Technogenic contamination or geogenic enrichment in Regosols and Leptosols? Magnetic and geochemical imprints on topsoil horizons

Determination of the origin (geogenic or technogenic) of magnetic and geochemical imprints in soils and the role of weathering processes is crucial for the assessment of relationship between technogenic magnetic particles (TMPs) and potentially toxic elements (PTEs), iron and manganese. Our primary aim is to distinguish the most reliable methods for the appropriate interpretation of measured parameters strictly attributed to particular soil horizons (including bedrock), which is not yet very common in soil magnetometry. We examined six soil profiles developed from/on five different rock types (sedimentary and igneous rocks). The magnetic parameters reflect the concentration, type and grain size of the magnetic particles. Geochemical analysis consists of the determination of PTEs (Co, Cr, Cu, Ni, Pb and Zn), Fe and Mn. Quantification of soil contamination is based on calculating the index of geoaccumulation and enrichment factors. Interpretation is based on plots and magnetic and geochemical data, evaluated by principal component analysis for particular soil horizons. The results show that the volume magnetic susceptibility of the topsoil (mineral horizon Ah) in areas where the soil is developed from strongly magnetic bedrock is primarily influenced by geogenic magnetic particles (GMPs) derived from weathering processes. In cases of magnetic susceptibility determined on organic subhorizons (Oea or Oa), the magnetic parameters indicate the occurrence of TMPs. Moreover, the ratios of saturation isothermal remanent magnetization over magnetic susceptibility, and magnetic susceptibility over iron content help in distinguishing both geogenic and technogenic magnetic contributions. The variations of magnetic susceptibility and iron concentration reflect the presence of the strongly magnetic fraction in total soil iron content. Lead and zinc exhibit unambiguous technogenic influx into topsoil horizons. We conclude that the technogenic contamination dominates over geogenic enrichment in soils developed from/on weakly magnetic bedrock, but also strongly magnetic bedrock (without weathering effects).