Environmental Migration of Radionuclides ( 90 Sr , 137 Cs , 239 Pu ) in Accidentally Contaminated Areas of the Southern Urals

In the late 1940s, the facility Mayak Production Association (Mayak PA) for weapon grade plutonium production was put into operation in the vicinity of the town of Kyshtym. The technology used in plutonium production involved generation of high-level waste. A number of accidents that occurred at the plant were associated with inadequate radioactive waste storage techniques. In 1949-1956, radioactive waste with total activity of about 1.8 1017 Bq (4.9 MCi) was discharged into the Techa River which resulted in contamination of all river system components. Currently, at late time after the beginning of contamination, 90Sr and 137Cs still remain essential dose-forming radionuclides on the Techa River. In 1957, the East-Urals Radioactive Trace (EURT), and in 1967 the Karachai Radioactive Trace (KRT), were formed. A distinguishing feature of the radionuclide composition of the releases on the EURT at late time is the prevalence of 90Sr and a minimum content of 137Cs. The composition of radioactive dust on the 1967-Trace is represented primarily by Cs and Sr isotopes in less accessible biological forms compared to those observed on the EURT (Fig. 1). In contaminated areas, measurements of soil contamination levels, analysis of the patterns of radionuclide migration, changes in their biological accessibility, transfer of radionuclides from soil to vegetation, milk and vegetable produce have been conducted on regular basis. Specific activity of 90Sr measured in cross sections of the river at most of the riverside villages has decreased to permissible values since the start of observations in 1960. In flooded areas of the bank line, the processes of deepening of radionuclides into soil and a more uniform distribution of radionuclide contents over the soil layer at a depth of 1.5 m were observed. Mean content of 90Sr in milk produced in the riverside villages has declined to permissible values. On EURT and KRT, of the total radionuclides contained in the soils, 80% remain deposited in the upper 20-cm layer. Biologically accessible and insoluble forms of 90Sr and insoluble forms of 137Cs are prevalent. Reduction in radionuclide content in milk has taken place over the first 1-2 years due to deepening of radionuclides into soil and a decrease in their biological accessibility. The main factor that caused cleansing of radionuclides from food chains was radioactive decay and reduced biological accessibility of radionuclides in soils.


Introduction
In the late 1940s, the facility Mayak Production Association (Mayak PA) for weapon grade plutonium production was put into operation in the vicinity of the town of Kyshtym.The technology used in plutonium production involved generation of high-level waste.A number of accidents that occurred at the plant were associated with inadequate radioactive waste storage techniques.In 1949In -1956, radioactive waste with total activity of about 1.8 10 17 Bq (4.9 MCi) was discharged into the Techa River which resulted in contamination of all river system components.Currently, at late time after the beginning of contamination, 90 Sr and 137 Cs still remain essential dose-forming radionuclides on the Techa River.In 1957, the East-Urals Radioactive Trace (EURT), and in 1967 the Karachai Radioactive Trace (KRT), were formed.A distinguishing feature of the radionuclide composition of the releases on the EURT at late time is the prevalence of 90 Sr and a minimum content of 137 Cs.The composition of radioactive dust on the 1967-Trace is represented primarily by Cs and Sr isotopes in less accessible biological forms compared to those observed on the EURT (Fig. 1).In contaminated areas, measurements of soil contamination levels, analysis of the patterns of radionuclide migration, changes in their biological accessibility, transfer of radionuclides from soil to vegetation, milk and vegetable produce have been conducted on regular basis.Specific activity of 90 Sr measured in cross sections of the river at most of the riverside villages has decreased to permissible values since the start of observations in 1960.In flooded areas of the bank line, the processes of deepening of radionuclides into soil and a more uniform distribution of radionuclide contents over the soil layer at a depth of 1.5 m were observed.Mean content of 90 Sr in milk produced in the riverside villages has declined to permissible values.On EURT and KRT, of the total radionuclides contained in the soils, 80% remain deposited in the upper 20-cm layer.Biologically accessible and insoluble forms of 90 Sr and insoluble forms of 137 Cs are prevalent.Reduction in radionuclide content in milk has taken place over the first 1-2 years due to deepening of radionuclides into soil and a decrease in their biological accessibility.The main factor that caused cleansing of radionuclides from food chains was radioactive decay and reduced biological accessibility of radionuclides in soils.

Natural-climatic characteristics of the affected territory
As a result of the accidents at the Mayak AP, a number of rivers, water basins and lands of the southern and middle zone of the Trans-Urals region were contaminated.The EURT occupies over 3/4 of the forest and forest-steppe part of the Trans-Urals region where there are numerous lakes, swamps, all kinds of depressions and pits, wood lands and forest outliers which account for non-uniformity of radioactive fallouts.The most common are chernozemic-meadowy and meadowy-chrnozemic soils.The Trans-Urals region has a typical continental climate which is formed by the air masses coming from the Atlantic Ocean.The wind conditions of the region are characterized by prevalence of westerly winds.Whirlwinds are not an infrequent phenomenon.Species of wood prevailing in the forest zone include pines and the main hardwood species -birches and aspens.The floodplain vegetation includes grassy and woody-shrubby species.Birches and willows are encountered in the floodplain.Miscellaneous herbs are characteristic of the Techa floodplain in the middle and lower reaches of the river.the lower reaches of the river, the width of the valley ranges from 240 m to 2 km.The floodplain is meadowy, loamy, usually flooded during high water.The watercourse is moderately winding.The river is mostly supplied with snow water.The swampy floodplain of the upper reaches retain a considerable amount of thaw water.The river's tributaries are water-short.Floods usually occur in April.Low water lasts till mid-October.Water discharge during the low water periods increases along the river length from 0.84 m 3 /d near Muslyumovo to 2.62 m 3 /d near the village Klyuchevskoye.The coefficient of ground water supply accounts for 10-30% of the total river drainage.

