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Radioecology

Properties of radiocesium

Radiocesium in the environm.

Abstract of research results

Cs-137 in forest soils

Cs-137 in plants

Cs-137 in mushrooms

Cs-137 in deer truffle

Cs-137 in wildlife

Sr-90 in envirm. samples

Pb-210 and Ra-226

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             Dynamics of radiocesium (Cs-137) in the animal model                                

Introduction

Radiocesium can reach the organism of animals through the skin (percutaneous resorption), through the respiratory tract (inhalation) and through the gastro-intestinal tract (ingestion). By far the most important pathway is the ingestion (HVIDEN and LILLEGRÄVEN 1961; LANGHAM 1960). For example, WHICKER et al. (1965) examined mule deer (Odocoileus hemionus) in Colorado, and could show, that food ingestion contributes with 97% to the Cs-137 contamination of the animals. In the following it is for this assumed, that the Cs-137 activity is caused by ingestion only. The Cs-137 activity of the animals is determined by input and output of radionuclides, i.e. by the type and quantity of the ingested food and the kinetics of Cs-137 in the animal’s organism (figure 5).

Fig. 1: Schematical model for intake and kinetics of radiocesium in the animal’s body, with the assumption of 2 storages for Cs-137 in the organism.

In the present radioecological model the activity intake of game is calculated as follows:

                                                               (11)

    Am(t):    Activity intake of an animal m (Bq/d)

    Nm:         Number of food components

    CF,k(t):   Activity concentration (Bq/kg) of food component k

    Im,k(t):   Rate of food intake (kg/d) of animal m with respect to food component k

Because the food components ingested by the animals living in wilderness are plants it can be set CF,k(t) = CP,k(t).

 

The kinetics of Cs-137 in the animal’s body

Roe deer and red deer are ruminants. They have stomachs with several chambers (rumen,  reticulum, omasum and abomasums). Microorganisms help to decompose cellulose from ingested plants and other hard to digest components. The end products, short-chained fatty acids, are resorbed quickly.

Roe deer belong to the “leaf browsers” (Konzentratselektierer according to HOFMANN)) due to the morphology of the gastrointestinal system and their alimentation (HOFMANN und STEWART, 1972; HOFMANN, 1989). Browsers are adapted to the digestion of plants with easy to digest cellular substances (TIXIER et al., 1997). In relation to their weight they have a small gastro-oesophageal vestibule with little capacity and few subdivisions, but large openings between the sections of the gastro-oesophageal vestibule (HOFMANN, 1988). This leads to a rapid passage and a short retention time of the particles (HOLAND, 1994) and is the reason, why biological processing of hard to digest plant fibers is limited.

In terms of nutrition physiology red deer are between the browsers and the pure roughage eaters. They belong to the intermediary ruminant type. Their food comprises easy to digest plants as well as cellulose-rich plants. (HOFMANN, 1989). Red deer have a relatively large gastro-oesophageal vestibule, which has many sub-chambers and where villi etc. delay the passage of food particles. This results in a long retention time of the nutrition mash in the gastro-oesophageal vestibule, so that there the microbes can decompose and transform the plant cells thoroughly. The ability of ruminants to digest cellulose decreases from those ruminants, which eat grass to those, which are browsers. Red deer can process cellulose better than roe deer.

Wild boar are omnivores, with stomachs consisting of one chamber only (monogastric animals). While in the ruminants roe deer and red deer, bacteria in the gastro-oesophagial vestibule decompose a large portion of the ingested food, in wild boar the food has essentially to be decomposed by enzymes in saliva, stomach and small intestine.

Cesium is due to the high solubility of its salts quickly resorbed in the gastro-intestinal tract of mammals, and it is transported with the bloodstream to the tissue. Like potassium it is concentrated intra-cellular, in particular in the muscle tissue. In monogastric animals the main location of resorption is the small intestine (MOORE and COMAR, 1962), while ruminants take up cesium additionally through the rumina (HOOD and COMAR, 1953; ILIN and MOSKALEV, 1957). The portions of resorption are:

    Monogastric animals

    85-100%  (MOORE and COMAR, 1962; VOIGT et al., 1989)

    Ruminants

    50-80% (HOEK, 1976 (64-79% for sheep); ILIN and
    MOSKALEV, 1957 (50% for cow); MC CLELLAN et al., 1962 (50-80% for sheep);   SANSOM, 1966 (70-80% for cow)].

