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                     Strontium-90 (Sr-90) in environmental samples, Germany  

Exposition
Sr-90 doesn’t naturally exist in the environment. The Sr-90 currently present  in the bio-sphere mainly comes from above ground nuclear weapons tests. In comparison the contamination emanating from the Chernobyl fallout was in comparison fairly low for most areas in Germany - the proportion of Cs-137:Sr-90 was about 100:1 ( in the nuclear weapons testing-fallout in contrast about 1.6:1)

In the metabolism strontium follows the essential bioelement calcium and is preferentially taken up by the mineral substance of bones. The biological half-life of  the nucleid in this tissue is ca. 50 years. The physical half-life is 28.1 years. Extensive investigations were conducted on the amount of Sr-90 in human tissue and in food sources over the  past three decades (ex. DEHOS 1985). In contrast the Sr-90 contamination of local wildlife species or their habitats is barely known. The goal of this investigation was to  find out if the concentrations  in the lower jaw of deer (Fig. 1) would reliably represent the course of contamination of their habitats.

   Fig. 1. lower jaw of a roe deer
 

Deer were chosen because, their feeding habits and their abundance in their habitats make them good bioindicators ( ex. HECHT 1994). Additionally the Sr-90 activity in various plants was to be identified. As bone tissue lower jaws were chosen, because specimens are still available from collections put together  before the reactor incident. Therefore, there was the possibiliy of investigating the Sr-90 contamination restrospectively.

The Sr-90 amount which accumulates in the bone mainly depends on its mineral content and the Sr-90 concentration in the ingested food. The mineral metabolism changes according to the age of the animals; during the juvenile stage after birth it is very high , in the following years it is lower. Correspondingly more or less Sr-90 is incorporated into the bones. Because different bone tissues have varying mineral metabolisms, the Sr-90 is unevenly spread in the skeleton (and even within an individual bone). The ingested amount of Sr-90 also varies according to the contamination of the habitat.

 

 

Sample origin and  preparation
The lower jaws of 23 deer which came from the hunting district of the state forestry department Bodenmais, Bavaria, between 1967 -1991 were analysed for their Sr-90 concentrations. The specimens were generously made available by the state forestry department of Bodenmais. An exact description of the research area is presented in FIELITZ (1994). The age identification of the jawbones was done with the help of the tooth filing method (the age is identified by counting the cement zones in the 1. molar).

Additionally the activity of Sr-90 was measured in 6 plant specimens. The samples came from the 100m x 100m  permanent sample area B1. The sampling procedure was the same as that for the research on radiocesium . The radio chemical preparation of the bone samples is briefly described here:

The lower jaw bones were cleaned from remaining tissue and fat, cut into ca. 1 cm pieces, dried, and then ground in a swing grinder. The samples were ashed at 400° C and broken down with the nitric acid method. By precipitation in numerable steps calcium, radium, barium, and finally the  ions with a valence of 3 were removed from the solution. Afterwards the Yttrium-90 was separated and the solution was left to settle a week. During this time Sr-90 recreated its daughter Y-90 by radioactive decay. At the beginning of the separation process natural strontium was added to the sample solution as isotope carrier to determine the loss in yield during the investigations. The measuring of the beta activity of Y-90 was done with a Low-Level-Beta counter. The minimum limit was at 0.01 Bq. After  measuring the Y-90 activity the Sr-90 activity of the sample was calculated by taking the rediscovery rate of the isotope carrier into consideration. For the calcium classification of the bones the ash was disolved in HCl, mixed with HNO3 and titrated with EDTA-solution.
 


Fig. 2: Preparation of bones for Strontium-90 classification


The preparation of the plant specimens was done according to the method given by the BMI (1983). The activity measurement of the samples was carried out in the Isotope laboratory for biological and medical research at the University of Göttingen.

 

Results and discussion
In table 1 the results of the Sr-90 and Y-90 measurements  of the plant samples are listed. The leaves of the single plants differ in their Sr-90 and also in their Y-90 concentrations. The highest activity of both nucleids was observed in Dryopteris carthusiana (thorn fern). From a comparison of the proportions of Cs-137 to Sr-90,  it is obvious that the  plant species take up each nucleid differently. Regarding the Cs-137 activity  blueberry (Vaccinium), for ex., has accumulated 3 times as much Sr-90 as the thorn fern (Dryopteris).
 

