ANÁLISE COMPARATIVA DE METODOLOGIAS DE DETERMINAÇÃO DE VALORES DE REFERÊNCIA PARA SEDIMENTOS DE CORRENTE

Abstract

The determination of reference values in geochemical data set has been a major challenge in studies related to environmental geochemistry. Old methods were based on statistical parameters, the construction of tables, graphs and histograms. However, these techniques do not define these values properly, or show very different values for the same region. This research tested different methods the determination of reference values for stream sediments: mean +2x standard deviation, median +2x MAD, boxplot UIF, fractal analysis, spatial analysis with the IDW interpolation model and associated methodologies, in the Iron Quadrangle, Brazil, known worldwide for diversity of minerals and rock types. The results show that ancient methods of determination of reference values, notably the mean + 2x standard deviation and median + 2x MAD not to be effective for the separation of anomalies, because overestimate or underestimate the reference range, may cause errors considerable when discuss aspects related to environmental legislation. In other hand, the methodologies “fractal analysis” and “boxplot UIF” show, for most of the elements, very close values, confirming that the reference range is correct and, demonstrating that both are appropriate for complex geological environments. However, some elements, mainly Cu, Fe, Pb and Ti showed a great difference of between the methodologies, around 30 to 46%. For these elements, the reference values obtained via fractal analysis proved to be more consistent with the spatial distribution of the elements, and also presented the outliers in places where observed rock types rich in the element and environmental impacts that able to release it to the environment.