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Migration of 137Cs and 134Cs in different forest soil layers
J. Environ. Radioactivity, Vol. 33, No. I, pp. 63-75, 1996 Copyright 0 1996 Elsevier Science Limited Printed in Ireland. All rights reserved 0265-931X/96 ELSEVIER
$15.00 + 0.00
0265-931X(95)00069-0
Migration of 13’Cs and 134Cs in Different Forest Soil Layers
W. Riihm, Federal
L. Kammerer,
L. Hiersche
& E. Wirth
Office of Radiation Protection, Institute for Radiation Hygiene, Ingolstgdter Landstr. 1, D-85764 Neuherberg, Germany (Received
10 March
1995; accepted
29 September
1995)
ABSTRACT Caesium-134 and ‘37Cs measurements in about 250 samples from L-, Qf’, Oh-, Ah- and B-horizons of a Bavarian forest from 1987 to 1994 are analysed with respect to migration by using a compartment model. The derived ecological halJllives are 28 years, 38 years, 4.4 years and 7.7 years, respectively. By using these results, caesium behaviour can be predicted for about 2.5 years. The resulting profile is similar to that found nowadays for weapons fallout caesium, migrating within about 25 years in the same forest. Therefore, the model is suitable for the estimation offuture behaviour of radiocaesium in the forest investigatedfor a period of about 25 years after deposition. Copyright G 1996 Elsevier Science Limited
INTRODUCTION After the reactor accident at Chernobyl on 26 April 1986, a significant contamination with radionuclides occurred in Bavaria due to heavy rain showers at the end of April and the beginning of May. The ‘37Cs deposition reached up to 100kBq/m2 in the eastern part of Bavaria (Federal Health Office, 1991). In the years following the accident, it turned out that root uptake by agricultural plants was small and therefore concentrations of less than 1 Bq/kg fresh weight were measured in 1990 (Federal Office of Radiation Protection, Institute for Radiation Hygiene 1990). This behaviour could 63
64
W. Riihm et al.
not be observed in plants grown in natural ecosystems. Even nowadays around Munich, understorey vegetation of forests such as berry plants and mushrooms show unchanged high concentrations in radiocaesium of up to 15 kBq/kg fresh weight found in some species of mushrooms (Kammerer et al., 1994). Similarly, concentration of 137Cs in game meat from Bavaria is very high achieving seasonal dependent maxima of up to 6000 Bq/kg fresh weight (Fielitz, 1992). As forest ecosystems are important to man, it is of interest to analyse the reasons of this unchanged high contamination level of forest products and estimate the potential radiation exposure from their extensive use by the population. For this reason, a forest in Bavaria has been continuously investigated in the framework of several research programs since 1987. Radiocaesium measurements were carried out on about 250 soil samples, 120 plant samples and 350 mushroom samples. In this paper, the results of soil measurements including samples from 1994 are presented and discussed. A compartment model is developed to describe the dynamic behaviour of ‘34Cs and 137Csin different layers of the forest soil and the corresponding site-specific ecological half-lives are derived. The reliability of the model is tested using weapons fallout data from the same forest.
MATERIALS
AND METHODS
Since 1987, a coniferous forest located near the village of Hochstadt close to Lake Ammersee in Bavaria, has been investigated. It is characterized by a pure Norway spruce (Picea &es) stand aged about 100 years. The soil is characterized by the typical horizons of an undisturbed forest floor. The L-horizon is built up by yearly falling litter (needles, twigs, etc.), has an average thickness of about 0.7cm, and a density of 1.O kg (dry weight)/m2. The following Of-horizon with an average thickness of about 1.5 cm and a density of 1.5 kg/m2 is characterized by the early fermentation of litter material. In this layer, partly decomposed needles are baked together but the initial needle structure is still observable. After that in the so-called Oh-horizon the litter material is completely humified and only small quantities of structured material can be found. The thickness of this layer is about 18 cm, the density 2.9 kg/m*. These three layers form the organic component of the forest floor. The first mineral horizon (Ah-horizon, thickness about 1.I cm, density 5.2 kg/m*) shows a still high organic content, but the nutrient content is already low. It is followed by the B-horizon (6.4cm thick, density 37.2kg/m2), a pure mineral layer with small content of organic matter.
Migration of 137Csand 134Cs in dlxferent,forest .soil layers
65
The mineral part of the forest soil is a cambisol soil type on calcareous moraine. Since 1987, up to 16 plots per year were randomly chosen within the forest at places where no understorey vegetation such as moss or vascular plants could possibly influence radiocaesium migration. Cores, divided in the layers mentioned above, were taken. It should be mentioned that for the first 3 years only few samples were taken. The single soil samples were air-dried (70°C) and milled. Aliquots of l&lOOOg were taken and measured with germanium detectors. Dependent on the geometry used for the measurements, calibration of the detectors was performed by using commercial calibration sources supplied by the Federal Institute for Physics and Metrology (Physikalisch Technische Bundesanstalt, PTB). In the case of 134Cs,pure 134Csstandards were used to account for sum peak effects. Intercomparison runs showed that for a lot of different geometries, and all detectors used for the soil measurements, the results for ‘34Cs and ‘37Cs were in agreement with the expected values within about 3%. Therefore, the errorbars of our ‘j’Cs and 134Csmeasurements of the soil samples are estimated to be of the order of several per cent, provided sufficient counting statistics were obtained. In the case of samples from mineral horizons, errorbars may be larger due to lower counting rates. The runtime of the measurements was chosen in a way that at least a 2-sigma-error of less than 5% could be achieved, even for the measurement of ‘34Cs performed in 1994.