Tab.1: Sr-90 and Cs137 activities of different plant species from the sample area B1, Bodenmais, Bavaria.  Samples taken July 1993


Plant species (leaves)

Sr-90
[Bq/kg] DW

Cs-137
[Bq/kg] DW

Ratio
Cs-137:Sr-90


Dryopteris carthusiana


82.2


26420


321

Vaccinium myrtillus

78.8

  8730

111

Athyrium filix femina

68.7

11510

168

Rubus fruticosus

43.2

 5900

137

Prenanthes purpurea

29.0

 8190

282

Rubus idaeus

24.9

 5500

221

    

The variation in the Sr-90 and Cs-137 activities of the investigated plants probably is based on genetically differing acquisition capabilities for both nucleids, as is known for calcium and potassium.

In Table 2 the measured results of the bone samples are presented. The samples only differ slightly in their ash as well as in their calcium concentrations. The average concentration of the lower jaws was 0.362% per  gram ash, with a  standard deviation of 0.006%.   This value concurs well with the results of Farris et al. (1996), who determined an average of 37% / gram ash in investigations of white tailed deer from Colorado.

The Sr-90 activity of the lower jaws was, on average (+s) +537 +177Bq/kg. based on dry matter. The measured values ranged from 171 Bq/kg  to 904 Bq/kg. The median activity (+s) of Sr-90, based on gram calcium was 2.20+0.75Bq. The lowest value was 0.75 Bq, the highest 3.80 Bq. The coefficient of variation is 33%.
 

 Tab.2: Strontium-90 in lower jaws of deer from Bodenmais (n=20)

Year shot

Age

[Year]

Sr-90

[Bq/kg] DW

Sr-90

[Bq/g] Ash

g Ca per
g Ash

Bq Sr-90
per g Ca

 


1967


3


904 ± 20


1.34


0.352


3.80 ± 0.08

1968

4

880 ±  17

1.31

0.360

3.65 ± 0.07

1972

4

472 ± 15

0.70

0.350

2.01 ± 0.07

1981

3

470 ± 13

0.68

0.367

1.85 ± 0.05

1982

8

171 ± 15

0.27

0.359

0.75 ± 0.06

1983

3

500 ± 22

0.73

0.354

2.08 ± 0.09

1985

6

519 ± 20

0.75

0.358

2.09 ± 0.08

1989

0.5

646 ± 27

0.95

0.356

2.68 ± 0.11

1989

3

501 ± 23

0,72

0.358

2.00 ± 0.09

1989

4

728 ± 20

1.15

0.359

3.20 ± 0.09

1989

4

482 ± 14

0.70

0.361

1.93 ± 0.06

1989

6

413 ± 19

0.62

0.358

1.72 ± 0.08

 1990

0.5

351 ± 12

0.55

0.370

1.50 ± 0.05

1990

0.5

798 ± 23

1.26

0.359

3.52 ± 0.10

1990

2

538 ± 14

0.78

0.370

2.12 ± 0.06

1990

2

449 ± 18

0.69

0.371

1.86 ± 0.07

1990

5

497 ± 12

0.69

0.380

1.89 ± 0.05

1990

6

679 ± 18

0.99

0.364

2.72 ± 0.07

1991

0.5

321 ± 10

0.49

0.370

1.33 ± 0.04

1991

0.5

646 ± 16

0.69

0.367

1.89 ± 0.06

1991

3

368 ± 14

0.57

0.369

1.55 ± 0.06

1991

4

480 ± 17

0.77

0.368

2.08 ± 0.08

1991

5

501 ± 13

0,75

0,370

2,03 ± 0,05

 

 

 

 

 

 

 

In the following the results are compared with those of other investigations, whereby due to the unequal spread of Sr-90 in the skeleton, only the studies on the same type of bone are considered.

In the 1960’S LONGHURST et al. (1967) investigated the chronological course of Sr-90 in the lower jaws of 45 one year old black tailed deer (Odocoileus hemionus columbianus) from California. They determined an average activity (+s) of 2.07(+0.84) Bq Sr-90 / gram Ca. The in the present work determined values from 2 samples from the sixties are higher, with 3.80 respectively 3.65 Bq Sr-90 /gram Ca.

