| Abstract: | Inorganic toxicants in soils of North Rhine-Westphalia. I. Evaluation of multi-modal frequency distributions In 335 soil samples from North Rhine-Westphalia the contents of 18 elements were quantified (total contents, mobilizable fraction (extraction with EDTA cocktail), mobile fraction (extraction with NH4NO3). An inspection of the resulting histograms revealed that the distribution of element contents in soils is not normal and even not log-normal. The appearance of several peaks indicates the existence of multi-modal distributions with various independent sub-distributions. Two methods for analysing such data sets are presented. The parametric approach assumes several single lognormal components, which are fitted by the Expectation Maximization Algorithm. In a nonparametric approach the Kernel Density Estimation is used. With this procedure five to ten concentration levels can be determined per element for our data set with corresponding 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 elements can be derived. The parametric approach is lacking sharpness of separation in the case of more than four subdistributions. |
