Gerald Kirchner

Environmental processes affecting plant root uptake of radioactive trace elements and variability of transfer factor data: a review.

Soil-to-plant transfer factors are commonly used to estimate the food chain transfer of radionuclides. Their definition assumes that the concentration of a radionuclide in a plant relates linearly solely to its average concentration in the rooting zone of the soil. However, the large range of transfer factors reported in the literature shows that the concentration of a radionuclide in a soil is not the only factor influencing its uptake by a plant. With emphasis on radiocesium and -strontium, this paper reviews the effects of competition with major ions present in the soil-plant system, the effects of rhizosphere processes and soil micro-organisms on bioavailability, the factors influencing transport to and uptake by roots and the processes affecting long-term uptake rates. Attention is given to summarizing the results of recent novel electrophysiological and genetic techniques which provide a physiologically based understanding of the processes involved in the uptake and translocation of radiocesium and -strontium by plants.

Applicability of compartmental models for simulating the transport of radionuclides in soil

Abstract The applicability of multi-compartmental models to describe the transport of radioactive trace substances in soil is reconsidered. It is shown that the widely used serial compartmental model implicitly assumes convection-dominated flow and has to be replaced by a model with backflow for diffusion-controlled transport. A compartmental approach provides a valid model of the transport processes only if the number of compartments is set to a specific value which is determined by the physical flow conditions in the soil studied. It is demonstrated that describing diffusional transport by the commonly used serial compartmental model results in apparently time-dependent residence times. Modifications of compartment models made in order to account for complex sorption behavior of the radionuclides are discussed. Consequences for the design and interpretation of field studies are outlined.

Vertical migration of fallout 137Cs in agricultural soils from Southern Chile

Total inventories and depth distributions of atomic weapons testing fallout 137Cs were determined at four sites in Chile. Cesium inventories are always higher than previously estimated for the Southern Hemisphere and depend on annual rainfall. From the measured depth distributions cesium migration rates of < 0.5 cm/year were calculated. Modeling the 137Cs transport by the convection-dispersion equation yielded a better accordance with the measured depth distributions than the application of a compartmental model.

Accumulation of 210Pb, 226Ra and radioactive cesium by fungi

Fungi sampled in three areas in France were analyzed by gamma-spectrometry for their concentrations of 134Cs, 137Cs, 210Pb and 226Ra. In most of the samples radioactive cesium was detected with a maximum of 2860 Bq kg-1 (dry wt.). Activity concentrations of 210Pb were in the range < 1.76-36.5 Bq kg-1 (dry wt.). Activity concentrations of 226Ra were consistently lower, often by one order of magnitude. Models are developed to estimate the contributions of atmospheric 210Pb deposited onto the fruit bodies to the measured 210Pb concentrations and of the uptake of 222Rn soluted in soil pore water which subsequently decays into 210Pb. It is shown that both pathways are of only minor importance. Comparison with the soil-mushroom concentration ratios of stable lead, which were determined for some of the samples, confirmed that 210Pb in mushrooms mainly originates from direct uptake of 210Pb present in the soil. Despite of the high concentrations of 137Cs detected in most of the mushrooms, radiation doses to individuals due to mushroom consumption are dominated by 210Pb for the majority of the edible mushrooms sampled.

Seasonal variations in soil-to-grass transfer of fallout strontium and cesium and of potassium in North German soils

The uptake of 134Cs, 137Cs, 90Sr and 40K by grass plants growing on permanent pastures was studied between May 1990 and November 1992. Four experimental sites were chosen including both mineral and organic soils. At each site, some pasture plots were cropped repeatedly in order to simulate grazing of cattle, while other plots were cropped only once during the vegetation period. For all soils and radionuclides investigated, nuclide concentrations in grass showed marked seasonal fluctuations, though no regular pattern could be demonstrated in successive years or at different sites. No correlations with air temperature or rainfall were found, but cesium and strontium concentrations in plants tended to be negatively correlated with the soil moisture within the rooting zone which was calculated using the OPUS code. This result indicates that root uptake rates are controlled by concentrations of ions in solution which increase with decreasing moisture content of the soils. Differences in transfer factors between grass plants growing on different soils ranged up to a factor of about 100 for 137Cs, but were less than one order of magnitude for 90Sr. Transfer factors were higher if grass plants were cropped repeatedly. This effect might be attributed to the plant treatment: grass plants which are defoliated repeatedly develop a more shallow root system resulting in preferential uptake from the uppermost soil layers where concentrations of radionuclides are highest. Uptake of cesium by grass growing on the two organic soils was high enough to differentiate between Chernobyl and weapon fallout cesium. Transfer factors of Chernobyl cesium were consistently higher, and for one of the sites, showed a marked decrease with time. As the results of a simple model show, these observations can be explained by the impact of the differing depth distributions of the cesium fractions within the rooting zone of the grass plants. It is concluded that in interpreting soil-to-plant transfer data the distribution of the radionuclides in the plant rooting zone should receive much attention.

