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 analyses. 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 dating has been successfully applied to natural and artificial lakes representing a wide range in water-residence times and sediment-trapping characteristics. Sedimentation rates for the period 1986 to the time of coring (mid 1990s) vary as a function of distance to the inflowing rivers, between 0.5 and 2.5 cm yr(-1) (0.13-1.0 g cm(-2) yr(-1)) in Greifensee and between 1.4 and 3.5 cm yr(-1) (0.49-2.4 g cm(-2) yr(-1)) in Lake Biel. in reservoirs located along the major Swiss rivers Aare and 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 the identification of radiocesium deposition maxima in sediment cores that are related to the global deposition of radiocesium during atmospheric nuclear weapons testing (maximum, 1963) and the accident of the Chernobyl nuclear reactor (April 1986). Additional markers confirm age determinations based on radiocesium. They are based on either known discharges of radioniclides by nuclear reactors or known maxima of anthropogenic organic substances such as detergent-derived chemicals. In cases where the Cs dating technique fails (e.g., in turbulent river sections with disturbed sedimentation), human activities can be used as time 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 measured.