RESULTS AND DISCUSSION Phenomenological discussion
Figures l-5 show the measured ‘34Cs deposition in [Bq/m2] from 1987 to 1994 for the different horizons defined above, normalized to I May 1986. This caesium is Chernobyl caesium only because very small amounts of ‘34Cs were produced during weapons testing, and this amount has already decayed due to a short half-life of 2.1 years. In general, variations of up to a factor of 10 occur within one horizon and year. This reflects the well-known heterogenous horizontal contamination pattern in natural ecosystems due to various mechanisms such as interception by the canopy stem flow, decomposition of fruitbodies of fungi, etc. Interestingly, looking at data from 1993 and 1994, the scattering seems to become smaller in case of the L-horizon (Fig. 1). Future observations are necessary to investigate whether this phenomenon is due to a horizontal displacement of an initially heterogenous contamination.
W. Rtlhm et al.
Fig. 1.
Measured
and fitted
134Cs activity in [Bq/m’] L-horizon.
(corrected
for 1 May 1986) in the
In the L-horizon, the absolute deposition is decreasing by a factor of about 6 from 1500 Bq/m* in 1987 to 300 Bq/m* in 1994 (Fig. 1). This is due to transition into the Of-horizon, where the deposition is decreasing by a factor of 2 from 2700 Bq/m* to 1300 Bq/m* in the same time period (Fig. 2). In case of the Oh-horizon (Fig. 3), increase of activity due to ‘34Cs from the Of-horizon and decrease of activity due to loss of ‘34Cs to the Ah-horizon seems to be similar and accordingly a roughly constant ‘34Cs contamination of about 2500 Bq/m* can be observed. In the mineral layers the situation is different. From Fig. 4 it can be seen that the contamination of the Ah-horizon is increasing from low values of 800Bq/m* in 1988 to about 2000 Bq/m* in 1994. The contamination of the B-horizon is increasing from about 1000 Bq/m2 in 1988 to about 2000 Bq/m* in 1994.
DISCUSSION
USING A COMPARTMENT
MODEL
The qualitative discussion from above can be put on a more quantitative basis by using the compartment model described as follows.
Migration
of i37Csand ‘34Cs in d$ferent ,fbrest soil layers
67
Of-Horizon
Fig. 2.
Measured
and fitted
lx4Cs activity in [Bq/m’] Of-horizon.
(corrected
for 1 May 1986) in the
Model description
In order to analyse the dynamic behaviour of radiocaesium in soil, all data were corrected for physical decay to 1 May 1986. Half-lives of 2.06 and 30.17 years were used for 134Cs and ’37Cs, respectively. The remaining time-dependent effect is expressed in terms of the ecological half-life (eqns (1) and (2)). 1
ntotal = 1 /~I/2.ecol
lphys =
+
(1)
&cd
1/T1/2.mll
-
1 / T1.2 phys
(2)
To describe the decay-corrected behaviour of 134Cs in soil, a compartment model was developed. This model consists of five compartments, representing the L-, Of-, Oh-, Ah- and B-horizons of the forest soil. Provided litterfall is neglected as a source of activity of the L-horizon, as well ai root uptake by plants as a loss of activity in all layers, to keep the model as simple as possible, the corresponding differential equations can be written as follows: dL(t)/dt
= -A1 L(t)
(3)
W. Rtihm et al.
68
dOf(t)/dt
= &L(t)
dOh(t)/dt
= A,Of(t)
dAh(t)dt dB(t)dt
= &Oh(t) = &Ah(t)
- &Of(t)
(4)
- AsOh
(5)
- &Ah(t)
(6) (7)
I,(t) is the activity [Bq/m2] of ‘s4Cs in the L-horizon as function of time, Of(t) is the activity of ‘s4Cs in the Of-horizon as function of time, etc. This system of coupled differential equations (eqns (3)-(7)) can be solved analytically (Bronstein & Semendjajew, 1981). The resulting solution functions, i.e. L(t), Of(t), Oh(t), Ah(t) and B(r), describe the develsoil horizons with time. For example, opment of ‘34Cs in different L(t) = Lo,134exp(-A,t) is the solution for the L-horizon. By means of a least square fit, this function is adjusted to the experimental data shown in Fig. 1 using L0,tJ4, i.e. the initial contamination of the L-horizon in 1986, and ,I,, i.e. a constant describing the decrease of ‘34Cs with time in this horizon, as tit parameters. Taking all horizons into account, four time
Migration
Fig. 4.
Measured
qf 13’Cs and ‘34Cs in different .forest soil layers
and fitted
‘34Cs activity in [Bq/m*] (corrected Ah-horizon.
69
for I May 1986) in the
constants Ai-& are used as tit parameters, from which for each horizon ecological half-lives can be calculated by formula (8).
i
Tilz., = ln2/& Additionally, the initial ‘j4Cs contaminations Lo,i34, Of0,134, etc. of each horizon at 1 May 1986 serve as fit parameters. All Iits were performed with the program package STATGRAPHICS (STSC, 1991). It should be mentioned that no direct information on the neglected litterfall is available for the forest investigated, but the part of radiocaesium circulating yearly due to litterfall is expected to be small. For example, Belli (1996) reports of yearly falling litter containing about 0.5% of the total radiocaesium inventory of an Italian forest site. Therefore, input of radiocaesium due to litterfall is not included explicitly in eqn (3). Nevertheless, it is implicitly inherent in the half-life deduced from the data of the L-horizon (see over). Including litterfall in eqn (3) would introduce an additional source for the L-horizon and result in a smaller half-life. In this sense, the effect of root uptake if there is any included inherently in the deduced half-lives, too.
