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Cs-137 in forest soils

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                     Migration and dynamics of Cs-137 in forest soils

A long term research project on the behaviour of radiocesium in forest soils. In introduction to the complete project is given at page “abstract of research results”, papers are available at our Download site.

Background

Research results 2004

Research results 2001

 

 

Background

The spatial distribution of radiocesium in the soil depends in particular on its adsorption properties and the movement of soil water.The movement of water in the soil is determined by the amount of precipitation  as well as the number, size distribution and shape of pores in the soil.  The adsorption properties are determined by the proportion and composition of the clay mineral content of the soil, the organic substance, es well as the exchange capacity and pH-value of the soil.  These parameters vary according to site which fact make generalizations about the reaction of radioceesium in the soil impossible (COUGHRY and THORNE 1983, FRANKE 1982, KĂśHN 1982, PAVLOTSKAYA et al., 1967). 

The vertical rate of spreading of radiocesium in undisturbed soils is relatively low (with the exception of sandy soils and tropical laterites).  According to concurring results from several authors, at least 80% of Cs activity remains in the upper 15 cm of soil (ANPA 2000, KĂśHN 1982, RITCHIE and RUDOLPH 1970, SQUIRE and MIDDLETON 1966, WALTON 1963, ZIBOLD et al. 1997).

The cause of this low depth penetration is the fixation of cesium by the crystal lattices of clay minerals.  Clay minerals are leaflike structures composed of layered negatively loaded silicate platelets. In triple layered clay minerals cations are taken up between the layers to equalize ionic charges. According to TAMURA (1963) cesium and potassium ions fit esp. well into these interspaces due to their size and high polarity and so are fixated there.  Due to this specific binding, they are protected against leaching into deeper soil layers and are only limitedly available to plants (FREDRIKSSON et al 1958). 
 

 

In forest ecosystems decaying plant and tree material as well as to a lesser extent dead animal matter continually add to the organic layer of the soil.  This organic substance is decomposed by soil organisms into organic detritus and humus, and partially worked into the lower soil and mineralized.  The rate of this decompositon process mainly depends on the site conditions, i.e. the prevailing living conditons for soil organisms, along with the annual amount of litter produced and its composition.  Under favourable conditions ( high soil pH, good soil moisture and temperature conditions, good aeration) the organic substance released by litter decomposition are primarily worked into the upper  mineral soil so that noor very shallow humus layers arise. Under unfavourable site conditions (low soil Ph, poor drainage, low soil temperature, permanent dryness, nutrient poor soils, etc,) the rate of decompositon of organic matter is slowed down or greatly reduced which leads to the building up of thick humus layers (up to 30 cm).

Almost all forest herbs and most trees take up their nutrients from the uppermost10 cm of soil.  This corresponds to the layer in which Cs-137 is deposited over the long term and in which it is available to the plants.

In all investigations of the reaction of Cs-137 in the environment the problem of the extreme variations in the specific activity of the basic total quantity of this isotope arises. Due to small spatial differences in the distribution of precipitation Cs-137 from the Tschernobyl fallout of April 1986 was very unevenly deposited over the Federal Republic of Germany. This inhomogenous distribution is usually independent of the size of the observed system. For example, the variability of spatial contamination of soils in the Federal Republic of Germany is very distinct for each observed level:
 
                     National → regional → local site → micro site → microrange

 

 

 


Research results 2004

 

Vertical distribution of 137Cs in soil profiles of sample area B1

The soil depth of the 10 soil profiles, which were taken on the sample area B1 in 2004, was 30 cm. The box-plots in figure 15 show the dry bulk density as function of the soil depth. The dry bulk density in the profiles of area B1 (a 100x100 m permanent study plot, located in southern Bavaria) varies considerably as do other profiles, which were taken in the examination area. The Ah horizon changes to the Bv horizon in general between 12 and 14 cm. This change is indicated by the remarkable increase of the dry bulk density. 

Fig. 15: Dry bulk density of 10 soil profiles in the area B1, sampled in 2004

The vertical distribution of 137Cs in the soil is shown in figure 16 with series of single profiles and the corresponding mean value. The upper photo shows a drilling core of one of these series. The figure below and the photo above were scaled, so that the profile depths are matching.

The vertical distribution of 137Cs in the soil profiles varied considerably. The standard deviation of the measured values is approximately 50% of the mean value for each soil layer. The major part of the activity turned out not to be in the humus layer, as in the 1980s and 1990s, but in the approximately 8 cm thick zone between the lower part of the humus layer and the mineral soil. The activity within this zone is largely uniformly distributed; in each of the four layers of this zone are between 13.4% and 14.1% of the total activity and all four layers together contain 55.3% of the total activity. This means, that the migration of 137Cs into deeper soil layers is proceeding.

Fig. 16: Drilling core of a soil profile, sampled in the area B1, with the humus layer, the dark colored Ah horizon and the loam colored Bv horizon (upper picture). Vertical distribution of the 137Cs activity in 10 soil profiles on the sample area B1, July 2002. Mean value = black point symbol (lower picture).

