Incorporating soil structure and root distribution into plant uptake models for radionuclides: toward a more physically based transfer model

Most biosphere and contamination assessment models are based on uniform soil conditions, since single coefficients are used to describe the transfer of contaminants to the plant. Indeed, physical and chemical characteristics and root distribution are highly variable in the soil profile. These parameters have to be considered in the formulation of a more realistic soil-plant transfer model for naturally structured soils. The impact of monolith soil structure (repacked and structured) on Zn and Mn uptake by wheat was studied in a controlled tracer application (dye and radioactive) experiment. We used Brilliant Blue and Sulforhodamine B to dye flow lines and 65Zn and 54Mn to trace soil distribution and plant uptake of surface-applied particle-reactive contaminants. Spatial variation of the soil water content during irrigation and plant growth informs indirectly about tracer and root location in the soil profile. In the structured monolith, a till pan at a depth of 30 cm limited vertical water flow and root penetration into deeper soil layers and restricted tracers to the upper third of the monolith. In the repacked monolith, roots were observed at all depths and fingering flow allowed for the fast appearance of all tracers in the outflow. These differences between the two monoliths are reflected by significantly higher 54Mn and 65Zn uptake in wheat grown on the structured monolith. The higher uptake of Mn can be modelled on the basis of radionuclide and root distribution as a function of depth and using a combination of preferential flow and rooting. The considerably higher uptake of Zn requires transfer factors which account for variable biochemical uptake as a function of location.     

Vertical migration of 60Co, 137Cs and 226Ra in agricultural soils as observed in lysimeters under crop rotation

In most studies quantifying the migration parameters - apparent migration velocity and apparent dispersion coefficient - of radionuclides in the soil by model calculations, these parameters are determined for undisturbed soils. For soils disturbed by ploughing, however, no such data are available in the literature. Therefore, in the present study, the migration parameters of (137)Cs, (60)Co and (226)Ra were estimated for ploughed soils by means of a convection-dispersion model. The depth distributions of the radionuclides were determined in four lysimeters (area: 1m(2), depth of soil monolith: 0.75m) filled with artificially contaminated soils of different types in July 1990. The lysimeters were cropped with agricultural plants. The soil in each lysimeter was ploughed manually once a year until 1996 (plough depth 20cm). In July 1999, soil samples were collected from three pits in each lysimeter. The depth distributions of all radionuclides proved to be very similar in each soil pit. The spatial variability of the depth distributions of a given radionuclide within the lysimeters was about the same as their variability between the four lysimeters. Evaluation of the migration parameters revealed that the convective transport of the radionuclides was always rather small or even zero, while the dispersive transport caused a "melting" process of the initially sharp activity edge at the lower border of the Ap horizon. These results are explained by the high evapotranspiration (80-90% of the total precipitation plus irrigation) and the small amounts of seepage water during the observation period of 9 years.     

Examination of the effect of particle size on the radionuclide content of soils

Information on the distribution of radiocaesium as a function of soil particle size is fundamental for its use as tracer in soil transport. Since the processes involved in soil erosion are known to remove and transport finer particles with larger efficiency, the aim of this work was to obtain data on the particle size versus radionuclide content distribution regarding the reference site of a soil erosion study. The analysis done was based on more than 5kg of source material and the changes happened to the radionuclide content of the different size fractions between the individual separation steps have been carefully monitored. About 10% of the total amount of (137)Cs present was found to be trapped in the serrations of larger stone-fragments while another 10% is transportable only during heavier storm-events. Within the remaining 80%, physical weathering products and clay particles have a (137)Cs-activity concentration, transportability and mass ratio of about 1:10, 1:2 and 1:1, respectively.     

Vertical distribution, migration rates, and model comparison of actinium in a semi-arid environment

Vertical soil characterization and migration of radionuclides were investigated at four radioactively contaminated sites on Kirtland Air Force Base (KAFB), New Mexico to determine the vertical downward migration of radionuclides in a semi-arid environment. The surface soils (0-15 cm) were intentionally contaminated with Brazilian sludge (containing (232)Thorium and other radionuclides) approximately 40 years ago, in order to simulate the conditions resulting from a nuclear weapons accident. Site grading consisted of manually raking or machine disking the sludge. The majority of the radioactivity was found in the top 15 cm of soil, with retention ranging from 69 to 88%. Two models, a compartment diffusion model and leach rate model, were evaluated to determine their capabilities and limitations in predicting radionuclide behavior. The migration rates of actinium were calculated with the diffusion compartment and the leach rate models for all sites, and ranged from 0.009 to 0.1 cm/yr increasing with depth. The migration rates calculated with the leach rate models were similar to those using the diffusion compartment model and did not increase with depth (0.045-0.076, 0.0 cm/yr). The research found that the physical and chemical properties governing transport processes of water and solutes in soil provide a valid radionuclide transport model. The evaluation also showed that the physical model has fewer limitations and may be more applicable to this environment.     