W. Rh71 et al.
70
E-Horizon
Fig. 5.
Measured
and fitted
‘34Cs activity in [Bq/m*] (corrected B-horizon.
for 1 May 1986) in the
Discussion of results of the model
In Figs l-5 results of least squares fits of the experimental data on ‘34Cs activities in different horizons, using the solutions of eqns (3)-(7) are plotted. The qualitative behaviour of the experimental results is described very well by the fitted curves. Instead of discussing the fitted curves by looking at correlation coefficients (which are sometimes very small due to the wide scattering of the experimental data), a discussion of the resulting lit parameters (see Table 1) is used to rate the results of the fits. Caesium ,from Chernobyl
To describe the transport of caesium from the Chernobyl fallout, tits of ‘34Cs(t) for each horizon were used. From top of the forest floor, the resulting ecological half-lives are increasing with depth from 2.8 years in the L-horizon, 3.8 years in the Of-horizon, 4.4 years in the Oh-horizon and 7.7 years in the Ah-horizon. Activities in the B-horizon can be explained by assuming no loss into deeper layers (see also eqn (7)). Compared to these results Bunzl et al. (1989) report of 200-400day
Migration
of ‘37Cs and ‘34Cs in different,jkwest
soil layers
71
TABLE 1 Resulting Fit Parameters: Time Constants li with Error Bars Provided by STATGRAPHICS, Resulting Ecological Half-Lives Calculated with Eqn (8) and Amount of ‘34Cs Deposition on 1 May 1986 (Chernobyl Caesium Only) with Error Bars Provided by STATGRAPHICS
li [l/year] TI !2 bear1 Initial ‘34Cs deposition
L
Of
Oh
Ah
0.25 f 0.05 2.8 f 0.5 1900 I!=360
0.18 + 0.04 3.8 f 0.8 2800 & 650
0.16 f 0.04 4.4+ 1.2 1800 f 620
0.09 f 0.06 7.7 i 4.9 340 It 520
B
11ooi200
[Bqim’l
residence times inorganic layers of a spruce stand, measured within the first 600 days after the Chernobyl accident. On the other hand these authors mention that residence times derived from weapons fallout caesium are up to a factor 6 higher. Since we have only few data from the Iirst years after the Chernobyl accident, the derived ecological half-lives are mainly dominated by the long-term component of the residence time. In this context it should also be mentioned that the ecological half-lives derived are site-specific. The initial ‘s7Cs deposition in 1986 due to the Chernobyl accident is calculated by multiplying the extrapolated initial i34Cs contaminations (Table 1) by 1.75, which is the ‘37Cs/‘34Cs ratio characteristical for the Chenobyl fallout near Munich (Hotzl et al., 1987). This results in a total ‘37Cs deposition of 13.9 f 1.9 kBq/m2. This is in agreement with the German deposition map (Federal Health Office, 1991) which gives a ‘s7Cs contamination between 10 and 15 kBq/m* in the area of interest. Interestingly, the initial deposition was not only located in the L-horizon, but also in deeper horizons. According to the result of the Iits (Table 1), 24% of the initial deposition can be found in the L-horizon, 35% in the Of-horizon, 23% in the Oh-horizon, 4% in the Ah-horizon and 14% in the B-horizon, respectively. This can be explained by penetrating rain water transporting part of the initial fallout to deeper layers immediately after the deposition. Such a phenomenon could be demonstrated by laboratory experiments of Schimmack et al. (1994), who found up to 30% ‘37Cs in the mineral layers of undisturbed forest soil cores after artificial precipitation of up to 30 mm/h simulating a heavy rain shower. Vevjfication of the model by using fallout data
In order to deduce the distribution of weapons tests ‘37Cs from our data, a caesium ratio of 1.75 of the Chernobyl fallout in Bavaria in May 1986
W. Riihm et al.
12
(Hotzl et al., 1987) has to be used again. As an example, this approach for the L-horizon. L 137;fallout
--L
137.total,exp
-
eqn (9) elucidates
1.75&34,exp
By averaging the results from 1987 to 1994 for each horizon, an averaged depth profile for 137Cs from weapons fallout can be estimated (Table 2). A total 137Cs deposition of about 1.7 f 0.8 kBq/m2 is calculated. This is about one half of the expected value. According to Bunzl & Kracke (1988) who measured 137Cs deposition due to weapons fallout all over Bavaria and correlated it to the annual precipitation rate, a deposition of about 3.6 f 0.8 kBq/m2 is expected. For this estimation, a mean precipitation rate (averaged from 1951 to 1980) of 950mm per year is used for Hochstadt from data of the Germany Weather Service (Deutscher Wetterdienst DWD) gathered in Puch, located about 15 km from our sampling site (Mtiller-Westermeier, 1990). Taking into account a spread of a factor 2 in the data shown in Bunzl & Kracke (1988) and the errorbars, the two values are in agreement. Prediction
oj’the,future
behaviour
The measured 137Cs profile (weapons fallout) shown in Table 2 is valid for the time period 1987-1994, that is about 25 years after the maximum weapons fallout which occurred in 1963 and 1964. 137Cs deposition using the ecological half-lives derived above. One should bear in mind that the assumption of superficially deposited weapons fallout caesium might only be a crude estimation, since it does not account for caesium deposited before 1963. Both measured and calculated weapons fallout profiles agree well. This suggests that the used model works reliably for the chosen site, and the ecological half-lives derived seem to be reasonable. Therefore, a prediction of the future caesium behaviour in our forest, at least 25 years from the date of deposition, is possible. In this context it should be mentioned that from our data no difference in residence times of fallout and Chernobyl caesium can be deduced. Figure 6 shows the future development of the ‘34Cs activity based on Bq/m2, until 2036. It can be seen that half of the initially deposited caesium has left the horizons L to Ah and entered the B-horizon about 20 years after deposition, provided physical decay is not respected. In case of an increasing fixation of Chernobyl caesium with time, residence times will still increase in future and the result of 20 years represents a lower limit.