The average Sr-90 activity (±s) in the lower jaws of 4 deer killed in January 1987 in a forest close to Göttingen was 0.819±=0.111 Bq/g Ca. The animals were 0.7, 1.5, 1.5, and 7 years old FIELITZ (1987). In comparison  the bone samples from deer from Bodenmais, collected during the same investigation period, presented a significantly (p<0.01)  higher Sr-90 concentration. This is probably due to the differing  amounts of Sr-90  falling out with the precipitation: In Bodenmais it rains an average of 1.400 mm a year, which is about twice as much precipitation as in Göttingen (700 mm a year). Because the amount of Sr-90 deposition, from the nuclear weapons testing fallout, correlates positivly with the percipitation rate, the “contamination load” of the nucleids from the sixties in Bodenmais is approximately   twice as high as in Göttingen. Since sixteen times as much Sr-90 was deposited in a Bavarien research area through the Tschernobyl fallout, a higher contamination of the lower jaws is also to be expected. In figure 3 the activity in lower jaws from deer coresponding to the hunting year is shown.

Fig. 3: Sr-90 in lower jaws of deer from Bodenmais, 1967-1993


A statistical evaluation of the data was not conducted because of the age differences of the investigated bone samples. The highest Sr-90 concentration with 3.8Bq Sr-90 /g Ca or 3.65 Bq Sr-90 /g Ca occured in the samples, coming from the 60’s.  Both deer were born 1964, in the year where the Sr-90 deposition worldwide was greatest. Hence, the main growth phase of the bones coincided with a relatively high contamination over the food ingested. The animals killed during the years from 1972 to 1986 have lower Sr-90 activities in their lower jaws; during these years the Sr-90 deposition from the atmosphere was relativly low. The Sr-90 concentration in the plants at that time, must have mainly come through absorption over the roots and therefore was lower than in the early 60’s where Sr-90 was deposited directly on the surface of the plants. A further reduction of the activity was due to the radioactive decay of the nucleids.

After the Tschernobyl-fallout  higher amounts of Sr-90 were found in some samples, but on the whole the measured values were within the fluctuation range of the past 3 decades.

 

References:
BMI
(Bundesminister des Inneren); 1983: Leitstellen für die Überwachung der Umweltradioaktivität: Verfahren zur Bestimmung von Strontium-90 in Boden- bzw. Bewuchsproben.
Dehos R.; 1985: Bestimmung des Strontium-90-Gehalts in menschlichen Knochen, weichen Geweben und Ausscheidungen. Institut für Strahlenhygiene des Bundesgesundheitsamtes, Heft 59.
Farris G., Whicker F.W., Dahl A.H.; 1966: Effect of age on radioaktive and stable strontium accumulation in mule deer bone. Internat. Symp. on "Aspects of strontium metabolism". Chapelcross, Scotland: 93-102.
Fielitz U., 1994: Abschlußbericht über das Forschungsvorhaben "Radioaktivität in Wildtieren". Institut für Wildbiologie und Jagdkunde, Univ. Göttingen.
Fielitz U.; 1987: Cäsium-137 und Strontium-90 in Wildtieren, Boden und Vegetation nach Tschernobyl. Zwischenbericht an das Hessische Ministerium für Umwelt und Energie. Institut für Wildbiologie und Jagdkunde, Univ. Göttingen.
Hecht H.; 1993: Feststellung des Langzeitverhaltens von Schadstoffen im Biozyklus Boden - Pflanze - Wildtier. Umweltforschungsplan des Bundesministers für Umwelt, Naturschutz und Reaktorsicherheit. Forschungsbericht 116 08 052. Im Auftrag des Umweltbundesamtes.
Longhurst W..M., Goldmann M., Della Rosa R.J.; 1967: Comparison of the environmental and biological factors affecting the accumulation of Sr-90 and Cs-137 in deer and sheep. in: Aberg B., Hungate F.P.; 1967: Radiological concentration processes: 635-648.

 

Acknowledgements:
For the radio chemical preparation of the samples we want to thank Mr. Rainer Schultz (Isotope Laboratory for biological and medical research of the University of Göttingen).

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