Quantitative assessment of dissolved radiotracers in the English Channel: Sources, average impact of la Hague reprocessing plant and conservative behaviour

P. Bailly du Bois, P. Guéguéniat Continental Shelf Research ; FluxManche II Dedicated Volume, 19, p.1977-2002 (1999)

Knowledge of long-term movements of water-masses in the English Channel has been substantially improved using hydrodynamic modelling coupled with radio tracers studies; nevertheless, the precision of results so obtained is still largely dependent on measurement precision. New tools are now available to make more accurate determinations of radio tracer distribution: (1) Repositioning of station locations at the same tide reference time, giving a homogeneous spatial data set, coupled with the possibility of interpolating and quantifying the amounts of dissolved radioactivity flowing through the English Channel; (2) the first measurements of tritium (3H) in seawater on a large scale in the English Channel demonstrate that this fully conservative radionuclide is a clearly identifiable marker of industrial releases, (3) recent campaigns carried out during the FLUXMANCHE Il CCE (1994) programme show the general distribution of dissolved radionuclides 137Cs, 134Cs, 60Co, 125Sb, 106 Ru and 3H in the English Channel and the Irish Sea; and (4) the re utilisation of data from previous campaigns (1983, 1986, 1988) provides indications, at any given location in the English Channel, about the average dilution and distribution of releases derived from the La Hague reprocessing plant. Excesses and losses of radionuclides are now quantified with respect to known source terms, estimates of losses are provided for non conservative radionuclides, while an excess of 137Cs was observed in the English Channel during the period 1983 1994. This excess, which has the same order of magnitude as the quantities released from La Hague plant in the English Channel, could bc explained by about 1 % of the Sellafield reprocessing plant releases entering the Channel. These results confirm and give a more detailed picture of the previously known distribution of water masses in the English Channel. They lead to clear information about transit times and dilution at this scale, and provide directly comparable data for the validation of hydrodynamic models.