Migration
of i37Cs and “‘Cs
in different ,forest soil layers
TABLE 2 ‘j7Cs Activity Distribution (Weapons Fallout) Derived from Experimental (9) and corrected for 1 May 1986. Average of n Samples of Each Horizon 1994 Plus Corresponding Standard Deviation
n
“‘Cs [Bq/m’] Relative [%I Calculated [%]
Data (see Eqn From 1987 to
L
Of
Oh
Ah
B
49 -0.4 i 50 0 0
66 -16 i 570 0 3
38 300 i 200 18 12
46 510 f 290 31 33
44 850 i 500 51 52
Last row: calculated relative profile after 25 years assuming superficial horizon and using the ecological half-lives shown in Table 1.
/
Fig. 6.
73
I
Extrapolation
I
I,
*
7
I,
1,.
1,.
1,
of the fit results of lJ4Cs activity ([Bqim’] 1986) until the year 2036 for all horizons.
Totdi
depostion
I,,
/,
corrected
243 1660 + 840 100 100 on the L-
I,
for 1 May
In this context is should be emphasized that only transport processes such as decomposition, diffusion, leaching, etc. are described by the ecological half-lives discussed up to now. Physical decay is not taken into account since all data were corrected to 1 May 1986. Taking the physical decay of ‘34Cs into account as well, the loss from the system is much more effective. Due to the short half-life of 2.1 years, physical decay is the most important process when considering the loss of lT4Cs from the forest floor. As for ‘s7Cs, the situation is different due to the long physical half-life of
74
W. Rtihm et al.
30 years. In this case, physical decay and physical migration are of similar importance when looking at the decrease of the 137Cs contamination of forest floor.
CONCLUSIONS About 250 measurements of 134Cs and 137Cs in different layers of forest soil from a stand in Bavaria are presented. These measurements form a complete time series from 1987 to 1994. With the help of a compartment model where each compartment represents one soil horizon, ecological half-lives of 2.8, 3.8,4.4, and 7.7 years can be deduced for the L-, Of-, Ohand Ah-horizon, respectively. Taking measured 137Cs deposition in each horizon and correcting for 137Cs from Chernobyl, an averaged profile for 137Cs from weapons fallout could be deduced. This profile which developed within about 25 years from 1964 can be reproduced by extrapolating the Chernobyl contamination to about 25 years from the initial deposition in 1986. This shows that the derived ecological half-lives are reliable enough to predict future behaviour of radiocaesium in the soil of the forest investigated at least over 25 years. In the case of 134Cs, physical decay is the most dominant process when looking at the decrease of forest soil contamination. In the case of 137Cs, the ecological half-life describing the loss from the L-, Of-, Oh- and Ahhorizon is about 20 years and of the same order of magnitude as the physical half-life.
ACKNOWLEDGEMENT The present work was financially supported by the European Community under contract number FI3P-CT920050.
REFERENCES Belli (1996). Fluxes of Radiocaesium in forest environments. Paper in preparation. Bronstein, I. N. & Semendjajew, IS. A. (1981). Taschenbuch der Mathematik. Bunzl, K. & Kracke, W. (1988). Cumulative deposition of 137Cs, 238Pu, 239+240Pu and 241Am from global fallout in soils from forest, grassland and arable land in Bavaria (FRG). J. Environ. Radioactivity, 8, 1-14. Bun& K., Schimmack, W., Kreutzer, K. & Schierl, R. (19892. The migration of and of ’ 7Cs from weapons fallout 134Cs, 137Cs and io6Ru from Chernobyl testing in a forest soil. Z. Pjlanzenerntihr. Bodenk., 152, 34-39.
Migration of 137Csand “4Cs in different forest soil layers
75
Federal Health Office (1991). 137Cs Contamination Map of the Federal Republic of Germany. Institute of Water, Soil and Air Hygiene. Federal Office of Radiation Protection, Institute for Radiation Hygiene (1990). Report on radiation exposure in the year 1990. (In German.) Fielitz, U. (1992). Ausbreitung und Transfer von Radioctisium entlang des Pfades Boden-Pflanze Reh in zwei unterschiedlichen WaldcYkosystemen. Dissertation. (In German.) Hotzl, H., Rosner, G. & Winkler, R. (1987). Ground depositions and air concentrations of Chernobyl fallout radionuclides at Munich-Neuherberg. Radiochim. Acta, 41, 181-190. Kammerer, L., Hiersche, L & Wirth, E. (1994). Uptake of radiocaesium by different species of mushrooms. J. Environ. Radioactivity, 23, 135-150. Miiller-Westermeier, G. (1990). Klimadaten der Bundesrepublik Deutschland Zeitraum 1951-1980. Selbstverlag des Deutschen Wetterdienstes. Schimmack, W., Bunzl, K., Dietl, F. & Klotz, D. (1994). Infiltration of radionuclides with low mobility (13’Cs and 6oCo) into a forest soil. Effect of the irrigation intensity. J. Environ. Radioactivity, 24, 53-63. STSC, Inc. (1991). STATGRAPHICS Version 5.