 

The upper 2 cm of the soil contained with 1.4% nearly the same activity as the soil layer between 28 and 30 cm with 1.2%. The mean value of the 137Cs inventory is 72 828 Bq/m2, the minimum value 43 291 Bq/m2 and the maximum value 112 199 Bq/m2, which is an outlier value (profile 10). In the deepest soil layer of this profile 244 Bq 137Cs/kg were measured, which is a value one magnitude higher than in the other profiles.

The results show, that the nuclides have migrated deeper into the soil: with 31 323 Bq/m2 and 34 241 Bq/m2 the soil layers from 0 to 10 cm and from 10 to 20 cm contained nearly the same amount of activity. The activity in the zone from 20 to 30 cm was 7 264 Bq/m2.

 

 

 

Research results 2001

The area wide contamination of the soil with Cs-137 in the investigation area Bodenmais through atomic weapons testing and Tschernobyl fallout.  The areal soil contamination with Cs-137 in the study area Bodenmais was determined in 1989. For this purpose 59 grid samples were taken to a depth of 20 cm.  Within a radius of 50 m around each sample point 10 bore cylinders were extracted, each was separated according to humus or mineral soil layer and then each layer was combined to a mixed sample.  Fig. 1 presents the distribution of Cs-137 activity over the soil area as box plots.
 

Fig. 1 Area related Cs-137 activity of the soils in Bodenmais (n=59) on the May 1, 1986

The median of the measured values was 51,800 Bq/sq.m,  the average Cs-137 soil contamination was 53,690 + 26,690 Bq/sq.m (x +SD)  ranged, however, between 12,090 Bq/sq.m to 146, 480 Bq/sq,m. With these levels of Cs-137 in the soil the study area Bodenmais is one of the most highly contaminated areas in the Federal Republic of Germany.

The greatest proportionn of Cs-137 activity can be traced back to Tschernobyl fallout: in 1986 an average of 45,160 Bq/sq.m were deposited on the soil. A minimum of 6,740 Bq/sq.m and a maximum of 145,130 Bq/sq-m   were determined.  The median value was 8.080 Bq/sq.m. The average Cs-137 activity still present in the soil on May 1, 1986 due to global fallout amounted to 8,530 + 4,660 Bq/sq.m, the median was 8080 Bq/sq.m. The range of measured values was also relatively large here, between 1,330 Bq/sq.m and 23,040 Bq/sq.m.  The variation coefficient for Cs-137 for the pre-Tschernobyl fallout was 55%, for the Tschernobyl fallout it was 57%.

Cs-137 contamination of permanent sample plots
Due to the high variability of Cs-137 in the respective biomedia, the investigation of the spatial distribution and the long term behaviour of this nucleid in the soil, in plants, trees. and mushrooms was exclusively limited to the 100 x 100 m permanent sample plots established in 1987.  The sample plots were designated as follows:  B1 and B2 (Bodenmais), F1 (Fuhrberg), and G1 (Göttingen).  In correspondence with the north-south decline of Cs-137 contamination in the Federal Republic of Germany the area wide radioactivity of the soil was clearly higher for the sample plots B1 (98,450 Bq/sq.m) and B2 (83,370 Bq/sq.m) than for F1 (10,320 Bq/sq.m) and G1 (5,450 Bq/sq.m). 
 

 

Cs-137 distribution according to depth in soil profiles
Since 1989 soil profiles have been dug and evaluated on the sample plots B1 and F1 in intervals of several years and the vertical distribution of Cs-137 determined. The depth of the humus layer varies on the plots B1 and F1 between 2 and 8 cm with an average depth of 4 cm.  The Ol, Of. and Oh horizons are clearly distinguished. The sample plot G1 only shows an Ol horizon.

The total Cs-137 activity in the profiles, i.e. total level of Cs-137 per profile ranged from 5,450 Bq/sq.m  for G1 to 73,200 Bq/sq.m for B1. 

In the soil profiles of the 3 investigation areas an average of 56% of Cs-137 actlivity was found in the upper 10 cm in 2000/2001 and an average of 93& in the upper 20 cm.  Fig. 2 provides an example of the volume related depth distribution of Cs-137 based on the total activity of a soil profile on  sample plot B1. It can clearly be seen that the main accumulation of Cs-137 is in the upper 10 cm of soil. This is exactly the layer from which forest plants and many trees extract their nutrients.  Since cesium is much more available to plants in the humus layer than in the mineral layer, the transfer of this nucleid to plants on forest sites is relatively high over the long term. As many wild animals feed on these forest plants, they consequently also contain higher levels of radiocesium in Tschernobyl fallout contaminated areas.
 

Fig. 2:  Depth distribution of volume related Cs-137 activity in soil profile V on the permanent sample plot B1 in the investigation area Bodenmais,  May 2001


In order to investigate the dynamics of the vertical migration of Cs-137 in the soil profile the distribution over depth of this nucleid was determined in intervals of several years.  Fig. 3 shows the distribution of Cs-137 in the soil profiles on sample plot B1 from 1989 to 2001. Since the areal radioactivity of the Cs-137 on this plot varied over several thousand Bq/sq.m, the respective percentage of this isotope per 2 cm layer and sq.m is presented. The scale on the abscissa is logarithmic.

