Journal o f Radioanalytical and Nuclear Chemistry, Articles, VoL 133, No. 2 (1989) 391-395

RADIONUCLIDES IN SEDIMENTS OF THE DANUBE RIVER

N. AJDACIC, M. MARTIC

Boris Kidri~ Institute of Nuclear Sciences, Belgrade (Yugoslavia)

(Received March 29, 1989)

The trend of variation of sediment radiocontamination, expressed as a function of space and time, and specific accumulation of particular radionuclides in some investigated localities, are discussed on the basis of experimental results of gamma spectrometric analyses of artificial radionuclide contents.

Introduction

Besides air, river waters appear to be the most significant means of radionuclides transfer over great distances in the contaminated biosphere.

After the Chernobyl accident contamination level of the atmospheric layer was extremely high during May 19861 but already in August it was within the tolerance limits. 2 In the latter period only expected, so called seasonal variations were ob- served but they were slightly more prominent than before the accident.

However, due to permanent inflow of waters from the contaminated territory of their water basins, water courses maintain increased contamination level because of

the presence of radioactive materials, which is to a large extent due to selective ac- cumulation of numerous radionuclides in the sediment and biota. 3 Also., larger

rivers are recipients of waste waters, which, due to their specific origin, are the cause of radiation loading of the fiver surface water by radionuclides from technologically

changed natural radiation sources (phosphate fertilizers and phosphoric acid plants,

brick plants, thermal power stations), by radionuclides of artificial origin, from

nuclear objects, or by different radioisotopes application in agriculture, medicine, hydrology, research, etc. Namely, all these are permanent sources of biosphere and especially water contamination on ~ e whole river basin territory. 4,s

Before the Chernobyl accident, the Danube river contamination control was re-

duced to detection of particular radidnuclides along the fiver water course, while

their migration and accumulation in the constituents of the fiver ecological system (water, suspended material, sediments, biota) was investigated by studying radio- nuclide concentrations along the water course and by determining distribution co-

Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

N. A.IDA(~I~, M. MARTI(~ : RADIONUCLIDES IN SEDIMENTS

efficients and concentration factors. 6 However, in post-accidental conditions when water contamination is also high, distribution coefficients are not particularly re- liable indicators of increased radionuclide accumulation in the sediment. Therefore, the problem which arises with respect to environmental protection is that of proper differentiation between the consequences of accidental and potential later contamin- ation of the river system. It is also important to see how to locate contamination sources in space and time especially in our conditions when sampling is performed only periodically at most of the control points.

Starting from the fact that the river mud preserves "information" on earlier river contamination for a determined period of time (before its surface becomes covered by new layers) we assumed that precise analysis of gamma.spectrometrically obtained data could provide evidence on eventual regularity in radionuclides be- haviour in the Danube river aquatic system. This possibility has been proved by analyzing in such a way the data obtained for a great distance (over 300 km) along the river course and for a fearly long period of time (about 3 years). Conclusions presented in this work are based on the authors' data, obtained at 6 experimental profiles in a period from the spring of 1986 (Chemobyl accident) to the end of 1988. Among numerous quantitatively determined radionuclides, the behaviour of two long life Cs isotopes, 134Cs and 137Cs, were followed as the representatives of nuclides obtained by activation, and of fission products, respectively, as they had also been determined even before 1986. 7 In the period from 1979 until the end of 1985, 134Cs could only partially be determined, in the concentrations close to detectability limits, whereas ~37Cs activity levels were at that time within the limit of 0.3 -+ 0.2 to 17.8 + 0.6 Bq/kg of dry sample. After May 1986, activity levels of 134Cs and 137Cs, respectively, are visible in Fig. 1, for the observed part of the Danube river course, from 848 km to 1162 km.

Experimental

Sampling was performed by collecting sediment samples from about 10 cm thick surface layer and up to several kg in weight with a dredger specially constructed for this purpose. Thus the sediment samples obtained were sieved in situ in order to eliminate admixtures. In laboratory conditions sediment was dried at 105 ~ milled and sieved again. Activity of samples 0.5 kg in weight was measured in contact geo- metry by using a HPG semiconductor detector and ND-2400 gamma spectrometer.

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N. AJDA~IC, M. MARTIC: RADIONUCLIDES IN SEDIMENTS

Results and discussion

High degree of correlation has been found by comparing numerous data on 134 Cs and 137Cs activities for different periods of sampling, irrespective of the fact that examinations were performed on a very dynamic and huge natural system. Graphical presentation of the correlation obtained is given in Fig. 1. Slopes of the straight lines thus obtained are the numbers which determine the isotope ratio (K) of the two observed radionuclides. In the natural medium this ratio results from their physical desintegration (the dominant factor!) as well as from biogeochemical processes de- veloping under given hydrological conditions in the presence of these nuclides up to the moment of sampling.

