Pollution distribution in floodplain structure visualised by electrical resistivity imaging in the floodplain of the Litavka River, the Czech Republic

Shape and position of modern polluted sediment bodies in the floodplain internal structure can provide insight into past and future erosional/deposition behaviour of fluvial systems. Extensive field works needed for such studies can be mitigated by the use of geophysical imaging, originally developed for engineering practice but still rather sparsely used in floodplain research. We demonstrate the performance of electrical resistivity tomography (ERT), dipole electromagnetic profiling (DEMP), and X-ray fluorescence spectroscopy for identification of subsurface sediment bodies of contrasting lithology, age, and pollution status; we found minor remnants of unpolluted (ancient) floodplain segments that would likely have been overlooked in arbitrary blind sampling. Pollution chemostratigraphy of floodplains based on the concentrations of certain risk elements can be affected by post-depositional migration. We thus identified signatures related to mining and smelting that are carried by coarser particles (gangue, fly ash) not expected to undergo post-depositional translocations in alluvium: the local enrichment factor of Zr (LEF Zr) and mass-specific magnetic susceptibility (chi). The LEF Zr differentiated pre mining floodplain sediments which originated from erosion of Zr-rich catchment loess soils from Zr-poor, gangue-derived material. chi exhibits a step-like increase above the threshold value of approximately 25.10(-8) m(3)/kg in severely polluted sediments; we assign that chi step to the replacement of charcoal and wood by coke in smelting technology during the late 19th century. The shape of the unpolluted sedimentary bodies, as well as bodies of historically changing pollution extent, indicate considerable recent lateral river instability. A mild aggradation flattened the floodplain surface and prevented the identification of buried sedimentary bodies by surface topography, but did not restrict the power of the geophysical imaging.