Helge E. Bjørnstad

A study on radionuclide association with soil components using a sequential extraction procedure

Measurements performed in 1986–1988 demonstrate that most of the radiocesium isotopes (137Cs and134Cs) deposited after the Chernobyl accident are still located in the upper soil layers (0–2 cm). The vertical migration appears to be slow, and only a small fraction of the radiocesium has been transferred into the biological cycle. Sequential extraction techniques have been utilized in order to investigate the degree of binding or association between deposited radionuclides (137Cs,134Cs and90Sr) and components in soil. The results indicate that a major fraction of the radiocesium is associated strongly with organic and mineral materials in the litter or upper soil layers: less than 10% is easily leachable. The distribution of137Cs throughout the fractions was similar to that determined for naturally occurring stable cesium (133Cs), implying that isotopic exchange had been extensive. For90Sr, the results show a relatively high leachable fraction. Therefore, present results indicate that radiocesium should be less mobile, and less available for root uptake, than90Sr in soil.

The effects of naturally occurring high and low molecular weight inorganic and organic species on the yolk-sack larvae of Atlantic salmon (Salmo salar L.) exposed to acidic aluminium-rich lake water

Yolk-sac larvae of Atlantic salmon (Salmo salar L.) were exposed to water from two acidic, aluminium rich lakes, Nepptjern and Lomstjern. The average pH of the lakes during the experiments were 4.85 and 5.24. The concentrations of total monomeric aluminium (Ala) were high in both lakes, i.e. 640 ± 22 and 451 ± 16 μg Al 1−1, but the total amount of organic carbon (TOC) differed significantly (1.89 ± 0.04 and 11.6 ± 0.1 mg Cl−1). The larvae were also exposed to cation exchanged and hollow-fiber ultrafiltered water from both lakes. Based on the hollow-fiber ultrafiltration, the toxic aluminium species present were as the inorganic, low molecular weight fraction (mol. wt. < 3 000 Da). Within this fraction, the highest mortality rates [LT50=24.6±0.5 h (n = 6)] were observed in the lake water having the highest amounts of the hydrolysis products, i.e. Al3+, Al(OH)2+ and Al(OH)2+. The aluminium-fluorides, AlOHF+, AlF2+, and AlF2+ were also found to be toxic, but less toxic [LT50=49.6±1.1 h (n=4)] than the aluminium hydrolysis products. Removal of high molecular weight species including inorganic and organic Al species, did not affect the toxicity or the mortality rates. No acute toxicity was observed when the larvae were exposed to the cation exchanged eluates. Thus, the organic aluminium-complexes seemed to have no acute toxic effect, at least at the concentration levels tested.

Use of an aluminium-26 tracer to study the deposition of aluminium species on fish gills following mixing of limed and acidic waters

Results are presented from an experiment designed to investigate the deposition of Al species onto fish gills following the mixing of limed and acidic natural waters. The natural waters were labelled with 26Al and the distribution between high and low molecular weight forms was determined by ultrafiltration of water samples. In labelled acidic waters, the 26Al was present predominantly in low molecular weight forms, whereas in labelled limed waters the major fraction of 26Al was present in a high molecular weight form. In mixing experiments, 26Al was only detectable on fish gills when the 26Al was initially present in a low molecular weight form (i.e., in labelled acidic waters). Aluminium-26 was not detected on the gills of fish exposed to labelled limed waters. These results support the hypothesis that Al on fish gills arises from polymerization of low molecular weight species and that, within mixing zones, the high molecular weight species do not play a significant role in the precipitation of Al onto the gill surface.

Fractionation of cesium isotopes and90Sr in snowmelt run-off and lake waters from a contaminated Norwegian mountain catchment

Cesium isotopes and90Sr have been determined in the inflow and outflow rivers of a Norwegian subalpine lake. The lake is situated in an area contaminated by Chernobyl fallout. Sampling was carried out during the spring peak discharge period associated with snowmelt. Transported coatse particulate plant material was collected by traps. Particles and colloids were removed from water samples by hollow fibre ultrafiltration. The results illustrate that run-off during the spring snowmelt is an important pathway for these radionuclides. The cesium isotopes are predominantly transported as colloids, while90Sr is present in the form of low molecular weight mobile species. Based on lake budget calculations, more than 50% of the cesium input is retained in the lake, while more than 90% of the90Sr is transported through the lake and into lower parts of the drainage system.

Winter transport of Chernobyl radionuclides from a montane catchment to an ice-covered lake

The amounts of 137Cs and 90Sr have been determined in the inflows and outflows of the Norwegian sub-alpine lake, Ovre Heimdalsvatn, in March/April during the period of ice-cover, when discharge is extremely stable. The lake is situated in an area contaminated by Chernobyl fallout. The transported course particulate plant material has been collected in traps; the particles and colloids have been removed from water samples by cross-flow ultrafiltration. On the basis of radionuclide inputs and outputs, lake budget calculations have been made for 137Cs and 90Sr during the period of ice-cover. Daily transport of radionuclides is considerably less than that observed during the spring snowmelt period when discharges are high. Size distribution patterns of Cs and Sr observed during winter are compared with previously published data from the same lake during the spring spate. The retention of 137Cs is similar in winter and spring, but retention of 90Sr is greater in winter.