Vertical migration of radionuclides in undisturbed grassland soils.

Literature data on numerical values obtained for the parameters of the two most popular models for simulating the migration of radionuclides in undisturbed soils have been compiled and evaluated statistically. Due to restrictions on the applicability of compartmental models, the convection-dispersion equation and its parameter values should be preferred. For radiocaesium, recommended values are derived for its effective convection velocity and dispersion coefficient. Data deficiencies still exist for radionuclides other than caesium and for soils of non-temperate environments.

Performance assessment studies of models for water flow and radionuclide transport in vegetated soils using lysimeter data

Increasingly the burial of nuclear waste in deep underground repositories is being regarded as a safe long-term solution for disposal. However, to support this safety assessment models of the associated risks are required. An important component of these models is the upward migration of radionuclides from a contaminated water table into arable and pasture crops. A five-year experiment to investigate the processes which control these transfers has been undertaken at Imperial College. Selected data from this experiment were made available to participants of the BIOMOVS II programme in order to allow them to perform blind hydrological and radionuclide transport simulations. The results show the importance of correctly characterising the soil hydrology and indicate that model conceptualisations derived from surface contamination studies may not adequately capture the various processes which influence the upward movement of radionuclides in the vadose zone. These include not only the water movement, but also chemical and biological processes. Finally, the difficulty and importance of a priori parameter selection is highlighted.

Worldwide isotope ratios of the Fukushima release and early-phase external dose reconstruction

Measurements of radionuclides (RNs) in air made worldwide following the Fukushima accident are quantitatively compared with air and soil measurements made in Japan. Isotopic ratios RN:137Cs of 131I, 132Te, 134,136Cs, are correlated with distance from release. It is shown, for the first time, that both within Japan and globally, ratios RN:137Cs in air were relatively constant for primarily particle associated radionuclides (134,136Cs; 132Te) but that 131I shows much lower local (<80 km) isotope ratios in soils relative to 137Cs. Derived isotope ratios are used to reconstruct external dose rate during the early phase post-accident. Model “blind” tests show more than 95% of predictions within a factor of two of measurements from 15 sites to the north, northwest and west of the power station. It is demonstrated that generic isotope ratios provide a sound basis for reconstruction of early-phase external dose rates in these most contaminated areas.

Effect of user interpretation on uncertainty estimates: examples from the air-to-milk transfer of radiocesium

Abstract An important source of uncertainty in predictions of numerical simulation codes of environmental transport processes arises from the assumptions made by the user when interpreting the model and the scenario to be assessed. This type of uncertainty was examined systematically in this study and was compared with uncertainty due to varying parameter values in a code. Three terrestrial food chain codes that are driven by deposition of radionuclides from the atmosphere were used by up to ten participants to predict total deposition of 137 Cs and concentrations on pasture and in milk for two release scenarios. Collective uncertainty among the predictions of the ten users for concentrations in milk calculated for one scenario by one code was a factor of 2000, while the largest individual uncertainty was 20 times lower. Choice of parameter values contributed most to user-induced uncertainty, followed by scenario interpretation. Due to the significant disparity in predictions, it is recommended that assessments should not be carried out alone by a single code user.

Behaviour of radionuclides in soil/crop systems following contamination

Publisher Summary This chapter discusses the behavior of radionuclides in soil or crop systems following contamination. Considerable progress has been made in understanding the behavior of radionuclides, particularly those of caesium and strontium, in soil or crop systems after the Chernobyl accident contaminated large areas of continental Europe. For most radionuclides, the soil is the primary long-term reservoir in agricultural ecosystems. As a result, soil-based countermeasures to reduce food-chain contamination can be designed to reduce the degree of soil–plant transfer significantly although undesirable side effects of some of these treatments, such as heavy fertilizer treatments, may be experienced. Experimental mass or activity balance studies have shown that absolute losses from soils of key radionuclides such as 137Cs by processes such as cropping and leaching are small: Radioactive decay can probably dictate the rate at which agricultural systems become decontaminated. The development of more rapid methods to clean-up soils is desirable and one of these is addressed in the chapter.