Soil-plant Transfer Research Articles

Effects of ammonium-ferric(III)-hexacyano-ferrate(II) and faeces addition on yield and soil-plant transfer of radiocaesium to rye-grass

AFCF[ ammonium-ferric(III)-hexacyano-ferrate(II) ] was shown to be an effective countermeasure against radiocaesium uptake by domestic animals. Following the addition of AFCF and faeces to a sandy farm soil, we evaluate here radiocaesium transfer to rye-grass as well as grass yield in a pot experiment under greenhouse conditions. Sheep faeces and/or soil were artificially contaminated with 137Cs or 134Cs, respectively. Radiocaesium from both soil and faeces was equally absorbed by rye-grass. AFCF, at concentrations of 1 g m −2 soil, was not deleterious for plant growth. AFCF is, furthermore, an effective countermeasure for radiocaesium soil—plant transfer. The AFCF applications at about 1 g AFCF m −2 on the sandy soil resulted in a fourfold reduction of the radiocaesium transfer to rye-grass.

The transfer of 137Cs through the soil-plant-sheep food chain in different pasture ecosystems

A grazing experiment with sheep was carried out in 1990-1993 on natural, semi-natural and cultivated pasture on clay soil. The pastures were located in Southern Finland and were moderately contaminated with 137 Cs by Chernobyl fallout. Natural pasture refers to forest pasture and serai-natural pasture to set-aside field pasture, the latter having been under cultivation about 15 years ago. The transfer of 137Cs to sheep was clearly higher from forest pasture than from the other two pastures and it was lowest from cultivated pasture. The transfer was higher to muscle and kidney than to liver and heart. The transfer of 137Cs to plants and to meat varied with years. Seasonal variation in the plant 137 Cs was followed-up on forest and set-aside field pasture; the activity concentration of plants reached a maximum in June, a lesser increase occurred later in the autumn. In 1993, which was considered an average year with respect to 137Cs transfer to plants, the mean soil-plant transfer factors of 137Cs for forest, set-aside field and cultivated pastures were 1.78, 0.36 and 0.09, and soil-meat aggregated transfer factors 11.0, 0.28 and 0.03, respectively.

Transfer of radiocaesium from podzol to ryegrass as affected by AFCF concentration

The possible use of ammonium-ferrichexacyano-ferrate (AFCF) as a countermeasure for soil-plant transfer of radiocaesium is evaluated. On a sandy agricultural soil, AFCF application rates of 10 and 100 g AFCF m−2 reduced radiocaesium transfer to ryegrass by a factor of 25 and 225, respectively, without affecting plant growth. Even additions of only 1 g AFCF m−2 resulted in a 4-fold reduction of the radiocaesium transfer factor. Additions of less than 0.1 g AFCF m−2 were not effective in reducing radiocaesium transfer to rye grass. Reductions in transfer with AFCF addition can be estimated and have been quantified using an appropriate regression equation.

Sorption of 137Cs from undisturbed forest soil in a zeolite trap

A method to study the time-integrated migration of radiocaesium (137Cs) from undisturbed coniferous forest soil cores is presented, which is based on a natural zeolite trap enclosed in a semi-permeable membrane that can be mounted into soil cores. Results show that higher zeolite trapping of 137Cs occurred in a forest soil with high soil-plant transfer parameters, low pH and high organic contents. Part of the biologically bound 137Cs that was associated within the soil microflora and microfauna appears to increase the migration of 137Cs after heating of the soil cores. Therefore, migration and mobility of 137Cs in forest soil seems to depend to a major extent upon the soil's physical and chemical parameters, particularly the activity concentration of 137Cs in soil solution, soil pH, percentage of soil organic matter (OM%) and soil mineral type.