Effect of pH, humus concentration and molecular weight on conditional stability constants of cadmium

Abstract The molecular weight distribution of DOC and cadmium in humic samples spiked with cadmium has been studied after changing the concentration and/or pH of the original humic sample. The ultrafiltration method, using standard Amicon ultrafiltration membranes with molecular weight cut-off values of 500, 10 3 and 10 4 , was used for the separation of different molecular weight fractions. 109 Cd was used for labelling the cadmium solutions. Humus concentration ranged from 2.7 to 13.6 mg DOC 1 −1 , pH was varied from 5 to 7. Molecular weight distribution of DOC was found to be very dependent on experimental conditions. Some of the results are consistent with earlier proposed humus conformation model. Relative binding strength of single molecular weight fractions was expressed per mole and per weight basis, the fraction with nominal molecular weight in the range 10 3 -10 4 being the strongest chelating agent. Conditional stability constants ( K ) of cadmium humus complexes were calculated. An empirical equation for the calculation of K was derived.

Determination of radionuclides associated with colloids in natural waters

Due to the interaction with components present in natural waters, radionuclides may be present in different physico-chemical forms, varying in size, charge and density. The distribution pattern will influence the transport, mobility and biological uptake of the radionuclides. Size fractionation based on hollow fiber is useful for the determination of the size distribution pattern of radionuclides in natural waters. Furthermore, a continuous mixing and separation system has been developed for the investigation of the association of radionuclides with naturally occurring colloids. Results based on radionuclides in waste water from the Forsmark nuclear power plant, Sweden, will illustrate the potential usefulness of the technique.

Determination of plutonium-239 + plutonium-240 and plutonium-241 in environmental samples using low-level liquid scintillation spectrometry

A radiochemical method for the simultaneous determination of 239Pu + 240Pu and 241Pu in environmental samples has been developed. In the course of the analysis a 236Pu tracer was used for estimating the chemical yield of plutonium isotopes. After suitable pre-treatment of the sample, the plutonium nuclides in solution were coprecipitated with iron(III) hydroxide and calcium oxalate and isolated further from impurities and interfering radionuclides by means of anion-exchange chromatography. Plutonium isotopes in the eluate (NH4I-HCI) were converted into nitrate form and then extracted with 20 ml of 5% bis(2-ethylhexyl) hydrogen phosphate extractive cocktail. The final organic solution was measured spectrometrically using an ultra-low-level liquid scintillation spectrometer, Quantulus (LKB, 1220 Wallac). The chemical yields of plutonium range from 25 to 50% for 100 I of sea-water and 30 to 60% for 40 g of dried soil sample. The counting efficiencies are nearly 100% for 239Pu + 240Pu and 48.8% for 241Pu, respectively. The detection limits were estimated to be 0.20 mBq for 239Pu + 240Pu and 2.2 mBq for 241Pu, respectively. The proposed procedure has been tested for the simultaneous determination of 239Pu + 240Pu and 241Pu in sea-water (Irish Sea, North Sea) and soils (Cumbrian coast, UK; Byelorussia, USSR).

Different approaches for estimating the deposition of radiocaesium on mountain pasture in Southern Norway

Different approaches to the estimation of radiocaesium deposition on different types of indigenous mountain pastures have been examined. In situ gamma spectrometric measurements performed using a portable multi-channel analyser (3 x 3 in Nal detector) with the detector placed on the ground, on a tripod at a height of 100 cm above the ground or fastened to a rucksack frame 90 cm above the ground were compared with conventional soil sampling. Field gamma-ray measurements in connection with soil sampling appear to be a promising tool for estimates of deposition on mineral soils in areas with different topography and vegetation.

2.2. The Characterization of Radiocaesium Transport and Retentions Nordic Lakes

Summary Fractionation studies of radiocaesium have been carried out in three Nordic lakes, Ovre Heimdalsvatn in Norway, Hillesjon in Sweden and Saarisjarvi in Finland. These lakes differ markedly in several aspects and provide insight into the factors determining radionuclide transport in a range of lake ecosystems. Transport of 137 Cs in plant material (Coarse Particulate Organic Matter, CPOM) was about 17 times greater to Ovre Heimdalsvatn than Saarisjarvi, although over 99% of the inflow CPOM was retained in both lakes. Inflows to Hillesjon were an order of magnitude lower than to Saarisjarvi and the net retention was only 71%, on account of the outflow of autochthonous production, largely water lily fragments. With regard to the water phase, the lakes differed in the activity of 137 Cs in the various molecular weight fractions. This was a function of catchment processes, resuspension and biological activity in the lakes. In Ovre Heimdalsvatn and Saarisjarvi 45% of the 137 Cs in the water phase was retained in the lake, while in Hillesjon ten times more 137 Cs flowed out than flowed in, due to resuspension of 137 Cs-rich sediments.