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 ^137Ñs 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	6.18×10^13	66,970		1.85×10^13		19,980
Kanev	9.44×10^12	14,060		3.33×10^12		4,810
Kremenchug	8.14×10^12	3,626
Dniprodzerzhinsk	1.3×10^12	2,294
Dniprovske	3.7×10^11	888
Kahovka	1.3×10^12	592
In sum	8.23×10^13			2.18×10^13*

*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 â 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.