Transfer of technetium in the soil-rice plant system

In order to assess the behavior of Tc in flooded soil-plant systems, laboratory experiments have been done using95mTc as a tracer. Two common soil types in Japan, Andosol and Gray lowland soils, were used. Soil-plant transfer factors of Tc in rice grain were very low, i.e. 5×10−5 for Andosol and 6×10−4 for Gray lowland soil. It was found that the Tc concentrations in rice plants were influenced by those in soil solutions. Concentrations of95mTc in both soil solutions decreased rapidly in the early period of cultivation. It was observed that redox-potential (Eh) also decreased markedly following flooding. A relationship was found between the decrease of the95mTc concentrations in soil solutions and the drop of Eh in the soils. The Tc (VII) added to soil was transformed to insoluble Tc (IV) under the reduced conditions existing in flooded soil.

Transfer of technetium from soil to paddy and upland rice.

Soil-plant transfer factors (concentration ratio between the plant and soil) of technetium in paddy and upland rice plants were obtained from laboratory experiments. The transfer factor is one of the most important parameters for environmental radiation dose assessment. Technetim tracer (95mTcO4-) was added to the soil prior to rice cultivation. The transfer factor of technetium for the hulled grains (brown rice) of paddy rice (< or = 0.0002) was much lower than for that of upland (0.021). The transfer factors for both types of hulled grains were much lower than in the leaves. The technetium decontamination rate from hulled grains by polishing was 34%, the percentage of the weight decrease being 12%. The concentration of technetium in the soil solution collected from the paddy rice soil (flooded conditions) decreased rapidly with time due to its adsorption on the soil. In the upland rice soil (non-flooded) solution, the decrease in the technetium concentration was fairly slow. The low transfer factors for the paddy rice plants could be explained by the immobilization of technetium in the flooded soil. The oxidation-reduction potentials (Eh) in the flooded soil decreased rapidly with time. We conclude that technetium tracer added as TcO4- to flooded soil is readily transformed to an insoluble form (e.g. TcO2) under the reducing conditions provided by flooding.

The relationship between concentrations of plutonium and americium in soil interstitial waters and their uptake by plants

Large soil cores (23.5 cm diameter), containing elevated concentrations of plutonium and americium derived from fuel reprocessing effluents (typically 2000–3000 but up to 10000 Bq kg −1 239,240Pu and 3000–5000 but up to 15 000 Bq kg −1 241Am), were collected from three zones of a salt marsh in west Cumbria. The standing vegetation was removed, the cores placed in a greenhouse and the vegetation allowed to regrow. After 2 months, the regrowth was harvested and the amount of plutonium and americium taken up determined. These data, combined with data from earlier, in-situ sampling of soil pore waters and from analysis of soil cores, allow estimation of soil-plant transfers. If these are expressed simply as the ratio of the activity concentrations in the plant tissues to those in the soil, the values lie in the range 10 −5−10 −6, comparable to those found by many other workers. However, the ratio of concentrations in plant tissues to those in the soil solution is much higher; 40–110 l kg −1 for 239,240Pu and 210–640 l kg −1 for 241Am. There appears to be a consistent relationship between soil-plant transfer and the in-situ solid-solution distribution coefficient (Rd).

Mobility and plant availability of radioactive Cs in natural soil in relation to stable Cs, other alkali elements and soil fertility

The mobility and plant availability of radioactive Cs from the Chernobyl accident in natural soil-plant systems of varying fertility were studied at three sampling locations situated in subalpine areas of central Norway. The soil samples included litter, humus (0–2 cm and 2–5 cm depth), and mineral soil (8–12 cm and 20–30 cm depth), and the plant species studied were Betula nana, Empetrum hermaphroditum and Juncus trifidus. The lichen Cetraria nivalis was also sampled. The sampling took place in the middle of the growth season during the period 1987–1989. The soil and vegetation samples werw analysed with respect to total radiocaesium ( 137Cs and 134Cs), Rb, stable Cs and exchangeable 137Cs (in soil only), K, Ca and Mg. The radiocaesium activity in the vascular plants and lichen decreased by at least 40% during the sampling period. The decrease in activity of E. hermaphroditum and C. nivalis is explained by growth of new low-activity material in 1988 and 1989, diluting material directly contaminated by the fallout in 1986. For B. nana and J. trifidus the decrease was caused by lower radiocaesium uptake by the root system. The uptake of radiocaesium in the latter two species seemed to be connected primarily with the radiocaesium content in the soil humus layer. Only in J. trifidus radiocaesium showed clear connection with the other soil parameters studied. Calculations of specific activities and soil-plant transfer coefficients showed that radioactive caesium in the soil was more available to plants than total stable caesium. Comparison of 137Cs/ 134Cs ratios in soil and plants indicated that the radiocaesium content in the soil is still influenced by 137Cs from nuclear weapons fallout.

