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4.3.2 Planning approach: Containment or blocking pathways

Contents
4.3.2.1 Enhanced attenuation
4.3.2.2 Physical barriers and liners
4.3.2.3 Surface barriers

4.3.2.1 Enhanced attenuation

Although contaminated media sometimes provide sufficient attenuation capacity, normally these attenuation mechanisms must be enhanced through technical measures. Methods of enhancement can consist of stimulating bio-degradation of organic compounds, improving soil retention capacities, for example improving the sorption capacity, or changing the geochemical environment, for example changing the bulk redox state, such that migration of metals is hindered. Enhancement of attenuation may be targeted at particular exposure pathways; for example, plant uptake may be minimized or blocked to prevent contaminants from entering the food chain [IAEA-2004b].

Simple ploughing or deep soil mixing is not an efficient means of reducing direct surface gamma exposure as such an approach will result in a dispersal of radionuclides over a larger area, thereby increasing the volume of contaminated soil [IAEA-2004b].

Studies subsequent to the Chernobyl accident found that deep ploughing with digging, combined with liming and potassium fertilizer application, can decrease caesium and strontium transfer from soil to plants by a factor of 3 to 4. The objective of deep ploughing is to skim off the upper 0 – 5 cm contaminated soil layer and burrow it beneath the turned over arable layer (30 – 50 cm), thereby preserving most of the soil fertility. Subsequent cultivation practices have to be limited to shallower depths to prevent the contaminated soil layer from being dug up or roots from reaching this layer. This quite cost effective countermeasure had only limited application after the Chernobyl accident because of the thin humus horizon of the predominantly light textured soils in the region [IAEA-2004b].

Changing the pH and redox conditions in contaminated zones can enhance attenuation, particularly in situations in which treatment is otherwise difficult, such as the presence of fractured rock. Oxidation or reduction can be achieved by injecting aqueous solutions of appropriate agents, or by bubbling gases through the contaminated zone. Long term sustainability is uncertain, and competing geochemical processes need to be evaluated carefully. Environments with relatively low redox potential and high organic matter content (e.g., wetlands) tend to trap metals naturally, a property that can be utilized (see Section 4.5.1.18) [IAEA-2004b].

The number of sorption sites may be increased by adding clay or zeolites to soils. The addition of reactive minerals, such as lime, apatite and its derivatives, such as bone meal, may lead to immobilization through the formation of sparingly soluble mineral phases incorporating the contaminating radionuclides [IAEA-2004b].

4.3.2.2 Physical barriers and liners

Applicable to radiological, non-radiological and mixed contamination, one of the most straightforward means of dealing with contaminated sites appears to be to isolate them from human and other receptors by constructing physical barriers [IAEA-2006b]:

  • Surface barriers, which are intended to minimize surface water infiltration into the contaminated soil of the site, to provide a barrier inhibiting direct contact and intrusion by plants and animals, and to inhibit inadvertent human intrusion. There are several general types of surface barrier, such as single layer covers, engineered multi-layer covers and biotic barriers (see Section 4.3.2.3).
  • In-situ barriers, which are constructed vertically or horizontally below ground level to contain contaminated material. Vertical barriers are comprised of low permeability trenches, walls or membranes to impede lateral migration, usually keyed into a naturally occurring low permeability basal stratum. Horizontal barriers are installed beneath contaminated soil of the site using in-situ techniques such as grouting or soil mixing.

Surface containment systems are fully accessible during construction, allowing checking and testing, i.e., comprehensive quality control. They may be constructed on an uncontaminated surface to act as a liner on to which contaminated material is placed or they may be constructed above contaminated material to act as a cover. Liners form the basis of a dedicated landfill or ‘containment cell’ and invariably are used in combination with covers, for total encapsulation.

A surface barrier alone may not provide sufficient containment or isolation of contaminants so that a combination of technologies may be required to control contaminant migration and/or exposure to contaminants. A cover system and active hydraulic control, i.e., a drainage system, will be needed to limit groundwater rise within the containment.

Physical containment can be used in an integrated fashion with other remedial methods. Excavation of hot spots may precede the construction of a covering system in order to reduce the size of the soil of the site to be contained. In-situ stabilization may be employed as a pre-treatment step to enhance immobilization of contaminants and to provide a stronger base to support a final cover, thus reducing the maintenance needs caused by subsidence. Alternatively, physical barriers can aid other forms of remediation by limiting the volume of contaminated material to be treated when using methods such as groundwater pump and treat.