不同水耕年限稻田土壤水分运动特征研究

利用田间染色示踪和室内样品分析相结合的方法,研究了江汉平原不同水耕年限稻田土壤水分运动特征及其影响因素,期望为制定合理的稻田水分管理措施提供科学依据。结果表明:新稻田(水耕17a)与老稻田(水耕大于100a)土壤理化性质差异显著,引发土壤剖面内不同水分运动特征。新稻田垂直渗漏较老稻田明显,表现为新稻田有更大的染色深度。老稻田染色区域主要集中在耕作层,染色面积为48.52%;犁底层和底土层染色面积较小,仅为3.17%和0.2%。除耕作层外,新稻田其他土层染色面积均大于老稻田,且优先流特征较老稻田明显。老稻田侧向水分运动较新稻田强烈,其侧流剖面的染色面积(15.35%)显著高于新稻田(8.45%),老稻田侧向水流主要发生在犁底层以上。为减少稻田水分渗漏损失,老稻田可以通过减少农田-田埂-沟渠过渡区侧渗实现,而新稻田可采取少量多次的灌溉原则。     

Modelling radioiodine transport across a capillary fringe

Due to its long radioactive half-life, iodine-129 is considered to be an important radionuclide in the context of underground radioactive waste disposal safety assessment. Iodine speciates as iodide (I-) in reducing conditions and iodate (IO3-) in oxidizing conditions. As iodate is more reactive, it is much less mobile than iodide. Consequently, in considering vertically upward transport within a soil profile, iodine will tend to accumulate at the top of the capillary fringe. In this paper, a model of iodine transport across a capillary fringe is developed by coupling equations for variably saturated flow, oxygen dynamics and rate-limited sorption. Model parameters are obtained by consideration of literature values, calibration on soil column data and other supporting laboratory experiments. The results demonstrate the importance of rate kinetics on the migration and bioavailability of radioiodine in the near-surface environment.     

Validating riverine transport and speciation models using nuclear reactor-derived radiocobalt

Risk assessment of intentional or accidental discharges of toxic substances into river systems requires combined hydraulic and chemical modeling. Periodic discharges of known volumes with low radioactivity by the Beznau nuclear reactor (Switzerland) serve as validation tracers for both river flow and chemical speciation simulation. Validation of the former has been achieved by comparison of modeled and measured arrival times of radiocobalt along a 65 km transect with a maximum reaction period of 24 hours. Modeled breakthrough curves coincide well with measurements collected during three field campaigns, in spite of the fact that sorption and sedimentation processes were not activated during simulation. This gives indirect evidence of inefficient cobalt sorption. Particle/solution distribution measurements carried out during breakthrough allow further validation of our speciation approach, which is based on Tableau setup of inorganic reactions combined with sorption and organic complexation. Modeled and measured speciation results confirm recent observations of enhanced complexation of cobalt with dissolved organic substances, which significantly reduces particle sorption. The large variability of conditional stability constants for sorption and complexation reactions, for sorption site densities, and for organic ligand concentrations explains the variability of published particle-solution distribution coefficients.     

90Sr migration to the geo-sphere from a waste burial in the Chernobyl exclusion zone

Results are presented from an ongoing field-scale experimental study (namely the Chernobyl Pilot Site project) aimed at characterization of processes controlling (90)Sr releases from a shallow trench containing nuclear fuel particles, and subsequent radionuclide transport in the underlying sandy aquifer at the Chernobyl nuclear power plant site. Microscopic analyses of waste material and leaching experiments have shown that 10-30% of the radioactive inventory is associated with chemically extra-stable Zr-U-O particles. The largest fraction of (90)Sr activity in the trench ( approximately 30-60%) is currently associated with relatively slowly dissolving non-oxidized UO(2) matrix fuel particles. The (90)Sr migration velocity in the eolian sand aquifer is retarded by sorption to approximately 9% of groundwater flow velocity (K(d) approximately 2 ml/g). The dispersivity values for non-reactive solute transport in the aquifer predicted by geostatistics (i.e. 0.8 6 cm) were confirmed by a natural gradient tracer test using (36)Cl. The observed negative correlation between hydraulic conductivity and K(d) of aquifer sediments suggests that (90)Sr could be subjected to larger dispersion in the subsurface compared with (36)Cl.     