$15.00 + 0.00
0265-931X(95)00069-0
Migration of 13’Cs and 134Cs in Different Forest Soil Layers
W. Riihm, Federal
L. Kammerer,
L. Hiersche
& E. Wirth
Office of Radiation Protection, Institute for Radiation Hygiene, Ingolstgdter Landstr. 1, D-85764 Neuherberg, Germany (Received
10 March
1995; accepted
29 September
1995)
ABSTRACT Caesium-134 and ‘37Cs measurements in about 250 samples from L-, Qf’, Oh-, Ah- and B-horizons of a Bavarian forest from 1987 to 1994 are analysed with respect to migration by using a compartment model. The derived ecological halJllives are 28 years, 38 years, 4.4 years and 7.7 years, respectively. By using these results, caesium behaviour can be predicted for about 2.5 years. The resulting profile is similar to that found nowadays for weapons fallout caesium, migrating within about 25 years in the same forest. Therefore, the model is suitable for the estimation offuture behaviour of radiocaesium in the forest investigatedfor a period of about 25 years after deposition. Copyright G 1996 Elsevier Science Limited
INTRODUCTION After the reactor accident at Chernobyl on 26 April 1986, a significant contamination with radionuclides occurred in Bavaria due to heavy rain showers at the end of April and the beginning of May. The ‘37Cs deposition reached up to 100kBq/m2 in the eastern part of Bavaria (Federal Health Office, 1991). In the years following the accident, it turned out that root uptake by agricultural plants was small and therefore concentrations of less than 1 Bq/kg fresh weight were measured in 1990 (Federal Office of Radiation Protection, Institute for Radiation Hygiene 1990). This behaviour could 63
64
W. Riihm et al.
not be observed in plants grown in natural ecosystems. Even nowadays around Munich, understorey vegetation of forests such as berry plants and mushrooms show unchanged high concentrations in radiocaesium of up to 15 kBq/kg fresh weight found in some species of mushrooms (Kammerer et al., 1994). Similarly, concentration of 137Cs in game meat from Bavaria is very high achieving seasonal dependent maxima of up to 6000 Bq/kg fresh weight (Fielitz, 1992). As forest ecosystems are important to man, it is of interest to analyse the reasons of this unchanged high contamination level of forest products and estimate the potential radiation exposure from their extensive use by the population. For this reason, a forest in Bavaria has been continuously investigated in the framework of several research programs since 1987. Radiocaesium measurements were carried out on about 250 soil samples, 120 plant samples and 350 mushroom samples. In this paper, the results of soil measurements including samples from 1994 are presented and discussed. A compartment model is developed to describe the dynamic behaviour of ‘34Cs and 137Csin different layers of the forest soil and the corresponding site-specific ecological half-lives are derived. The reliability of the model is tested using weapons fallout data from the same forest.
MATERIALS
AND METHODS
Since 1987, a coniferous forest located near the village of Hochstadt close to Lake Ammersee in Bavaria, has been investigated. It is characterized by a pure Norway spruce (Picea &es) stand aged about 100 years. The soil is characterized by the typical horizons of an undisturbed forest floor. The L-horizon is built up by yearly falling litter (needles, twigs, etc.), has an average thickness of about 0.7cm, and a density of 1.O kg (dry weight)/m2. The following Of-horizon with an average thickness of about 1.5 cm and a density of 1.5 kg/m2 is characterized by the early fermentation of litter material. In this layer, partly decomposed needles are baked together but the initial needle structure is still observable. After that in the so-called Oh-horizon the litter material is completely humified and only small quantities of structured material can be found. The thickness of this layer is about 18 cm, the density 2.9 kg/m*. These three layers form the organic component of the forest floor. The first mineral horizon (Ah-horizon, thickness about 1.I cm, density 5.2 kg/m*) shows a still high organic content, but the nutrient content is already low. It is followed by the B-horizon (6.4cm thick, density 37.2kg/m2), a pure mineral layer with small content of organic matter.
Migration of 137Csand 134Cs in dlxferent,forest .soil layers
65
The mineral part of the forest soil is a cambisol soil type on calcareous moraine. Since 1987, up to 16 plots per year were randomly chosen within the forest at places where no understorey vegetation such as moss or vascular plants could possibly influence radiocaesium migration. Cores, divided in the layers mentioned above, were taken. It should be mentioned that for the first 3 years only few samples were taken. The single soil samples were air-dried (70°C) and milled. Aliquots of l&lOOOg were taken and measured with germanium detectors. Dependent on the geometry used for the measurements, calibration of the detectors was performed by using commercial calibration sources supplied by the Federal Institute for Physics and Metrology (Physikalisch Technische Bundesanstalt, PTB). In the case of 134Cs,pure 134Csstandards were used to account for sum peak effects. Intercomparison runs showed that for a lot of different geometries, and all detectors used for the soil measurements, the results for ‘34Cs and ‘37Cs were in agreement with the expected values within about 3%. Therefore, the errorbars of our ‘j’Cs and 134Csmeasurements of the soil samples are estimated to be of the order of several per cent, provided sufficient counting statistics were obtained. In the case of samples from mineral horizons, errorbars may be larger due to lower counting rates. The runtime of the measurements was chosen in a way that at least a 2-sigma-error of less than 5% could be achieved, even for the measurement of ‘34Cs performed in 1994.