Radioactive contamination of water
During the initial period, the studies of radioactive contamination of water were based on measurements of -emitting nuclide activity.The most well-systematized data were presented in (Marey A.N., 1959).The highest level of -activity was observed in water in 1951; it was decreasing appreciably with advancing years and increasing distance from the site of releases (Fig. 2).The activity of -emitters in water was significantly lower.The dependences governing the changes in the concentration of these emitters are similar to those identified for -emitters.Reduced concentrations of radionuclides in the river water with increasing distance from the release site were accounted for primarily by the dilution processes in the water flow, sedimentation and radionuclide sorption by bottom sediments.Fig. 3. Changes in volumetric activity of -emitting radionuclides in Techa River water as a function of distance from LRW release site, 1952 (Marey A.N., 1959).
Nuclide composition of the river water sampled in the middle and lower reaches was for the first time determined in 1951 (Table 1).It was established that a significant proportion of activity of the radionuclides cesium, yttrium, cerium and plutonium is transported by the river stream down the rivercourse on clayey and sandy particles.The same applies to zirconium and niobium.The radionuclides strontium and ruthenium are transferred with river stream mostly in the dissolved state.The basic source of inflow of suspended particles is the surface-slope drainage from the catchment area.The results of the researches conducted in 1963 showed that small amounts of radionuclides (from 0.001 to 0.014%) (Yu.G. Mokrov, 2002) were carried by the bottom alluvium to the Techa River.
The construction of the Techa cascade of water reservoirs for storing low-level sewage water and re-directing medium-level waste to Karachai Lake resulted in reduced concentrations of radionuclides in river water and bottom sediments.By that time, the radionuclides 90 Sr and 137 Cs became the most important contaminants of the Techa River.The long-term dynamics of radionuclide content measured in river water (e.g., at Muslyumovo) up to 1990 was characterized by persistent reduction in 90 Sr and 137 Cs concentration.Instability and periodical increases in radionuclide concentrations have been observed in river water (Fig. 4) since 1994.In addition to that, 90 Sr concentration is stably exceeding the currently permissible level of 4.9 Bq•l 1 .Concentrations of 137 Cs in river water are less stable along the watercourse (Fig. 5), but the values of the volumetric activity of this radionuclide does not exceed the permissible concentration for drinking water (11 Bq•l -1 ), the role played by 137 Cs in radiation exposure of the riverside population is not very significant.(Table 2).The main source of radioactive contamination of river water is the Techa cascade of reservoirs.Additional contamination is accounted for by desorption of radionuclides from the contaminated floodplain and the river bottom sediments.
In 2009, specific activity of tritium was for the first time determined in water of the Techa river (Fig. 6).Presented in the figure are concentrations of 90 Sr and tritium in water of the Techa River over its total length down to its confluence with the Isset River measured in samples taken within a week's time in August 2009.(Yu.A. Izrael, 2000).

Zone
Volumetric activity (Bq l -1 )   Mokrov Yu.G., 2000); and also because under oxidizing conditions characteristic of surface waters, uranium existing in the form of uranyl-ion (UO 2 +2 ) is weakly sorbed by floodplain soils and the river's bottom sediments.The difference between the values of the ratio 3 H/ 90 Sr for river and TCR waters (10 and 1) is accounted for by dilution of TCR effluent seepage with bypass canal waters in the proportion 1 to 10.
In order to assess the role played by the catchment area of the upper reaches and the river bottom sediments (0-40 km) in contamination of water with 90 Sr, we applied the twocomponent mixing model (X M =X A ×f + X B ×(1-f), where X M is the end mixture, X A and X B are components, and f is the compound coefficient) using mixture parameters obtained for 3 H. 3 H and 90 Sr volumetric activities were measured at cross-sections located at 3.5 km and 40 km from dam 11, respectively.The values of 90 Sr volumetric activities for waters flowing into the river from the catchment area of the upper reaches range from 4.3 Bq L -1 to 19.3 Bq l -1 , the average value amounting to 9.24 Bq l -1 .It was concluded based on the calculations that entry of 70% of the total activity of 90 Sr into the Techa watercourse results from TCR effluent seepage drained through the bypass canal system.The proportion of 90 Sr activity contributed by washing out of radionuclides from the floodplain and by desorption from bottom sediments accounts for 30%.