Fig. 3:  Temporal course of the depth distribution of Cs-137 in soil profiles of the sample plot B1 from 1989 – 2001


Each symbol represents the average percentual distribution of this nucleid at the corresponding depth level of 3 profile series.  The amount of work involved in cutting soil profiles is enormous; for the results presented for sample plot B1 a total of 1,310 individual 2 cm soil layers were cut. During the investigation period the Cs-137 activity of each profile first increased for the first 3 layers, reached a maximum activity level at 4 – 6 cm and then decreased continually throughout the lower layers. The soil depth 0 to 6 cm corresponds roughly to the humus layer which ranges from 2 o 6 cm.

The distribution of Cs-137 in the humus layer was completely opposite to that of the mineral soil.  Here the activity increased with depth (Ol  Of  Oh horizons)  and then decreased with increasing depth of the mineral soil. 

The migration occurred through the successive displacement of Cs-137 ions over time from each 2 cm layer to the one below.  In the humus layer the loss of activity per soil layer was almost always greater than the increase from the preceding layer.  In contrast in the successive layers of the mineral soil the increase of Cs-137 activity is respectively higher than the transfer. 

The turning point in this migration process is in the soil layer between 6 – 8 cm in the sample plot B1. On average this is the upper soil region of the Ah horizon, the layer under the humus deposit. The areal activity in this layer scarcely changes. On B1 16 – 17% of the radioactivity of the whole profile is concentrated here over the long term.The constancy of these values, however, do not mean that there is no migration of Cs-137 ions in this layer; in contrast the dynamics of movement are greatest here comparable to the narrowest part of an hourglass.



References:
ANPA
2000: SEMINAT. Long-term dynamics of radionuclides in semi-natural environ-ments: derivation of parameters and modelling. Final report 1996-1999, European Commission-Nuclear Fission Safety Programme.
Cougthry P. J. , Thorne M. C., 1983: Radionuclide distribution and transport in terrestrial and aquatic ecosystems. Vol.I.: Caesium. Balkema, Rotterdam: 321-424.
Cremers A., Elsen A., De Preter P., Maes A., 1988: Quantitaive analysis of radiocesium retention in soil. Nature 335: 247-249.
Franke B., 1982: Transfer radioaktiver Stoffe aus dem Boden in Pflanzen. In: Arbeitsgemeinschaft für Umweltfragen (Hrsg.): Das Umweltgespräch. Tagungsprotokoll: Radioökologiesymposium vom 15/16 Okt. 1981, Univ. Stuttgart: 152-183.
Fredriksson L., Eriksson B., Rasmuson B., Gahne Bo., Edvarson D., Löw K., 1958: Studies on soil-plant-animal interrelationsships with respect to fission products. Part A: Plant uptake of Sr-90 and Cs-137 from soils. Geneva Conference Paper P/177: 449-470.
Kühn W., 1982: Ausbreitung radioaktiver Stoffe im Boden. In: Arbeitsgemeinschaft für Umweltfragen (Hrsg.): Das Umweltgespräch. Tagungsprotokoll: Radioökologie-symposium vom 15/16 Okt. 1981, Univ. Stuttgart: 76-99.
Pavlotskaya, F.I., et al. 1967: On the mobility of strontium and some other components of global fallout in soils and their accumulation in plants. in: Aberg B.; Hungate, F.P. (eds.): Radiological concentration processes. (Proceedings of an international symposium held in Stockholm, April 25-29, 1966). Pergamon Press, Oxford::  25-32
Ritchie J.C., Rudolph W.K., 1970: Distribution of fallout and natural gamma radionuclides in litter, humus and surface mineral soil layers under natural vegetation in the Great Smoky Mountains, North Carolinia-Tennessee. Health Phys. 18: 479-489.
Scheffer F., Schachtschnabel P., 1984: Lehrbuch der Bodenkunde. F. Enke Verlag. Stuttgart.
Squire H.M., Middleton L.J., 1966: Behavior of Cs-137 in soils and pastures. A long term experiment. Radiation Botany 6: 413-423.
Tamura T., 1963: Cesium sorption reactions as indicator of clay mineral structures. International clay conference. Proceedings of the conference held at Stockholm, Sweden, August 12-16.
Walton A., 1963: The distribution of radioactivity in soils from weapon tests. J. Geophys. Res. 68 (5):1485-1496.
Zibold G., Drissner J., Klemt E., Konopleva A.V., Konoplev A.V., Miller R.,: 1997: Biologische Verfügbarkeit von Cäsium-Radionukliden in Waldgebieten des nördlichen und südlichen Voralpenlandes. In: Honikel K.O., Hecht H. (Hersg.): Radiocäsium in Wald und Wild. 2. Veranstaltung, Kulmbach 10/11.06.1997.


This research was conducted  with funds of the Federal Ministery for Environment. Nature Protection, and Reactor Safety
This report reflects the views and opinions of the contractor and need not necessarily correspond to those of the sponsor.

 

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