The lower intake of cesium by ruminants in comparison to monogastric animals is caused essentially by the different gastro-intestinal system and the adsorptive properties of raw fibers in the food.

 

Transfer of 137Cs to the animal’s body

The resorption is a quick process: Within one to several hours maximum levels of Cs concentration are reached in the blood. The highest Cs contaminations appear at first in kidney and liver, which are the most active organs in terms of metabolism, whereas the concentration in the muscle tissue increases merely slowly. With chronic intake of cesium the equilibrium of the activity in the tissue is reached after some days. In spite of further cesium intake the activity does not change any more, because the rates of resorption, distribution and excretion have reached a constant level. When the state of equilibrium is reached, the transfer factor (TFl), which describes the transfer of nuclides from food to meat, can be calculated.

                                                                         (12)

    AFl  =      Activity of the nuclide in the meat [Bq/kg]

    AFu=       Activity of the nuclide in the food [Bq/kg]

    I    =       Daily amount of ingested food [kg/d]

When the intake of cesium stops, the tissue is emptied of the radionuclides with a biological half-life, which has a characteristic value for each animal species (Tbiol).

The transfer factor TFl is obviously dependent on the kind of digestion and the body mass of the animal species. For domestic animals NALEZINSKI et al. (1995) have found a negative correlation between the transfer factor feed ® meat and the body mass within the group of monogastric animals and ruminants: The transfer factor decreases with increasing body mass. Furthermore, monogastric animals have higher transfer factors than ruminants with comparable body mass. For game no corresponding investigations are available.

The transfer of radionuclides to the body of an animal is described in the model by means of the transfer factor food → animal. The integration of the time dependent food amount ingested by the animal leads under consideration of the biological half-life and the radioactive decay to the calculation of the activity concentration:

 

                       (13)

 

    CA,m(T):  Activity concentration (Bq kg-1) of animal m at time T

    :       Transfer factor (d/kg) for animal m

     Tbiol,m:   Biological half-life (d) for animal m

 

Food intake in “mushroom years” (only for roe deer modelling)

There are years, in which one or more of the aboveground fructifying mushroom species appear particularly numerous in the forests (“mushroom years“). In these years roe deer are ingesting more mushrooms than normal. Since mushrooms contribute considerably to the 137Cs input of roe deer, the model takes into account such mushroom years, as well as those years, in which few mushrooms are growing. Irrespective of the amount of ingested mushrooms, the total amount of food ingested by the animals remains unchanged. In years with particularly many or few mushrooms, not only the rate of mushroom intake has thus to be changed. It has also to be weighted the amount of the others food components. This is done by the following equation:

 

 

 

Dynamics of 137Cs in the plant model (animals nutrition plants)

After a single and temporarily limited deposition of radio-cesium, as happened after the Chernobyl accident, the long-term contamination takes place nearly exclusively by root uptake. Since for the root uptake only those radionuclides are available, which are not fixed in the soil, the activity of plants can be calculated with the above described soil model and by taking into account radioactive decay:

                                                (10)

    CP,k(t):   Activity concentration of a plant k (Bq kg-1)

    TFP,k:     Soil-to-plant transfer factor (activity per kg plant / activity per kg soil)

    Adep:      Deposited activity (Bq m-2) at time point t = 0

    r:           Density of the soil (kg m-3)

    L:          Depth of the root zone (m)

    n(t):              Time function

    t:         Half-life of radioactive decay (a)

 

In the soil model the rates for the migration out of the root zone (between soil compartment I and soil compartment III) as well as the rates of fixation and desorption (between soil compartment I and soil compartment II) may adopt different values for different plants. This can be ascribed to different soil conditions at the sites, where the plants are growing.  

 

 

 

This research was conducted  with funds of the

Federal Ministry for Environmental, Nature Protection and Reactor Safety.

This report reflects the views and opinions of the contractor and need not necessarily correspond to those of the sponsor.

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