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MODELLING THE TRANSPORT OF RADIONUCLIDES THROUGH THE FRESHWATER ENVIRONMENT

Objective

Although models of radionuclide transport through the aquatic environment are becoming more sophisticated, they still remain site specific in their application. Models developed for a specific river or lake system require considerable reworking and calibration in order to be made applicable to other sites. Major causes for this lack of portability are the variability of the adsorption coefficients (K ) and concentration factors (CF) used as lumped parameters in all models. Both parameters can vary by 3 to 4 orders of magnitude between sites. In a similar way the dominance of different processes in both the catchment and the lake or river varies from site to site in an, as yet, unpredictable way.

By studying the transport of several radionuclides, but mainly radiocaesium, in aquatic systems we hope to increase our understanding of the general properties of K , CF and process dominance and develop more portable models. The groups will focus on: the chemical processes underlying adsorption onto particles; the effects of water chemistry and the ecology of aquatic organisms on the uptake, storage and excretion of radionuclides; the relative importance of some newly identified pathways of radionuclides from catchment to water to sediments; processes of remobilization in sediments and at the interface between seawater and freshwater.
A single catchment area, chosen from a group identified as having high levels of secondary input of radionuclides, has been studied. The change in caesium activity with time was obtained for the lake and six contributory streams. Two streams had much higher activity levels than the others and significant relationships with flow and time form the initial Chernobyl fallout. The two emanated from saturated peat catchments; the others from either ranker or thin podsolic soils. Five mechanisms of caesium release have been suggested. Firstly, that the lack of illite in the catchment area prevents long term storing of caesium. Secondly, that flooding of peat soils allows the caesium to be washed out easily. Thirdly, that humic compounds in the peat soils chelate the caesium and hold it in solution. Fourthly, that low concentrations of ammonia in rainfall displace caesium from frayed edge sites on the small quantity of illite particles in the catchment area. Lastly, that low molecular weight humic acids, formed by oxidation of humic material, deform the frayed edge sites on the small quantities of illite particles, thus releasing the caesium.

Data on fish size and caesium activity have been obtained from several lakes in the United Kingdom. The data show considerable scatter and it is not possible to observe any trend after Chernobyl. A power relationship exists between activity per unit mass in the flesh and fish size (weight or length).

A study concerning the radiocaesium sorption behaviour in the freshwater sediments of the Tejo River was carried out. The characterization of the sediments included determination of: cation exchange capacity and exchangeable ions; number of specific sites; the selectivity pattern of caesium toward potanium, ammonium, calcium and magnesium; the site heterogeneity.

For the adsorption experiments 2 granulometries were used, and several conditions representative of water composition in the Tejo River. The experimental Kd values of radiocaesium agreed quite well with prediction. Radiocaesium sorption was governed by micaceous minerals and the potanium and ammonium concentrations in the liquid phase.

Cobalt-60 transfer in a simplified trophic chain was also studied. The chain consisted of a planktonic microalgae, Selen astrum capricornutum Printz (Chlorophyceae), a small filter crustacean, Daphnia Magna Strauss (Cladocera) and 2 fish, Tinca tinca hinnaeus (omnivorous) and Chondrostoma polylepis polylepis Steindaeker (omnivorous, but feeding mainly on macroalgae and microalgae). The water usedwas from the Fratel dam in the Tejo River; physicochemical characteristics of the water were known.

Transfer of cobalt-60 to D magma from water was carried out over 3 weeks. Equilibrium was reached at about 11 days for an initial radioactivity of 39 Becquerels per millilitre of water. The evaluated concentration factor (CF), referred to net weight, was 53 +/- 6. For Tinca tinca of average weight 1.7 +/- 0.3g equilibrium was reached after 43 days and the CF was 4.2 +/- 0.5. The retention experiment showed that data fitted an exponential model with 2 rate functions. For Chondrostoma polylepis polylepis of average weight 3.1 +/- 0.3 g with an initial water radioactivity of 25 Becquerels per millilitre, the CF was 3.6 +/- 04. and the retention experiment data fitted an exponential model with one rate function.

Cationic forms of colbat-60 were shown to be the most prevalent in water.

The short term and long term effects of radiocaesium behaviour in sediments were studied. Short term effects relate to the quantitative prediction of radiocaesium sorption and its variability in a variety of sedimentary materials. The long term effects relate to the potential desorption of radiocaesium and its remobilization in freshwater sediments or at the freshwater to marine interface. Particular emphasis was given to ageing processes and the kinetic aspects of the sorption and desorption process of radiocaesium in sediments.

A kinetic 3 box model was developed for describing the solid to liquid sorption and desorption dynamics in sediments. Tests showed that the main drawback of the approach was the need to obtain the rate of the immobilization process in the lattice indirectly, on the basis on radiocaesium measurements in the liquid phase. In response to this, a method was developed to quantify the time dependence of the radiocaesium immobilization process.

The effect on the Rhone river of liquid effluents from nuclear installation was compared with fallout from the Chernobyl accident. Aquatic vegetation proved to be a good indicator of radioactivity; the bryophytes in particular responded quickly to fluctuation of radioactivity in the water. Levels of radioactivity in fish, although lower in concentration, still reflected the composition of the power station effluent. Chernobyl fallout was deposited either during rain or by the washing out of aerosols, and had a significant effect on all trophic compartments for a period of 2 years.

