Geoelectric, magnetic susceptibility, and geochemical survey as a tool to clarify the origin of Bronze Age water reservoirs at the Stepanov hillfort, Czechia

Geophysical prospecting methods can reveal structures and objects connected with past settlement in a noninvasive manner and thus contribute significantly to understanding a fortified settlement's function. It is challenging to discriminate properly between geogenic and anthropogenically influenced soil features. In highly magnetic soil that is locally influenced by water, this is not straightforward and more detailed study is needed. We propose to address this challenge by combined measurements of electrical resistivity tomography, magnetic susceptibility, and elements content on vertical soil profiles. Pedological description is completed by examining the content of soil organic matter (OM) and active soil reaction. We found this combination of methods to enable distinct identification of water reservoirs (WR) of artificial versus natural origin. Soils in the vicinity of the three WRs reflect physicochemical mineral changes by magnetic susceptibility (chi), its frequency dependence parameters (chi(FD), and chi(FD,)%), and differences in element concentrations. The strongly magnetic Cambisol and Cambisol with gleying features at the Stepanov hillfort reveal the best links between chi(F), chi(FD), and concentrations of the elements Fe, Mn, Ti, Sr, and Ca. The most enriched elements in soil were Pb, S, K, Rb, Si, and Zr compared to parent rock. Topsoil accumulates Pb and S originated from recent human activities. Zinc positively correlates with OM and magnetic parameters in the acropolis and near WR1. Stagnant water has a significant effect on links between magnetic susceptibility and element concentrations, but chi significantly correlates with Ti, Al, Ca, Sr, and Cr. This study provides new insight into the applicability of magnetic susceptibility for the identification of past settlement at highly magnetic sites in discriminating between natural and anthropogenic particles.