Forming barriers in-situ by injection (see Figure 4.9) from the surface can reduce construction and waste disposal costs and can be useful for replenishing barriers that have lost their effectiveness over time. Development of barrier emplacement methods that do not involve soil excavation is a significant advantage of this technology.

4.3.2.3 Surface barriers

Surface barriers, often referred to as (landfill) caps, are a common form of remediation for many types of contamination because they are a conceptually easy to understand and fairly inexpensive way to manage some of the risks associated with a contaminated site, such as direct exposure of humans and release of contaminants. They usually also enjoy a high public acceptance, as they seem to indicate visibly that something ‘is being done’. Surface barriers or caps can be used to:

  • Minimize direct exposure on the surface of the contamination from both radioactive and other hazardous substances;
  • Prevent vertical infiltration of water into contaminated zones and wastes that would produce contaminated leachate;
  • Contain waste while treatment is being applied;
  • Control gas emissions from underlying contaminated materials that might be hazardous by themselves (e.g., radon and volatile organic compounds) or act as a carrier for contaminants, for example, 210Pb and 210Po;
  • Create a land surface that can support vegetation and/or be used for other purposes.

The design of surface barriers is site specific and depends on the intended functions of the system. Surface barriers can range from a one layer system of vegetated soil to a complex multilayer system of soils and geo-synthetic products, see Figure 4.4. In general, less complex systems are required in dry climates and more complex systems are required in humid climates. The materials used in the construction of surface barriers include low permeability and high permeability soils and low permeability geo-synthetic products. The low permeability materials divert water and prevent its passage into the contaminated zone. The high permeability materials carry away water that percolates into the barrier. Other materials may be used to increase slope stability.
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Figure 4.4 Generic layout of surface capping [IAEA-2006b]
Figure 4.4 Generic layout of surface capping [IAEA-2006b]

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Low permeability barrier layers are either natural clays and other low permeability soils or geo-synthetic clay liners. Soils used as barrier materials are generally clays that are compacted to a hydraulic conductivity no greater than 1 × 10-8 m.s-1. Compacted soil barriers are generally installed in lifts of at least 15 cm to achieve a thickness of 0.5 m or more. A flexible synthetic geo-membrane (plastics) liner is placed on the top of this layer. The candidate list of polymers commonly used is lengthy, and includes polyvinyl-chloride, poly-ethylenes of various densities, reinforced chloro-sulphonated polyethylene, poly-propylene, ethylene interpolymer alloy (EIA) and many new materials. Geo-membranes are usually supplied in large rolls and are available in several thicknesses (0.5 – 3.6 mm), widths (3 – 30 m) and lengths (60 – 275 m). A composite barrier uses both soil and a geo-membrane, taking advantage of the properties of each. The geo-membrane is essentially impermeable, but, if a leak develops, the soil component prevents significant leakage into the underlying waste. Inspections of existing geo-membranes have, however, shown that their functionality cannot be guaranteed even a few years after their installation. Differential settlement and imperfect seams during installation are the main causes [IAEA-2006b]. In addition, there is no experience with the really long term stability of synthetic materials, as these have been in existence for generally less than 50 years.

For barriers placed over degradable contaminants, the collection and control of methane and carbon dioxide, which are potent greenhouse gases, must be part of the design and operation of the surface barrier. It is, however, generally accepted wisdom that degradable materials should not be emplaced into engineered landfills.

Surface barriers may be temporary or final. Temporary barriers can be installed before final closure to minimize generation of leachate until a better/the final remedy is selected and implemented. They are usually used to minimize infiltration when the underlying contaminant mass is undergoing settling. A more stable base will thus be provided for the final cover, reducing the cost of post-closure maintenance. Surface barriers may also be applied to residue and waste masses that are so large that other treatments are impractical. At mining sites, for example, surface barriers can be used to minimize the infiltration of water to contaminated tailings piles and to provide a suitable base for the establishment of vegetation. In conjunction with water diversion and retention structures, surface barriers may be designed to route surface water away from the waste area while minimizing erosion [IAEA-2006b].