RESULTS AND DISCUSSION Phenomenological discussion
Figures l-5 show the measured ‘34Cs deposition in [Bq/m2] from 1987 to 1994 for the different horizons defined above, normalized to I May 1986. This caesium is Chernobyl caesium only because very small amounts of ‘34Cs were produced during weapons testing, and this amount has already decayed due to a short half-life of 2.1 years. In general, variations of up to a factor of 10 occur within one horizon and year. This reflects the well-known heterogenous horizontal contamination pattern in natural ecosystems due to various mechanisms such as interception by the canopy stem flow, decomposition of fruitbodies of fungi, etc. Interestingly, looking at data from 1993 and 1994, the scattering seems to become smaller in case of the L-horizon (Fig. 1). Future observations are necessary to investigate whether this phenomenon is due to a horizontal displacement of an initially heterogenous contamination.
W. Rtlhm et al.
Fig. 1.
Measured
and fitted
134Cs activity in [Bq/m’] L-horizon.
(corrected
for 1 May 1986) in the
In the L-horizon, the absolute deposition is decreasing by a factor of about 6 from 1500 Bq/m* in 1987 to 300 Bq/m* in 1994 (Fig. 1). This is due to transition into the Of-horizon, where the deposition is decreasing by a factor of 2 from 2700 Bq/m* to 1300 Bq/m* in the same time period (Fig. 2). In case of the Oh-horizon (Fig. 3), increase of activity due to ‘34Cs from the Of-horizon and decrease of activity due to loss of ‘34Cs to the Ah-horizon seems to be similar and accordingly a roughly constant ‘34Cs contamination of about 2500 Bq/m* can be observed. In the mineral layers the situation is different. From Fig. 4 it can be seen that the contamination of the Ah-horizon is increasing from low values of 800Bq/m* in 1988 to about 2000 Bq/m* in 1994. The contamination of the B-horizon is increasing from about 1000 Bq/m2 in 1988 to about 2000 Bq/m* in 1994.
DISCUSSION
USING A COMPARTMENT
MODEL
The qualitative discussion from above can be put on a more quantitative basis by using the compartment model described as follows.
Migration
of i37Csand ‘34Cs in d$ferent ,fbrest soil layers
67
Of-Horizon
Fig. 2.
Measured
and fitted
lx4Cs activity in [Bq/m’] Of-horizon.
(corrected
for 1 May 1986) in the
Model description
In order to analyse the dynamic behaviour of radiocaesium in soil, all data were corrected for physical decay to 1 May 1986. Half-lives of 2.06 and 30.17 years were used for 134Cs and ’37Cs, respectively. The remaining time-dependent effect is expressed in terms of the ecological half-life (eqns (1) and (2)). 1
ntotal = 1 /~I/2.ecol
lphys =
+
(1)
&cd
1/T1/2.mll
-
1 / T1.2 phys
(2)
To describe the decay-corrected behaviour of 134Cs in soil, a compartment model was developed. This model consists of five compartments, representing the L-, Of-, Oh-, Ah- and B-horizons of the forest soil. Provided litterfall is neglected as a source of activity of the L-horizon, as well ai root uptake by plants as a loss of activity in all layers, to keep the model as simple as possible, the corresponding differential equations can be written as follows: dL(t)/dt
= -A1 L(t)
(3)
W. Rtihm et al.
68
dOf(t)/dt
= &L(t)
dOh(t)/dt
= A,Of(t)
dAh(t)dt dB(t)dt
= &Oh(t) = &Ah(t)
- &Of(t)
(4)
- AsOh
(5)
- &Ah(t)
(6) (7)
I,(t) is the activity [Bq/m2] of ‘s4Cs in the L-horizon as function of time, Of(t) is the activity of ‘s4Cs in the Of-horizon as function of time, etc. This system of coupled differential equations (eqns (3)-(7)) can be solved analytically (Bronstein & Semendjajew, 1981). The resulting solution functions, i.e. L(t), Of(t), Oh(t), Ah(t) and B(r), describe the develsoil horizons with time. For example, opment of ‘34Cs in different L(t) = Lo,134exp(-A,t) is the solution for the L-horizon. By means of a least square fit, this function is adjusted to the experimental data shown in Fig. 1 using L0,tJ4, i.e. the initial contamination of the L-horizon in 1986, and ,I,, i.e. a constant describing the decrease of ‘34Cs with time in this horizon, as tit parameters. Taking all horizons into account, four time
Migration
Fig. 4.
Measured
qf 13’Cs and ‘34Cs in different .forest soil layers
and fitted
‘34Cs activity in [Bq/m*] (corrected Ah-horizon.
69
for I May 1986) in the
constants Ai-& are used as tit parameters, from which for each horizon ecological half-lives can be calculated by formula (8).
i
Tilz., = ln2/& Additionally, the initial ‘j4Cs contaminations Lo,i34, Of0,134, etc. of each horizon at 1 May 1986 serve as fit parameters. All Iits were performed with the program package STATGRAPHICS (STSC, 1991). It should be mentioned that no direct information on the neglected litterfall is available for the forest investigated, but the part of radiocaesium circulating yearly due to litterfall is expected to be small. For example, Belli (1996) reports of yearly falling litter containing about 0.5% of the total radiocaesium inventory of an Italian forest site. Therefore, input of radiocaesium due to litterfall is not included explicitly in eqn (3). Nevertheless, it is implicitly inherent in the half-life deduced from the data of the L-horizon (see over). Including litterfall in eqn (3) would introduce an additional source for the L-horizon and result in a smaller half-life. In this sense, the effect of root uptake if there is any included inherently in the deduced half-lives, too.
