|
Environmental Studies |
|
|
|
 |
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Introduction In the area of Crossen these slagheaps bordered directly on agriculturally used land. This posed the question of how much radioactivity is released from these slagheaps into the surrounding envirornment. In addition to resuspension and emanation of radionucleids the path from soil to plant accounts for one of the main ways by which these isotopes can enter the environment.
The goal of this investigation was to gain an idea of the extent of the distribution of Pb-210
and Ra-226 in the soil, in vegetation, and in animals. For this purpose the specific activities of these nucleids in the soil, plants, and trees on slagheap sites as well as on adjacent fields
were determined. In order to find out whether Pb-210 and Ra-226 are enriched in long living plant tissues, needles and branches of 3 spruce trees, separated according to age, were
gamma-spectrometrically examined. The accumulation of the nucleids in animal organisms was investigated via organ samples taken from 3 roe deer.
Fig. 1 House at the “uranium slagheap” in Crossen
Transfer factors were calculated by dividing the activity per kg of plant dry matter by the activity per kg of soil (dry matter). Results In samples of wheat (n = 5) and rye (n = 5) taken from the same sites as the soil samples the Pb-210 and Ra-226 activities were below the minimum discernible limits. Soils from slagheaps had an average specific activity of 1,000 + 620 Bq/kg Pb-210 and 2,200 + 1,900 Bq/kg Ra-226 (n = 20). The range of the measured values for Pb-210 spanned 1,960 Bq/kg from the lowest measured specific activity of 140 Bq/kg to the highest of 2,100 Bq/kg both taken from a slagheap near Johanngeorgenstadt. The minimum (160 Bq/kg and maximum (7,200 Bq/kg) values for Ra-226 were also determined at that site. The measurements of various plant species on slagheap sites are summarized in Table 1. Due to the unfavourable site conditions only few species grew as a rule on these sites and of these there were only a few individuals. Tab. 1 Activities of Pb-210 and Ra-226 as well as transfer factors (TF) soil to plant for vegetation (leafs) on slagheap sites in the region of the Erzgebirge (Iron Mountains). Sampling date August 1991
The highest Pb–210 and Ra-226 activities in plant organs were found in grass (370 Bq/kg) and
birch bark (770Bq/kg) growing on the most highly contaminated soils in Johanngeorgenstadt. For all plant tissues an average transfer factor of 0.076 for Pb-210 and 0.089 for Ra-226 was
determined. If the transfer factors are related to the fresh weight of the plant organs, the calculated results are reduced by 30%.
On slagheaps and directly adjacent sites in Johanngeorgenstadt trees were felled and the wood and bark were investigated via gammaspectrometrical analysis for relevant
radionucleids. Fig. 3 shows one of the sampled slagheap areas.
The results are summarized in Table 2. In the bark and wood of larch, pine, and spruce that
came from the vicinity of a slag heap the average Ra-226 activity was lower than the minimum discernible limit (DL), the average Pb-210 activity ranged between 5 and 26 Bq/kg. Clearly
higher activities were shown by the wood and bark of birch and alder that were growing on the slagheap. Here it was obvious that for Ra-226 the transfer factor for bark was about one
magnitude greater than that for wood In contrast for Pb-210 the transfer factor for wood was about twice as high as that for bark. Tab. 2: Activities of Pb-210 and Ra-226 as well as transfer factors (TF) for trees from slagheap sites in the Iron Mountains region , Sampling date 1992
In figure 4 the activities of Pb-210 and Ra-226 are presented for 5 different specimens of birch.
Fig. 4: Activities of Pb-210 and Ra-226 in 5 birches, slagheap site in Johanngeorgenstadt, Sampling date: 1992
The long term behaviour of Pb-210 and Ra-226 in trees was investigated in various needle and branch samples according to age groups from 3 spruce trees. (Sampling date: October, 1992). Figure 5 presents the results from one spruce tree. The spruce trees grew on differently contaminated sites in Johanngeorgenstadt. The Pb-210 and Ra-226 concentrations in the soil around the root zone of the trees (0 – 20 cm) were as follows: Tree I 56Bq/kg and 160Bq/kg, tree II 2,200 Bq/kg and 5,300 Bq/kg, tree III 68 Bq/Kg and 130 Bq/kg. The branches of the 3 spruce differed distinctly in their Pb contents, whereby no correlation to the amount of this nucleid in the soil was evident. There was a tendency for the activities to increase from old to middle aged whorls and then to decrease again in the youngest whorls. The Pb-210 activity in needles did not differ significantly among the trees. Older needles had higher concentrations than younger ones. Presumably this is due to the longer time in which these needles were exposed . This result was also obtained by [2] in studies on the element content of spruce in Solling: with increasing needle age the lead contents increased.
Fig. 5: Activities of Pb-210 and Ra-226 in various branches (above) and needle age groups (below) of a spruce (I), slag heap site in Johanngeorgenstadt, Sampling date : 1992 It can be concluded from the study results that the Pb-210 contamination of the needles and branches is mainly caused by deposition from the air, while the transfer of activity from the soil seems to play a subordinate role. It is apparent that the branches and needles of a tree contain approximately the same amounts of Ra-226. The branches of spruce I contained an average of 79 Bq/kg, the needles 74 Bq/kg; spruce II respectively 630 Bq/kg and 710 Bq/kg, and Spruce III 36 Bq/kg and 26 Bq/kg. With increasing Ra-226 concentrations in the soil of the root zones of the 3 spruce trees the concentrations in the branches and needles also increased. In contrast to Pb-210 the Ra-226 activity in the needles and branches is mainly caused by uptake through the roots. For branches as well as for needles the lowest activities were observed in the respectively youngest age groups.
The investigations of the organs of 3 roe deer did not show any discernible activity for Pb-210 and Ra-226 in the sampled muscles, kidneys and livers. For roe deer I 12 Bq/kg activity for Pb-210 and 66 Bq/kg activity for Ra-226 were found in the carpal bone. For roe deer II the Pb-210 activity was below the minimum discernible limit and the Ra-226 activity was 27 Bq/kg. In a mixed sample of roe deer feces collected on the slagheap the activity for Pb-210 amounted to 100 Bq/kg and for Ra-226 to 99 Bq/kg.
References
Top of Transfer of lead-210 and radium-226 in environmental samples page
|
