Radionuclides in the Loire river estuary (France) : Sources, transport and fate

¹ EDF R&D, Département Laboratoire National d'Hydraulique et Environnement, 6 quai Watier, 78401 Chatou, France
² Laboratoire des Sciences du Climat et de l'Environnement, UMR 1572 du CEA/CNRS, Domaine du CNRS, 91198 Gif-sur-Yvette cedex, France
³ Département Analyse, Surveillance, Environnement, Service Radioanalyse, Chimie et Environnement, CEA/DAM Ile de France, BP. 12, 91680 Bruyères-le-Châtel, France

Abstract

Anthropogenic radionuclide inputs in the Loire estuary (French Atlantic coast) consist of radioactive releases from 14 nuclear reactors located along the Loire river basin, and of fallout from nuclear weapon tests and from the Tchernobyl accident. To estimate to what extent radionuclides associated with sediment accumulate in the estuary, three complementary approaches were used: field surveys, laboratory experiments and numerical modelling. Sampling of bottom sediments, water and suspended solids was carried out at 8 different dates over a 15 month-period. Analysis covered ¹⁴C, ⁹⁰Sr, ³H, the naturally occurring gamma-emitters (uranium and thorium decay chains, ⁷Be and ⁴⁰K), and the artificial gamma emitters (mainly cobalt and cesium isotopes). Processes of radionuclide transport and exchange between dissolved and particulate phases were included in a previously developed estuary specific 2D-hydrodynamic model. Equations of sorption and desorption kinetics were derived from laboratory experiments conducted at different salinities. Simulations carried out for two river discharge conditions (low summer flow, high winter flow) allowed to follow radionuclide desorption in the estuary. For long term simulations, a simplified model was developed. It provided estimates of the amount of radionuclides expelled out of the estuary under dissolved and particulate forms, of the transit time for both forms and of the variations in radionuclides concentrations in the fluid mud. Based on computed results and observations, contributions from different origins (natural, military, industrial) to the inventory of radioactivity in the estuary are presented.