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4.3.1 Planning approach: monitored non-intervention

Contents
4.3.1.1 Introduction
4.3.1.2 Natural attenuation
4.3.1.3 Physical processes
4.3.1.4 Chemical processes
4.3.1.5 Biological processes
4.3.1.6 Alternative land uses and agricultural countermeasures

4.3.1.1 Introduction

A variety of naturally occurring physical, chemical and biological processes in the subsurface reduce contaminant concentrations at a given point in space and time without human intervention [IAEA-2006b]. The combination of these processes is known as natural attenuation or “non-intervention” approach. This depends on the natural processes of retention (sorption), retardation (physical, chemical and biological), and radioactive decay.

Consideration of this option requires modelling and evaluation of contaminant degradation rates and pathways to demonstrate that natural processes will reduce contaminant concentrations below regulatory standards before exposure through various pathways can occur [IAEA-1999].

A decision not to intervene in site clean-up implies reliance on the capacity of natural media (rocks, soils, sediments and groundwater) to retard contaminant migration (i.e., natural attenuation) or on physical, chemical and biological processes to reduce activity levels to below those of concern (i.e., dilute and disperse). In either case, environmental monitoring will be required to verify that such an approach is effective for the system under investigation. It should be noted that, ultimately, all remediation options that do not entail complete removal of the contaminant source would de facto revert to this solution if the half-lives of the radioactive species exceed a few hundred years [IAEA-2004b].

It is also important to draw a distinction between those radionuclides that occur naturally and those that do not, such as caesium, technetium and the transuranics. In the case of the former, reference may be made to the known geochemistry of the element in a given environmental medium (see Section 3.2.4). This provides a degree of confidence in predicting future migration behaviour. For artificial radionuclides, experience can be limited to laboratory data or small scale field trials.

In general, natural attenuation is considered a viable option when it can be determined that contaminants are degrading or becoming immobilized at a rate faster than the rate of migration and are not expected to reach human or ecological receptors. Doing ‘nothing’ may be considered the baseline option in any remediation case. In terms of expenditure on actual remediation activities, this is certainly the cheapest option. Nevertheless, it may entail a variety of other costs, including social and economic, at a later stage. Most notably cost for monitoring would arise. The cost efficiency of active remediation would be compared with this baseline option, taking all cost elements into account for all possible remediation options. The advantages of natural attenuation include reduced generation of remediation waste and possible reductions in the cross-media transfer of contaminants. The disadvantages include slower clean-up, the creation of transformation products that may be more toxic than the original contaminants, more costly site characterization, a reliance on uncertain institutional controls to ensure long term monitoring, and the chance that subsurface conditions will not support natural attenuation as long as necessary [IAEA-2006b]. Also from the point of influence on workers health and safety non-intervention is it the best approach.

When natural attenuation is considered as a remediation option, monitoring is performed to assess contaminant migration, degradation and retardation. This is often referred to as monitored non-intervention. The purpose of monitoring is to ascertain compliance with regulatory requirements and to recognize emerging problems well in advance and thus to be able to implement contingency plans in good time. An approach relying on monitored natural attenuation consists of the following three main elements: a site assessment and monitoring programme, a model to predict the site development and a contingency plan. These three elements are developed interactively, whereby modelling results are used to optimize the monitoring programme while the model in turn is refined using the monitoring and site assessment data. The contingency plan is periodically revised on the basis of conclusions from the other two elements. Mathematical methods to deal with spatial and temporal parameter uncertainty in this context have been developed.

The physical, chemical and biological processes as well as the rate and extent to which these natural attenuation processes occur depend on the contaminant and site hydro-geological and geochemical conditions. These processes are typically categorized as either destructive or non-destructive. Destructive processes reduce the potential risk from a contaminant by converting it to a less toxic form and include bio-degradation and hydrolysis. Bio-degradation is by far the most prevalent destructive mechanism. Non-destructive processes reduce potential risk from a contaminant by reducing its concentration and thus its bio-availability in groundwater or surface water.

Non-destructive processes include hydrodynamic dispersion and dilution, and adsorption, which reduce the mobility and solution concentration by binding to soil minerals and organic matter.