Contamination of bottom sediments with 90 Sr and 137 Cs
According to the data of the first investigations, the highest concentrations of radionuclides in bottom sediments were observed in the reaches close to the release site and in the area of Assanov swamps (30 km from the release site).The lowest level of contamination was registered over the last 40 km stretch down to the outfall.A large amount of activity was accumulated in the surface layer (Table 4) (Glagolenko Yu.G., 1966).
Distance  (Saurov M.M., 1968). 90Sr and 137 Cs contamination densities in bottom sediments in the early 1990s are shown in Table 5.During spring floods, silt sediments contaminate the surface of the floodplain which maintains the high levels of contamination of the flooded riverside valley.The levels of silt and water radionuclide contamination are interdependent over the river course.The contents of radionuclides in silt and in water, from dam 11 up to the river outfall are steadily decreasing (Figure 7).Specific activity of 137 Cs in silts is about 5-fold higher than in water, that of 90 Sr if 3-fold higher.Compared to 90 Sr, 137 Cs contamination densities for silts are about 2-fold higher over the total length of the river.
Fig. 7. 90 Sr and 137 Cs contamination densities measured in Techa River silts at different distances from dam 11.
Four decades after termination of intensive discharges of radioactive waste into the river, radionuclides deposited in sandy and silty soils migrated to the depth of over 35 cm.The results of the vertical distribution of the radionuclides of interest in the upper reaches of the river are presented in Figures 8 and 9. Compared to 137 Cs, the distribution of 90 Sr in the bottom soil profiles is more uniform.Maximum values of contamination densities for these radionuclides are in general observed in 0-10 cm layers of soil. 137Cs is characterized by a more dramatic decline of contamination density values in lower layers of soil (at the depth of 20-35 cm).In the upper reaches, additional inflow of radionuclides due seepage from TCR and washout of radioactivity from the lands adjacent to the river is observed; in the mid-stream area the contribution of desorption processes from the upper layer is larger.
With distance from the release site, the proportion of exchangeable and mobile forms of 90 Sr is increasing, on the contrary, the proportion of poorly-accessible forms of 90 Sr is decreasing.Spring overflows of the Techa, and particularly the flood of 1951 contributed to intensive radioactive contamination of the riverside area.The studies of the contents of radioactive substances in floodplain soils started in 1951.The results obtained allowed an insight into the patterns and intensity of the riverside contamination (A.N.Marey, 1959).The width of the floodplain where radioactive contamination was detected did not usually exceed 150-200 m (Table 6).In the upper reaches, in the Assanov swamps area, the overflow reaches 3000 m.A consequent decrease in the levels of floodplain contamination with increasing distance from the release site, and a decrease in the 137 Cs/ 90 Sr ratio should also be noted (Table 7).The first maximum level of floodplain contamination density is observed in the area of Assanov swamps: from dam 11 to the distance of 7.5 km.The second peak takes place in lower reaches, at the distance of 37 km from the dam in the area of Muslyumovo swamps.In these areas, the incessant winter run-off of radioactive substances with TCR waters is accumulated.Most thoroughly the floodplain soils are washed during high waters.Surface-downslope waters wash upper layers of the soil, flood waters wash upper layers of the flooded river bank, and ground waters wash deeper layers of the floodplain soils.
With distance along the watercourse, the levels of floodplain contamination with 137 Cs are appreciably decreasing.In the swamps of the upper reaches, the values of contamination with the radionuclides of interest amount to 150-550 Ci⋅km -2 , the values for the middle reaches are 20-30 Ci km -2 , and the respective value for the area close to the estuary is 5 Ci⋅km -2 .The dynamics of reduction in 90 Sr contamination density values assumes a more monotonous character.The results of measurements of 137 Cs and 90 Sr contamination densities conducted by us in the floodplain in 2005 are presented in figure 10.It can be seen that the level of floodplain contamination with 137 Cs is higher than the respective value obtained for 90 Sr, with the exclusion of the last locality (village Zatechanskoye).The study of 137 Cs distribution by layers of the floodplain along the watercourse (Table 8) has demonstrated that the upper 20 cm layer is contaminated to the highest extent.However, the measurements conducted in the village Zatechenskoye where sandy soils predominate, the concentrations of radionuclides in the lower and surface layers are actually the same.As has been shown by measurements at most of the sampling sites, the upper 0-5 cm soil layer contains a smaller amount of 137 Cs as compared to the underlying layer.This fact indicates that there has been no intensive contamination of the floodplain with 137 Cs in the recent years.An extreme non-uniformity in the distribution of 90 Sr by soil profiles (Table 9) in different reaches of the river can be accounted for by different physical-chemical and morphological properties of the floodplain soils.Most of the floodplain soils (Assanov Bridge, Nadyrov Bridge) are characterized by depletion of the upper 0-10 cm layer, presence of maximum 90 Sr concentrations at the depth of 10-30 cm with a dramatic drop in concentrations measured deeper along the profile.This is caused by washing of the upper layer with the surface waters.In the locality of Muslyumovo, the distribution of 90 Sr by soil layers is influenced by the continuous run-off from the overlying swampy layers.Further along the watercourse, the processes of cleaning radionuclides from the upper soil layers and a more uniform distribution along the depth of the soil become prevalent.(Nizhnepetropavlovskoye and Zatechenskoye).From 1991 through 2005, the total content (reserve) of 137 Cs at the sampling site (Fig. 11) decreased from 574.5 MBq·m -2 to 52.8 MBq·m -2 , accumulating mainly at the depth of 30 cm, and lower, at 40 cm above the aquifer.The total content of 90 Sr in soil samples taken in the floodplain (Fig. 12) decreased from 14.4 to 8.8 kBq·m -2 .Over this period, equalization of the contamination was noted at the depth of 55 cm, while in the lower layer an increase in contamination level was observed, with radionuclides moving all the way to the aquiferous layer, obviously due to intensive seasonal washouts of the bog soils.
In 2009, samples were taken in the floodplain area close to Assanov swamps at the depth of 80 cm within 30, 70, 106 and 250 m of the shoreline.Below, an aquifer composed of blue clay was situated.It was shown that 137 Cs and 90 Sr contamination densities in the floodplain soil (figure 13) were declining depending on distance from the shoreline.The reserve of 137 Cs exceeds that of 90 Sr at any distance, and their ratio changed at different distances from 22-2 times.The distance from 30 to 70 m is characterized by the lowest levels of contamination due to wash-off by ground waters.In samples of floodplain taken in the area of Assanov swamps at different depths and sites, prevalent are water-soluble forms of radionuclides deposited in deep layers (70-80 cm) of riverside soil (up to 26%) in localities where they migrate in the direction of the watercourse.A slight increase in exchangeable and mobile forms of 137 Cs in the lower layer was observed.
In 2005 and 2006, samples of ground and surface waters were taken in the area of Assanov and Muslyumovo swamps with the aim to determine the actual scope of influence exerted by deposited radionuclides on concentrations of radioactivity in swamp water.During cold seasons, when migration processes are at their minimum, concentration of 90 Sr in flow channels is high, reaching 187 Bq l -1 , the concentration level registered in ground water is 14-19 Bq•l -1 .In spring, in the portion of the swamp area, where there is no distinctly outlined water flow, 90 Sr concentrations in water amount to 80-100 Bq•l -1 in localities far removed from the river banks.In flow channels with a sufficiently dynamic water flow, radionuclide concentrations reach about 40 Bq•l -1 .In Muslyumovo swamps, water sampled in dead channels contains 11-25 Bq•l -1 of 90 Sr with radionuclide concentration level in river water of 6.2 Bq•l -1 .The data obtained point out to the fact that it is mainly 90 Sr which is leached from the floodplain to water, the same applies to 137 Cs, but to lesser extent.
Thus, it was demonstrated that the floodplain and bottom deposits are the key source of secondary contamination of the river water with radionuclides.The level of radioactive contamination of these river components is, in its turn, determined by the radioactive runoffs from the Techa cascade of reservoirs.

