Satisfacción de los visitantes con respecto a la planta turística

Animals take up radionuclides through contaminated forage and direct soil ingestion. Milk and meat were major contributors to the internal radiation dose to humans after the Chernobyl accident, both in the short term, due to 131_{I, and in}

the long term, due to radiocaesium. In intensively managed agricultural ecosystems, high levels of contamination of animal food products can be expected only for a few weeks, or at most a few months, after a pulse of fallout. In these circum- stances the extent of interception and retention on plant surfaces largely determines both the duration

and the level of contamination of animal derived food products. An exception is found where very high deposition occurs or where plant uptake is high and sustained, both of which occurred in some areas after the Chernobyl accident.

The levels of radiocaesium in animal food products can be high and persist for a long time, even though the original deposition may not have been very high. This is because: (a) soils often allow significant uptake of radiocaesium; (b) some plant species accumulate relatively high levels of radio- caesium, for example ericaceous species and fungi; and (c) areas with poor soils are often grazed by small ruminants, which accumulate higher caesium activity concentrations than larger ruminants [3.35].

The contamination of animal products by radionuclides depends on their behaviour in the plant–soil system, the absorption rate and metabolic pathways in the animal and the rate of loss from the animal (principally in urine, faeces and milk). Although absorption can occur through the skin and lungs, oral ingestion of radionuclides in feed, and subsequent absorption through the gut, is the major route of uptake of most radionuclides. Absorption of most nutrients takes place in the rumen or the small intestine at rates that vary from almost negligible, in the case of actinides, to 100% for radioiodine, and varying from 60% to 100% for radiocaesium, depending on the form [3.31].

After absorption, radionuclides circulate in the blood. Some accumulate in specific organs; for example, radioiodine accumulates in the thyroid, and many metal ions, including 144_Ce, 106_{Ru and}

110m_{Ag, accumulate in the liver. Actinides and}

especially radiostrontium tend to be deposited in the bone, whereas radiocaesium is distributed throughout the soft tissues [3.36, 3.37, 3.50, 3.59, 3.60].

The transfer of radionuclides to animal products is often described by transfer coefficients defined as the equilibrium ratio between the radio- nuclide activity concentration in milk, meat or eggs divided by the daily dietary radionuclide intake. Transfer coefficients for radioiodine and radio- caesium to milk, and for radiocaesium to meat, are generally lower for large animals such as cattle than for small animals such as sheep, goats and chickens. The transfer of radiocaesium to meat is higher than that to milk.

The long term time trend of radiocaesium contamination levels in meat and milk, an example of which is displayed in Fig. 3.29, follows that for

1986 1988 1990 1992 1994 1996 1998 2000 2002 Year 20 15 10 5 0 TF ((Bq/kg)/(kBq/m 2))

FIG. 3.27. Dynamics of the 90_{Sr TF into natural grass from}

soddy podzolic soil in the CEZ [3.39].

100.00 10.00 1.00 0.10 0.01 1 3 5 7 9 11 13 15

Years after fallout

(1) (2) Tag2 = 0.30 exp(–ln 2t/4.0) + 0.11 Tag1 = 18 exp(–ln 2t/4.1) + 3.8 Tag (Sr) (10 –3 m 2/kg) Tag3 = 0.12 exp(–ln 2t/3.3) + 0.034 (3)

FIG. 3.28. Dynamics of the 90_{Sr aggregated TF for natural}

grasses (1: sandy and sandy loam soil, Bryansk region, Russian Federation) and cow’s milk (2: sandy and sandy loam soil, Bryansk region, Russian Federation; 3: chernozem soil, Tula and Orel regions, Russian Federa- tion) [3.56].

vegetation and can be divided into two phases [3.55, 3.57, 3.58]. For the first four to six years after the deposition of the radiocaesium there was an initial fast decrease with an ecological half-life of between 0.8 and 1.2 years. For later times, only a small decrease has been observed [3.55, 3.56].