Each contaminant tends to be unique in the way different environmental processes affect its fate, so making generalizations that apply to all contaminants is inappropriate. Especially significant is the difference between organic and inorganic contaminants. The fate of organic and inorganic contaminants is controlled by a combination of physical, chemical and biological processes. The physical processes control the rate and direction of travel as contaminants migrate through soil away from the source. The chemical and biological processes determine the extent to which the initial compounds will be transformed in the soil. Although organic contaminants may be completely degraded to carbon dioxide and water, some intermediate degradation products may pose a greater risk than the original contaminant. For example, vinyl chloride is more persistent, more mobile and more toxic than its parent chlorinated compounds. Some inorganic contaminants are amenable to destructive attenuation, for example, oxy-anions, nitrate, sulphate, chromate and arsenate. The resulting products, however, may or may not be of lesser concern: for instance, nitrogen gas, ammonia and Cr3+. In general, inorganic contaminants may be transformed by non-destructive processes to forms that have lower mobility’s or bio-availabilities.

It is important to note that inorganic contaminants persist in the environment because chemical elements are not amenable to attenuation by destructive processes, except for radioactive decay.

The presence of a contaminant mixture can enhance or inhibit natural attenuation of any one component of the mixture. In some cases the presence of co-contaminants may be aiding natural attenuation reactions to occur, but in other cases co-contaminants can interfere with these processes. For example, the presence of fuels can enhance the bio-degradation of chlorinated solvents, whereas the degradation reactions that reduce pH can mobilize radionuclides and metals. Conversely, the presence of metals, including radionuclides, can inhibit bio-degradation.

The non-intervention does not need to mean only a “to do nothing” approach. It can also include some human activities to facilitate the natural mechanisms and processes of retardation, retention and decay of contaminants, especially when large areas of landscape are treated.

4.3.1.2 Natural attenuation

The concept of natural attenuation has received a great deal of attention in recent years. It constitutes the least invasive approach to environmental restoration. The concept is not new; for example, it forms an integral component of the design criteria for geological repositories that depend on geochemical processes to retard radionuclide migration to the biosphere. It is not entirely without financial cost. Reliance on natural attenuation requires adequate monitoring, owing to the evolution of natural systems with time and the incomplete understanding of the processes operating. The effects of any change in land use or in water abstraction would also need to be assessed, hence the increased use of the term ‘monitored natural attenuation’ in the literature [IAEA-2004b].

A large number of processes can contribute to natural attenuation, as discussed below. Figure 4.2 illustrates the effect of some of these processes on the migration and concentration distribution of radionuclides. In order to be effective, they must prevent or delay the arrival of a radionuclide at a receptor until such time that it will have decayed to an insignificant level.

Figure 4.2 Transport mechanisms effecting dilution and attenuation
Figure 4.2 Transport mechanisms effecting dilution and attenuation

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Whether to intervene or to rely on natural attenuation can only be determined on a site by site basis [IAEA-2004b]. Factors militating against intervention include:

  • The areal extent of the contamination;
  • The accessibility of the site;
  • The proximity to sensitive receptors;
  • The radionuclide inventory;
  • The time frame;
  • The presence or absence of co-contaminants;
  • The chemical and mineralogical characteristics of the material;
  • In the case of surface deposits, the geotechnical stability;
  • The transmissivity of the host medium.

A comprehensive site investigation programme is essential to determine these factors.

The degree of confidence that can be ascribed to natural attenuation in preventing harmful exposure or environmental damage is proportional to the level of characterization of that site. Developing an understanding of the physical, chemical and biological processes operating is more crucial in the case of natural attenuation than if the contamination were to be removed physically from the site.

Figure 4.3 Schematic presentation of the principle of natural attenuation
Figure 4.3 Schematic presentation of the principle of natural attenuation

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A decision to apply monitored natural attenuation (Figure 4.3) as the preferred management strategy will invariably be made by considering a combination of scientific, economic and political criteria. Ideally it should be based on a prior risk analysis of the specific site and follow an established technical protocol. Given a backdrop of scarce resources, various initiatives are under way to promote the acceptance of natural attenuation as a part of a cost effective and environmentally sound solution for radioactively contaminated sites worldwide [IAEA-2004b].

4.3.1.3 Physical processes

Physical processes, such as volatilization and dispersion may also contribute to natural attenuation. The transport and retention mechanisms for dissolved organic contaminants are largely the same as for inorganic constituents.

In some instances of contamination, concentrations of non-miscible organic compounds may be so high that they form a three phase system together with the solid substrate and the groundwater, often referred to as non-aqueous phase liquids (NAPL). In cases where the vapour pressure is high at ambient temperatures, even a four phase system may develop, with a separate gas phase. In the unsaturated zone a four phase system may be present in the sense that in-phase polar liquids fill some of the pore space.