W. Rh71 et al.
70
E-Horizon
Fig. 5.
Measured
and fitted
‘34Cs activity in [Bq/m*] (corrected B-horizon.
for 1 May 1986) in the
Discussion of results of the model
In Figs l-5 results of least squares fits of the experimental data on ‘34Cs activities in different horizons, using the solutions of eqns (3)-(7) are plotted. The qualitative behaviour of the experimental results is described very well by the fitted curves. Instead of discussing the fitted curves by looking at correlation coefficients (which are sometimes very small due to the wide scattering of the experimental data), a discussion of the resulting lit parameters (see Table 1) is used to rate the results of the fits. Caesium ,from Chernobyl
To describe the transport of caesium from the Chernobyl fallout, tits of ‘34Cs(t) for each horizon were used. From top of the forest floor, the resulting ecological half-lives are increasing with depth from 2.8 years in the L-horizon, 3.8 years in the Of-horizon, 4.4 years in the Oh-horizon and 7.7 years in the Ah-horizon. Activities in the B-horizon can be explained by assuming no loss into deeper layers (see also eqn (7)). Compared to these results Bunzl et al. (1989) report of 200-400day
Migration
of ‘37Cs and ‘34Cs in different,jkwest
soil layers
71
TABLE 1 Resulting Fit Parameters: Time Constants li with Error Bars Provided by STATGRAPHICS, Resulting Ecological Half-Lives Calculated with Eqn (8) and Amount of ‘34Cs Deposition on 1 May 1986 (Chernobyl Caesium Only) with Error Bars Provided by STATGRAPHICS
li [l/year] TI !2 bear1 Initial ‘34Cs deposition
L
Of
Oh
Ah
0.25 f 0.05 2.8 f 0.5 1900 I!=360
0.18 + 0.04 3.8 f 0.8 2800 & 650
0.16 f 0.04 4.4+ 1.2 1800 f 620
0.09 f 0.06 7.7 i 4.9 340 It 520
B
11ooi200
[Bqim’l
residence times inorganic layers of a spruce stand, measured within the first 600 days after the Chernobyl accident. On the other hand these authors mention that residence times derived from weapons fallout caesium are up to a factor 6 higher. Since we have only few data from the Iirst years after the Chernobyl accident, the derived ecological half-lives are mainly dominated by the long-term component of the residence time. In this context it should also be mentioned that the ecological half-lives derived are site-specific. The initial ‘s7Cs deposition in 1986 due to the Chernobyl accident is calculated by multiplying the extrapolated initial i34Cs contaminations (Table 1) by 1.75, which is the ‘37Cs/‘34Cs ratio characteristical for the Chenobyl fallout near Munich (Hotzl et al., 1987). This results in a total ‘37Cs deposition of 13.9 f 1.9 kBq/m2. This is in agreement with the German deposition map (Federal Health Office, 1991) which gives a ‘s7Cs contamination between 10 and 15 kBq/m* in the area of interest. Interestingly, the initial deposition was not only located in the L-horizon, but also in deeper horizons. According to the result of the Iits (Table 1), 24% of the initial deposition can be found in the L-horizon, 35% in the Of-horizon, 23% in the Oh-horizon, 4% in the Ah-horizon and 14% in the B-horizon, respectively. This can be explained by penetrating rain water transporting part of the initial fallout to deeper layers immediately after the deposition. Such a phenomenon could be demonstrated by laboratory experiments of Schimmack et al. (1994), who found up to 30% ‘37Cs in the mineral layers of undisturbed forest soil cores after artificial precipitation of up to 30 mm/h simulating a heavy rain shower. Vevjfication of the model by using fallout data
In order to deduce the distribution of weapons tests ‘37Cs from our data, a caesium ratio of 1.75 of the Chernobyl fallout in Bavaria in May 1986
W. Riihm et al.
12
(Hotzl et al., 1987) has to be used again. As an example, this approach for the L-horizon. L 137;fallout
--L
137.total,exp
-
eqn (9) elucidates
1.75&34,exp
By averaging the results from 1987 to 1994 for each horizon, an averaged depth profile for 137Cs from weapons fallout can be estimated (Table 2). A total 137Cs deposition of about 1.7 f 0.8 kBq/m2 is calculated. This is about one half of the expected value. According to Bunzl & Kracke (1988) who measured 137Cs deposition due to weapons fallout all over Bavaria and correlated it to the annual precipitation rate, a deposition of about 3.6 f 0.8 kBq/m2 is expected. For this estimation, a mean precipitation rate (averaged from 1951 to 1980) of 950mm per year is used for Hochstadt from data of the Germany Weather Service (Deutscher Wetterdienst DWD) gathered in Puch, located about 15 km from our sampling site (Mtiller-Westermeier, 1990). Taking into account a spread of a factor 2 in the data shown in Bunzl & Kracke (1988) and the errorbars, the two values are in agreement. Prediction
oj’the,future
behaviour
The measured 137Cs profile (weapons fallout) shown in Table 2 is valid for the time period 1987-1994, that is about 25 years after the maximum weapons fallout which occurred in 1963 and 1964. 137Cs deposition using the ecological half-lives derived above. One should bear in mind that the assumption of superficially deposited weapons fallout caesium might only be a crude estimation, since it does not account for caesium deposited before 1963. Both measured and calculated weapons fallout profiles agree well. This suggests that the used model works reliably for the chosen site, and the ecological half-lives derived seem to be reasonable. Therefore, a prediction of the future caesium behaviour in our forest, at least 25 years from the date of deposition, is possible. In this context it should be mentioned that from our data no difference in residence times of fallout and Chernobyl caesium can be deduced. Figure 6 shows the future development of the ‘34Cs activity based on Bq/m2, until 2036. It can be seen that half of the initially deposited caesium has left the horizons L to Ah and entered the B-horizon about 20 years after deposition, provided physical decay is not respected. In case of an increasing fixation of Chernobyl caesium with time, residence times will still increase in future and the result of 20 years represents a lower limit.
