Applying heatmaps in interpretation of geochemical baseline data on urban soils in Finland

Abstract

Urban soil geochemical mapping was carried out, using the same sampling and analytical protocol, in seven Finnish towns in 2012–2017. All samples were taken from urban topsoil (0–10 cm), dried at <40 °C and sieved to <2 mm grain size. Aqua regia-extractable concentrations of 24 elements and total carbon content were assessed in this statistical study. Soil geochemical results are a typical example of compositional data. Thus, the data were isometric log-ratio (ilr) transformed before correlation analyses and heatmap production. Heatmapping involves colour coding of correlations, with different colours for positive and negative correlations. Elements were re-arranged for heatmaps by clustering the correlation matrix, using hierarchical clustering with distances based on the correlations. Heatmaps were produced for four general soil classes derived from samples taken from the urban topsoil: natural fine-grained sediments, natural coarse-grained sediments, man-made minerogenic soil material and man-made soil material rich in organic matter. The heatmaps revealed two distinct groups in all urban soil parent materials: elements related to abundance of trioctahedral mica minerals and elements bound to soil organic matter. Some man-made soil materials reflecting a single source of pollutants were also identified in the heatmaps.