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4.5.5 Biological techniques (CT)

Contents
4.5.5.1 Biological barrier walls (Bio-walls)
4.5.5.2 Phyto-stabilisation
4.5.5.3 Constructed wetlands

4.5.5.1 Biological barrier walls (Bio-walls)

Biological barrier walls, often called “bio-walls” represent an in-situ barrier that relies on biological processes to restrict the migration of radionuclides; the principle is shown in Figure 4.16. The application of the technology is most appropriate to geological formations with significant permeability (e.g., sands, sandstone and permeable limestones) and no preferential flow paths such as open cracks and fissures. A bio-wall can be emplaced downstream from the contaminated site or constructed in-situ via the formation of bio-films and bio-colloids [IAEA-2004b]. The development of a bio-wall requires the introduction of suitable micro-organisms and the provision of nutrients and essential elements to further their propagation. Adjustments to the pH or redox potential may also be required to initiate bacterial growth.
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Figure 4.16 The principle of a bio-wall [IAEA-2006b]
Figure 4.16 The principle of a bio-wall [IAEA-2006b]

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The effectiveness of bio-walls results from [IAEA-2006b]:

  • The physical reduction of permeability and hence groundwater flow by the microbial population. This effect can be enhanced by the use of ultra-microcells (less than 100 nm). In the course of growth by metabolism they increase in size and may completely fill the pore space.
  • The generation of metabolites capable of restricting the migration of radionuclides through the barrier wall. Such metabolites are mainly extracellular polymeric substances (EPS), commonly termed ‘slimes’, which the microbial cells use for attaching themselves to the substrate. These extracellular polymeric substances also fill pore spaces and thus reduce permeability.
  • The sequestering of radionuclides from groundwaters by complexation, although it should be noted that subsequent mobilization of these colloidal species could constitute an additional transport mechanism. Microbial action can also be utilized to modify groundwater chemistry (e.g., sulphate reducing bacteria) to immobilize redox sensitive species such as uranium or to prevent the formation of acid drainage [IAEA-2004b]. The latter two methods would act in a similar way to an inorganic reactive wall. Several microbial strains are commercially available.

The application of bio-walls in fractured rocks might be a problem, partly because their hydraulic behaviour is difficult to predict, and partly because comparatively high flow velocities along the fractures may make attachment of micro-organisms difficult.

4.5.5.2 Phyto-stabilisation

The development of a stable and permanent vegetation cover is called by the term “phyto-stabilization”. Such treatment reduces the risk of erosion of contaminated soil from sparsely or non-vegetated land, thus reducing waterborne and dustborne exposure pathways. In addition to preventing erosion, this technique may change the mobility of potentially toxic elements by reducing concentrations in the soil water and on freely exchangeable sites within the soil matrix. Both processes alter the speciation of soil metals, reducing the potential environmental impact. The technologies draw upon fundamental plant and soil chemical processes as well as established agricultural practices. The development of a stable and self-perpetuating ecosystem as a result of this type of treatment may have additional benefits, as in some circumstances plant root activity may also influence metal speciation by changes in redox potential or the secretion of protons and chelating agents. The micro-flora associated with root systems may also be involved in these processes. The rainwater infiltration rate is reduced by plant induced evapo-transpiration, thus reducing the potential for leaching and acid drainage generation. Two applications of phyto-stabilization approaches are presented below: (1) the contaminated area close to the Chernobyl NPP site and (2) the uranium mining dump near Schlema, Germany [IAEA-2004b].

Soil stabilization is very important on certain types of arable land to prevent horizontal radionuclide migration due to water and wind erosion. The 137Cs activity in topsoil in valleys may be increased by 30 – 80 % as a result of run-off of fine soil particles, as shown in field experiments in Belarus. Crop rotation with perennial grasses covering up to 50 – 80 % of the cultivated area and avoiding row crops reduces contaminated topsoil loss from 10 – 20 t/ha to 2 – 3 t/ha. On slopes, deep soil tillage without overturning the arable layer is needed. Conventional ploughing with overturning of the arable horizon should only be carried out to destroy and plough in old turf. Good cultivation practices, such as ploughing parallel to the slope, rather than up and down, will also reduce erosion [IAEA-2004b].

Wind erosion may occur on sandy soils and on drained peaty soils. It is recommended to eliminate root crops on soils for which soil loss amounts to 8 – 15 t/ha or more. The major area (50 – 80 %) of crop rotation should be under perennial grasses. A smaller area can be allocated to winter and spring cereals and to annual grasses. In any case, soils should be under vegetation cover throughout the year, preferentially under perennial grasses. In such a manner soil loss due to wind erosion can be reduced to 2 t/ha [IAEA-2004b].

Practical application of the phyto-stabilisation method close to the Chernobyl NPP site within the Dnieper catchment system is shown in Figure 4.17. This and adjacent drainage basins form a wide area from which contaminated waters flow and sediments are transported downstream through the Pripyat and Dnieper Rivers across Ukraine and to the Black Sea. Phyto-stabilization techniques could in this context also be considered as remedial options. Three phyto-rehabilitation approaches involving willow plantations have been studied [IAEA-2004b].