I000 - /

01986 -0([- �9 1987

<" 6oo- / /

400 -

200 - ^ ~

n-dIIiNiF'-- I I I I I I I ~ I =~ "0 200 400 600 800 1000 1200 I/.00 1600 1800 2000

A,.cs ,Bq/kg Fig. 1. Correlation diagram of ~37Cs and * S4Cs activities in Danube river sediment sampled on

the locality from 838 km to 1162 km, in the period from May 1986 to the end of 1988

A 7 - �9 K

, I , I , I I0 20 30

At, month Fig. 2. Functional dependence of the factor K = A 137cs/A134Cs on the time of sediment

sampling

393

N. AJDA(~I~, M. MARTIC: RADIONUCLIDES IN SEDIMENTS

As most natural processes follow regularities which can be mathematically ex-

pressed as first order kinetic reactionsl by expressing K for 35 available samples as a function of sampling time, we have obtained the diagram presented in Fig. 2, and it appears to be exactly as it could be assumed. Surprising however is an almost in- credible regularity of the experimentally found dependence, similar to that which

might have been expected in a most carefully planned and performed laboratory

experiment! The equation:

K = 1.98 e 9"02 " 10-" t (1)

is obtained by mathematical presentation of the results with reliability value of R 2 = 0.99.

In Eq. (1) K represents A137cs/A134Cs ratio for each analyzed sample, on all loca- tions from 838-1162 km, in the period from June 1986 to the end of 1988, and t

is time, in days, from April 26,.1986 to the moment of sampling. It follows from this equation that, at the time of the Chernobyl accident, the

lSTCs/1S4Cs activity ratio was 1.98. In the observed experimental period (somewhat

less than 3 years) our K values were within the range of 2.01 to 4.75. The only K

value which did not fit in this range, K = 6.89, drew our attention to the possible

cause of deviation.

After critical observation of all experimentally obtained results and all relevant

observations on the sampling locations, we have found that the isotope ratio remains

unchanged in agreement with the natural processes characteristic for the observed

water course and in spite of evident temporary increase of both investigated radio-

nuclide activities in the sediment on particular locations. Therefore, independently of the lSTCs and 134Cs activity levels obtained, their ratio in the sediment appeared

to be an important factor for proper estimation of the river system contamination under post-accidental Conditions. Thus, for example, on the localities at 1076 km and 1145 km of the Danube river course in downflow direction from the mouth of

the two large tributaries, the Morava and the Sava, seasonal increase of absolute 137Cs and ~3aCs activity values was observed long after primary contamination, but

still the ratio of these activities followed the established regularity. This is quite understandable if we have in mind the fact that large tributaries bring contaminated water from the territory of their water-course. Therefore, in this case, the observed periodical increase of sediment contamination is no doubt the consequence of acci- dental contamination of biosphere in May, 1986. On the other side, prominent deviation of K value, obtained for the Sediment sample collected at 849 km in Oc- tober 1988, from Eq. (1), doubtless points to the presence of radioactive materials

of artificial origin as if contamination sources were located upstream from the sam-

394

N. AJDA(~IC, M. MARTIC: RADIONUCLIDES IN SEDIMENTS

piing place. This is also confirmed by other factors, primarily, and as most important,

by the presence of other radionuclides of fission origin, in the concentrations which

exceed radioactivity that possibly remained from 1986. Besides the possibility of discovering additional contamination sources of the water basin, factor K can also

be used to estimate the "age" of the fission mixtures as such sources. In our case,

according to the value of K = 6.89, the sediment sample collected at 849 km pre-

served the "information" on additional contamination of biosphere by the fission

mixture older than the one originating from the Chernobyl reactor. (According to

equation (1) the value of K ~ 7 should not be expected before the end of 1989!)

As indicated by the parameter K value, there is no doubt that the equilibrium estab-

lished in the Danube river by natural processes is disturbed by new emissions of radiopollutants.

Conclusion

In spite of the fact that radionuclide transport and accumulation in a river system result from numerous processes on the molecular level, in its final phase radionuclide

accumulation in the river water sediment appears to be a regular function of-time, irrespective of the complexity and spaciousness of the observed experimental me-

dium. Experimental results enabled mathematical formulation of the functional depend-

ence of laTCs and 134Cs activity ratios in the sediment sampled in a large area of

the Danube river watercourse during a long period of time. The possibility of using the established equation to discover the sources of addi-

tional contamination of the water basin by fission products and to evaluate the age of fission mixtures additionally introduced into the river has been discussed.

Radioactivity control of the Danube river should unquestionably be more fre-

quent and should represent a joint care of all Danubian countries in an effort to

avoid radiocontamination of its water. This is especially valid for the Danube course on Yugoslav territory, because of specific geomorphological characteristics of the

river banks and of the presence of the accumulations Djerdap I and Djerdap II.

References

1. N. AJDACIC, M. MARTIC J. Radioanal. Nucl. Chem., 128 (1988) 309. 2. N. AJDACIC, M. MARTIn, God izve~taj za 1986., Int publ., IBK-Vin~a, 1987. 3. M. MARTfC, N. AJDA~IC, J. Radioanal. Nucl. Chem., 120 (1988) 239. 4. N. AJDACIC et al., 5 tn Intern. Congress of the IRPA, Jerusalem, Israel, March 1980. Book of

papers, VoL3, p. 201. 5. N. AJDACIC, M. MARTIC, XII Jugoslovenski simpozijum o za~titi od zra~enja, Ohrid, 31.

maj-3, juni 1983. 6. M. MARTIC N. AJDACIC Godignii elaborati za 1983., 1984., 1985., Int. publ. IBK-Vinga. 7. M. MARTIC, N. A.AJDA(~IC, J. Radioanal. Nucl. Chem., 133 (1989) 325.

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