Contamination assessment of heavy metals in urban soil of former industrial city (Ózd, Hungary)

Iron and steel works activities are an important industrial source of pollutants that change soil composition with a wide range of substances containing heavy metals. Determination of the multi-element composition of urban soil contributes to an accurate assessment of the quality of the urban environment. Therefore, concentrations of 15 metals (Ti, V, Cr, Mn, Fe, Co, Ni Cu, Zn, Mo, Ag, Sn, Sb, Pb and W) were measured in Ózd, a former industrial city located at the northern part of Hungary. It has been exposed to contaminants for almost two centuries by different anthropogenic activities such as transportation, coal mining, iron and steel works. In this study, 56 urban soil samples were collected from playgrounds, kindergartens, parks and roadsides, as well as 1 local coal, 1smelter slag and 2 steel slags samples.

In this study, we determined cluster distribution of the samples using compositional data analysis clr-transformed (clr-biplot), k-means cluster analysis (CA) and calculates enrichment factors (EF). To observe the relationships among the 15 metals, the clr-biplot was performed in CoDaPack software and k-means in R statistics, following recommendation in the literature [1].  The results of k-means were overlapped on the clr-biplot and plot on a map. Enrichment factors were calculated for every cluster with the formula: EF=[M/Fe]sample/[M/Fe] background, where (M) metals concentration and Fe was used for normalization. Background values were taken from brown forest soil.

The average concentration of 15 elements in (mg kg^-1) for the 57 samples are Fe(27204), Mn(842), Zn(225), Ti (135),  Pb (81), Cr(41,3), Cu(30.2), V(24.2), Ni(21), Co(7.34), Sn(4.22), Sb(1.41), Mo(1.19), W(0.726) and Ag (0.268), respectively.

The optimal number of clusters are 4, where the most samples in the first cluster are distributed on the northwest side of the city where agricultural activity is a common occasion. Samples forming the second cluster are characteristic in the area of the former iron and steel factory. Samples of the third cluster are located at the new industrial park (northeast side of the city). The fourth cluster samples are derived from the surrounding area the new industrial area and covers most of the city south side. The results of average enrichment factor (EF)>5, which represent significant or very significant enrichment are 1^stcluster Ti, 2^ndcluster W>Ti>Sn>Ag>Cr>Pb>Sb>Zn>Cu, 3^rdcluster W>Cr>Ti, and 4^thcluster Ti>W.

The differences between cluster distributions and enrichment of each metallic element show complexity of the study area, which suggested areas with features associations of elements to natural sources, hybrid (natural and anthropogenic) and industrial areas.