Environmental contamination : soil, water, forest, plants

 

Soil contamination. All soil used anywhere in the world for agriculture contains radionuclides to a greater or lesser extent. Typical soils contain approximately 300 kBq/m3 of 40K to a depth of 20 cm. This radionuclides and others are then taken up by crops and transferred to food, leading to a concentration in food and feed of between 50 and 150 Bq/kg. The ingestion of radionuclides in food is one of the pathways leading to internal retention and contributes to human exposure from natural and man-made sources. Excessive contamination of agricultural land, such as may occur in a severe accident, can lead to unacceptable levels of radionuclides in food.

The radionuclides contaminants of most significance in agriculture are those which are relatively highly taken up by crops, have high rates of transfer to animal products such as milk and meat, and have relatively long radiological half-lives. However, the ecological pathways leading to crop contamination and the radioecological behaviour of the radionuclides are complex and are affected not only by the physical and chemical properties of the radionuclides but also by factors which include soil type, cropping system (including tillage), climate, season and, where relevant, biological half-life within animals. The major radionuclides of concern in agriculture following a large reactor accident are 131I, 137Cs, 134Cs and 90Sr . Direct deposition on plants is the major source of contamination of agricultural produce in temperate regions. While the caesium isotopes and 90Sr are relatively immobile in soil, uptake of roots is of less importance compared with plant deposition. However, soil type (particularly with regard to clay mineral composition and organic matter content), tillage practice and climate all affect propensity to move to groundwater. The same factors affect availability to plants insofar as they control concentrations in soil solution. In addition, because caesium and strontium are taken up by plants by the same mechanism as potassium and calcium respectively, the extent of their uptake depends on the availability of these elements. Thus, high levels of potassium fertilisation can reduce caesium uptake and liming can reduce strontium uptake.

The releases during the Chernobyl accident contaminated about 125 000 km2 of land in Belarus, Ukraine and Russia with radiocaesium levels greater than 37 kBq/m2, and about 30 000 km2 with radiostrontium greater than 10 kBq/ m2. About 52 000 m2 of this total were in agricultural use; the remainder was forest, water bodies and urban centres. While the migration downwards of caesium in the soil is generally slow, especially in forests and peaty soil, it is extremely variable depending on many factors such as the soil type, pH, rainfall and agricultural tilling.

Plant contamination. The radionuclides are generally confined to particles with a matrix of uranium dioxide, graphite, ironceramic alloys, silicate-rare earth, and silicate combinations of these materials. The movement of these radionuclides in the soil not only depends on the soil characteristics but also on the chemical breakdown of these complexes by oxidation to release more mobile forms. The bulk of the fission products is distributed between organomineral and mineral parts of the soil largely in humic complexes. The Exclusion zone has improved significantly partly due to natural processes and partly due to decontamination measures introduced. There were also large variations in the deposition levels. During 1991 the 137Cs activity concentrations in the 0-5 cm soil layer ranged from 25 to 1 000 kBq/m3 and were higher in natural than ploughed pastures. For all soils, between 60 and 95% of all 137Cs was found to be strongly bound to soil components. Ordinary ploughing disperses the radionuclides more evenly through the soil profile, reducing the activity concentration in the 0-5 cm layer and crop root uptake. However, it does spread the contamination throughout the soil, and the removal and disposal of the uppermost topsoil may well be a viable decontamination strategy.

The problem in the early phase of an accident is that the countermeasures designed to avoid human exposure are of a restrictive nature and often have to be imposed immediately, even before the levels of contamination are actually measured and known. These measures include the cessation of field work, of the consumption of fresh vegetables, of the pasturing of animals and poultry, and also the introduction of uncontaminated forage. Unfortunately, these measures were not introduced immediately and enhanced the doses to humans in Ukraine.

Furthermore, some initial extreme measures were introduced in the first few days of the accident when 15 000 cows were slaughtered in Ukraine irrespective of their level of contamination, when the introduction of clean fodder could have minimised the incorporation of radiocaesium. Other countermeasures, such as the use of potassium fertilisers, decreased the uptake of radiocaesium by a factor of 2 to 14, as well as increased crop yield. In some podzolic soils, lime in combination with manure and mineral fertilisers can reduce the accumulation of radiocaesium in some cereals and legumes by a factor of thirty. In peaty soils, sand and clay application can reduce the transfer of radiocaesium to plants by fixing it more firmly in the soil.

The radiocaesium content of cattle for human consumption can be minimised by a staged introduction of clean feed during about ten weeks prior to slaughter. A policy of allocating critical food production to the least contaminated areas may be an effective common sense measure.

In 1993, the concentration of 137Cs in the meat of cows from the Kolkhoz in the Sarny region, where countermeasures could be implemented effectively, tended to be much lower than that in the meat from private farms in the Dubritsva region. The meat of wild animals which could not be subjected to the same countermeasures had a generally high concentration of radiocaesium.

Decontamination of animals by the use of Prussian Blue boli was found to be very effective where radiocaesium content of feed is high and where it may be difficult to introduce clean fodder. Depending on the local circumstances, many of the above mentioned agricultural countermeasures were introduced to reduce human exposure.

