IN-SITU CONSOLIDATION OF LAKE DEPOSITS - AN EMPIRICAL-MODEL TO RECONSTRUCT POLLUTION HISTORY

In the past, much effort has been put into the development of sophisti cated mathematical models to describe settlement and consolidation of water deposited sediments. Such models often aim at completeness and a ccuracy in modeling the physical processes involved. However, as a res ult of the generality of the descriptions, the models often fail to co mpute local circumstances satisfactorily. In specific cases, the empir ical approach may prove to be highly competitive and reliable. In larg e water bodies in the central part of the The Netherlands, the alluvia l 'IJsselmeer-deposit'' is a common type of fresh water sediment. Its deposition and settlement started in 1932, when anthropogenic activiti es changed the physical and chemical conditions of the lakes drastical ly. Five representative cores of this sediment were taken in deep zone s of the lakes. Periodic water depth surveys over the last sixty years at these locations provided information on the net sedimentation rate and the total thickness of this Ijsselmeer-deposit at known time inte rvals. In order to calculate a time-equivalent of the depth scale, cor rection factors for soil consolidation are introduced. A decrease in t he total thickness of individual sediment layers is proportional to th e decrease of its volume, which is derived from in situ characteristic s. Correction factors are based on a simplification of various stages of compression (i.e., 0 percent, 30 percent, and 45 percent). A factor n, which represents changes of water content of the sediment as a dep endence of clay content, is derived for each layer, allowing an invers e calculation procedure to determine the initial, uncompressed thickne ss of each layer. Hence, a fairly reliable time scale in depth can be reconstructed. Furthermore, the radionuclide activity was measured in some cores and the degree of organic and inorganic pollution was deter mined in numerous layers of all cores. Cs-isotopic tracers (Cs-137, Cs -134) give a good reconstruction of the last six decades of physical c hanges of the sediment and the degree of pollution, assuming post depo sitional redistribution and transformation of pollutants to be negligi ble. The results showed close coherence to calculated time-scales.