, Bilthoven, The Netherlands, 13 pp. (In Dutch.)

Munich-Neuherberg’, Radiochimica Acra, 41, 181 - 190.

catchment, Devon, England’, CATENA, 14,201-212.

from south-eastern Australia’, Zeirschrft fur Geomorphologie, 33, 2, 235-250.

in a Canadian prairie landscape’, CATENA, 14, 5, 439-451.

soil‘, Ausr. J . Soil. Res., 18, 119-128.

Processes and hndforms. 11, 49 1 ~ 504.

144, 3,209-217.

Rogowski, A. S. and Tamura, T. 1970. ‘Erosional behaviour of Caesium-137, Health Physics, 18,467-477. Rogowski, A. S., Khanbilvardi, R. M., and DeAngelis, R. J. 1985. ‘Estimating erosion on plot, field, and watershed scales’, in El-Swaify,

Schouten, C. J., Rang, M. C., and Huigen, P. M. J. 1985. ‘Erosie en wateroverlast in Zuid-Limburg’. Landschap, 2, 118-132. Statham, I. 1981. ‘Techniques for the measurement of surface water erosion processes on slopes’, in Goudie, A. (Ed.), Geomorphological

Techniques, George Allen & Unwin, London. Van Der Helm, P. P. M., De Roo, A. P. J., and Schouten, C. J. 1989. Eodemerosie en wareroverlasr, Milieuefiecten van het ruimtegebruik

in streekplangebied Zuid-Limburg. Dee1 C. University of Utrecht. Vanden Berghe, 1. and Gulink, H. 1987. ‘Fallout I3’Cs as a tracer for soil mobility in the landscape framework of the Belgian Loamy

Region’, Pedologie, 37-1, 5-20. Van Deursen, W. P. A. and Kwadijk. J. 1990. ‘Using the WATERSHED tools for modelling the Rhine catchment’, in Harts, J. J., Ottens.

H. F. L.. and Scholten, H. J. (Eds), Proceedings of EGIS ‘90: First European Conference on Geographical Information Systems, Amsterdam, 254-262.

Walling, D. E. and Bradley, S. 8 . 1988. ‘The use of caesium-137 measurements to investigate sediment delivery from cultivated areas in Devon, UK’, in Sediment Budgets (Proceedings of the Porto Alegre Symposium). IAHS Pub]., 174, 325-335.

Walling, D. E., Rowan, J. S., and Bradley, S. B. 1989. ‘Sediment-associated transport and redistribution of Chernobyl fallout radionuclides’, in Hadley, R. F. and Ongley. E. D. (Eds), Sedimenr and the Environmenr. Proceedings of rhe Baltimore Symposiuni,

Wischmeier, W. H. and Smith, D. D. 1978. ‘Predicting rainfall erosion losses - a guide to conservation planning’, US. Departmenr of

Young, A. 1972. Slopes, in Geomorphology texts, edited by K. M. Clayton, Edinburgh, 288 pp. Xinbao, Zhang. Li Shaolong, Wang Chenghua, Tan Wanpei, Zhao Qingchang, Zhang Yiyun, Yan Meiqiong, Liu Yalun, Jiang Jingjiang,

Xia Jule. and Zhou Jie 1990. ‘Use of Caesium-137 measurements to investigate erosion and sediment sources within a small drainage basin in the loess plateau of China’, Hydrological Processes, 3, 317-323.

S. A., Moldenhauer, W. C., and Lo, A. (Eds), Soil Erosion and Conservation, Soil Cons. Soc. of Amer., Ankeny, Iowa, 149-166.

IAHS Pub/.. 184. 37-45.

Agriculture, Agriculrural Handbook, 531, 58 pp.