INORGANIC TOXICANTS IN SOILS OF NORTH-RHI NE-WESTPHALIA - I - EVALUATION OF MULTIMODAL FREQUENCY-DISTRIBUTIONS

In 335 sail samples from North Rhine-Westphalia the:contents of 18 ele ments were quantified (total contents, mobilizalbe fraction:(extractio n with EDTA cocktail);mobile fraction (extraction with NH4NO3). An ins pection of the resulting histograms revealed that the distribution of element contents in soils is not normal and even not log-normal. The a ppearance of several peaks indicates the existence of multi-modal dist ributions with.various independent sub-distributions. Two methods for analysing such:,data sets are presented-The parametric approach assume s several single lognormal components, which are fitted by the Expecta tion Maximization Algorithm. In a nonparametric approach the Kernel De nsity Estimation is used. With this procedure five to ten concentratio n levels can be determined per element for our data set with correspon ding groups of soil samples being characterized by common features (e. g., parent material, region of origin, specific source of pollution). This way,e.g:, background concentrations for potentially toxic element s can be derived. The parametric approach is lacking sharpness of sepa ration in the case of more than four subdistributions.