Since July 1986, the dose rate from external irradiation in some areas has decreased by a factor of forty, and in some places, it is less than 1% of its original value. Nevertheless, soil contamination with 137Cs, 90Sr and 239Pu is still high and in Belarus, the most widely contaminated Republic, eight years after the accident 2640 km2 of agricultural land had been excluded from use. Within a Exclusion Zone nature reserve have been excluded from use for an indefinite duration.

The uptake of plutonium from soil to plant parts lying above ground generally constitutes a small health hazard to the population from the ingestion of vegetables. It only becomes a problem in areas of high contamination where root vegetables are consumed, especially if they are not washed and peeled. The total content of the major radioactive contaminants in the 30-km zone has been estimated at 4.4 PBq for 137Cs, 4 PBq for 90Sr and 32 TBq for 239Pu and 240Pu.

However, it is not possible to predict the rate of reduction as this is dependent on so many variable factors, so that restrictions on the use of land are still necessary in the more contaminated regions in Belarus, Ukraine and Russia. In these areas, no lifting of restrictions is likely in the foreseeable future. It is not clear whether return to the 30 km exclusion zone will ever be possible, nor whether it would be feasible to utilise this land in other ways such as grazing for stud animals or hydroponic farming. It is however, to be recognised that a small number of generally elderly residents have returned to that area with the unofficial tolerance of the authorities.

In Europe, a similar variation in the downward migration of 137Cs has been seen, from tightly bound for years in the near-surface layer in meadows, to a relatively rapid downward migration in sandy or marshy areas. For example, the greatest deposition in Switzerland and the soil there has fallen to 42% of the initial 137Cs content in the six years after the accident, demonstrating the slow downward movement of caesium in soil (OF93). There, the 137Cs from the accident has not penetrated to a depth of more than 10 cm, whereas the contribution from atmospheric nuclear weapon tests has reached 30 cm of depth.

In the United Kingdom, restrictions were placed on the movement and slaughter of 4.25 million sheep in areas in southwest Scotland, northeast England, north Wales and northern Ireland. This was due largely to root uptake of relatively mobile caesium from peaty soil, but the area affected and the number of sheep rejected are reducing, so that, by January 1994, some 438 000 sheep were still restricted. In northeast Scotland, where lambs grazed on contaminated pasture, their activity decreased to about 13% of the initial values after 115 days; where animals consumed uncontaminated feed, it fell to about 3.5%. Restrictions on slaughter and distribution of sheep and reindeer, also, are still in force in some Nordic countries.

The regional average levels of 137Cs in the diet of European Union citizens, which was the main source of exposure after the early phase of the accident, have been falling so that, by the end of 1990, they were approaching pre-accident levels. In Belgium, the average body burden of 137Cs measured in adult males increased after May 1986 and reached a peak in late 1987, more than a year after the accident. This reflected the ingestion of contaminated food. The measured ecological half-life was about 13 months. A similar trend was reported in Austria. In short, there is a continuous, if slow, reduction in the level of mainly 137Cs activity in agricultural soil.

About 1.4 million of people are living on 30 000 km2 of land contaminated higher than 185 kBq/m2, and 130 000 people are living in areas where the contamination is higher than 555 kBq/m2. For the territories where the annual dose is lower than 1mSv, life is considered as normal. When the annual dose is higher than 1 mSv per year, people receive social compensations. In Russia, some districts were declassified in January 1998, and this decision was accepted badly by the affected populations.

In early 2001, 2217 settlements are still under radiological control in the Ukraine. In fact, only 1316 need permanent controls, but the population of the 901 remaining settlements refuse the declassification of their areas because this could be associated with the end of financial and social compensation.

In the Exclusion Zone, the impact on fauna and flora is characterised by the extremely heterogenous deposition of radioactive particles, which produces a wide range of doses to which the biota were subjected. In some cases, even in very small geographic areas, the impacts differed by an order of magnitude.

Some consequences of the accident for the natural plant and animal populations are determined by secondary ecological factors resulting from changes in human activities. For example, the forbidding of hunting alters the types and numbers of birds. In general, animal numbers have greatly increased compared to adjacent inhabited areas. These favourable conditions for large numbers of commercially hunted mammal species will be preserved.

The transfer of radionuclides by water and wind, by extreme seasonal weather conditions has not led to long term contamination beyond the Exclusion Zone. In the Exclusion Zone, the future radioactive contamination will be reduced slowly through radioactive decay.

 

 

Forests contamination. Forests are highly diverse ecosystems whose flora and fauna depend on a complex relationship with each other as well as with climate, soil characteristics and topography. They may be not only a site of recreational activity, but also a place of work and a source of food. Wild game, berries and mushrooms are a supplementary source of food for many inhabitants of the contaminated regions.

Timber and timber products are a viable economic resource. Because of the high filtering characteristics of trees, deposition was often higher in forests than in agricultural areas. When contaminated, the specific ecological pathways in forests often result in enhanced retention of contaminating radionuclides. The high organic content and stability of the forest floor soil increases the soil-to-plant transfer of radionuclides with the result that lichens, mosses and mushrooms often exhibit high concentrations of radionuclides.