Reduction of soil-plant transfer factors: mechanical aspects

In order to reduce food-chain contamination after a nuclear accident, mechanical actions may be taken to remove the radionuclides, or to prevent their uptake. Some of these actions have been reviewed by a working group in the REACT workshop. The conclusions of the group are briefly reviewed using tables to summarised available methods.

Transfer of PCDD/PCDF from contaminated soils to food and fodder crop plants

Field studies were conducted on PCDD/PCDF transfer from contaminated soils to lettuce, potato plants and hay. Contamination of crops by soil particles proved to be an important source of PCDD/PCDF in plants. When this pathway was excluded, there was little correlation between soil concentrations and plant concentrations of PCDD/PCDF. PCDD/PCDF concentrations only increased in unpeeled potato tubers with increasing levels of soil contamination. In shoots of plants air-plant transfer of PCDD/PCDF by far exceeds soil-plant transfer.

Tracer Experiments on the Transfer of Technetium from Soil to Rice and Wheat Plants.

Laboratory experiments on the transfer of Tc from soil to rice and wheat plants have been carried out by using a 95mTctracer, added as TcO4-. The soil-plant transfer factors of Tc for rice plants were ≤50.005 for the grains (hulled) and about 1.1 for the lower leaf blade. In contrast to this, higher transfer factors were found for wheat plants, i. e. 0.027 for the grains (hulled) and 230 for the lower leaf blade. The Tc concentrations in the grains of both plants were lower than in the leaves. The level of Tc in the soil solution collected from the flooded soil was found to decrease rapidly with time. In the case of wheat plants grown in a non flooded soil, the decrease in the Tc level in the soil solution was rather slow. It was suggested that the Tc tracer added as TcO4- to the paddy soil was readily transformed to insoluble forms under the reducing conditions in the flooded soil. The low transfer factor observed for the rice plants could be explained by a fixation of Tc in the soil.

Predicting the radionuclide fraction transferred to consumption through grain foods.

Data on the root uptake by wheat of three activation products, 54Mn, 65Zn and 63Ni, and two fission products, 90Sr and 137Cs, were obtained in a long-term field study. Soil-plant transfer factors calculated from these data were used to determine the amount of various radionuclides that may reach the humans through grain foods. The temporal variation of this contribution showed that the fraction of 54Mn and 65Zn transferred to human consumption during the first year was considerable, but owing to decay and decreasing uptake the gradients diminished and the accumulated fraction flattened to a plateau. In contrast, the fractions of the long-lived 63Ni, 90Sr and 137Cs transferred to human consumption were smaller and with nearly constant gradients. This study shows that the long-lived radionuclides are transferred to the same extent as 54Mn and 65Zn. In the long-term, the greatest doses from grain products are contributed by the long-lived radionuclides.