Migration
of i37Cs and “‘Cs
in different ,forest soil layers
TABLE 2 ‘j7Cs Activity Distribution (Weapons Fallout) Derived from Experimental (9) and corrected for 1 May 1986. Average of n Samples of Each Horizon 1994 Plus Corresponding Standard Deviation
n
“‘Cs [Bq/m’] Relative [%I Calculated [%]
Data (see Eqn From 1987 to
L
Of
Oh
Ah
B
49 -0.4 i 50 0 0
66 -16 i 570 0 3
38 300 i 200 18 12
46 510 f 290 31 33
44 850 i 500 51 52
Last row: calculated relative profile after 25 years assuming superficial horizon and using the ecological half-lives shown in Table 1.
/
Fig. 6.
73
I
Extrapolation
I
I,
*
7
I,
1,.
1,.
1,
of the fit results of lJ4Cs activity ([Bqim’] 1986) until the year 2036 for all horizons.
Totdi
depostion
I,,
/,
corrected
243 1660 + 840 100 100 on the L-
I,
for 1 May
In this context is should be emphasized that only transport processes such as decomposition, diffusion, leaching, etc. are described by the ecological half-lives discussed up to now. Physical decay is not taken into account since all data were corrected to 1 May 1986. Taking the physical decay of ‘34Cs into account as well, the loss from the system is much more effective. Due to the short half-life of 2.1 years, physical decay is the most important process when considering the loss of lT4Cs from the forest floor. As for ‘s7Cs, the situation is different due to the long physical half-life of
74
W. Rtihm et al.
30 years. In this case, physical decay and physical migration are of similar importance when looking at the decrease of the 137Cs contamination of forest floor.
CONCLUSIONS About 250 measurements of 134Cs and 137Cs in different layers of forest soil from a stand in Bavaria are presented. These measurements form a complete time series from 1987 to 1994. With the help of a compartment model where each compartment represents one soil horizon, ecological half-lives of 2.8, 3.8,4.4, and 7.7 years can be deduced for the L-, Of-, Ohand Ah-horizon, respectively. Taking measured 137Cs deposition in each horizon and correcting for 137Cs from Chernobyl, an averaged profile for 137Cs from weapons fallout could be deduced. This profile which developed within about 25 years from 1964 can be reproduced by extrapolating the Chernobyl contamination to about 25 years from the initial deposition in 1986. This shows that the derived ecological half-lives are reliable enough to predict future behaviour of radiocaesium in the soil of the forest investigated at least over 25 years. In the case of 134Cs, physical decay is the most dominant process when looking at the decrease of forest soil contamination. In the case of 137Cs, the ecological half-life describing the loss from the L-, Of-, Oh- and Ahhorizon is about 20 years and of the same order of magnitude as the physical half-life.
ACKNOWLEDGEMENT The present work was financially supported by the European Community under contract number FI3P-CT920050.
REFERENCES Belli (1996). Fluxes of Radiocaesium in forest environments. Paper in preparation. Bronstein, I. N. & Semendjajew, IS. A. (1981). Taschenbuch der Mathematik. Bunzl, K. & Kracke, W. (1988). Cumulative deposition of 137Cs, 238Pu, 239+240Pu and 241Am from global fallout in soils from forest, grassland and arable land in Bavaria (FRG). J. Environ. Radioactivity, 8, 1-14. Bun& K., Schimmack, W., Kreutzer, K. & Schierl, R. (19892. The migration of and of ’ 7Cs from weapons fallout 134Cs, 137Cs and io6Ru from Chernobyl testing in a forest soil. Z. Pjlanzenerntihr. Bodenk., 152, 34-39.
Migration of 137Csand “4Cs in different forest soil layers
75
Federal Health Office (1991). 137Cs Contamination Map of the Federal Republic of Germany. Institute of Water, Soil and Air Hygiene. Federal Office of Radiation Protection, Institute for Radiation Hygiene (1990). Report on radiation exposure in the year 1990. (In German.) Fielitz, U. (1992). Ausbreitung und Transfer von Radioctisium entlang des Pfades Boden-Pflanze Reh in zwei unterschiedlichen WaldcYkosystemen. Dissertation. (In German.) Hotzl, H., Rosner, G. & Winkler, R. (1987). Ground depositions and air concentrations of Chernobyl fallout radionuclides at Munich-Neuherberg. Radiochim. Acta, 41, 181-190. Kammerer, L., Hiersche, L & Wirth, E. (1994). Uptake of radiocaesium by different species of mushrooms. J. Environ. Radioactivity, 23, 135-150. Miiller-Westermeier, G. (1990). Klimadaten der Bundesrepublik Deutschland Zeitraum 1951-1980. Selbstverlag des Deutschen Wetterdienstes. Schimmack, W., Bunzl, K., Dietl, F. & Klotz, D. (1994). Infiltration of radionuclides with low mobility (13’Cs and 6oCo) into a forest soil. Effect of the irrigation intensity. J. Environ. Radioactivity, 24, 53-63. STSC, Inc. (1991). STATGRAPHICS Version 5.