East-Urals Radioactive Trace
3.1 Radioactive contamination of soil, vegetation, food products with 90 Sr and 137 Cs in the early years after the 1957 accident A distinguishing feature of the radioactive emission on the EURT is the presence in it of all basic uranium fission products and a minimum content of the 137 Cs ( The settling of the radioactive mixture from the cloud which was wind-drawn in the northeastern direction from the explosion site resulted in the formation of the East-Urals Radioactive Trace (EURT).The Trace encompassed parts of Chelyabinsk, Sverdlovsk and Tyumen oblasts.The length of the EURT is 300 km, its width is 30-50 km.According to refined data, (Korsakov Yu.D. et al., 1996), in 1957 the area with contamination density (η) for 90 Sr >2 Ci km -2 was 560 km 2 , that with η>12 Ci km -2 was 230 km 2 , with η>50 Ci⋅km -2 -120 km 2 , with η>200 Ci⋅km -2 -50 km 2 , with η>800 Ci⋅km -2 -16 km 2 , and with η> 2 000 Ci⋅km -2 -8 km 2 .There were over 200 populated localities in the Trace area, including several towns and industrial communities.At early time after the accident, the population of the EURT area was exposed to radiation-related hazards, including, in the first place, external exposure to γ-radiation due to prevalence of γ-emitting nuclides in the deposited mixture and, in the second place, internal exposure resulting from intakes of radionuclides contained in food products produced in the localities.As the activity of γ-emitting nuclides (which decayed almost completely 6-7 years after the accident) decreased, the radiation hazards were mostly determined by the radionuclide 90 Sr.
The first measurements conducted on the contaminated territory showed that the γradiation dose rate was proportional to the distance from the accident site.According to measurements conducted during the first year after the accident in the area with contamination density amounting to 1 Ci km -2 for 90 Sr, the dose in air due to γ-radiation was 1R (G.N. Romanov, 1963).Direct exposure to -radiation only occurred in areas with contamination density of over 1 500 Ci⋅km -2 for 90 Sr.According to data presented in (A.Ya. Kogotkov, 1968), a decrease in relative contents of 90 Sr in the composition of the mixture depending on distance from the accident site, and, respectively, an increase in the contents of 144 and 137 Cs.Soil contamination densities in some localities may differ by an order of magnitude, or greater.
The coniferous woods were affected most heavily.At the distance of 12.5 km from the contamination source, a total loss of pine woods was registered in the summer of 1958.Mass loss of birch forests were only observed in areas with contamination densities of over 4 000 Ci⋅km -2 .Migratory birds were only affected in the spring of 1958, after dose rates in the tree crowns had decreased 10-fold.A reduced number of bird's nests was noted in the areas with contamination level of over 2 000 Ci⋅km -2 for 90 Sr.No loss of animals was registered.In the ensuing years, due to the fact that the contaminated area was made into a sanitaryprotection zone, the number of hares, roes and elks increased considerably.
During the first days after the accident, radioactive contamination of grass estimated relative to 1 Ci/km 2 of soil contamination was 1.5⋅10 6 decay min -1 ⋅kg -1 .Radioactive contamination of individual food products was very high (Table 11).Since cattle and other domestic animals were fed contaminated forage, contamination of milk and meat was of structural rather than superficial nature.It should be noted that contamination of milk was registered as early as the first 2-3 days after the accident, and that of meat on days 10-12 (R.M. Alexakhin et al., 2001).
During the first days after the accident, radioactive contamination of grass estimated relative to 1 Ci/km 2 of soil contamination was 1.5⋅10 6 decay min -1 ⋅kg -1 .Radioactive contamination of individual food products was very high (Table 11).Since cattle and other domestic animals were fed contaminated forage, contamination of milk and meat was of structural rather than superficial nature.It should be noted that contamination of milk was registered as early as in the first 2-3 days after the accident, and that of meat on days 10-12 (R.M. Alexakhin et al., 2001) Food products were the main contributor to radiation exposure of the population.The main cause of the reduction in contents of radionuclides in food products over time is the reduction of radionuclide contents in soils.Table 12 shows the basic composition of radionuclides observed in food products and the dynamics of cleaning the food from radionuclides.Among those radionuclides, 144 Ce+ 144 Pr prevailed during the first 3 years, later on 90 Sr gained priority.
Beginning from the spring-summer season in 1958, an additional contamination of vegetation and agricultural plants occurred due to a downwind migration of radionuclides from areas with a higher level of contamination density.The proportion of surface contamination of grain crops accounted for 10-15% (P.P. Lyarsky, 1962).
Contamination levels measured for food products of the first post-accident harvest reaped in the fields situated in the Trace zone with contamination densities of 0.2-0.5 Ci⋅km -2 for 90 Sr or higher, were higher than the permissible limit legally valid at that time: 1300 decay min -1 ⋅kg -1 (22 Bq⋅kg -1 ).

Current levels of soil contamination
The studies conducted in 2006-2009 in the villages Allaki, Bagaryak, Bulzi, Tartar Karabolka and Yushkovo located around the perimeter of the EURT of sanitary-protection zone.The soils of the pasture lands adjacent to these villages are mainly dark-grey clay and leached chernozem.
The mean dose-rate value of gamma-radiation in the localities of Karabolka, Musakayeva and Bagaryak is 0.12 µSv•hr -1 which is comparable to natural background values for Chelyabinsk oblast.The mean pasture land contamination density for Sr 90 in the localities of Tatarskaya Karabolka and Musakayeva is 5.9 kBq•m -2 , that for Bagaryak is 2-2.5 times higher.Pasture land contamination density for 137 C is 12.9-24.8kBq•m -2 .Mean contamination densities measured in soils of kitchen gardens attached to houses in Bagaryak was 77.5 kBq•m -2 .Mean specific activity in grass measured in different areas along the EURT axis ranged from 11 to 15 thousand Bq•kg -1 for Sr 90 and from 3 to 60 Bq•kg -1 for 137 C. Forms of Sr 90 and 137 C encountered in soils sampled in the frontal portion of the Trace (close to Alabuga Lake) have been identified (Table 13).Attention is drawn to the increased number of exchangeable forms of Sr 90 and mobile forms of 137 C.During the early years after the fallout, the total amount of Sr 90 was only encountered in soluble state.A decade later, the proportion of exchangeable forms accounted for 65-75%, and the value has not changed since then.
According to data presented by V.V. Martyushov et al. (1996) 36 years after the contamination, the content of exchangeable forms of 137 C and plutonium did not exceed 3% and 1%, respectively.The proportion of poorly accessible forms of 137 C and plutonium reached 95-98%.The content of water-soluble forms of 137 C and plutonium accounts for less than 1%.Water-soluble forms of Sr 90 are mostly found in cationic compounds (72-76%).At a distance of 30 km from the Mayak PA in an area along the Trace axis samples of soils were taken at a depth of 10 cm. 90Sr contamination density was found to range from 2.2 kBq/m 2 to 55.9 kBq/m 2 , that for 137 Cs ranged from 2.2 to 50.7 kBq/m 2 .At the periphery of the Trace, contamination density for 137 Cs exceeded that for 90 Sr 2-5-fold.The contribution made by the debris layer varies significantly.In afforested and steppified areas with well developed steppe debris layer, the contribution of the debris layer to the contamination density for 90 Sr reaches 37%, for 137 Cs -8.8%.