The transfer of radionuclides to wild game in this environment could pose an unacceptable exposure for some individuals heavily dependent on game as a food source. This became evident in Scandinavia where reindeer meat had to be controlled. In other areas, mushrooms became severely contaminated with radiocaesium.

Forest occupies 30-40% of the contaminated areas and initially played the role of a filter in intercepting the fallout. The significant part of fall-out 1986 year was intercepted by the foliage. Characteristically, contamination of the foliage on periphery of the forest 10-20% higher then to its centre. Half of the radioactive material intercepted by the foliage reached the ground within one month of the accident. The leaf litter is now the most contaminated part of the forest ecosystem, as 50-90% of the forest contamination is concentrated in it. The litter constitutes a relatively closed medium, since the movement of radionuclides towards the sub-soil and towards the arboreal vegetation is estimated at less than 2% per year.

In 1998 the total activity accumulated by the vegetation is assessed at 2-3% of the radioactive deposition in the forest as a whole. Deciduous trees have a tendency to accumulate more radioactive material than conifers. The main contribution to human dose from the contamination of the forests remains, however, the consumption of contaminated mushrooms, which may give rise to a significant fraction of the internal dose in certain regions of Belarus, the Russian Federation and Ukraine. The transfer of caesium to mushrooms is very variable, and can range from 0.001 to 0.1 (Bq.kg-1)/(Bq.m-2).

Different strategies have been developed for combating forest contamination. Some of the more effective include restriction of access and the prevention of forest fires. One particularly affected site, known as the Red Forest, lies to the South and West close to the site. This was a pine forest in which the trees received doses up to 100 Gy, killing them all. An area of about 375 ha was severely contaminated and in 1987 remedial measures were undertaken to reduce the land contamination and prevent the dispersion of radionuclides through forest fires. The top 10-15 cm of soil were removed and dead trees were cut down. This waste was placed in trenches and covered with a layer of sand. A total volume of about 100 000 m3 was buried, reducing the soil contamination by at least a factor of ten. These measures, combined with other fire prevention strategies, have significantly reduced the probability of dispersion of radionuclides by forest fires. The chemical treatment of soil to minimise radionuclide uptake in plants may be a viable option and, as has been seen, the processing of contaminated timber into less contaminated products can be effective, provided that measures are taken to monitor the by-products.

Changes in forest management and use can also be effective in reducing dose. Prohibition or restriction of food collection and control of hunting can protect those who habitually consume large quantities. Dust suppression measures, such as re-forestation and the sowing of grasses, have also been undertaken on a wide scale to prevent the spread of existing soil contamination.

 

 

Water contamination. Surface water

Radioactive contamination of water bodies occurred in the primary forms like the fallout of radioactive aerosols on the surface of water basins, contact of the contaminated air masses with water surface, and as a result of the secondary effects such as radioactivity washout of the surface of water catchement areas, inflow of contaminated water from the more contaminated water bodies and areas into the less contaminated ones, mass exchange between bottom sediment and aquatic masses, discharge of contaminated subsurface water into the surface water bodies, etc.

The initial composite of the water contamination in May 1986 was developing from the several dozens of nuclides, basic of which as to their various contribution into the exposure rate of biocenosis were radionuclides 141Ce, 144Ce, 103Ru, 140Ba, 131I, 95Zr, 95Nb, 140La, 134Cs, 137Cs, and other uranium fission products. Since 1987, however, 137Cs and 90Sr have made a major contribution into the dose for account of their migration over water-ways.

The highest levels of contamination were registered during the period of maximal fallout in the first decade of May 1986, after which the quantities of water contamination started to decline. The overall radioactivity in the Pripyat water decreased from 10-8 - 10-7 Ci×l-1 (about 106 Bq×l-1) during the first days of the accident to 104 - 103 Bq×l-1 by the beginning of June. The maximal level of 90Sr in the Pripyat registered by the research and industrial company Typhoon was as much as 20 Bq×l-1, with 100 Bq×l-1 and more having been detected in the water bodies of the nearest zone. The highest concentrations of 137Cs and 239Pu in the Pripyat were about 103 Bq×l-1 and 1 Bq×l-1, respectively, in the first days of May, with the subsequent decrease by an order of magnitude by August 1986. A sharp increase of 131I concentration in rivers, the Dnipro dammed bodies of water and other even distant water storages was registered immediately after the accident and regularly monitored by Kiev sanitary services.

Chernobyl radionuclides were detected in various rivers of the USSR and Western Europe. Recovery of pre-accidental levels of background radioactive contamination was rather slow in many rivers, whereas it has not occurred in some rivers even 12 years after the fallout. Notwithstanding the current very low levels of contamination in these rivers, the content of 137s and 90Sr is found to increase regularly at rains and spring run-offs.

 

Heavy radioactive fallout in a southerly direction began on April 29, 1986 and it covered the aquatic areas of the Dnipro dammed reservoirs. The Kiev and Kanev reservoirs were the most severely affected ones. The fallout eventually went on for the whole May 1986, with the greatest intensity having occurred in the period from May 1 to May 3.

 

 

 

 

 

 

Table 3-1. Amounts of the area-averaged 137Cs and 90Sr aerosol fallout on the surface of the Dnipro reservoirs after the Chernobyl accident in 1986.