The volatilisation and sorption of (129)I in coniferous forest,grassland and frozen soils

129I is a potentially important radionuclide in safety assessments of proposed deep geological radioactive waste repositories due to its radiotoxicity, high mobility and long physical half-life (15.7 million years). In soils, iodine is present both in an inorganic form and in organohalide complexes, some of which are volatile under natural environmental conditions.This study has examined volatilisation, sorption and the effect of freezing on sorption and loss of (125)I (physical half-life 60.2 days), as a surrogate for (129)I, within coniferous forest and grassland soils. The results do not suggest that volatilisation from these soils is a significant pathway for the transport of (129)I. Strong and specific sorption of iodine to humic substances has been demonstrated, which is reduced at freezing temperatures. It is hypothesised that rapid sorption to soil humic substances can significantly reduce volatilisation rates. The effect of freezing conditions on iodine extractability from soils suggests a microbially mediated sorption process.     

Distribution and migration of 95Zr in a tea plant/soil system

(95)Zr is a primary radionuclide in the radioactive liquid efflux from a pressurized water reactor and one of the main radionuclides released after nuclear accidents. The fission yield of (95)Zr is as high as 6.2%, however, its environmental behavior has not been well documented. An experiment was conducted to evaluate the accumulation and distribution of (95)Zr in a tea plant/soil system. (95)Zr was accumulated primarily in the trunk of tea plants after being taken up from the soil. The radioactivity concentration of (95)Zr in the trunk increased slowly with time, then it reached a dynamic equilibrium 14 days after application. The radioactivity concentration of (95)Zr in the other parts of the tea plant was very low; only slighter greater than the detection limit. The results indicated that (95)Zr was not readily translocated in the tea plant. About 98.9% of applied (95)Zr was found to concentrate in the upper 5 cm layer after being sprayed onto the soil surface. The results indicated that (95)Zr could not readily move downwards with percolating water due to strong adsorption to surface soil.     

A new generic sub-model for radionuclide fixation in large catchments from continuous and single-pulse fallouts, as used in a river model

This paper presents a new general sub-model for fixation in catchment areas to be used within the framework of a river model for substances such as radionuclides and metals from continuous and single-pulse fallouts. The model has been critically tested using data from 27 European river sites covering a very wide geographical area and contaminated by radiocesium and radiostrontium from the Chernobyl accident and from the nuclear weapons tests (NWT fallout). This modelling approach gives radionuclide concentrations in water (total, dissolved and particulate phases) at defined sites on a monthly basis. The overall river model is based on processes in the upstream river stretch and in the catchment area. The catchment area is differentiated into inflow (approximately dry land) areas and outflow (approximately wetland) areas. The model has a general structure, which can be used for all radionuclides or substances. It is simple to apply in practice since all driving variables may be readily accessed from maps and standard monitoring programs. The driving variables are: latitude, altitude, catchment area, mean annual precipitation and fallout. Note that for large catchments, this model does not require data on the characteristic soil type or the percentage of outflow areas (wet lands) in the catchment, as in most previous models, since in practice it is very difficult to obtain reliable data on characteristic soil type or percentage of outflow areas, especially in large and topographically complex catchments. Modelled values have been compared to empirical data from rivers sites covering a wide domain (catchment areas from 3000 to 3,000,000 km2, precipitation from 400 to 1700 mm/year; fallouts from 1600 to 280,000 Bq/m2; altitudes from 0 to 1000 m.a.s.l. and latitudes from 41 degrees to 72 degrees N). The river model with its sub-model for fixation predicts close to the uncertainty factors given by the empirical data, which have been shown to be about a factor of 1.6 for 137Cs and a factor of 2.2 for 90Sr in river water. The obtained characteristic uncertainty factors for 137Cs from the Chernobyl fallout is 2.4, for 137Cs from the NWT fallout it is 1.3 and for the 90Sr results from the NWT fallout it is 3 using the new model.     

A method for determining the analytical form of a radionuclide depth distribution using multiple gamma spectrometry measurements

When characterizing environmental radioactivity, whether in the soil or within concrete building structures undergoing remediation or decommissioning, it is highly desirable to know the radionuclide depth distribution. This is typically modeled using continuous analytical expressions, whose forms are believed to best represent the true source distributions. In situ gamma ray spectroscopic measurements are combined with these models to fully describe the source. Currently, the choice of analytical expressions is based upon prior experimental core sampling results at similar locations, any known site history, or radionuclide transport models. This paper presents a method, employing multiple in situ measurements at a single site, for determining the analytical form that best represents the true depth distribution present. The measurements can be made using a variety of geometries, each of which has a different sensitivity variation with source spatial distribution. Using non-linear least squares numerical optimization methods, the results can be fit to a collection of analytical models and the parameters of each model determined. The analytical expression that results in the fit with the lowest residual is selected as the most accurate representation. A cursory examination is made of the effects of measurement errors on the method.     

