Distribution of super(7)Be, super(2) super(1) super(0)Pb and super(1) super(3) super(7)Cs in watersheds of different scales in the Seine River basin: Inventories and residence times

The activity of environmental radionuclides ( super(7)Be, super(2) super(1) super(0)Pb and super(1) super(3) super(7)Cs) was monitored in nested catchments, inside the Seine River basin. Suspended matter data was collected at 8 different watersheds, ranging from order 1 to order 7, and ranging in size over 4 orders of magnitude. Suspended matter was analyzed for super(2) super(1) super(0)Pb, super(1) super(3) super(7)Cs and super(7)Be, and used to calculate the flux of sediments out of each watershed. Monthly atmospheric flux data of super(2) super(1) super(0)Pb and super(7)Be was analyzed to assess the input flux of each into the watersheds, taking into account the rainfall during sampling periods. Taking advantage of the different half-lives of super(7)Be (53 days) and super(2) super(1) super(0)Pb (22 years), a two-box model was built for each of the catchments following a methodology previously developed by Dominik et al. [Dominik J, Burrus D, Vernet JP. Transport of the environmental radionuclides in alpine watershed. Earth Planet Sci Letters 1987; 84: 165-180.]. The model divides the watershed into a soil box and a rapid reservoir and provides insight into the removal rate of suspended matter from the surrounding watershed. The model enables the assessment of the surface area and the residence time of slow and rapid reservoirs to describe the fate of contaminants of atmospheric origin inside the river basin. The model was improved by considering the dissolved fraction in the total flux and adding the super(1) super(3) super(7)Cs inventory as an additional constraint. The effects of these changes are discussed. Residence times in the soil box, characterized by low transport velocity, range between 4800 years at Melarchez (order 1) to about 30000 years at Andresy and Poses (order 7). They remain constant in each watershed over a large range of variation of atmospheric fluxes of super(7)Be and super(2) super(1) super(0)Pb during the whole study, but are sensitive to SM variations. The residence time in the rapid box, which includes the surface of the river and immediate surroundings, is less than one year, while its surface area is in the range 0.6% to 2.2% of the total catchment area. They are sensitive to super(7)Be atmospheric flux variations. The two-box model was used to estimate the amount of the radionuclides in each reservoir. Inventories appear to be constant from one watershed to the next. The super(7)Be inventory ratio in the rapid and slow bo