 

Reservoir

137Cs

90Sr

 

Bq

Bq×m-2

Bq

Bq×m-2

 

Kiev

 

6.18×1013

 

66,970

 

1.85×1013

 

19,980

Kanev

9.44×1012

14,060

3.33×1012

4,810

Kremenchug

8.14×1012

3,626

 

 

Dniprodzerzhinsk

1.3×1012

2,294

 

 

Dniprovske

3.7×1011

888

 

 

Kahovka

1.3×1012

592

 

 

In sum

8.23×1013

 

2.18×1013*

 

 

*Estimates were made only for the Kiev and Kanev reservoirs.

 

Amounts of 90Sr and 137Cs (Table 3-1.) that fell down on the water surface of the Kiev and Kanev reservoirs were estimated by the maps-charts of the aerosol fallout densities. The maps laid out by Gosgidromet of Ukraine on contamination of the areas adjoining to the reservoirs were used. The quantities of density fallout on the reservoirs downstream the Dnipro were estimated with the outcomes of the analysis of coastal samples collected by UkrNIGMI in 1988-1991. In the following years, the content of the radionuclides and their forms in the Dnipro water system have undergone redistribution under hydrodynamic and internal processes in the reservoirs including the physico-chemical transformation of radionuclides.

Since the initial fallout, the radiation situation in the Dnipro water system has been determined by the amount of the radionuclide inflow brought about by the rivers from the contaminated areas. The principal providers of contamination have been and still remain the Pripyat, Desna, and the upper Dnipro. A contribution made by other rivers into the overall radioactive inflow is insignificant. Regardless of its own 137Cs and 90Sr the upper Dnipro provides the most conspicuous watering down for the Pripyat radioactive runoff. The Desna contributes significantly into watering down the radioactive runoff that flows through the Kiev Hydro into the reservoirs of the Dnipro middle course.

An estimation of the average of data for the last ten years shows that in case of a full intermix of the Pripyat and the Dnipro waters in the basin of the Kiev reservoir, the concentrations of 137Cs and 90Sr drop as much as 1 - 1.5 and 2 times respectively. The same outcome is observed in the upper part of the Kanev reservoir where the Dnipro is intermixed with the less contaminated water of the Desna, reducing the amount of 137Cs by a factor of 1.24 and that of 90Sr by a factor of 1.23. The given coefficients of watering down have insignificantly varied during the post-accident years, that allows to use their values in estimating the transformation of radionuclide concentrations in the Dnipro lower course.

During the post-accident years the seasonal variations in the contamination of the reservoirs have been determined by the cycles of washing out the radionuclides of the watersheds, floodplains and the ChNPP area. A marked trend is the annual reduction of the radionuclide inflow. In 1987 the amount of 137Cs inflow due to the Pripyat was 5 times as great as that in 1997, while 90Sr reduction was minor. Accordingly, the share of 90Sr in the radioactive inflow has augmented (in 1987 90Sr/137Cs ratio was 0.8 with 4.0 in 1997.)

It is worth be noted that all extreme increases of 90Sr in the Pripyat (due to floods in the river valley nearby the ChNPP in 1988, 1991, 1993 and 1994) were reflected in the level of radioactive contamination all over the Dnipro cascade of reservoirs.

A general temporary trend towards a reduction of the radionuclide inflow is evident on comparing the levels of 1992 and 1995 radioactive contamination that were similar both as for the runoff and the conditions of its formation.

The trend is the result of a summative effect of two processes: a) the on-going fixation of radionuclides by the soil particles at water collection areas; b) the diminution of the exchangeable forms of radionuclides due to infiltration, burying in the lower layers of soil and washing out into the river channels.

The reduction of 137Cs was unaffected by water variations. The share of 137Cs transported with the suspension had augmented from 29% in 1987 to 47% in 1991. The monitoring data exhibit a stabilization of the share, though 137Cs transportation into the river channels with solid particles is more actual than its inflow in a dissolved form. The outcome of sporadic observations carried out by UkrNIGMI at the Pripyat and Dnipro invariably show the larger amount (50-60%) of 137Cs to have been transported by the suspended drifts in 1994-1995.

The post-accident studies have shown the contribution of all rivers of the Exclusion Zone into contamination of the Kiev reservoir to have been 15% for 137Cs and 45% for 90Sr in the total amount of radionuclides discharged by the Pripyat and Dnipro in 1987-1991. The contribution may vary with years and seasons depending on the wet or dry kind of the period, a character of a high water flow over the Exclusion Zone, an extent of submerge of the contaminated floodplain, an amount of snow in the Zone and its melting rate. Calculations made with the use of the UkrNIGMI monitoring outcomes show that at least 70% of 137Cs is contributed into the Pripyat by the vast watersheds of the Ukrainian and Belorus Polesje. The figure is close to that (77%) computed exclusively on the basis of the RCNPS observations. This is indirectly confirmed by the close values of the specific activity of the sediment loads discharged by the Pripyat and Upper Dnipro whose watershed does not encompass areas of the Exclusion Zone. The average specific activity of the suspended sediments transported by the Pripyat into the Kiev reservoir in 1987-1992 was 109×10-9 Ci×g-1, with 96×10-9 Ci×g-1 discharged by the Dnipro. From June 1986 to December 1995 the rivers had transported about 3,800 Ci of 137Cs and 4,200 of 90Sr into the upper part of the Dnipro cascade of reservoirs. The Pripyat accounts for 66% of 90Sr in the sediments load, with its contribution having raised from 49% to 75% between 1987 and 1994.

