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
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
Furthermore,
some initial extreme measures were introduced in the first few days of the
accident when 15 000 cows were slaughtered in
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
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
In
In the
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
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
In
early 2001, 2217 settlements are still under radiological control in the
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
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
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 “
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
Heavy radioactive fallout in a southerly direction began on
Table 3-1. Amounts of the area-averaged 137Cs
and 90Sr
aerosol fallout on the surface of the Dnipro
reservoirs after the
|
Reservoir |
137Cs |
90Sr |
||||
|
|
Bq |
Bq×m-2 |
Bq |
Bq×m-2 |
||
|
|
6.18×1013 |
66,970 |
1.85×1013 |
19,980 |
||
|
Kanev |
9.44×1012 |
14,060 |
3.33×1012 |
4,810 |
||
|
|
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
Amounts
of 90Sr and 137Cs
(Table 3-1.) that fell down on the water surface of the
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,
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
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
Underground water.
The impact of
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
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
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
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
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
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
·
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 |
|
|
|
Total |
|
137Cs |
740 |
690 |
660 |
2090 |
|
90Sr |
440 |
|
|
440 |
|
Total |
1180 |
690 |
660 |
2530 |
Table 4-2: Statistics for
stockbreeding
|
Type of CM |
|
|
|
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.
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.
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.
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.
These methods are connected with changes in the system of maintenance
and feeding of animals.
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 -
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.
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
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).
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.
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 bull’s 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 |
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
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