Modalites de la contamination de deux chaines trophiques dulcaquicoles par le cobalt 60: I: Contamination directe des organismes par l'eau

Abstract

1. Introduction

In order to evaluate Cobalt 60 behavior in freshwater ecosystems, two food chains including different nutritionally interdependent species in a natural environment were defined: CHLORELLA → DAPHNIA → CARP Chlorophyceae Crustacea Fish ELODEA → LYMNAEA → CRAYFISH Phanerogamia Mollusca Crustacea

This article will cover the dynamics of water-borne contamination in the above species.

2. Experimental Procedures

• 2.1. EXPERIMENTAL CONDITIONS

  Table I outlines the basic experimental conditions.

  In each case, Cobalt 60 was introduced into water in chloride form in HCI solution. Except for Chlorella culture medium, radioactivity levels of water were readjusted at regular intervals throughout the investigation.

• 2.2. SAMPLING METHODS

  Water samples were filtered to 0.45 μm and activity levels were measured separately in the soluble and particulate forms.

  Chlorella were separated by centrifugation then rinsed in non-radioactive water and centrifuged again. Radioactivity was measured in the living cells. The algae were then oven-dried and their dry weight recorded: fresh weight was estimated at five times the measured dry weight.

  For the other species, radioactivity was measured on living specimens and related to fresh weight.

• 2.3. RADIOACTIVITY MEASUREMENTS

  They were carried out by y-spectrometry, with suitable allowance for the detection system efficiency and geometry effects.

3. Results and Discussion

• 3.1. CHLORELLA

  It was observed that Cobalt 60 accumulation was fast; peak value was reached within 24 h (figure 1), with a mean concentration factor versus water of 565 (Table II).

• 3.2. CARP

  Environmental equilibrium conditions were reached on the 32nd day (Figure 1), with a mean concentration factor of 3.4 (Table II).

• 3.3. ELODEA

  Contamination kinetics are shown on Figure 2. Under equilibrium conditions mean concentration factors ranged from 4037 to 4410 according to activity levels in water (Table II).

• 3.4. LYMNAEA

  Equilibrium was reached after four to six weeks, with mean concentration factors between 401 and 1038 according to radioactivity levels in water (Figure 2 and Table II).

• 3.5. CRAYFISH

  The equilibrium concentration factors ranged from 40, in group A, to above 394 for individuals in group A, to above 394 for individuals in group B, (Figure 2). These variations might be related to the condition of the exoskeleton between molts.

  Our experimental result were compared with other published data (Tables III and IV)

4. Conclusion

Concentration factors obtained in laboratory conditions are of limited value for extrapolation to environmental contamination; however, such experimental data do have a relative value. Generally the most highly contaminated species were the primary elements in the food chain, particularly macrophytes. Radioactivity levels in animal species decreased directly with increasing zoological complexity, which in turn generally correspondended to a higher trophic level.

This factor must be considered during subsequent research on Cobalt 60 transfer in ecosystem through the food vector.