The Role of Floodplain Soils as a Barrier to Radionuclide Migration: An Example of the Techa–Iset' River System

The contents of ⁹⁰Sr and ¹³⁷Cs and the pattern of their redistribution in the soil and plant cover of floodplain ecosystems have been assessed. It is shown that the radionuclide distribution across the floodplain and along the river flow is determined by the formation of a barrier to their migration near the river channel, at which less mobile ¹³⁷Cs accumulates. The soil and plant cover of the central floodplain are enriched with ⁹⁰Sr. Differences in radionuclide migration in floodplain soils and their input into plants are determined by the relationship between the processes of their immobilization and migration with soil water.     

The influence of a coal-fired power plant operation on radionuclide concentrations in soil

Fifty-two soil samples in the vicinity of a coal-fired power plant (CFPP) in Figueira (Brazil) were analyzed. The radionuclide concentration for the uranium and thorium series in soils ranged from <9 to 282 Bq kg(-1). The range of 40K concentration in soils varied from <59 to 412 Bq kg(-1). The CFPP (10 MWe) has been operating for 35 years and caused a small increment in natural radionuclide concentration in the surroundings. This technologically enhanced natural radioactivity (TENR) was mainly due to the uranium series (234Th, 226Ra and 210Pb) and was observable within the first kilometer from the power plant. The CFPP influence was only observed in the 0-25 cm soil horizon. The soil properties prevent the radionuclides of the 238U-series from reaching deeper soil profiles. The same behavior was observed for 40K as well. No influence was observed for 232Th, which was found in low concentrations in the coal.     

Migration of Cs and Sr in undisturbed soil profiles under controlled and close-to-real conditions

Migration of ¹³⁷Cs and ⁹⁰Sr in undisturbed soil was studied in large lysimeters three and four years after contamination, as part of a larger European project studying radionuclide soil–plant interactions. The lysimeters were installed in greenhouses with climate control and contaminated with radionuclides in an aerosol mixture, simulating fallout from a nuclear accident. The soil types studied were loam, silt loam, sandy loam and loamy sand. The soils were sampled to 30–40 cm depth in 1997 and 1998. The total deposition of ¹³⁷Cs ranged from 24 to 45 MBq/m², and of ⁹⁰Sr from 23 to 52 MBq/m². It was shown that migration of ¹³⁷Cs was fastest in sandy loam, and of ⁹⁰Sr fastest in sandy loam and loam. The slowest migration of both nuclides was found in loamy sand. Retention within the upper 5 cm was 60% for both ¹³⁷Cs and ⁹⁰Sr in sandy loam, while in loamy sand it was 97 and 96%, respectively. In 1998, migration rates, calculated as radionuclide weighted median depth (migration centre) divided by time since deposition were 1.1 cm/year for both ¹³⁷Cs and ⁹⁰Sr in sandy loam, 0.8 and 1.0 cm/year, respectively, in loam, 0.6 and 0.8 cm/year in silt loam, and 0.4 and 0.6 cm/year for ¹³⁷Cs and ⁹⁰Sr, respectively, in loamy sand. A distinction is made between short-term migration, caused by events soon after deposition and less affected by soil type, and long-term migration, more affected by e.g. soil texture. Three to four years after deposition, effects of short-term migration is still dominant in the studied soils.     

Transfer of 137Cs and 60Co from irrigation water to a soil-tomato plant system

An experiment has been performed at the nuclear power plant of Garigliano (Caserta, Italy), aiming at the measurement of transfer factors of 137Cs and 60Co radionuclides from the irrigation water to a soil-plant system, with particular attention to the influence on such transfers of the irrigation technique (ground or aerial). Tomato plants were irrigated weekly with water contaminated with 137Cs and 60Co (about 375 Bq/m2 week), using both irrigation techniques. After 13 weeks, fruits, leaves, stems, roots and soil were sampled, and radionuclide concentrations were measured by high-resolution gamma spectroscopy. It was found that the activity allocated to the plant organs is significantly dependent upon the irrigation technique, amounting to 2.1% and 1.6% of the activity given in the cultivation for aerial treatment and 0.4% and 0.3% for the ground treatment, for 137Cs and 60Co respectively. The activity absorbed by plants is allocated mainly in leaves (> 55%), while less then 10% is stored in the fruits, for both irrigation techniques. Transfer factors (soil-plant and irrigation water-plant) of tomato plants and of weeds have been determined for 137Cs and 60Co, as well as for natural 40K in the soil.     