Volatilization removes contaminants from groundwater or soil by transfer to a gaseous phase, eventually reaching the unsaturated zone. For highly volatile organic compounds such as benzene, volatilization may account for 5 – 10 % of the total mass loss at a site, with most of the remaining mass loss due to bio-degradation. For less volatile organic compounds, the expected mass loss due to volatilization would be lower, of course. Volatilization and transfer into the unsaturated zone may actually enhance bio-degradation of certain organic compounds [IAEA-2006b].

Where a separate phase of non-miscible organic compounds exists, two cases can be distinguished:

  1. The density of the organic liquid is lower than the density of water. In this case the contaminant will float on the groundwater table.
  2. The density of the organic liquid is higher than the density of water. In this case the organic liquids will collect at the bottom of an aquifer, often referred to as dense non-aqueous phase liquids (DNAPL).

The potential for attenuation by physico-chemical processes is lower for those lighter, and also more volatile, organic phases. They may readily migrate as liquid or gas phase. Conversely, the denser liquids collect in depressions at the bottom of the aquifers and remain rather stationary, also due to the typically rather higher viscosity. This, indeed, makes them rather inaccessible to pump and treat remediation techniques. While the bulk of the contaminant may remain stationary, a small fraction may dissolve in the water and thus lead to a persistent source term. Natural bio-degradation processes may give rise to a continuing source term of degradation products that may be of concern. It is further possible that such dense non-aqueous phase liquids act as an in-situ solvent extraction process, concentrating heavy metals, including radionuclides. On the other hand, lighter organic phases are often more amenable to bio-degradation.

Lighter-than-water organic liquids floating on the water table may become entrapped in the capillary fringe due to a fluctuating groundwater table. The migration and retention processes in the four phase system of the type soil solids – pore water – soil gas – liquid organic are rather complex and controlled inter alia by the surface tension of the organic liquid and its vapour pressure.

The dispersion of dense non-aqueous phase liquids is initially driven by gravity and controlled by the capillary forces in the unsaturated zone. Once they reach the saturated zone, a three phase system develops. The further downward movement is controlled by the surface tension of the organic phase and the hydrodynamics in the aqueous phase. These factors may result in dispersion of the organic phase. If the amount of dense non-aqueous phase liquids is not sufficient for a complete in-phase flow, droplets of the organic phase may become trapped and isolated in pores due to their surface tension. This in turn will reduce the permeability of the aquifer concerned. The trapped droplets can act as a long term source for small releases of organic contaminants and are not amenable to removal by techniques such as pump and treat.

  • Radioactive decay. The half-lives of radionuclides now present in the environment range from microseconds to many millions of years. For higher members of the natural series (234U, 235U, 238U, 232Th), together with some transuranics (e.g., 239Pu) and fission products (e.g., 99Tc, 129I), no substantial decay will have occurred even on the longest assessment timescale. However, many other isotopes produced by nuclear fission (e.g., 60Co, 90Sr, 137Cs) will not persist beyond a few hundred years. Clearly, it is therefore essential that a detailed radionuclide inventory be compiled before deciding to adopt natural attenuation as a management policy at any given site. The extreme fractionation between members of a decay series caused by chemical processing precludes the assumption of secular equilibrium in the majority of cases [IAEA-2004b].
  • Dilution and dispersion. Radioactive materials are discharged routinely into the air and into surface waters, both fresh and marine, from nuclear facilities worldwide.
    The effectiveness of dilution in aqueous media is critically dependent on the speciation of the radioelement under the prevailing environmental conditions. This will control factors such as solubility, adsorption to surfaces, bio-availability and toxicity. Many radiologically important elements may be concentrated by geochemical and/or biological processes, leading to secondary sources of potential contamination. Similarly, physical dispersion of solids may not always be effective if the size and density of the particles differ significantly from the ambient environment [IAEA-2004b].
    There is no doubt that, even where not proscribed by legislation, the dilute and disperse option is opposed by regulators, environmental groups and the public at large.
  • Filtration. In most situations the dominant exposure pathway is via flowing water. Resistate minerals (e.g., monazite, zircon and barite), other insoluble materials, for example cement, or particulate matter on to which radionuclides have become bound may be retarded by filtration. This will depend on the relative size of the particles and the pore distribution of the host medium, although even small colloids may be removed by fine grained clay matrices or fibrous peat. In the case of aquifer transport, adequate characterization of the hydro-geological flow regime (permeability, hydraulic conductivity, heterogeneity, fracture distribution) is a prerequisite for a quantitative assessment. Variably saturated conditions and geotechnical issues also have to be taken into account for surface deposits.
  • Volatilization. Radon produced by decay of parent radium isotopes will escape from well ventilated soils or heaps, and hence the progeny will be subject to atmospheric dispersion. Methylated and permethylated forms of bismuth, lead, polonium and selenium microbially generated in the subsurface can also be volatilized [IAEA-2004b].

