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Cs-137 in forest soils

Cs-137 in plants

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               Cs-137 in Plants   

Results
The problem of the extreme fluctuations in the total amount of Cs-137 activity had to be taken into consideration for all investigations on the behaviour of this nucleid in plants and mushrooms growing in the forest.
The investigations on the behaviour of Cs-137 in plants and mushrooms were exclusively conducted on 100 x 100 m permanent sample plots (B1, B2, F1) in the study areas (see also abstract of the research results of 1988 – 2000). The results of the investigations from 1998 to 2000 showed that the average Cs-137 content of the leaves of the individual plant species on each sample plot varied whereby the sequence of contamination of B1, B2, and F1 is the same or at least similar.  Basically the leaves of ferns were on average most contaminated while those of raspberries and blackberries were least contaminated.  Fig. 1 presents the Cs-137 activity of the various plant species on sample plot B2.


Fig.1  Cs-137 activity in the leaves of various plant species on B2 (Bodenmais, Bavaria)
 n = 9 each, 1998–2000.  Average, minimum, and maximum values. Logarithmic presentation


The ferns typically belong to the most highly contaminated native plant species. For B2 the measured values for narrow buckler-fern (Dryopteris carthusiana) ranged from 3,950 Bq/kg and 12,690 Bq/kg with an average value of 7,560 Bq/kg. In contrast the leaves of raspberry (Rubus idaeus) averaged only 473 Bq/kg,  blackberry (Rubus fruticosus) 403 Bq/kg, with maximum values of respectively 1,290 Bq/kg and 1,045 Bq/kg.  Sedge (Carex brizoides) and lady fern (Athyrium filix femina) were similarly highly contaminated with average values of 2,240 Bq/kg and 2,340 Bq/kg. The Cs-137 contents of many of the plant species fluctuated seasonally during the investigation period.  The greatest differences in activity during the vegetation period were shown by  the fern  Pteridium aquilinum on F1 in which the Cs-137 content increased from spring to fall every year.  In June of 1999 and 2000 1,000 Bq/kg were recorded and in October of the same years an average of  2,500 Bq/kg.  Narrow buckler-fern on B2 and F1 and lady fern on B2 also showed an increase in Cs-137 content over the vegetation period though to a lesser extent. A decrease in Cs-137 activity over the year were demonstrated by the leaves of blackberry on B2 and F1. of raspberry on all sample plots and of the grass Molinia caerulea on F1.

 

 

In Fig. 2 the temporal distribution of Cs-137 in the leaves based on dry weigth (DW) content are shown for narrow buckler-fern, Great Wood-rush (Luzula sylvatica) , and blackberry on B1 and B2 for the time period 1989 to 2003. Each point represents the measured value of a mixed sample of 20 to 40 individual plants of aech species.  Data were recorded for all the years between 1989 and 2003 except for the year 1996 for the sample plots B1 and B2.  

 

Fig. 2:  Temporal distribution of Cs-137 in the leaves of  Dryopteris carthusiana, Luzula sylvatica, and Rubus fruticosa on the permanent sample plots B1. B2, and F1, 1989 – 2000

For all investigated plant species Cs-137 activity clearly decreased between 1987 and 2000 (only Pteridium aquilinum did not show such a distinct reduction). The average Cs-137 content in 1989 of thorn fern leaves on B1 was 56,600 Bq/kg, in 200 it was only 10.210 Bq/kg.  On F1 the recorded activity decreased on average from 4,500 Bq/kg to 1,010 Bq/kg, on B2 from 3,200 Bq/kg to 610 Bq/kg, and on F1 from 940 Bq/kg to 150 Bq/kg.

From the radio-ecological point of view it is of particular interest fo find out at what point in time the Cs-137 contamination of products from a defined forest ecosystem is reduced by half.  This time period is designated the effective half-life (Teff) .  This effective half-life takes all factors into consideration that cause a decrease in contamination such as radioactive decay, changes in contamination due to mass reproduction of the plants, changes in the biological availability of this nucleid in the soil, etc.,  According to the principle of radioactive decay the reduction of specific activity is calculated as follows:

                                                 C/C0 = exp (-λ eff * t)
 
Whereby the elimination constant eff  is the product of the physical decay constant, the biological elimination constant, and the elimination constants for population explosions. 

In this report the effective half-life was calculated by drawing a linear regression through the actual logarithmetized Cs-137 data and by deducing the slope coefficient (eff ) of the regression line. In Table 1 the effective halflives of Cs-137 for various plant species on the 3 permanent sample plots are presented. .
 

Tab.1  Effective half-lifes (Teff) of forest plants on the sample plots B1, B2, and F1 1989 – 2000

Spezies

 

Teff [Jahre]

 

 

Teff [Jahre]

 

B1

B2

F1

 

Mittelwert

Pteridium aquilinum

 

 

23,8

 

23,8

Dryopteris carthusiana

4.7

6.2

5.2

 

5.4

Rubus fruticosus

 

4.8

4.6

 

4.7

Vaccinium myrtillus

7.9

7.2

6.0

 

7.0

Rubus idaeus

3.1

2.9

4.7

 

3.6

Molinia coreulea

 

 

4.8

 

4.8

Carex brizoides

 

5.3

 

 

5.3

Athyrium filix-femina

8.4

7.3

5.9

 

7.2

 Luzula sylvatica

 

4.0

 

 

4.0

 

Pterydium aquilinum has the longest effective halflife of Cs-137, 23.8 years.  For the other plants the values range between 2.9 and 8.4 years.
The half-lifes of the individual plant species correspond relatively well for the 3 sample plots. Raspberry always has the shortest effective halflive with an average value of 3.6 years. The leaves of Athyrium filix femina and Vaccinium myrtillus always have longer halflives, on average 7.2 and 7.0 years respectively.
The effective half-lifes were calculated for the time period 1989 to 2000. However, Cs-137 activity did not decrease uniformly in the plants. For all species investigated except P. aquilinum the rate of activity reduction was relatively quick from 1989 to about 1994. During the following phase from 1005 to 2000 some species showed a distinct slowdown in reduction (for ex.blackberry and raspberry on F1, D.carthusiana on B1 and B2), no change (for ex. V. myrtillus on B1 and B2 and L. sylvatica on B2), or an increase of Cs-137 activity (for ex. P. aquilinum on F1 and raspberry on B1).  Due to the brevity of the investigation period and the small changes in Cs-137 activity these trends are statistically not significant.
 

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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