  1. The effect of willow plantations on vertical migration of radionuclides;
  2. The effect on the stabilization of the Chernobyl cooling pond sediments; and
  3. For lateral erosion control.

The area of interest for studying the vertical migration control by willows was an extremely contaminated zone of 16 km2 on the left bank of the Pripyat River (between 3.7 and 18.5 TBq/km2 90Sr and 137Cs and 0.37 TBq/km2 Pu), which is partly protected from spring floods by a dam. Through modelling exercises it was shown that, due to their high evapo-transpiration rate, willow short rotation coppice (SRC) stands are expected to lower the groundwater table level by 100 – 200 cm in fertilized stands. Without fertilization, a lowering of the groundwater table level of less than 50 cm was predicted. Since the immobilization potential of 137Cs and 90Sr in the willow wood is limited, the influence of plant uptake on migration remains low.

Following the closure of the Chernobyl nuclear power plant, the water level of the cooling pond (22.5 km2; depths between 1.5 and 15 m; with about 111 TBq 137Cs and 37 TBq 90Sr) will drop by 4 – 7 m, and 15 km2 of the sediments will become exposed and may be in need of stabilization. To this end the SALIMAT option was investigated. SALIMATs consist of a roll of willow rods (stems) rolled around central disposable tubes that are unwound by dragging them across the lagoon. Small scale tests have demonstrated that SALIMATs establish well on contaminated pond sediments and produce a full vegetation cover during the second year. The approximated cost of the phyto-stabilization option ranges between € 0.8 million and € 1.9 million for the reclamation of 15 km2 of sediments, which is low compared with the prospected cost of removal of the sediments ($ 6 million, transport and disposal costs not included) or maintenance of the present water level ($ 200 000 per year) [IAEA-2004b].
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Figure 4.17 Phyto-stabilization approaches at the contaminated area of the Dnieper close to the Chernobyl nuclear power plant
Figure 4.17 Phyto-stabilization approaches at the contaminated area of the Dnieper close to the Chernobyl nuclear power plant

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The project area for horizontal erosion control was on the right bank of the Pripyat River, which was significantly less contaminated than the left bank and is not protected by a dam. After inundation, part of the activity is eroded and transported to the Pripyat River with the withdrawing water. It was calculated that even in the event of extremely high flooding, a dense willow plantation will effectively decrease horizontal soil erosion and the concomitant transport of radionuclides into the Dnieper River system.

Vegetation or re-vegetation is a commonly employed measure for the capping of engineered waste disposal facilities and mining residues such as spoil heaps or tailings ponds. The final step in closing out an impoundment for uranium mill tailings is the design and placement of a cover that will give long term stability and control to acceptable levels radon emanation, gamma radiation, erosion of the cover and tailings, and infiltration and precipitation into the tailings and heaps. Surface vegetation can be effective in protecting tailings or a tailings cover from water and wind erosion [IAEA-2004b].

Plants chosen within the phyto-stabilisation technique should match the local climatic conditions. From an agro-biological perspective, the nature of the ore and the milling process will largely determine whether uncovered tailings are capable of sustaining growth. Considerable efforts to improve unfavourable properties such as low or high pH values and low plant nutrient content will usually be required before tailings can sustain growth. Depending on the substrate, re-vegetation requires preparation and amelioration of the topsoil, including removal, for example, of acid generating minerals. Techniques and strategies to overcome such difficulties have been developed, for example hydro-seeding or the use of compost from organic household refuse. The method may be limited to low contaminant concentrations, owing to the (root) toxicity of higher concentrations. An adequate soil cover may need to be established [IAEA-2004b].

Another example of the application of the phyto-stabilization remedy technique is at a 35 year old reclaimed site on a uranium mining dump near Schlema, Germany. It was concluded that vegetation cover could reduce infiltration by 40 – 60 % due to interception by the canopy (25 – 40 %) and increased transpiration. It was further found that of the 165 000 g/ha of uranium in the soil (30 cm depth), only 4 g/ha was in the above ground plant parts and 510 g/ha in the below ground plant parts. Since most (90 %) of the uranium taken up during the growing season is recycled (returned to the soil) with pine needles, uranium dispersion by uptake through vegetation is minimal. It may be concluded from these preliminary results that forest vegetation may reduce the infiltration rate and will disfavour radionuclide dispersion [IAEA-2004b].

The proper design of tailings covers is crucial to ensure their long term stability with respect to plant intrusion. Since plant roots can penetrate compacted sealing layers (trees can have roots reaching down 3 – 4 m) and since trees need to have a certain degree of mechanical support in order to minimize the probability of uprooting, a vegetation substrate depth of at least 1.5 m is required. The vegetation substrate layer must be such that the critical suction is not exceeded at the top of the clay seal. It must be thick enough for plants to find sufficient water and nutrients so as to prevent the generation of a high suction at the seal. Cracks resulting from such suctions become accessible to roots and can be widened as further water is extracted [IAEA-2004b].