Radionuclide
At a distance of about 40 km from the Mayk PA at a right angle to the Trace axis, 90 Sr and 137 Cs contamination density distribution was determined for the ploughed layer (0-20 cm).
As of the date of the measurements, there were only arable lands that had no sod cover were available for measurements.The highest measured contamination density was 162 kBq/m 2 for 90 Sr and 24.4 kBq/m 2 for 137 Cs Samples of sod-podzol soil were taken on the Trace axis close to Bolshoi Irtysh Lake (about 55 km from the Mayak PA).Contamination densities in the upper 0-20 cm layer and in the ground litter was 308 kBq/m 2 for 90 Sr, and 20 Bq/m 2 for 137 Cs.It is noteworthy that 11.2% of 90 Sr and 18.0% of 137 Cs are contributed by the ground litter.The total contamination densities in soil (0-20 cm) and ground litter along the Trace axis are presented in Figure 14.
The studies performed have shown that soil contamination densities close to the EURT axis are still high, even at the present time; besides, in a number of cases a considerable portion www.intechopen.com of radionuclides is contained in forest debris layer or steppe litter.Contamination levels mostly depend on the distance from the contamination source, i.e. from the Mayak PA.The highest levels of radioactive contamination were identified in the 0-5 cm layer of soil.Below the root-inhabited layer, 2.7%-57.2% of 90 Sr and 28.4%-41.1% of 137 Cs are deposited.In all types of soil, 90 Sr is encountered in biologically accessible forms, and, 137 Cs is contained in poorly-soluble compounds.

Vertical migration of 90 Sr and 137 Cs through soil profiles
To allow assessment of soil contamination levels, two main parameters are used, viz., specific activity of the radionuclide in soil (Bq•kg -1 ) and contamination density (Bq•m -2 ) which takes into account the total contamination density in all n soil layers.There is no direct relationship between these two parameters.It was established that over the time period since the 1957 accident the radionuclides had migrated to a significant depth.Actually, samples taken at all the sampling sites showed that the highest level of contamination with 137 Cs and 90 Sr was detected in the upper level of soil and in debris layer (in meadow soils it was found in sod cover and in thick felt of the steppe).The ratio of radionuclide specific activity in the 0-10 cm layer to activity in the 10-20 cm layer did not depend on the summarized contamination density.The value of this ratio is mostly influenced by the type of the ecosystem: the mean value of this ratio for 137 Cs in forest ecosystems is 20.4±4.4,and in meadows it is 2.9±1.6.For 90 Sr, the differences are insignificant: 3.4±0.9 in forests, and 2.8±1.5 in meadows.The ratio of specific activity of 137 Cs in debris cover to activity in the 0-10 cm soil layer amounted on an average to 0.5±0.1, and for 90 Sr to 1.5±0.2.Although the specific activity of 137 Cs and 90 Sr in the litter layer is sufficiently high, it does not exert substantial influence on the summarized contamination density since the volume weight of the litter layer is by two orders of magnitude lower than the volume weight of soil.Sod-podzol soils which most often are found in the northern part of the Trace, are formed under coniferous woods.The distribution of 90 Sr and 137 Cs along the profile of sod-podzol soils is comparable to the distribution observed for gray forest soils.The highest specific activity of 90 Sr (1.0×10 4 Bq•kg -1 ) and 137 Cs (1.2×10 3 Bq•kg -1 ) was found in the lower part of the forest waste, the highest contamination density for 90 Sr (39.6%) and 137 Cs (40.4%) was measured in the upper 0-5 cm soil layer.The forest waste and soil layer up to 20 cm in depth contain 96.2% of 90 Sr and 58.9% of 137 Cs.At the depth of over 20 cm 3.7% of 90 Sr and 41.1% of 137 Cs are deposited below 20 cm.As can be seen from the comparison of the distributions, the mobility of 90 Sr in the sod-podzol soils differs insignificantly from that registered in gray forest soils, the mobility manifested by 137 Cs is significantly higher.Already at the depth of 25-30 cm, the specific activities of 90 Sr and 137 Cs actually differ one from the other, however, in lower layers 137 Cs takes the first position.
There occur in steppefied areas of the EURT weak northern or leached chernozems which have been ploughed up for a long time period.It should have been expected, therefore, that the distribution of radionuclides through the 0-20 cm plough-layer would be more uniform and speedy.The site for taking soil samples from the chernozem profile is located close to the EURT axis, however, the contamination density for 90 Sr turned out to be low, viz., 17.8 kBq•m -2 for 90 Sr and 40.2 kBq•m -2 for 137 Cs over the whole profile. 137Cs was found to be significantly prevalent in each layer.
The highest specific activity for 90 Sr (92.2 Bq•kg -1 ) and 137 Cs (161 Bq•kg -1 ) was observed in the debris cover which was characteristic of other soil types too.The highest contamination density was measured in the 0-5 cm layer for 90 Sr (10.4%) and in the 5-10 cm layer (10.9%), and for 137 Cs in the 0-5 layer (21.4%).At the depth of over 20 cm, 57.2% of 90 Sr, and 37. 4% of 137 Cs were deposited.