Synopsis of French Experimental and in Situ Research on the Terrestrial and Marine Behavior of Tc

Terrestrial environment studies have been essentially concerned with the evolution of soil deposition and soil-plant transfers. Experimentally determined coefficients of distribution in soils are low: 60-80% of Tc remains hydrosoluble during the first months. Technetium emissions resulting from microbiological activity have been quantified. Antagonistic effects on Mo and Tc retention by soils are dependent on their respective concentrations. Four areas of soil-plant transfers have been studied: 1) root absorption kinetics relative to deposition of Tc, 2) interaction of stable Mo (environmental parameter) with the transfer of Tc to plants, 3) interaction of some long-lived radioisotopes (effluent parameters) with the transfer of Tc to plants, and 4) long-term soil-plant transfer and aging of deposited material. Of aquatic systems, only the marine environment has been studied. Under anoxic conditions in the presence of reducing sediments, the distribution coefficients (Kd) were very high (10(3)). Concentration factors (CF) from water to organisms were generally very low (1 to 10); however, CF greater than 1000 have been observed for some biota such as macrophytic brown algae, worms and lobsters. Biochemical analysis showed that Tc was essentially free and partially bonded to proteins. The transfer factors between sediments and species were very low (TF less than 0.5). The biological half-time was determined in some marine organisms that had accumulated Tc from water, food or sediments; the loss is biphasic. Uptake in edible parts was low. The physiochemical form affects the accumulation and loss of Tc. Analyses have quantified 99Tc in marine fauna and biota in the English Channel in relation with releases of the reprocessing plant of La Hague. Brown algae are the best bioindicators for following 99Tc dispersion in marine ecosystems.

Tracer experiments on transfer of radio-iodine in the soil — rice plant system

In order to study the transfer of 129I from soil into rice, tracer experiments have been carried out in the soil-rice plant system. The soil-plant transfer factors, which are defined as ‘concentrations of the nuclide in a plant organ’ divided by ‘concentrations of the nuclide in dry soil’, were 6.0 × 10−3 for brown rice, 3.2 for the first leaf blade (flag leaf), 5.1 for the second leaf blade, 2.6 for stem, 1.4 for rachis and 9.0 for root on a dry weight basis at the time of harvest. The ratio of radio-iodine in brown rice to polished rice was about 1:0.3. The transfer factor of radio-iodine from soil to polished rice was estimated to be about 2 × 10−3. It was observed that the concentration of radio-iodine in the soil solution increased with the plant growth. This was attributed to the reduced condition in the soil caused by the effects of root and/or micro-organisms. Release of I from rice plants into the air in an organic form was found. The chemical form of radio-iodine in the soil solution appeared to be iodide, and the radio-iodine in the plant tissue was found in a low molecular weight fraction by gel-filtration. The vertical distribution of radioiodine in the soil was also studied.

Plant uptake of activation and fission products in a long-term field study

The root uptake by wheat and timothy of three activation products, 54Mn, 63Ni and 65Zn, and two fission products, 90Sr and 137Cs, was the object here of a long-term field experiment. Some of the soils used were sampled in the vicinity of four nuclear power plants, while the others represented some of the agricultural soils which occur most frequently in Sweden. The aim of the investigation was to calculate the soil-plant transfer factors of these nuclides and to study the parameters which mainly influence root uptake. Transfer factors were highest in poor, acid soils and uptake was influenced mostly by the soil organic matter, calcium and potassium contents. The investigation provides new information on the uptake of 63Ni, which is found to be strongly correlated with the calcium content of the soil. The uptake of 137Cs is found to be dependent on the organic matter and potassium contents of the soil. The distribution of the nuclides in different soil fractions was also investigated, confirming that competition occurs between 63Ni and 90Sr on the one hand and calcium.

Long-term transfer from soil to plant of radioactive corrosion products

The uptake by swards of three activation products ( 54Mn, 57Co and 63Ni) was studied in a long-term field experiment. Some of the soils used in the investigation were sampled in the vicinity of four Swedish nuclear power plants, while the others represented the most frequent types of agricultural soils occuring in Sweden. Data on the uptake by clover and timothy during a 9-year period were used for the calculation of soil-plant transfer factors of these nuclides. The temporal distribution of the transfer factors and the variations exhibited by the different nuclides were examined, as well as the differences between crops. The increased uptake at the early stage of the experiments (first year effect) observed in clover is discussed. Regression analyses employed to quantify the dependence of uptake on soil parameters showed that the soil characteristics that most influenced the uptake are the pH and the soil contents of calcium and potassium.