There are differing rates of 137_{Cs transfer to}

milk in areas with different soil types, as demon- strated over nearly two decades after the accident (Fig. 3.30) in milk from the Bryansk, Tula and Orel regions of the Russian Federation, where few countermeasures have been used. The transfer of

137_{Cs to milk is illustrated using the T}

ag, which

normalizes the data for different levels of soil contamination; this makes comparison among soil types easier. The transfer to milk declines in the order peat bog > sandy and sandy loam > chernozem and grey forest soils. Both the dynamics of 137_{Cs activity concentration in milk and its}

dependence on soil type are similar to those in natural grasses (see Fig. 3.26) sampled in areas where cattle graze.

Similar long term data are available for comparing the transfer of 137_{Cs to beef in the}

Russian Federation for different soil types. They also show higher transfer in areas with sandy/sandy loam soils compared with chernozem soils (Fig. 3.31); there has been little decline in 137_Cs

transfer over the past decade.

The long term dynamics of 90_{Sr in cow’s milk}

sampled in Russian areas with dominant soddy podzolic and chernozem soils (see Fig. 3.28) are different from those of 137_{Cs. The graphs for} 90_{Sr in}

milk do not contain the initial decreasing portion with an ecological half-life of about one year, as shown in the graphs for 137_{Cs, which are presumed}

to reflect fixation of caesium in the soil matrix. In contrast, the 90_{Sr activity concentration in cow’s}

milk gradually decreases with an ecological half-life of three to four years; the second component (if any) has not yet been identified. The physical and chemical processes responsible for these time dynamics obviously include diffusion and convection with vertical transfer of 90_{Sr into soil, as}

well as its radioactive decay. However, the chemical interactions with the soil components may differ significantly from those known for caesium.

Meat Milk 10 000 1000 100 10 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 Year g k/ q B

FIG. 3.29. Changes with time in mean 137_{Cs activity}

concentrations in meat and milk produced in contaminated districts of the Bryansk region of the Russian Federation (Bq/kg) [3.55]. 10.00 1.00 0.10 0.01 0 2 4 6 8 10 12 14 16 18

Years after fallout (1) (2) Tag2 = 0.34 exp(–ln 2t/1.6) + 0.03 Tag1 = 13 exp(–ln 2t/1.6) + 0.78 Tag (Cs) (10 – 3 m 2/kg) (a) 10.0 1.0 0.1 0 2 4 6 8 10 12 14 16

Years after fallout (1) (2) Tag2 = 3 exp(–ln 2t/1.8) + 0.09 Tag1 = 7 exp(–ln 2t/1.7) + 0.12 Tag (Cs) (10 –3 m 2/kg) (b)

FIG. 3.30. (a) Dynamics of the 137_{Cs aggregated TF for}

cow’s milk. 1: peat bog soil, Bryansk region, Russian Federation; 2: chernozem soil, Tula and Orel regions, Russian Federation [3.56]. (b) Dynamics of 137_{Cs aggre-}

gated TF for cow’s milk (sandy and sandy loam soil, Bryansk region, Russian Federation). 1: 137_{Cs soil deposi-}

tion <370 kBq/m2_{; 2:} 137_{Cs soil deposition >370 kBq/m}2

By combining information on radionuclide transfer with spatially varying information in geographic information systems, it is possible to identify zones in which a specified average activity concentration in milk is likely to be exceeded. An example is shown in Fig. 3.32.

A significant amount of production in the former USSR is confined to the grazing of privately owned cows on poor, unimproved meadows. Owing to the poor productivity of these areas, radiocaesium uptake is relatively high compared with that on land used by collective farms. As an example of the difference between farming systems, changes in 137_Cs

activity concentrations in milk from private and collective farms in the Rovno region of Ukraine are shown in Fig. 3.33. The activity concentrations in milk from private farms exceeded the action levels until 1991, when countermeasures were implemented that resulted in a radical improvement.