The area of interest to study the vertical migration control by willow was an extremely contaminated zone of 16 km2 at the left bank of the Pripyat (between 3.7 and 18.5 TBq km-2 90Sr and 137Cs and 0.37 TBq km-2 Pu), which is partly protected from the spring floods by a dam. Through modeling exercises, it was shown that, due to their high evapotranspiration rate, willow SRC stands are expected to decrease the groundwater table level by 100 to 200 cm in fertilized stands. Without fertilization, only a decrease of groundwater table level of less than 50 cm was predicted. Because the immobilization potential of 137Cs and 90Sr in the willow wood is limited, the influence of plant uptake on migration remains low.

Following the effective closure of the ChNPP, the water level of the cooling pond (22.5 km2; depth between 1.5 and 15 m, with about 111 TBq 137Cs and 37 TBq 90Sr) will drop with 4 to 7 m, and 15 km2 of the sediments will come to surface and may be in need for stabilization. In this respect, the SALIMAT option was investigated. SALIMATs consist of a roll of willow rods (stems) rolled around a central disposable tube; these are unrolled by dragging them across the lagoon. Small tests have demonstrated that SALIMATs establish well on contaminated pond sediments and produce a full vegetation cover during the second year following establishment. The approximate cost of the phytostabilization option ranges from 0.8 to 1.9 106 EUR for the reclamation of 15 km2 of sediments. This is a low cost compared with the prospective cost of removal of the sediments ($6,000,000, transport and disposal costs not included) or the maintenance of the present water level ($200,000 per year).

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

29.3.9 Uranium Mining Tailings and Debris Heaps

(Re)vegetation is a commonly employed measure on the capping of engineered waste disposal facilities and on mining residues such as spoil heaps [82,83] 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 of 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 tailing cover from water and wind erosion.

Factors affecting the effectiveness of surface revegetation on impoundments can be broadly classed into climatological and agrobiological factors. Plants should be chosen to match the local climate conditions. Concerning agrobiological factors, the nature of the ore and the mill processes will largely determine the tailing characteristics from the point of view of sustaining growth. Considerable efforts to correct adverse characteristics, 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, revegetation requires preparation and amelioration of the topsoil to remove, for instance, acid-generating minerals [84,85]. Techniques and strategies to overcome such difficulties have been developed [86] — for instance, hydroseeding or the use of compost from organic household refuse [87]. 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.

Water and wind erosion are the primary causes of erosion of tailings or tailing cover material. A vegetation cover may decrease the erosion hazard. However, vegetation surfaces may raise concerns: the vegetation can promote radon emanation by drying out the tailing or tree roots may penetrate the contaminated material and break the cover integrity. Given the increased evapotranspiration rate and interception of precipitation following vegetation establishment, the vegetation cover alters the water household of the tailings and may decrease seepage. The effect of a vegetation cover on the radionuclide dispersion through an alteration of the water balance and also potentially because of the effect of plant roots on the physicochemical characteristics of the tailing material (biologically driven acidification of the tailing material) has not been studied intensively thus far.

For a 35-year-old reclaimed site on a uranium mining dump near Schlema (Saxony, Germany), it was concluded that the biomass could reduce infiltration by 40 to 60% due to interception by the canopy (25 to 40%) and increased transpiration [88]. Of the 165,000 g ha-1 U in soil (30 cm depth), only 4 g ha-1 was in the above-ground plant parts and 510 g ha-1 in the below-ground plant parts. Of the uranium taken up during the growing season, 90% is recycled (returned) with the needles. U-dispersion by uptake through vegetation is thus minimal. It may be concluded from these preliminary results that forest vegetation may reduce infiltration rate and will not favor radionuclide dispersion.

Kistinger et al. [89] evaluated some design criteria for tailing coverage regarding vegetation aspects. Because the plant roots can penetrate the compacted sealing layer (trees have roots up to 3 to 4 m) and because the trees should also have a certain degree of mechanical support in order to minimize 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 not to generate high suction at the seal. Cracks resulting from such suctions become accessible to roots and can be widened as further water is extracted.

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