 

Underground water.

The impact of Chernobyl accident on underground hydrosphere exhibited in the initial contamination of underground water from the first subsurface aquifer (in Quaternary deposits) to the deeper aquiferous horizons, not only in the Exclusion Zone, but also in the areas a good deal away from the NPP. Measurable concentrations of 137Cs and 90Sr were found practically in each water sample.

Assaying of underground water was accomplished at the following depth: 2 - 18 m (Quaternary aquiferous horizon), 45 - 65 m (Eocene aquiferous horizon), 80 - 150 m (Cenoman-Callovian aquiferous horizon), 200 - 300 m (Bajocian aquiferous horizon.) About 600 measurements of 137Cs and 400 those of 90Sr were made during 1992-1996. The concentrations of both radionuclide vary from a unit up to hundreds of MBq×l-1.

 

Table 3-2. Correlation of concentrations of radionuclides in KUA underground water (in %).

 

The age of aquiferous horizon

Concentration of 137Cs, MBq×l-1

 

Concentration of 90Sr MBq×l-1

 

 

< 10

10-50

51-150

> 150

< 10

10-50

> 50

Quaternary

24%

56%

14%

6%

53%

40%

7%

Eocenic

45%

41%

9%

5%

71%

21%

8%

Cenoman-Callovian

46%

31%

13%

10%

80%

20%

-

Bajocian

47%

28%

15%

10%

62%

35%

3%

 

During the first years after the accident, the Chernobyl origin of the radionuclides was proved by solitary findings of 134Cs in water samples. However, a control sampling of underground water in akin hydrogeological conditions but outside the contaminated territory (less than 20 kBq×m-2), did not bring out any noticeable amounts of above-mentioned radionuclides.

To find out the pathways of radionuclides is of paramount importance. The vertical downward ways of migration were found to play a major role in the contamination of the multilayer system of aquiferous horizons. Lateral paths of migration in the regional processes of the contamination of underground water are collateral because of the small rate of lateral filtering.

Alongside with the natural pathways of radionuclide migration, there were experimentally discovered the technogenic pathways of their penetration attributed to the technical imperfection of the wells and their annular spaces. A contribution of technogenic migration into the overall radioactive contamination of aquiferous horizons is small, though it may provide a distorted account of radionuclide concentrations that are measured at episodic assaying of underground water.

 

Foodstuff Contamination

All soil used anywhere in the world for agriculture contains radionuclides to a greater or lesser extent. Typical soils contain approximately 300 kBq/m3 of 40K to a depth of 20 cm. This radionuclide and others are then taken up by crops and transferred to food, leading to a concentration in food and feed of between 50 and 150 Bq/kg. The ingestion of radionuclides in food is one of the pathways leading to internal retention and contributes to human exposure from natural and man-made sources. Excessive contamination of agricultural land, such as may occur in a severe accident, can lead to unacceptable levels of radionuclides in food.

The radionuclide contaminants of most significance in agriculture are those which are relatively highly taken up by crops, have high rates of transfer to animal products such as milk and meat, and have relatively long radiological half-lives. However, the ecological pathways leading to crop contamination and the radioecological behaviour of the radionuclides are complex and are affected not only by the physical and chemical properties of the radionuclides but also by factors which include soil type, cropping system (including tillage), climate, season and, where relevant, biological half-life within animals. The major radionuclides of concern in agriculture following a large reactor accident are 131I, 137Cs, 134Cs and 90Sr. Direct deposition on plants is the major source of contamination of agricultural produce during first year after accident, after that the main pathway is uptake of roots.

The releases during the Chernobyl accident contaminated about 125 000 km2 of land in Belarus, Ukraine and Russia with radiocaesium levels greater than 37 kBq/m2, and about 30 000 km2 with radiostrontium greater than 10 kBq/m2. About 52 000 km2 of this total were in agricultural use; the remainder was forest, water bodies and urban centres.

While the migration downwards of caesium in the soil is generally slow, especially in forests and peaty soil, it is extremely variable depending on many factors such as the soil type, pH, rainfall and agricultural tilling.

The problem in the early phase of an accident is that the countermeasures designed to avoid human exposure are of a restrictive nature and often have to be imposed immediately, even before the levels of contamination are actually measured and known. These measures include the cessation of field work, of the consumption of fresh vegetables, of the pasturing of animals and poultry, and also the introduction of uncontaminated forage. Unfortunately, these measures were not introduced immediately and enhanced the doses to humans in Ukraine.

Countermeasures efficiency. According to the concerned zones, and by the way to the level of contamination, and to the field of application, different categories of countermeasures were and are implemented. Out of settlements in which specific methods were applied, four categories of countermeasures are distinguished in the agricultural and natural environment, related to the domain of application:

        Countermeasures in agricultural production,

        Countermeasures in natural and semi-natural ecosystems,

        Countermeasures in hydrological ecosystems,

        Technological countermeasures.