In addition to the mechanical effects of soil stabilization and water management, re-vegetation has aesthetic benefits and sometimes cultural connotations, in particular on native or aboriginal lands. The choice of vegetation cover may also affect some sort of institutional control; for example, converting contaminated agricultural land into forestry reserves interrupts a potential exposure pathway via the food chain. It has to be ascertained, however, that no other exposure pathway is opened up, for instance via burning contaminated firewood [IAEA-2004b].

4.5.5.3 Constructed wetlands

Constructed wetlands are engineered, human-made ecosystems specifically designed to treat wastewater, mine drainage and other waters by optimizing the biological, physical and chemical processes that occur in natural wetland systems. Constructed wetlands can provide an effective, economical and environmentally sound treatment of wastewater, and serve as wildlife habitats. The conceptual design the method, displayed in Figure 4.18, leads to either the (permanent) fixation of the contaminants in-situ or to plant uptake with the view to harvesting shoots later for further treatment and disposal.
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Figure 4.18 Schematic cross-section of a constructed wetland
Figure 4.18 Schematic cross-section of a constructed wetland

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The concept for such constructed wetlands was originally developed to treat domestic effluents for residual organic material, for example following mechanical and biological (activated sludge) treatment steps, and has found widespread application in particular for the treatment of (acid) mine effluents [IAEA-2004b].
Constructed wetland systems are grouped into three main types [IAEA-2004b]:

  1. Free water surface systems, or soil substrate systems, consist of aquatic plants rooted in a soil substrate within a constructed earthen basin that may or may not be lined, depending on the soil permeability and groundwater protection requirements (Figure 4.19). Free water surface systems are designed to accept preliminarily treated, low velocity wastewater, in plug flow, over the top of the soil media or at a depth of between 2 and 45 cm [IAEA-2004b].
  2. Subsurface flow systems are typically gravel substrate systems that are similar to free water surface systems; however, aquatic vegetation is planted in gravel or crushed stone and wastewater flows approximately 15 cm below the surface of the media. The aggregate typically has a depth of between 30 and 60 cm. No visible surface flow is evident in subsurface flow systems [IAEA-2004b].
  3. Aquatic plant systems are also similar to free water surface systems, but the water is located in deeper ponds and floating aquatic plants or submerged plants are used. Where available, natural ponds may be used. Where they exist, natural wetlands and bogs can be used as traps for radionuclides and other metals, although this might be better classed as bio-sorption or natural attenuation, since it is mostly the decaying organic matter that effects retention. Studies on natural analogues for radionuclide migration have demonstrated this mechanism to be effective for thousands of years [IAEA-2004b].
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Figure 4.19a Image of a constructed wetland [IAEA-2004b]
Figure 4.19a Image of a constructed wetland [IAEA-2004b]

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Early research revealed that phyto-extraction via constructed wetlands (used to purify water) was ineffective because it was difficult to remove inorganic elements that precipitated from the water into the sediments. In addition, floating plant systems, with subsequent biomass harvesting, were determined to be inefficient and uneconomic [IAEA-2004b].

As an example of application of the technique is a pilot constructed wetland to treat the mine water from the flooded Pöhla Tellerhäuser mine at Wismut, Germany. It was shown that the system removed iron, arsenic, manganese and radium. Removal processes were based on the geochemical characteristics of the contaminants. For manganese and 226Ra, removal was also partially through bio-film formation. Uranium was not removed, given the high pH and the presence of high bicarbonate concentrations. It is hence clear that process effectiveness in constructed wetlands depends on the speciation of the radionuclides concerned and hence on the control of the governing parameters in the surface and pore waters, such as pH, and that waters may need to be subject to enhancement by additives or pre-treatment [IAEA-2004b].

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Figure 4.19b Plan view of a constructed wetland [IAEA-2004b]
Figure 4.19b Plan view of a constructed wetland [IAEA-2004b]

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Wetlands may be constructed with the main objective of excluding atmospheric oxygen from material that would generate acid from the oxidation of pyrite and other sulphides [IAEA-2004b]. This method, however, is likely to be effective only in regions where precipitation is higher than evapo-transpiration (i.e., in temperate and humid tropical climates). Climatic conditions limit the general applicability of wetlands. Extended periods of deep frost as well as arid conditions are unfavourable. If, however, effluents only arise during frost free periods, it may be possible to operate wetlands in fairly high latitudes or altitudes.

Passive water treatment technologies such as constructed wetlands at abandoned mining sites may be appropriate for small contaminant loads. However, long term stability and resilience with respect to external disturbances and recovery are of major concern for both wetland operators and regulators. Technical guidance for designing and operating constructed wetlands may be limited, owing to a lack of long term operational data. Potential seasonal variability and impact on wildlife may negatively affect system operation and securing permits, respectively. Relatively large parcels of land are required and water consumption is high, owing to large evapo-transpiration rates in some areas [IAEA-2004b].