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The analysis of 90 Sr distribution patterns in the EURT soils were made using data of researches conducted in 1963-2008 (figures 15-17).It can be seen that the distribution of 90 Sr in the profile of the 30 cm layer is well described by the following exponential function: y=ae -bx , where y is the content of radionuclides calculated as percentage of the total contamination density in the 30-cm layer, x is reference number of sample taken in the 5-cm soil layer.Using the coefficient b it becomes possible to calculate the depth at which a decrease in contamination density to a preset level takes place. 90Sr which settled on the soil surface is slowly migrating to deeper layers, and the coefficient b is decreasing (Table 14).
In gray forest soils, the value of coefficient b correlates with the number of years that have passed since the accident (r = -0.94,= 0.02).In sod-podzol soils, especially intensive 90 Sr migration is going on.
So far, no shift in maximum 90 Sr and 137 Cs activities down the soil profile has been observed in any soil types of interest (gray forest, sod-podzol, chernozem).The highest specific activity of both 90 Sr, and 137 Cs in natural and fallow lands is retained in the lower layers of forest litter and or steppe debris.High specific activity of 90 Sr and 137 Cs is also retained in 0-5 soil layer.In deeper soil layers, the activity of these radionuclides is decreasing rapidly and reaches the minimal values in the 25-40 cm layers of eluvial horizons.In illuvial horizons, radionuclide activity is slightly decreased.In general, the specific activity of 90 Sr and 137 Cs in natural soils, beginning at a depth 20-25 cm is relatively stable.The calculations have shown that the 180-300 cm soil layer contains about 28% of 137 Cs and 18% of 90 Sr of the total contamination density in soil layer 0-300.Most of the researchers engaged in studies of radionuclide distribution in soil have noted that in the first years after atmospheric fallouts the highest amounts of 90 Sr and 137 Cs settled in sod cover or forest litter.It is known, however, that vegetable waste of herbaceous type decays within one season, while leaf wood waste decays within 3-4 years.It takes longer for waste of coniferous forests to decay, however, over the 50 years since formation of the EURT the forest litter contaminated by atmospheric fallouts should have decayed long ago, while the steppe cover of chernozem did not develop until 1991.Thus, the high specific activity currently observed in the forest litter is due not only to the initial fallouts in 1957 and 1967, but also to a high rate of local radionuclide turnover and continuous fallouts of activity.
The reserve of radionuclides in the vegetation mat and upper soil layers is constantly replenished due to vegetation waste.The waste cover is, in its turn, contaminated due to atmospheric fallouts and transfer of radionuclides to the top by the root system.Since actually the total surface biomass of vegetation is transferred to the waste cover at the end of the year, the yearly carry-over of 90 Sr to the surface is determined by the annual productivity of the ecosystem.
Waste of woody vegetation accounts for just a portion of the yearly increment, in addition to leaves and grass, the waste composition includes slowly decaying wood, needles and strobiles.That is why radionuclides are deposited not only in soil but also in wood and litter cover.Mineralization of decayed biomass involves an increase in the relative contents of radionuclides in the waste litter.Specific activity of 90 Sr in the ground litter was 2.2±0.7 times higher than in grass at a distance of 20 km from the contamination source, and 6.4±36 times higher at a distance of 30 km; the respective values for 137 Cs were 14±8 and 12±8.Table www.intechopen.com15 presents specific activity values for radionuclides deposited in litter cover and the upper layer of soil. 90Sr activity measured in the upper layer of the litter cover is actually the same as that in the upper layer of the soil, and activity in the lower layer is substantially higher.The difference between levels of 137 Cs activity in the upper and lower layers of the litter cover is even more substantial than that found for 90 Sr. Ratio of 90 Sr specific activity to the activity in the 0-10 cm soil layer at a distance of 20 km from the Mayak PA is 0.7±0.3, and that at 30 km is 2.1±1.0.The ratio for 137 Cs is 0.08±0.04 and 0.4±0.4.Since contamination levels of grass are higher at longer distances, it can be assumed that currently the uptake of 90 Sr and 137 Cs by the biomass is going on through the root system. 90Sr activity in leaves which contribute the largest portion of the vegetation mat is actually the same as that identified in the 0-5 soil layer.However, as was mentioned above, the weight of the vegetation waste per 1 m 2 was not large.Therefore, the plant litter vegetation waste contributes an insignificant proportion of contamination density of the plant litter and upper layer of soil.
It was found out as a result of measurements of the proportions of radionuclides occurring in different forms and having different degrees of accessibility to plants that 90 Sr is mostly encountered in exchangeable form (64-85%), a larger amount of exchangeable strontium being deposited in sod-podsol soils than in gray forest and chernozem.The largest amount of 137 Cs is strongly bound together and it occurs in acid-soluble form (4-36%) or as a solid residue (27-82%).In all cases, the content of 137 Cs accessible to plants in the 5-10 layer proved to be higher, and the content of 137 Cs in the form of solid residue was significantly lower.
Thus, as of today, the studies of all the soil types have shown that the major part of 90 Sr and 137 Cs activity is deposited in the upper layer.In 2005-2006, samples of grass and soils taken in the most heavily contaminated areas of the EURT were processed, following which proportionality factors were assessed (PF).Specific activity of 90 Sr in grass ranged from 70 to10940 Bq•kg -1 , that of 137 Cs from 10 to 997 Bq•kg -1 .Proportionality factor (Bq/kg in grass/Bq/kg in soil) was within the range 0.2-2.1, the mean value of transfer factor calculated as a ratio of specific activity of radionuclides in grass to soil contamination density, Bq•kg -1 / Bq•kg -2 , was 15.9±8.5 for 90 Sr, while for 137 Cs it was lower: 3.1±3.4.The difference in transfer factor values may depend on soil type and diversity of plant species composing the cover layer.It should be noted that transfer factor values are slightly higher for sod-podzol soils in the EURT zone.
Contamination with 90 Sr of all sampled wild-growing berries exceeds the permissible limit 3.5-13.5times, 90 Sr proportionality factor is higher than that determined for vegetables and grain (Table 16).Specific activity of 137 Cs in grass and berries does not exceed the permissible limit.It should be taken into consideration that wild-growing berries in the EURT zone present the highest hazard compared to other food products in view of the contribution they can make to dietary intakes of 90 Sr.

Sampling site Species
Specific activity of fresh berries,Bq

93
Potatoes take the second position after milk in terms of 90 Sr contributed by food produce.The dynamics of reduction in specific activity of 90 Sr in potatoes has been the sane over the total period of observations, viz., from 7.0 to 0.9 Bq⋅kg -1 , the respective value for 137 Cs is from 1.5 to 0.7 Bq⋅kg -1 .Within a number of years, stable values of transfer factor have been established for agricultural products ( Values of proportionality factor decrease overtime and, as a result, it becomes possible to grow agricultural product with admissible level of conatimation in soils with a higher contaminated level.
In 1967 about 97% of the total 137 Cs deposited in pasture soils settled in the upper 0-3 cm layer.Currently, 38 years after the fallout, 137 Cs is accumulated in the soil layer at a depth of 13 cm, and 90 Sr is mostly deposited in the soil layer to a depth of 0-20 cm (89.5%).A small portion of radionuclides which settled in the layer at a depth of 0-70 cm migrated to a depth of 70 cm.
In 1967, the transfer factor for contents of 90 Sr in grass and soil was 0.09 Bq⋅kg -1 / kBq⋅m -2 .A sharp decline in transfer factor values for 137 Cs within the soil-grass chain occurred one year later after it had been cleaned of surface contamination, and in the subsequent years no changes in transfer factor values were noted.During early years, the uptakes of the radionuclide were going on from sod cover and soil, and transfer factor values fluctuated between 0.0025 and 0.005 Bq/kg : Bq/m 2 .
Long-term studies of 90 Sr and 137 Cs transfer from soil to grass were conducted in the grazing land of Sarykulmak village where dairy cattle was grazing (Figure 25).A reduction in specific activity of 137 Cs in the pasture grass over the period from 1967 through 2009 is described by the two exponential dependences: 1/2 was 1.8 years in the initial period after deposition of radionuclides, and in the subsequent period it was 3.8 years.The first exponent describes the cleaning of grass from surface contamination, the second describes the reduction in uptakes of radionuclides by grass from soil.Over the period from 1967 through 2009, the specific activity of 137 Cs in grass decreased 100-fold , that of 90 Sr 10-fold.
The initial specific activity of 90 Sr in milk late in April, 1967, was 140 Bq/l, that of 137 Cs -237 Bq/l.From 1967 through 2009, the average values of 90 Sr and 137 Cs specific activity in milk decreased 30-fold.The decrease in 90 Sr and 137 Cs levels in milk was determined based on the decrease in grass contamination.Beginning from 1970, the period of half-cleaning of milk from 90 Sr was 20 years, and from 137 Cs 10 years.Proportionality factors for 90 Sr in the soilmilk chain estimated on the Karachai Radioactive Trace were on the average 5-fold lower than the values estimated for the EURT for the same time periods elapsed after the radioactive fallouts on the soil.Evidently, the long period for which 90 Sr and 137 Cs remained deposited in silt and soil on the shores of Karachai led to a reduction in biological accessibility of radionuclides to plants and, as a result, to a reduced uptake of radioactivity by milk.Specific activities of 90 Sr in milk are more correctly described by the log-normal distribution.After the accident, the contents of 137 Cs in potatoes was insignificant, and overtime it decreased about 10-fold, while the contents of 90 Sr decreased 3-fold.