4.3.1.4 Chemical processes

Examples of naturally occurring chemical processes in the subsurface that might reduce contaminant concentrations at a given point in space and in time without human intervention are:

  • Precipitation. Relatively few natural series radio-elements and no artificial isotopes will exist in sufficient mass concentrations to precipitate as a pure phase from surface, pore or groundwaters: the exceptions are uranium, lead and thorium.
    Uranium is a relatively mobile element in the near surface zone, owing to the stability of U(VI) aqueous complexes. However, it may be precipitated by reduction to U(IV) or in the form of uranyl minerals, principally phosphates, silicates, arsenates, vanadates and oxy-hydroxides, several of which may occur simultaneously at the same locality. It follows that the amount of uranium released to groundwaters or surface waters from these secondary sources will depend on the solubility and dissolution rate of the phases as a function of pH and water composition. Unfortunately, too few quantitative data exist at present to allow predictive modelling an issue that needs to be addressed [IAEA-2004b].
    Lead, for example, may precipitate as the insoluble sulphide galena (PbS) that will incorporate 210Pb by isotopic substitution.
    Thorium occurs only in the tetravalent state and is substantially insoluble except at very low pH. Where mobilized, for example in acid mine drainage (AMD), fixation occurs rapidly, often within a few micrometres, via the formation of silicates or, in the absence of silica, oxy-hydroxides.
  • Co-precipitation. Radionuclides present at very low mass concentrations can nevertheless form solid phases by co-precipitation in mineral lattices. An important example is the high selectivity shown by radium for barite, a mineral that has been very well characterized and is also exploited in a remediation context. It is likely that transuranic isotopes would be similarly incorporated in uranium and lanthanide bearing minerals. Establishing the geochemical controls on migration of artificial radio-elements is the major challenge to workers involved in the remediation of legacy nuclear sites [IAEA-2004b].
    Co-precipitation on ferric oxy-hydroxide flocks is an extremely efficient removal mechanism for a large number of radio-elements in solution. As the contaminants tend to be released upon crystallization to goethite, the process is often classified under the more general heading of sorption.
  • Sorption. Sorption, the process by which particles such as clay and organic matter ‘hold onto’ liquids or solids, retards migration of some organic compounds. This increases the time for bio-degradation to occur before contaminants can migrate to a potential receptor. Sorption is controlled by the organic content of soil, soil mineralogy and grain size.
    In its strictest sense, sorption refers to the non-specific and reversible uptake of ionic species at charged surface sites. Used loosely, it has come to encompass aspects of co-precipitation, ion exchange and a number of ion specific interactions that are more appropriately termed complexation. The distinction is not made here other than in the case of co-precipitation, described above, as the latter clearly extends beyond the surface, resulting in the formation of a defined mineral phase.
    Certain functional groups, notably the carboxylic or phenolic groups, on organic molecules will dissociate to a certain degree when the substance is dissolved in water. Such substances are termed ‘polar’. These groups, being anionic in nature, will give the molecules an overall negative charge and thus in general disfavour attenuation by hydrolysed mineral surfaces that are also negatively charged. There may be, however, more complex interaction mechanisms via hydrogen bonds or whereby metal ions act as bridges between the hydrolysed mineral surfaces and the charged molecule. In addition, complex soil organic constituents that are attached to the mineral surfaces can act as intermediates [IAEA-2006b].
    The interaction between non-polar organic molecules, i.e., those that do not dissociate in water, and solid mineral surfaces is much more complex. Such molecules may form surface coatings on clays, for example, and hence become immobile.
    Clay minerals typically show a strong affinity for radionuclides in the cat-ionic form. Geological media with high clay mineral content are therefore more likely to affect attenuation. Adsorption and ion exchange would be expected to play an important role in retarding the migration of soluble monovalent and divalent ions. Examples include the pronounced retention of caesium on zeolites (e.g., clinoptilolite) and the substitution of strontium for interlayer cat-ions in smectites. Surface sorption is an important transient for multivalent ions in the formation of new mineral phases [IAEA-2004b].
  • Complexation by organics. A number of radionuclides exhibit significant migration potential in the presence of aqueous, low molecular weight organic compounds. Equally, however, immobile organic matter in the form of peat or organic rich horizons in soils and sediments may provide an excellent substrate for radionuclide retention. These phenomena have been studied extensively in the context of ‘natural analogue’ studies for the performance assessment of radioactive waste repositories. Uranium approaching percentage levels has been reported in peat from Canada and northern Europe, whereas iodine, often considered to be a conservative tracer in such assessments, has been shown to be fixed in organic rich lacustrine deposits [IAEA-2004b].

