Deposition of anthropogenic radioactivity in Switzerland (see Table) can be related to (1) atmospheric atomic bomb testing with a maximum in 1963, (2) the Chernobyl accident in 1986, and (3) nuclear power plants (e.g. NPPM = Mühleberg 1971-94; B = Beznau 1982-95). Focus is given here on 137Cs and 60Co, both particle reactive metals, with half-lifes of 30 and 5.3 years, respectively. The deposition of these radionuclides as a function of time (flux rates) can be evaluated in archives of relatively undisturbed systems like Lake Brienz or the Fiescherhorn glacier. Ideal systems are those with a continuous radionuclide flux and a known relationship between deposition and source area. Resuspension or lateral transport should not occur. In ice-cores, wind drift of snow and particles may lead to over- or underestimation of radionuclide fluxes. In lakes catchment input requires a detailed study of catchment geology and pedology and the physical processes that govern transport of radionuclides in solution and bound to particles. On one hand areal radioactivity assessment requires ideal archives, on the other hand particle transfer across system boundaries and the disturbances of systems by different exogenic processes can be studied based on the radioactivity distribution. Several lake and river systems will be presented to illustrate such approaches:
Source (1) (2) NPPM NPPB
Bq 137Cs 1.48x1015 1.50x1015 3.5x1011 8.5x1010
Bq 60Co 4.3x1011 3.9x1011
The change in particle and radionuclide flux from the catchment area to a lake is related to soil-cover, soil stability and soil erosion. Radionuclide budgets and sediment radioactivity allow evaluation of both the present situation as well as historic changes. As a test site we have chosen a small lake in the Swiss Alps, located between 2500 and 3000 m altitude. Erosion and sedimentation rates increased during the last 20 years (10-20 cm/ka and 0.3-0.55 cm/a, respectively). This increase was a consequence of a landslide that occurred in the early 1960s.
Riverine suspended particle transport and sedimentation and the importance of particles controlling anthropogenic radionuclides have been studied on the basis of waste discharges by nuclear reactors. Particle/water distribution of 60Co increase as a function of distance to the discharge site. The distribution of 60Co and 137Cs in river sediments shows the connection between radionuclide transport, sedimentation, local particle fluxes and hydrodynamics. Examples will be given from river sediment cores taken in the Swiss rivers Aare and Rhine at locations below nuclear power plants.
Distribution of river water and suspended particles within a lake have been evaluated on the basis of radionuclide distribution in the sediments. The entrainment of river water into lakes, therefore the water and particle residence times are controlled by density. The radionuclide transfer from the water column to the sediments is a kinetic process related to particle composition and concentration and water and particle residence times. In the case of Lake Biel, situated 18 km downstream of the nuclear reactor Mühleberg seasonal variations are considerable. During the cold period the radionuclide-bearing Aare waters dive to the deepest part of the lake due to their higher density relative to lake water. Residence times during this period can reach up to 120 days. During the warm period the river water remains largely in the epilimnion and the residence time can be as low as 6 days.
These examples have been depicted to illustrate the strength of a joint radioactivity surveillance and environmental science program. An efficient surveillance program must rely on a large number of scientific information regarding the transport of radionuclides and their carrier phases. A more in depth understanding of these processes can be achieved when taking advantage of defined input terms of radioactivity releases and the time component associated with their decay.