Conclusion
1. Major radiation accidents that took place in the Southern Urals during the period 1949-1967 brought about contamination of vast territories with radionuclides and exposure of the local population.The situation on the Techa River involved contamination of river water, bottom sediments and the floodplain with 137 Cs and 90 Sr.On the East-Urals Radioactive Trace 90 Sr was the prevalent contaminant, while on the Karachai Trace biologically poorly-accessible compounds of 137 Cs and 90 Sr prevailed.2. The key mechanisms by which contamination of the environment can be eliminated include radioactive decay, reduction in biological accessibility and deepening of radioactive substances in soil.Pronounced sorption capabilities and poor solubility hamper migration of 137 Cs in the environment, while 90 Sr is more mobile.
3. Due to a number of protection measures implemented from 1965 through 2004 on the Techa River, concentrations of 90 Sr and 137 Cs in river water have decreased, and currently the specific activity of 90 Sr amounts to 10-15 Bq•l -1 , and that of 137 Cs to 0.5-1.5 Bq•l -1 .Radionuclides deposited in bottom sediments migrated to the depth of over 35 cm.Contamination density in the 0-10 cm layer depends on the concentration of radionuclides in the watercourse. 90Sr and 137 Cs are revealed in floodplain soil at a depth of over 70 cm where a decrease and averaging of concentrations overtime has been observed.At the present time, it is expedient to retain the constraints on the use of river water.
After the use of river water was banned in 1956, the major pathway for contribution of radioactivity to dietary intakes has been made by milk (87-95 %) and vegetables.Since 1967, the content of 90 Sr in milk, with rare exception, has not exceeded the permissible limit (25 Bq•l -1 ).
4. The rate of vertical migration of 90 Sr in EURT soils ranges from 0.25 to 0.35 cm•year -1 , the largest amount of radionuclides which remains deposited in the upper part of the soil profile (0-20 cm) is decreasing with increasing depth.Nonmobile 137 Cs is mosty retained in the upper 10-cm layer.Biological accessibility of 90 Sr has decreased over the past period 7-10 times and it has not actually changed over the recent years.Currently, 40 years after the accident, the content of fixed forms of 90 Sr in soil has reached 34%, that of 137 Cs and plutonium 95-98%.
The period of half-cleaning of milk from 90 Sr was 2-3 years during the first years after the accident, and during the subsequent period it amounted to 15 years.5. On the Karachai Trace, due to the prevalence of 137 Cs and 90 Sr with poor biological accessibility, the reduction in contamination of soils, grass and food products was going on more speedily (than that registered on the EURT).Specific activity of 137 Cs and 90 Sr in milk exceeded the permissible limits only during the first month after the accident.
6.The prognosis for the further development of the radiation situation on the Techa River is determined predominantly by the radioactive runoff from the Techa cascade of reservoirs.With the lapse of time, the part of the EURT territory where the use of agricultural lands is restricted, will diminish.There are no such restrictions in the Karachai Trace area.

Fig. 1 .
Fig. 1.Schematic map of radiation accidents in the Southern Urals

Fig. 5 .
Fig. 5. Specific activity of 137 Cs in water of the river cross-section opposite Muslyumovo.Data on contents of plutonium isotopes in Techa River water are scarce.Measurements of specific activity of238,239,240 Pu in river water was initiated by the URCRM researchers in 1993

Fig. 8 .
Fig. 8. Distribution of radionuclides along the depth of sandy bottom sediments in the upper reaches of the river.

Fig. 11 .
Fig. 11.Contamination densities for 137 Cs in the Techa floodplain area at sampling site "Assanov swamps" within 6.5 km of dam 11, 10 m from the shoreline.

Fig. 14 .
Fig. 14.Contamination density of soil (0-20 cm) and litter layer along the EURT axis as a function of distance from the source of contamination.

Fig. 15 .
Fig. 15.Dynamics of 90 Sr distribution in 30-cm layers of gray forest soil

Fig. 25 .
Fig. 25.Levels and dynamics of decrease in radionuclide contents measured in pasture grass

Table 1 .
Radiochemical composition of the river water as of 5.08.1951, %

Table 2
Concentrations of90Sr in water measured over the river length has changed from 12 to 1.6 Bq/l, tritium activity ranged from 140 to 17 Bq/l, which is 4-fold higher than the global level.The mean value of 3 H/ 90 Sr ratio in river water is 10.9±1.2.The total estimated carryover of 90 Sr with the Techa river run-off into the Isset River over the period from 1958 through 2001 amounted 2.2×1014 Bq . Specific activity of238, 239, 240Pu in Techa River water in 1994-2002, Bq⋅m -3 Fig.6.Content of 90 Sr and 3 in water of the Techa River www.intechopen.com

Table 3 .
Summarized statistical data on volumetric activities of 90 Sr, 3 H, 137 Cs in water bodies of different origin

Table 3
shows summarized statistical data on volumetric activities of the radionuclides measured in the Techa River, TCR, surface and ground waters of the catchment area in the upper reaches of the river for the period from 2008 through 2010.In our subsequent calculations, the median values of 90 Sr and 3 H were used for TCR water, as well as the 3 H/ 90 Sr ratio equal to 1, median values of 90 Sr and 3 H for Techa River water, 3 H/ 90 Sr It was established that radioactivity is best of all accumulated by silt and clay material, while sandy soils manifest a lower rate of accumulation.It was demonstrated that137Cs and plutonium isotopes were intensively sorbed by all varieties of soils. 90Sr, too, is actively sorbed by soils, but it easily enters into exchange reactions which determines its high migration potential

Table 6 .
Specific -activity in the surface layer in 1952 -1954, kBq⋅kg -1(Marey A.N., 1959)Changes in the width of the riverside area is, as a rule, determined by soil relief; maximum concentrations of radionuclides are registered in lowland areas.