4.3.1.5 Biological processes

Bio-degradation is a process or collection of processes (e.g., bio-mineralization, ‘bio-sorption’ and microbially mediated phase transfer) in which naturally occurring micro-organisms such as yeast, fungi and bacteria break down organic substances into less toxic or non-toxic compounds. The ability of micro-organisms to metabolize nutrients depends on the chemical composition of the environment. In most organisms, the metabolic process requires the exchange of oxygen and carbon. Bio-degradation can occur in the presence or absence of oxygen. Nutrients and essential trace elements must be available in sufficient quantity in order for the micro-organisms to break down all of the organic contaminant mass. The complex bio-geochemical processes effecting the fixation or mobilization of metals, including radionuclides, in various types of soil ecosystems have been studied with increased intensity in the aftermath of the Chernobyl accident and in other remediation contexts [IAEA-2004b].

In general there are three bio-degradation processes:

  • Those where the contaminant is used by the microbes as the primary food source;
  • Those where the contaminant is used to transfer energy;
  • Those where the bio-degradation occurs in response to a chain reaction between the contaminant and an enzyme produced during an unrelated reaction (termed co-metabolism).

For fuel hydrocarbons, the first process is dominant. The full degradation of chlorinated solvents requires all three processes. Until recently, scientists believed that chlorinated organic compounds were generally highly resistant to bio-degradation in the environment, but in the past two decades a variety of biological processes have been discovered that can transform these compounds in nature [IAEA-2006b]. It is worth noting that many microbial communities are very adaptable to the local circumstances and in the absence of other readily available energy sources may evolve to utilize highly resilient organic compounds. These processes are extremely complex and not yet fully understood, but are a topic of a significant body of research:

  • The contaminant is used as the primary food source. In the presence of oxygen, bacteria are able to use the carbon in organic contaminants as their primary food source. This relatively rapid process has greater potential for fuels and chlorinated solvents with few chlorine atoms per molecule. Highly chlorinated organic compounds are less susceptible to this type of degradation. In the absence of oxygen, micro-organisms can sometimes still use contaminants as their primary food supply. This form of degradation under anaerobic conditions depends not only on the compound but also on temperature, pH and salinity. In breaking down chlorinated solvents, bacteria use nitrate, iron, sulphate and carbon dioxide to help metabolize the carbon in the organic contaminants. If degradation is complete, the products are usually carbon dioxide, water and chlorine.
  • The contaminant is used to transfer energy. All living organisms respire in that they use organic substances and other nutrients by breaking them down into simpler products. In the absence of oxygen, micro-organisms may use chlorinated compounds as an aid to respiration rather than as a food source. This is accomplished through an electron transfer process. Where carbon in a contaminant is the food source, the contaminant is an electron donor. In the case where food is obtained from a different source, the contaminant sometimes aids this transfer by accepting electrons that are released during respiration. The most common anaerobic process for degrading chlorinated compounds is an electron transfer process termed reductive de-chlorination. In this process, hydrogen atoms are sequentially substituted for chlorine atoms in the contaminant molecule. The major requirement for reductive de-chlorination is the presence of other organic compounds that can serve as the food source.
  • Co-metabolism. In co-metabolism microbes do not degrade the contaminant directly, but the contaminant degrades by enzymatic reactions that occur during metabolism of other substrates. Reductive de-halogenation occurs only under anaerobic conditions, although some chlorinated compounds can be biologically degraded by other mechanisms in aerobic environments. Aerobic co-metabolism requires the presence of electron donor compounds, such as methane, toluene, phenol or other organic compounds, that leads to production of the enzymes.