An experimental study of the transfer mechanisms of silver-110m in a simplified freshwater trophic chain was also carried out. It was concluded that sediments played a major role in the distribution of silver-110m in the aquatic environment, acting as a persistent source of pollution.

Caesium-137 content has been determined at the molecular level for eel, with reference to its economics value as food. Subcellular fractionation was done on eel liver and muscle. The main radioactivity was found in the soluble fraction in the liver (87%) and in the muscle (79%). There was a very limited trend for radiocaesium to be fixed by the cytosolic proteins.

Suitable points for sampling water and suspended particles in the Stella river were identified on the basis of its hydrodynamic and chemicophysical characteristics. The area affected by the saline wedge was identified. A molecular selective water samples developed to examine the radionuclide distribution between water and suspended particles was adapted for use in brackish water.

It was shown that the radionuclide distribution in water and in suspended particles could be characterized as a function of the salinity value in the river.

A mathematical model has been developed for the physical and chemical speciation of radionuclides in saltwater and freshwater environment. The model calculates the aqueous specification of the major and minor elements among both inorganic and organic ligands, accounts for adsorption/desorption and dissolution/precipitation processes, and takes into consideration chemical reaction kinetics. The most important sources of error in the model are the inaccuracies in the values of the constants that describe the reactions (including protonation constants and complex stability constants). The model has proved to be of great value in the design and analysis of experiments concerning the availability and uptake of metals in defined saltwater and freshwater systems.

A mathematical model has been developed for the biological availability of radionuclides in saltwater and freshwater environments. The firststep involves the fixation of the element on the cellular surface of the solution to body interface. The second step involves the translocation of metals across the solution to body interface into the cells. The third step involves the transfer of the radionuclide to other parts of the body, including the eneretory system.

Experimental work has been carried out on the biological availability of radionuclides to determine the quantitative role of the 3 steps. The first 2 steps were shown to be fast processes. The form of the metal in the environment and the hydrogen concentration in the solution were the 2 most important factors determining availability of the metals in an aquatic environment.

A study was carried out to determine the main characteristics and to compare biochemical profiles of aerobic bacteria isolated from the sediment and from the water column in the Meuse river.

The uptake of cobalt-60 and caesium-134 by the free bacterial community of the water column was investigated, in the presence of increasing radiocontamination.

The release of cobalt-60 and caesium-134 by the bacterial community was investigated, in relation to environmental parameters such as the temperature and pH of the water column.
Adsorption coefficients.

Chemical properties of sediments and particulate material from several sites throughout Europe (IFE, LNETI, ENEA, LBR) will be measured and compared with the results of clay mineral masking techniques (ULe) to assess the relative importance of illite in the adsorption of radiocaesium. K factors will be described in terms of underlying theory.

Concentration factors.

Models of fish growth and feeding habits (IFE, CEN, LNETI) will be combined with physiological information on the intake pathways, storage and excretion of individual radionuclides in different forms (LBR) and in competition with other trace metals (RUCA). Complementary physiological work on algal uptake of radionuclides will be undertaken by LBR and EMU and on bacterially mediated uptake and release of radionuclides by ULi.

Transport processes.

Major differences in the runoff rate of radiocaesium from clay rich and clay deficient organic catchments will be studied in order to elucidate the storage medium on the latter and the physical chemistry of the loss processes (IFE, LNETI, ULE in collaboration with the University of Linkoping, Sweden). The chemical studies of the properties dictating the magnitude of Kd will be extended to try and clarify the importance of organic matter, particularly algal material, as a transporter of radionuclides from the water column to the sediments.

Remobilization.

Processes driving the release of radiocaesium from sediments will be studied by the analysis of pore water and adsorbed caesium concentrations in cores from several sites (IFE, ENEA, ECN, ULe) using very sensitive counting facilities (ECN). The release of radionuclides at the interface between freshwater and seawater will also be studied (Ule, ENEA, LBR) in order to test recent theories of remobilization under these circumstances (ULe, ENEA, LBR).

Modelling.

Where advances in understanding can be described mathematically they will be incorporated into existing models (IFE, CEN).

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

NATURAL ENVIRONMENT RESEARCH COUNCIL
Address
Far Sawrey The Ferry House
LA22 OLP Ambleside
United Kingdom

Participants (9)

COMMISSARIAT A L'ENERGIE ATOMIQUE
France
Address
Rue De La Federation 31-33
75015 Paris
ENERGY RESEARCH CENTRE OF THE NETHERLANDS
Netherlands
Address
Westerduinweg 3
Petten
Ente per le Nuove Tecnologie, l'Energia e l'Ambiente (ENEA)
Italy
Address
Via Vitaliano Brancati 48
00144 Roma
KATHOLIEKE UNIVERSITEIT LEUVEN
Belgium
Address
Oude Markt 13
3000 Louvain / Leuven
NATIONAL LABORATORY FOR ENGINEERING AND INDUSTRIAL TECHNOLOGY
Portugal
Address
Estrada Do Paco Do Lumiar 22
1649-038 Lisboa
UNIVERSIDAD DE MALAGA
Spain
Address
S/n,plaza De El Ejido S/n
29071 Estepona (Malaga)
UNIVERSIT DE NANTES
France
Address
Quai De Tourville 1
44035 Nantes
UNIVERSITE DE LIEGE*ULG
Belgium
Address
Place Du 20 Aout 7
4000 Liege
Universitair Centrum Antwerpen
Belgium
Address
12,Buekenlaan
2020 Antwerpen