Table 7
. Changes in contents of 137 Cs and 90 Sr in floodplain soils along the river course, Ci⋅km -2(Marey A.N., 1959)

Table 8 .
Distribution of 137 Cs by floodplain soil layers over the length of the river, kBq•m -2 www.intechopen.com

Table 9 .
Distribution of 90 Sr by floodplain layers along the watercourse, kBq•m -2

Table 10 .
Table 10).Characterization of the radioactive releases and the initial reserve of radionuclides on the EURT outside the Mayak PA industrial site(Avramenko V.I. et al., 1977)

Table 12 .
Composition of radionuclide observed in food products at different time after the accident

Table 13 .
Forms of radionuclides identified in the upper 5-cm layer of soil in the vicinity of Alabuga LakeIn 2009, soil samples were taken at 16 sampling sites at a distance of 20 km from the Mayak PA perpendicular to the EURT axis.Five of those sites are situated in a birch forest.The total contamination density in the soils of the rhizogenic layer and forest litter was, on the average, equal to 737 kBq/m 2 for 90 Sr, and 41.2 kBq /m 2 for 137 Cs.It should be noted that 10.5% of the contamination density for 90 Sr and 14.1% of contamination density for 137 Cs are contributed by the debris layer.At 10 points situated in the hayfield extending from the forest up to Alabuga Lake 90 Sr contamination density ranged from 161 to 350 kBq/m 2 , 137 Cs contamination density measured in the hayfield ranged from 34 to 93 kBq/m 2 .Contamination densities measured in the wood were higher than those measured in the hayfield.
Distribution of90Sr and 137 Cs in soil profiles was studied in 2008-2010 for 3 types of soils: gray forest, sod-podzol and chernozem.Gray forest soils are most prevalent in the foreststeppe zone.They are formed in leaf woods and mixed woods of the Trans-Urals region.Distribution of 90 Sr and 137 Cs in gray forest soil profiles was determined at a sampling site within 20 km of the Mayak PA.The highest specific activity of 90 Sr (1.6×10 4 Bq•kg -1 ) and 137 Cs (1.3×10 3 Bq•kg -1 ) was measured in the lower layer of the forest litter which contains a half-decayed tree waste.90Sractivity in the upper layer of the forest litter is actually equal to activity found in the 0-5 cm soil layer,137Cs activity is 2.7 times lower.However, the volume weight of the forest litter is low that is why contamination density summarized for its upper and lower layers is lower than that in the underlying 0-5 cm soil layer where the main amounts of 90 Sr (60%) and 137 Cs (50%) are deposited.The tree waste of 2008 sampled on October 31 demonstrated an increase in contamination density of the tree waste by 2.4 kBq•kg -m -2 for 90 Sr which accounts for 2.1% of the tree waste contamination measured in spring of 2008.Over the 50 years since the accident, no shift of the maximum values with increasing depth through the soil profiles has taken place.At the same time, both 90 Sr, and 137 Cs, though in small amounts, have at least, reached the depth of 170-175 cm.It should be added that 137 Cs which is considered to be less capable for vertical migration, is evidently migrating more actively than 90 Sr, 2.7% of 90 Sr, and 28.4% of 137 Cs are deposited at a depth of over 20 cm.

Table 14 .
Parameters of equations describing 90 Sr distribution in the profile of the 30-cm soil layer www.intechopen.com

Table 15 .
On the EURT axis, within 20 km of the Mayak PA, samples of 2008-vegetation waste were taken in a birch forest.Specific activity of 90 Sr measured in the waste was 5904 Bq•kg -1 , that of 137 Cs 54 Bq•kg -1 .The fall of the aboveground phytomass to the forest floor contributed 2.3 kBq•kg m -2 of 90 Sr and 028 kBq•kg m -2 of 137 Cs.This accounted for 2.1% and 3.4% of contamination density in forest litter for 137 and 90 Sr, respectively, and for 0.36% and 0.8% of the total contamination density in the 0-20 cm layer of forest litter and soil for 90 Sr and 137 Cs, respectively.Soil and litter contamination levels, Bq •kg -1

Table 16 .
Contamination of wild-growing berries with90Sr and 137 Cs Table17presents data on levels of contamination of fresh mushrooms.In 2008, specific activity of 90 Sr and 137 Cs in all samples was found to be significantly lower than the permissible limits (50 Bq•kg -1 for 90 Sr and 500 Bq•kg -1 for 137 Cs).Proportionality factors and transfer factors were significantly lower than those obtained for grass, vegetables and grains sampled in 2007.It can be assumed that mushrooms growing in the EURT zone present no hazards for the population as their contribution to internal dose is insignificant.

Table 17 .
Contamination of mushrooms with 90 Sr and 137 Cs

3.7 Migration of radionuclides along food chains
Levels and dynamics of90Sr specific activity in milk produced in villages at the periphery of theEURT in 1958EURT in  -2006are presented in figure24.A dramatic decrease in contents of 90 Sr in milk had occurred before 1963 while later on a slow decrease was observed overtime.Since 1960 the period of a 2-fold decrease in 90 Sr contents in milk ( 1/2 eff. ) was 23 years.The content of 137 Cs in milk produced in the villages studied was 2 times lower as compared to that of 90 Sr.Currently, this value is1.1 ±0.4 Bq/l for 137 Cs.
-1 /Bq•m -2 .A reduction in the transfer factor resulted from a decrease in biological accessibility of the radionuclide in the soil-pasture grass chain.www.intechopen.com

Table 18 .
Mean values of proportionality factors for 137 Cs and 90 Sr measured in the portion of cash crop grown in 0-20 cm layer of gray forest soils www.intechopen.com