The bio-degradation process most frequently observed at sites where natural degradation of chlorinated solvents occurs is reductive de-halogenation, where microbes use the chlorinated compounds for energy metabolism and remove a chlorine atom. For example, reductive de-halogenation can transform tetra-chloro-ethene (PCE), which has four chlorine atoms, to tri-chloro-ethylene (TCE), which has three, and then transform tri-chloro-ethylene to cis-dichloro-ethene (cis-DCE), with two chlorine atoms. Cis-dichloro-ethene can then be reduced to vinyl-chloride, which can be further reduced to ethylene, an essentially harmless compound. A potential risk of this process is a build-up of intermediate transformation products, such as vinyl-chloride, that are more toxic than the parent compound.

Natural attenuation of chlorinated compounds is a slow process and may not occur at all at a given site. Thus it is not likely to be an appropriate strategy at sites where rapid and sure clean-up of contamination is required. Monitoring for natural attenuation can also be costly. Nevertheless, the presence of intermediate and final degradation products indicates that at some sites natural degradation processes do take place. A primary advantage, however, is that it can eliminate the need for an engineered solution that may disrupt the site or it can reduce the size of an area requiring treatment with an engineered system. Engineering intervention, such as supplying nutrients to stimulate the natural degradation processes, can greatly enhance the attenuation [IAEA-2006b].

4.3.1.6 Alternative land uses and agricultural countermeasures

When extensive areas have been contaminated, many of the discussed remediation methods may be too expensive to carry out or too intrusive. In particular, when the land was used for agricultural purposes, alternative uses may need to be considered. Such alternative uses may range from switching to different crops to turning to completely different uses, such as parkland [IAEA-2004b].

Many studies have been targeting possible agricultural countermeasures in response to concentration levels in food and agricultural crops exceeding the applicable standards. Most studies have been conducted to test the effect of different physical and chemical countermeasures. However, information on the long term effect of countermeasures, and especially of a change to non-food crops, is still scarce.

When investigating alternative crops, the principal questions to be addressed are:

  • Can an alternative crop be found that is suited to the climate and soil conditions prevailing in the contaminated area?
  • What is the fate of the radionuclide in the cultivation system and along conversion routes?
  • How does the radionuclide in question behave during biomass processing and what is the expected radionuclide concentration in the end products?
  • What is the exposure during biomass cultivation and processing?
  • Would production and utilization of the alternative crop be economically feasible?
  • What are the overall prospects for the chosen alternative crop as an alternative land use for large contaminated areas?

In order to understand the fate of the various radionuclides and their distribution in products, residues and waste, one needs to know the various radionuclide fluxes. These depend on the initial deposition levels, crop accumulation factors, which in turn depend on plant and soil characteristics, and the radionuclide accumulation in the produce (e.g., wood, rape or beetroot). Whether residues and waste need to be treated as radioactive waste depends on the radionuclide concentration and the applicable exemption limits.

Crops used for liquid bio-fuel (oils, alcohol) production, such as rape, wheat, sugar beet, barley, potatoes and winter rye, may be suitable alternative crops.

The data in Table 4.1 indicate that crops with a low transfer factor (TF) to the useable product can be found and that the resulting liquid bio-fuels are almost free from activity, and that 137Cs levels in the waste and residues are generally of no concern.

Crop Plant component Caesium TF (10-3 m2/kg)

Spring wheat Straw 0.23-0.36
Grain 0.13-0.16

Winter wheat Straw 0.27-0.44
Grain 0.08-0.18

Rye Straw 0.43-0.60
Grain 0.17-0.29

Spring Rape Green mass 0.33-0.81
Straw 0.38-0.92
Seeds 0.27-0.66

Brassicaceae Seeds 0.037-3.4
Peas Seeds 0.69-1.25
Straw 0.82-1.45

Leguminosae Seeds 94 (12-750)

Sugar beet Root 0.43

Root crops Root 0.025 (1.1-110)

Green vegetables Leaves 0.07-4.86
Leaves (peaty) 260 (25-2700)

Sunflowers Straw 1.48-2.88
Seeds 0.43-0.82

Table 4.1 Caesium transfer factors to different plant parts of some potential bio-fuel crops [IAEA-2004b].

Examples from Belarus, however, show that caesium levels in oil cake from rapeseed oil (~2000 t/ha) and the pulp and vines from sugar beet (~4000 t/ha) may be too high for use as animal fodder and for incineration and that they may have to be disposed of as radioactive waste. On the other hand, the production of rapeseed and processing to edible rapeseed oil are profitable technologies and the levels of caesium and strontium in the rapeseed oil after three filtrations and bleaching are below the detection limit [IAEA-2004b].