The implementation of countermeasures in agriculture on contaminated area originated as a result of the accident of the Chernobyl Nuclear power Plant in year 1986 is one of the main element in system of radiation protection of the territories.

There are some reasons for such situation. First of all, the internal consumption, as a consequence of consumption of radionuclides containing food products, contributed approximately for 40%-50% of the total dose burden in Chernobyl affected zone. In certain zones of this area, where soil cover is presented by low fertile soil type, soddy-podzolic and peaty, the contribution of internal irradiation is higher, up to 70-80%.

Secondly, the exclusion of production of agricultural food where radionuclide content exceeds the permissive level is extremely important in social and psychological aspect to prevent the spreading of the radiophobia of population, especially rural.

Thirdly, the decrease of dose due to internal irradiation is in many cases more economically efficient than the decline by external irradiation.

Really in the Chernobyl affected zone the agricultural countermeasures deal with practically all the agricultural branches: plant breeding, animal husbandry, and countermeasures in natural and semi-natural ecosystems, and so on.

A countermeasure could be seen under three aspects. First it describes the special protection measures leading to reducing contamination in the environment and the food chain, expressed by reduction factors (RF). These measures are implemented compulsorily to reach the final goal of reducing to population, expressed in term of averted doses. And the third aspect of countermeasures is the economic one: the cost of irradiation dose saved, as a result of agricultural countermeasure application should be associated to a man.sievert averted.

The following classification has been established for countermeasures after Chernobyl accident, the categories correspond to the different spheres of application:

        countermeasures in crop production and in stockbreeding;

        countermeasures in forest ecosystem and on natural and semi-natural ecosystems;

        countermeasures for hydrological system;

        technological treatments and culinary processing as countermeasures.

The first categories correspond to countermeasures in Agricultural, and are briefly described in the following chapter. The three last ones are more specific and are not treated in this presentation.

Countermeasures in crop production. The complex of countermeasures in crop production may be classified on two main groups:

        organisational

        agrotechnical.

Countermeasures in stock-breeding. In this category concerns technologies for producing fodder and animal foodstuffs. The system of countermeasures in animal production includes four families of methods:

        restrictive,

        organisational,

        zootechnical,

        veterinary.

Assessing of countermeasures efficiency. A great number of data have been gathered in the three countries for rating the corresponding countermeasures. The two below tables (table 4-1 and table 4-2) present the amount of data collected for crop production and animal husbandry.

Figure 4-1 shows repartition according to soil types and soil granulometry.

Table 4-1 : Statistics for plant production

Country

Belarus

Russia

Ukraine

Total

137Cs

740

690

660

2090

90Sr

440

 

 

440

Total

1180

690

660

2530

 

Table 4-2: Statistics for stockbreeding

Type of CM

Belarus

Russia

Ukraine

by type

Restrictive

 

1

 

1

Organisational

1

3

 

4

Veterinary

251

197

179

627

Zootechnical

31

18

 

49

by country

283

219

179

681

 

By soil type

By granulometric composition

Figure 4-1: Statistics according to soil type and granulometry

Countermeasures in crop production

Organisational countermeasures

These measures are mainly based on the changes in land use:

                        increasing the area crops characterised by low accumulation of radionuclide,

                        abandoning of land for agricultural production (areas of very high deposition),

                        substitution of the existing crop by hay, grain, potatoes or grazing animals.

Agrotechnical countermeasures

In this group are included:

                        change of crops;

                        mechanical soil cultivation;

                        agrochemical measures;

                        plant protection.

Change of crops can be used for medium or long term, only for long life radionuclides when the others are not appropriated.

Select of the crop species that accumulate low levels of radionuclide, according to their rate of uptake of radionuclide from the soil and the distribution of radionuclide between edible and non-edible parts.

Mechanical soil cultivation

A major advantage is that, because of their low mobility, the majority of deposited radionuclides can be removed effectively by skimming off a relatively shallow layer of topsoil.

Countermeasures based on mechanical soil cultivation included removal of soil, ploughing or disking and rotary cultivation of soil upper layer.

Removal of soil upper layer (0-10 cm) can not be applied on soil with thin humus horizon.

Ploughing , standard or deep, is used, so that skim and burial ploughing and disking and rotary cultivation of soil upper layer.

Agrochemical measures include

                   liming of acid soils;

                   application of increased doses of K and P-K fertilisers;

                   supplementing soils with natural sorbents (different kinds of clay minerals);

                   use of organic fertilisers.

The development of agrochemical countermeasures is based on reducing biological availability of radionuclides. Radionuclides in the topsoil are potentially available for uptake by plant roots, although plant uptake of radionuclides is controlled by a variety of factors. The rate of uptake differs substantially for different radionuclides and soil types and depends on the physicochemical processes in the soil that govern their availability for the plants.

Plant protection countermeasures from insect, diseases and weeds in contaminated areas with 137Cs deposition 555 - 1480 kBq/m2 are based on the restricted assortment of protection means. The application of plant protection means results in the improvement of the plant physiological processes.

Countermeasures in stock-breeding

This category concerns technologies for producing fodder and animal foodstuffs.