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.     

Post-irrigation impact of domestic sewage effluent on composition of soils,crops and ground water--a case study

Long-term irrigation with sewage water adds large amounts of carbon, major and micro- nutrients to the soil. We compared the spatial distribution of N, P, K and other micronutrients and toxic elements in the top 0.6 m of an alluvial soil along with their associated effects on the composition of crops and ground waters after about three decades of irrigation with domestic sewage effluent as a function of distance from the disposal point. Use of sewage for irrigation in various proportions improved the organic matter to 1.24-1.78% and fertility status of soils especially down to a distance of 1 km along the disposal channel. Build up in total N was up to 2908 kg ha(-1), available P (58 kg ha(-1)), total P (2115 kg ha(-1)), available K (305 kg ha(-1)) and total K (4712 kg ha(-1)) in surface 0.15 m soil. Vertical distribution of these parameters also varied, with most accumulations occurring in surface 0.3 m. Traces of NO3-N (up to 2.8 mg l(-1)), Pb (up to 0.35 mg l(-1)) and Mn (up to 0.23 mg l(-1)) could also be observed in well waters near the disposal point thus indicating initiation of ground water contamination. However, the contents of heavy metals in crops sampled from the area were below the permissible critical levels. Though the study confirms that the domestic sewage can effectively increase water resource for irrigation but there is a need for continuous monitoring of the concentrations of potentially toxic elements in soil, plants and ground water.     

Distribution and mobilization of U, Th and 226Ra in the plant-soil compartments of a mineralized uranium area in south-west Spain

The activity concentrations of natural uranium isotopes (238U and 234U), thorium isotopes (232Th, 230Th and 225Th) and 226Ra were studied in soil and vegetation samples from a disused uranium mine located in the Extremadura region in the south-west of Spain. The results allowed us to characterize radiologically the area close to the installation and one affected zone was clearly manifest as being dependent on the direction of the surface water flow from the mine. The activity concentration mean values (Bq/kg) in this zone were: 10,924, 10,900, 10,075 and 5,289 for 238U, 234U, 230Th and 226Ra, respectively, in soil samples and 1,050, 1,060, 768 and 1,141 for the same radionuclides in plant samples. In an unaffected zone, the activity concentration mean values (Bq/kg) were: 184, 190, 234 and 7251 for 235U, 234U, 230Th and 226Ra, respectively, in soil samples and 28. 29, 31 and 80 in plant samples. The activity concentrations obtained for 232Th and 228Th showed that the influence of the mine was only important for the uranium series radionuclides. The relative radionuclide mobilities were determined from the activity ratios. Correlations between radionuclide activity concentrations and stable element concentrations in the soil samples helped to understand the possible distribution paths for the natural radionuclides.     

The influence of the development of temperate fruit tree species on the potential for their uptake of radionuclides

This paper reviews the published literature that describes the phenological development of above and below ground organs of temperate fruit trees (top fruit), particularly with respect to apple (Malus domestica). Critical information is presented which is considered appropriate in developing an understanding of the potential for top fruit species to take up radionuclide contaminants from the atmosphere and the soil. Information is cited on how climatic and edaphic factors influence the growth and development of temperate fruit trees, the phenological production of their leaf area and the development and growth of their fruit and hence the potential for foliar and fruit uptake of radionuclides from the atmosphere. The study also reports on the importance of the distribution and phenological development of roots in the soil and the potential for their uptake of radionuclides from the soil. The effects of above and below ground management procedures, within temperate fruit orchards, on potential radionuclide uptake are also considered. It is concluded that the potential for the uptake of radionuclides by temperate fruit tree species will depend on a number of phenological and physiological factors. For uptake from the soil these factors include; root distribution and density in the soil profile, seasonal changes in the production and distribution of roots, and the presence and amount of water in the soil. These factors are themselves influenced by rootstock type and its growth vigour, scion type and its growth vigour, tree age, spacing of trees in the orchard, orchard management practices (presence or absence of weeds or grass under the trees) and soil type and depth. Direct uptake by the shoot, however, will be influenced by the climatic conditions at the time of exposure and the presence of foliage. Deposition and uptake are likely to change with leaf area development and the ability of radionuclides to penetrate the cuticle of the leaf changes with seasonal development. Transport of radionuclides to the fruit may also depend on the time of season, as the importance of the xylem and phloem transport routes can change with the growth and development of the fruit.