The valorisation of contaminated land by willow short rotation coppice (SRC) for energy production has been addressed is another possibility. Coppicing is a method of vegetative forest regeneration by cutting trees at the base of their trunk at regular time intervals. Fast growing species of the Salix genus (willows) are frequently used in a coppice system because of the ease of their vegetative reproduction and the large biomass produced. The harvested biomass is converted into heat or power (with an appropriate off-gas treatment). As such, this non-food industrial crop is a potential candidate for the valorisation of contaminated land that has use restrictions. Short rotation coppice may be preferred over traditional forestry since revenues come sooner after establishment and more regularly (every 3 – 5 years). Short rotation coppice yields are also high on good agricultural soils, and its use is not a drastic change in land use; short rotation coppice is easy to introduce and it is easy to return the land to the production of food crops. Short rotation coppice may also be considered as complementary to forestry, given the different culture requirements of both vegetation systems. Forests perform well on sandy soils, whereas short rotation coppice requires soils with a sufficient water retention capacity. Short rotation coppice has additional potential advantages in a contamination scenario: since it is a perennial crop, dispersion of radionuclides will be limited. Harvest can be in winter, when the soil may be covered by snow, resulting in radiation protection of the workers. Finally, short rotation coppice cultivation is not too labour intensive, which is also an advantage with respect to exposure.

Willow short rotation coppice may be a suitable rehabilitation tool for highly contaminated land, but only if the radionuclide levels in the wood are below the exemption limits for fuel wood, if the average yearly dose received during coppice cultivation and coppice wood conversion is acceptable, if short rotation coppice can be grown successfully in the contaminated territories (soils, climate), if the cultivation of short rotation coppice is technically feasible and if short rotation coppice production and conversion are economically profitable.
For soils with a medium to high fixation (finer textured soils) and sufficient potassium availability, the transfer ratio of concentration in plant biomass to concentration in soil is < 10-5 m2/kg, and wood can be safely burnt and the ashes can be disposed of without concern [IAEA-2004b]. For light textured soils, however, with a low radio-caesium fixation and low soil potassium, the transfer to wood is around 10-3 m2/kg, and concentrations in wood may be elevated enough that the prevailing exemption limits are reached. Given that transfers for common forestry and for straw of winter wheat and rape are comparable, the same applies for burning wood or straw for energy.

Short rotation coppice has generally a high annual yield of about 12 t/ha, but sandy soils are only suitable for short rotation coppice production if well fertilized and irrigated. Only during the conversion phase and when burning highly contaminated wood (3000 Bq/kg) do doses in the vicinity of ash collectors exceed the level of 1 mSv/a for a member of the general public [IAEA-2004b]. Contributions from other possible exposure pathways are negligible (external exposure during cultivation and transport, inhalation dose in the combustion plant and doses to the public following wood burning).

Crop yield and the capital cost of the conversion units are among the most important parameters affecting system profitability. At the production site, a minimum yield of 6 t/ha/a is required for Belarus production conditions and of 12 t/ha/a for western European conditions, if all other parameters are optimal [IAEA-2004b]. Heating schemes may be a viable option for wood conversion in Belarus, whereas electricity generation schemes are not.
Subsidies would be required in Europe to make wood conversion economically feasible. It has also been concluded that the existence of a contamination scenario does not necessarily hamper the economic viability of the energy production schemes studied. The cost associated with the disposal of contaminated ashes was estimated as less than 1 % of the bio-fuel cost and will not affect economic feasibility.

Forestry can also be considered to be an adequate alternative land use [IAEA-2004b]. Soil to wood transfers to coniferous and deciduous wood are around 10-3 m2/kg and are hence comparable with the transfers to willow wood observed for low fertile soils with limited caesium fixation. They are high compared with the transfers observed for willow in finer textured soils and soils with an adequate potassium status. Moreover, the annual biomass increase is only 6 t/ha for forests and may attain 12 t/ha for short rotation coppice grown on soils with an adequate water reserve and fertility status. Short rotation coppice may hence be a more promising land use option on these types of soil than traditional forestry. On soils with a low water reserve (e.g., sandy soil), however, willow yield without irrigation is too low to be economically feasible, and forestry may hence be the preferred option [IAEA-2004b].

