MAXIMUM POLLUTANT CONCENTRATIONS AT THE WRONG DEPTH - A MISLEADING POLLUTION HISTORY IN A SEDIMENT CORE

A sediment core was collected in the western Dutch Wadden Sea (location Mokbaai) to construct pollution history profiles. Sedimentological inspection revealed a relatively undisturbed sedimentation pattern. Dating of the core was performed by Pb-210 and Cs-137 analysis. Owing to the Chernobyl accident, Cs-134 could also be detected. Subsample slices of the sediment core were chemically analyzed (total sediment) for eight trace elements: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc). Pollutant concentrations were normalized to grain size and organic matter content of the sediment. The results indicate that the contaminant profiles apparently correspond to the expected patterns; at larger depths only background concentrations were observed for the contaminants, while a rather steep increase could be observed from there on, which was attributed to an increase in pollutant loadings in the middle part of this century. A maximum concentration was found at about 15 cm depth. Both the Pb-210 and Cs-137 datings gave similar results; 6.5 and 6 mm year-1 net sedimentation rate, respectively. Unfortunately this would mean, that the maximum input of (all) pollutants was at about 1963, and not 10-15 years later, as would be expected on the basis of input data. With the use of a mathematical model 'WADSEDI', which also takes hioturbation into account, the sediment profiles were reconstructed. It is shown that the above initial interpretation was indeed erroneous. Bioturbation effectively caused a rapid mixing of the top layers of the sediment. The underestimation of the role of (bio)turbation on the top layers of the sediment was responsible for the initial misinterpretation of the pollution record.