Land filling does not lessen toxicity, mobility or volume of mixed contamination but does mitigate migration. Surface barriers are most effective where most of the underlying contamination or waste materials are above the water table. A surface barrier, by itself, cannot prevent the horizontal flow of groundwater through the contaminated material, only the vertical entry of water into it. In many cases, surface barriers are used in conjunction with subsurface barriers, such as vertical walls, to minimize horizontal flow and migration.

Criteria Quality
Effectiveness in remediation of the contamination
Ease of implementation
Cost associated with the remediation programme
Occupational safety and health risks associated with the technology
Potential secondary environmental impacts (collateral damage)
Prior experience with the application of the technology
Socio-economic considerations

Legend: x low; xx medium; xxx high.

Table 4.2 Evaluation of remediation criteria for given technique

The effective life of physical barrier components can be extended by long term inspection and maintenance. In addition, precautions must be taken to ensure that the integrity of the cap is not compromised by land use activities [IAEA-2006b].

As indicated in Table 4.2 individual criteria of a remediation technique may be evaluated, if data for the technique are available.

The humus content of a soil is important because of its tendency to form co-ordinate bonds with calcium and strontium, which are stronger than the binding by ion exchange sites on soil minerals. Organic matter addition has resulted in strontium transfer reductions by a factor of 1.2 to 7. The latter value was obtained after the addition of 15 % organic matter to a sandy soil. Field experiments on a podzoluvisol (loamy sand) soil in Belarus that increased humus content from 1.5 % to 3.5 % resulted in a reduction of 137Cs and 90Sr activity in perennial grass by a factor of 2 [IAEA-2004b].

Chemical amendments, such as zeolites, ammonium hexa-cyanoferrate (AFCF) or clay minerals, also reduce radionuclide uptake by plants, since the radionuclides are trapped and so rendered less available for plants. A reduction factor of 4.6 in 90Sr transfer has been obtained for a sandy soil after the addition of 1 % zeolites, and a factor of 25 by applying 10 g ammonium hexa-cyanoferrate per square metre. However, the investigation of zeolites and clay amendments in field trials on a loamy sand soil in Belarus resulted in only a rather low reduction of activity in cereals [IAEA-2004b].

A more radical improvement of private hay land and meadows in all Chernobyl affected rural settlements of Belarus, the Russian Federation and Ukraine has been recommended. This countermeasure combines the liming of acid soils, fertilization (including the basic application of organic fertilizers), destruction of old turf, sowing of new grass stand and regulation of soil water (drainage), if needed; for example, radical meadow improvement has resulted in a reduction of grass activity by a factor of 1.7 to 3.5, but other applications have achieved reduction factors for 137Cs of up to 16 – 20. The reduction factor of surface meadow improvement is lower, and is 3.5 on average [IAEA-2004b].

Although strictly speaking not remediation techniques, certain livestock management measures are effective in reducing public exposures. Such measures include feeding complexants, such as Prussian blue, to dairy animals to prevent 137Cs transfer into the milk, or changes in pasture or fodder at critical times to reduce uptake. Achieved reduction factors vary widely between 2 and 15 [IAEA-2004b].

Food processing can significantly reduce radionuclide concentrations in products. The efficiency depends on the type of processing and varies widely, removing 50 – 98% of the 137Cs or 90Sr during the production of butter or casein from milk [IAEA-2004b].

Activity reduction in rape products
137Cs 90Sr

Liming to 6 t/ha 14% 42%

Application of N90P90K180 fertilizer 42% 27%

Liming to 6 t/ha + N90P90K180 fertilizer 45% 59%

Variety selection 2.5 times 3.0 times

Rapeseed oil processing (crude oil) 250 times 600 times

Table 4.3 Effects of various countermeasures on rape radioactivity [IAEA-2004b].

The relative efficiency of different agricultural countermeasures can be seen from the experiments in which rape was grown on radioactively contaminated land in Belarus (Table 4.3). The effect of liming is mainly due to a rise of the soil pH and hence the increased availability of exchangeable calcium. Choosing a rape variety with less uptake offers activity reductions of up to three times. The most efficient removal of activity is offered by oil processing, resulting in a reduction of up to 600 times. Concentrations of radionuclides after a three stage filtration and bleaching are below the limits of detection. The combination of oil seed processing with several agricultural countermeasures therefore allows the production of food grade oil practically free from radionuclides and produces a valuable protein by-product (cake as animal fodder) with permissible concentrations of radionuclides [IAEA-2004b].