The system of countermeasures in animal production includes four families of methods (table 2.4-3):

                   Restrictive;

                   Organisational;

                   Zootechnical;

                   Veterinary.

Restrictive countermeasures

In this family are included a ban on keeping dairy cattle and a ban or restriction on the use of milk foodstuffs produced in areas with high levels of contamination (especially in the private sector).

Organisational countermeasures

Re-specialisation of animal production branches, inventory of fodder land by the contamination density, exclusion of the fodder land from the agricultural use or implementation of special agrotechnical measures for their improvement, replacement of animal foodstuffs by less contaminated ones.

Zootechnical countermeasures

These methods are connected with changes in the system of maintenance and feeding of animals.

Veterinary countermeasures

These countermeasures are connected with application of Cs-binder preventing from the absorption of radionuclides in the gastrointestinal tract are introduced into the diet of animals. The well-known caesium selective sorbents are Prussian blue, ferrocine, Giese salt, Nigrovitch salt and clay minerals, zeolites, alginate.

Criteria for rating countermeasures

As previously written, different criteria should be used for assessing efficiency of countermeasures.

First we have the radioecological criterion, based on the decrease of concentration in agricultural products and in the food chain.

Then the radiological (or dosimetric) criterion is placed its reliance on both individual and collective doses averted to population.

The economic criterion is built up upon cost of spared doses by countermeasure implementation.

Last criteria, more difficult to approach, are human ones, according to the social and psychological perception and acceptance of implemented countermeasures.

In the work , we only take into account the radioecological point of view, based on reduction factors (RF) in agricultural products and the food chain.

 

Factors influencing countermeasures effectiveness

Numbers of factors have an influence on the efficiency of counter measures :

                   Radionuclides composition (half-life, type of fallout, mobility, physico-chemical form);

                   Post-accidental time;

                   Environmental conditions (soils, landscape);

                   Food practices and import/export ratio;

                   Financial applicability;

                   Ecological acceptability;

                   Social perception;

Figure 4-2 : Dynamics of radionuclide accumulation in hay of perennial grasses on soddy-podzolic loamy sand soil - Mogilev region, Belarus

This diagrams (figure 4-2) show for perennial grasses the rather constant content of 90Sr and a regular decreasing for 137Cs. Keeping the same relative efficiency, absolute effectiveness of a countermeasure shall be in this case greater for strontium that for caesium because applied to a higher content.

 

Figure 4-3: Dynamics of 137Cs and 90Sr TF for hay of sown cereal grasses

In this example (figure 4-3), the influence of soil type appears clearly for 137Cs. Peat-bog soil causes a faster decreasing in Transfer Factor (TF) than soddy-podzolic. For 90Sr no significant difference appears between the two soil types. Countermeasures to be implemented in this case must be take into account this fact

Efficiency of countermeasures in crop production

The below results for different categories of countermeasures come from the 2530 data gathered in the frame of the SP5 of the French-German Initiative. They are given in mean values of the Reduction Factor (RF). The range of variation for the efficiency is within about 50% in most of the cases.

Change of crops

For medium or long term, changes in land use, including crop selection, can be effective but with social or economical long term consequences. The most severe the change is the highest long-term potential consequences could be. That is why these measures should be implemented only for long life radionuclides when the others are not appropriated. The comparison cost/benefits has to be considered. An example is given below (figure 4-4) for different rape varieties.


Figure 4-4: Reduction factor for different rape varieties with respect with one of them

Mechanical soil cultivation

Removal of soil upper layer (0-10 cm)

This measure is an effective technique for soil decontamination if its application does not deteriorate soil fertility and water regime. Efficiency can be very high (about 80%) according to the fact that most of the contamination, almost in the first time after accident, is in this upper layer. But the great disadvantages of the method are high procurement costs and problem of burial of a large body of radioactive wastes.

Ploughing

The efficiency depends on the type and depth of soil and on the type of crops, especially the rooting depth.

Deep ploughing greatly reduces the uptake of radionuclide is recommended exclusively for soils with a powerful productive (humus horizon) layer (for example, chernozem and peaty soils).

Agrochemical measures

Effectiveness of liming is high but varying with initial soil pH; average reduction factor for 90Sr is 2 but could be higher (up to 10) , for 137Cs average 1.8 (up to 3). Liming reduces radiocæsium and radiostrontium uptake into plants by increasing the cation exchange capacity of the soil. It is usually inexpensive and easy to apply.

Liming is the most effective in suppressing root uptake of radiocaesium when applied in conjunction with potassium. It is most effective when applied to organic soils.

The application of potassium and organic fertiliser is inexpensive and comparatively easy to carry out. Due to the addition of potassium fertiliser 137Cs intake in farm crops from different soils reduces on an average of 2 but up to 3-5 times. Effectiveness is high when applied to soil with low levels of available potassium.

