6. Cesium-137 mass and volume activities in marine environments

The cesium 137 measured in marine environments around mainland France, outside the influence of discharges from nuclear facilities, comes from radioactive deposits on the water surface during past fallout events (mainly dissolved cesium 137), and from soil drainage, particularly via river inputs (mainly in particulate form). Unlike freshwater, cesium is present in seawater mainly in dissolved form. It is transferred to marine organisms by adsorption in the case of phytoplankton, and by adsorption and absorption (ingestion in particular) in the case of animal organisms, with a tendency for mass activities to increase as one moves up the food chain, from zooplankton to large predators.

Due to the highly dispersive nature of the marine environment, caesium-137 activity volumes due to past radioactive fallout in the seas and oceans off the French coast are fairly homogeneous....