Agricultural impact
All soil used anywhere for agriculture contains radionuclides
to a greater or lesser extent. Typical soils (IA89a) contain
approximately 300 kBq/m3 of potassium-40 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 iodine-131, caesium-137,
caesium-134 and strontium-90 (IA89a). Direct deposition
on plants is the major source of contamination of agricultural
produce in temperate regions.
While the caesium isotopes and strontium-90 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.
Within the former Soviet Union
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 (Ri95). While the migration downwards of caesium
in the soil is generally slow (Bo93), 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 radionuclides are generally confined to particles with a matrix
of uranium dioxide, graphite, iron-ceramic 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 30-km 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 caesium-137 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 per cent
of all caesium-137 was found to be strongly bound to soil components
(Sa94). 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 (Pr95).
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 caesium-137 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 (Pr95).
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
(Al93). 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 per cent of its original value. Nevertheless,
soil contamination with caesium-137, strontium-90 and plutonium-239
is still high and in Belarus, the most widely contaminated Republic,
eight years after the accident 2,640 km2 of agricultural land
have been excluded from use (Be94). Within a 40-km radius
of the power plant, 2,100 km2 of land in the Poles'e state 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 caesium-137, 4 PBq for strontium-90 and
32 TBq for plutonium-239 and plutonium-240.
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
(Al93). 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.
Within Europe
In Europe, a similar variation in the downward migration of caesium-137
has been seen, from tightly bound for years in the near-surface
layer in meadows (Bo93), to a relatively rapid downward
migration in sandy or marshy areas (EC94). For example,
Caslano (TI) experienced the greatest deposition in Switzerland
and the soil there has fallen to 42 per cent of the initial caesium-137
content in the six years after the accident, demonstrating the
slow downward movement of caesium in soil (OF93). There,
the caesium-137 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 (Ma89), where lambs grazed
on contaminated pasture, their activity decreased to about 13
per cent of the initial values after 115 days; where animals consumed
uncontaminated feed, it fell to about 3.5 per cent. Restrictions
on slaughter and distribution of sheep and reindeer, also, are
still in force in some Nordic countries.
The regional average levels of caesium-137 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 (EC94).
In Belgium, the average body burden of caesium137 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 (Ha91).
In short, there is a continuous, if slow, reduction in the level
of mainly caesium-137 activity in agricultural soil.
Environmental impact
Forests
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.
In 1990, forest workers in Russia were estimated to have received
a dose up to three times higher than others living in the same
area (IA94). In addition, some forest-based industries,
such as pulp production which often recycle chemicals, have been
shown to be a potential radiation protection problem due to enhancement
of radionuclides in liquors, sludges and ashes. However, harvesting
trees for pulp production may be a viable strategy for decontaminating
forests (Ho95).
Different strategies have been developed for combatting 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"
(Dz95), 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 (Ko90). 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 bodies
In an accident, radionuclides contaminate bodies of water not
only directly from deposition from the air and discharge as effluent,
but also indirectly by washout from the catchment basin. Radionuclides
contaminating large bodies of water are quickly redistributed
and tend to accumulate in bottom sediments, benthos, aquatic plants
and fish. The main pathways of potential human exposure may be
directly through contamination of drinking-water, or indirectly
from the use of water for irrigation and the consumption of contaminated
fish. As contaminating radionuclides tend to disappear from water
quickly, it is only in the initial fallout phase and in the very
late phase, when the contamination washed out from the catchment
area reaches drinking-water supplies, that human exposure is likely.
In the early phase of the Chernobyl accident, the aquaeous component
of the individual and collective doses from water bodies was estimated
not to exceed 1-2 per cent of the total exposure (Li89).
The Chernobyl Cooling Pond was the most heavily contaminated water
body in the exclusion zone.
Radioactive contamination of the river ecosystems (Figure 8) was
noted soon after the accident when the total activity of water
during April and early May 1986 was 10 kBq/L in the river Pripyat,
5 kBq/L in the Uzh river and 4 kBq/L in the Dniepr. At this time,
shortlived radionuclides such as iodine-131 were the main contributors.
As the river ecosystem drained into the Kiev, then the Kanev and
Kremenchug reservoirs, the contamination of water,sediments, algae,
molluscs and fish fell significantly.
In 1989, the content of caesium-137 in the water of the Kiev reservoir
was estimated to be 0.4 Bq/L, in the Kanev reservoir 0.2 Bq/L,
and in the Kremenchug reservoir 0.05 Bq/L. Similarly, the caesium-137
content of Bream fish fell by a factor of 10 between the Kiev
and Kanev reservoirs, and by a factor of two between the Kanev
and Kremenchug reservoirs to reach about 10 Bq/kg (Kr95).
In the last decade, contamination of the water system has not
posed a public health problem. However, monitoring will need to
be continued to ensure that washout from the catchment area which
contains a large quantity of stored radioactive waste will not
contaminate drinking-water.
A hydrogeological study of groundwater contamination in the 30-km
exclusion zone (Vo95) has estimated that strontium-90 is
the most critical radionuclide, which could contaminate drinking-water
above acceptable limits in 10 to 100 years from now.
Outside the former Soviet Union, direct and indirect contamination
of lakes has caused and is still causing many problems, because
the fish in the lakes are contaminated above the levels accepted
for sale in the open market. In Sweden, for instance, about 14,000
lakes (i.e., about 15 per cent of the Swedish total) had
fish with radiocaesium concentrations above 1,500 Bq/kg (the Swedish
guideline for selling lake fish) during 1987. The ecological half-life,
which depends on the kind of fish and types of lakes, ranges from
a few years up to some tens of years (Ha91).
In the countries of the European Union, the content of caesium-137
in drinking-water has been regularly sampled and reveals levels
at, or below, 0.1 Bq/L from 1987 to 1990 (EC94), which
are of no health concern. The activity concentration in the water
decreased substantially in the years following the accident due
largely to the fixation of radiocaesium in the sediments.
Chapter VI
AGRICULTURAL AND ENVIRONMENTAL IMPACTS
In summary,
www.oecdnea.org
and at www.nea.fr
You may find that one of the two sites has a much faster access for you.