Fibre crops are also potential alternative crops for agricultural land with restricted use. Potentially suitable crops are the annual fibre crops hemp (Cannabis sativa L.) and flax (Linum usitatissimum L.). Hemp and flax are well known arable crops that have been cultivated for centuries. Ukraine has a legacy of flax and fibre hemp cultivation, but in Belarus there is only some flax production. Since the early 1990s the acreage for production of flax and hemp has declined dramatically in Ukraine. Establishment of fibre crops on contaminated arable land is generally of no radiological concern. The transfers observed to hemp fibres are a factor of 4 to 50 higher than the transfers observed to flax. Cultivation is hence generally restricted to not too contaminated areas (< 1000 kBq/m2). For both crops it holds that contamination levels in the waste products (oil seed cake, chaff, ash after burning of straw) may, however, be high enough that they should be considered as radioactive waste. The economics of this land use has not, however, been investigated [IAEA-2004b].

The introduction of alternative crops in a contamination scenario may be a feasible and adequate remedial option. Although there are some scenarios in which energy production from short rotation coppice and potentially other alternative crops on contaminated arable land is radiologically safe and economically feasible, installing this cultivation system on a large scale requires extensive logistics, infrastructure and initial investment. Implementation is likely only to be successful with adequate political support.
There are a number of additional types of alternative land use, such as the creation of parkland. Such measures, however, would largely be administrative and would amount to ‘institutional control’.

Many studies have been concerned with possible agricultural countermeasures in response to concentration levels in foodstuffs and agricultural crops exceeding the permissible levels in the wake of the Chernobyl. To this end, a database of 5261 experiments carried out during 1987-1999 and their respective results was compiled by participants from Belarus, the Russian Federation and Ukraine [IAEA-2004b]. The main evaluation criterion was the efficiency of experimental treatments in reducing radionuclide concentrations in final products as compared with untreated controls. It is important to note, however, that the majority of countermeasures do not intend to influence soil or groundwater concentrations, but aim to break exposure pathways.

Countermeasures can be based on a selection of crops that exhibit smaller radionuclide uptake than crops used previously, on food processing to reduce radionuclide contents or on choosing non-food crops, resulting in either case in a produce from the contaminated land that is radiologically acceptable [IAEA-2004b].

Assessments have shown that substituting crops and fertilization are the most effective countermeasures in plant production. The efficiency of countermeasures, expressed by the reduction factor of radionuclide concentration in final products, was found to be of the order of 3 to 9, depending on the soil and individual crops. Substituting crops may not be expensive, but its viability depends on a variety of economic conditions [IAEA-2004b].

Fertilizer application will suppress the uptake of certain radionuclides, mainly due to competitive effects. Thus potassium dosages will generally decrease the soil to plant transfer of 137Cs, certainly when the soil is low in potassium. Reported reduction factors have varied between studies, but overall reduction factors ranging between 1.1 and 5.0 have been obtained. The efficiency of potassium additions strongly depends on the exchangeable potassium content in the soil. For soils with a low to optimal potassium content, high dosages of potassium fertilizer are very effective and profitable. For soils with high potassium content, only moderate dosages of potassium fertilizer are recommended to replace the potassium removed with crop yields [IAEA-2004b].

The behaviour of 90Sr and its uptake by plants are controlled by its similarity to calcium. Many investigators have found a significant correlation between strontium transfers and the reciprocal of the exchangeable calcium content [IAEA-2004b]. Consequently, much of the research and actions to reduce strontium uptake by plants has centred on the use of lime as a soil based countermeasure. The use of lime has reduced strontium uptake by up to 40 %, the use of limed compost by up to 60 %. Generally, the reduction factor of radionuclide uptake by agricultural crops varies widely, from 1.1 to 3, depending strongly on the initial soil pH. The liming effect is most pronounced for acid soils [IAEA-2004b].

Countermeasures that aim to provide the optimum (from the plant production point of view) rates of fertilizer application are the most viable, since the investment on fertilizer is paid back in the form of additional crop yields, and frequently profits are made. It has to be noted that the addition of nitrogen fertilizer should be moderate, as high dosages appear to stimulate the accumulation of 137Cs and 90Sr in plants. Phosphorus dosages should be in accordance with crop responses and the phosphorus content of the treated soil, and should be crop specific [IAEA-2004b].

In Table 4.1 Caesium transfer factors to different plant parts of some potential bio-fuel crops [IAEA-2004b]: Use CROP
– by Rafael Garcia-Bermejo Fernandez about 6 years ago