RADIOCESIUM DATING OF SEDIMENTS FROM LAKES AND RESERVOIRS OF DIFFERENT HYDROLOGICAL REGIMES

Important information on the transport and deposition of anthropogenic substances in rivers and lakes can be assessed using sediment analyse s. Dating is a necessary prerequisite to calculate fluxes, to account for varying sedimentation rates, and thus to allow comparison between different localities. For sediments not older than 40 years, Cs-137 da ting has been successfully applied to natural and artificial lakes rep resenting a wide range in water-residence times and sediment-trapping characteristics. Sedimentation rates for the period 1986 to the time o f coring (mid 1990s) vary as a function of distance to the inflowing r ivers, between 0.5 and 2.5 cm yr(-1) (0.13-1.0 g cm(-2) yr(-1)) in Gre ifensee and between 1.4 and 3.5 cm yr(-1) (0.49-2.4 g cm(-2) yr(-1)) i n Lake Biel. in reservoirs located along the major Swiss rivers Aare a nd Rhine, sedimentation rates are in the same range from 1.8 cm yr(-1) (0.69 g cm(-2) yr(-1)) in Augst (Rhine) to 3.4 cm hr(-1) (1.8 g cm(-2 ) yr(-1)) in Klingnau (Aare). Sedimentation rates from the period 1963 -1986 are in some cases significantly lower. The dating is based on th e identification of radiocesium deposition maxima in sediment cores th at are related to the global deposition of radiocesium during atmosphe ric nuclear weapons testing (maximum, 1963) and the accident of the Ch ernobyl nuclear reactor (April 1986). Additional markers confirm age d eterminations based on radiocesium. They are based on either known dis charges of radioniclides by nuclear reactors or known maxima of anthro pogenic organic substances such as detergent-derived chemicals. In cas es where the Cs dating technique fails (e.g., in turbulent river secti ons with disturbed sedimentation), human activities can be used as tim e markers to estimate sedimentation rates, as documented for sediments from the Aare and Rhone rivers, where sedimentation rates between 1.7 (Niederried reservoir) and 13 cm yr(-1) (Chancy reservoir) were measu red.