Sum up of the efficiency of agricultural countermeasures

The general table of the average reduction of accumulation of radionuclides for crop production countermeasures is presented below in table 4-4

Table 4-4 : Average efficiency of crop production countermeasures

Family of countermeasure

90Sr

137Cs

1 - Agricultural

 

 

1.1 Use of mineral fertilising

1.6

2.0

1.2 Liming

2.0

1.8

1.3 Ploughing (or other mechanical treatments)

1.4

1.4

1.4 Crop selection

4

4

Some comments could be done about these data:

For this category of countermeasures, the most efficient for 137Cs is selection of crop species. The range is wide (0,54 to 20,3) but the mean value is about 4, which a very high level of efficiency. The selection of crop varieties is just after for its efficiency on 137Cs accumulation. The less efficient methods are probably application of clay minerals and biological active matter.

For 90Sr, the efficiency of the agricultural countermeasures is generally lower than for 137Cs or at least at the same level, except for liming because of competitive action of calcium. The efficiency range is narrower than for 137Cs: from 0,5 up to 5,29. The most efficient method seams also to be the selection of crop varieties. The other ones are in the same range, about 1.4 to 2 in mean.

Efficiency of countermeasures in stock-breeding

Nowadays 681 data have been collected on RN reduction related with stockbreeding. Most of them are related to 137Cs and only few for 90Sr.

Veterinary countermeasures

Within the joint project realised by IAEA, FAO, AG.University of Norway, Radiation Hygiene Institute of Norway, RIARAE, UIAR and BelRIAR the investigation was carried out with ferrocine in form of boli, salt licks and mixed with concentrates.

Application of Cs-binders like ferrocine , as a part of boli or as a part of salt licks, administration to cows resulted in 2-5,4-fold decrease in 137Cs concentration in milk within 2 month and in bulls muscle by a factor of 1.7-2.9.

For the use of sorbents like "Trepel" in Belarus, after 30 days of experiment 137Cs activity concentration in milk reduced by a factor of 1.8-1.9 compare to control animals.

The general results for stockbreeding countermeasure efficiency is presented in table 4-5.

The general comments are that this categories of methods leads to a wide range of reduction factors (1,4 up to 37,2 in mean). The highest one is obviously the selection of fodder in animal diet and use of "clean" fodder for pre-fattening of animals.

Table 4-5: Average efficiency of stockbreeding countermeasures

Family of countermeasure

90Sr

137Cs

Animal breeding

 

 

Nutrition

3

5

Sorbents (ferrocine)

-

6

Other type of countermeasures

For example of the other categories of Countermeasures, one in semi-natural ecosystems and the other from processing agricultural products, are presented below in table 4-6 some first results of amelioration of meadows and processing of milk to butter.

Table 4-6: Average efficiency of some other countermeasures

Type of countermeasure

90Sr

137Cs

Semi-natural ecosystems

 

 

Amelioration of meadows

3

5

Processing agricultural products

 

 

Milk to butter

5

7.5

 

It appear that this categories of countermeasures leads to high reduction factor, especially for processing milk.

The below diagram (figure 4-5) shows as an example influence of a countermeasure, amendment of soil in natural meadows. Implementation of the countermeasure provides a high gain in dynamics of decrease of Transfer Factor (TF). The time of decreasing by a factor 2 for TF is 4.3 years without amendment and is reduced to 0.87 year when applying amendment. This provides a TF about to times lower after four years after applying countermeasure.

 

Figure 4-5: Dynamics of 137Cs from soddy-podzolic soil to cereal grasses of natural meadow

Data still to be gathered in the frame of sub-project 5 of FGI until the end of the Initiative shall precise the range of efficiency but it is still evident that some of it are of high effectiveness on a radioecological point of view.

The sum up of countermeasures efficiency presented in the above paragraphs is gathered in the diagram (figure 4-6) below

Figure 4-6: Sum up of countermeasures average efficiency in agricultural environment

 

 

 

References

 

 

1.     V.Poyarkov, V.Bar'yahtar, V.Kholosha, N.Steinberg, V.Kukhar', V.Shestopalov, I.Los', The Chernobyl Accident. Comprehensive Risk Assessment.; English edited by G.Vargo; Battelle Press, Columbus, Richland, USA, ISBN 1-57477-082-9

2.     V.Poiarkov, D.Robeau, Laccident de Tchernobyl, in book : Catastrophes et Accidents Nucleaires Dans Lex-Union Sovietique, EDP Sciences, 2001

3.     Proceeding of International Conference "Fifteen Years after the Chornobyl Accident. Lessons Learned", ISBN 966 7600-01-7, Kyiv, 2001

4.     Present and future environmental impact of the Chernobyl accident, IAEA-IPSN Study, 1AEA-TECDOC-1240, IAEA, Vienna, 2001.

 

5.     F. Bréchignac, L. Moberg, M. Suomela, Long-term environmental behaviour of radionuclides, CEC-IPSN Association final report, 2000.

 

6.     V.A. Shevchenko et al, Reconstruction of radiation doses in population and radiation workers applying cytogenetic techniques. Proceedings of the International Conference on Radioactivity of Nuclear Blasts and Accidents. Moscow, 24-26 April 2000.

 

7.     The Human Consequences of the Chernobyl Nuclear Accident A Strategy for Recovery. A Report Commissioned by UNDP and UNICEF with the support of UN-OCHA and WHO, 25 January 2002.

 

8.     French-German Initiative for Chernobyl, Project 2, Radiological Consequences, 2004