Abstract
Floodplains are one of the most complex sedimentary environments used for the reconstruction of human impacts on fluvial deposition and catchment pollution. Studies of polluted floodplains require an interdisciplinary approach, including tools from geomorphology, geophysics, and geochemistry.
The spatial distribution of pollutants can reflect not only pollution history but also post-depositional transfers of pollutants. The Litavka River (SW Czech Republic) was impacted by historical mining and processing of Ag-Pb-Zn-Sb sulfidic ores, which resulted in a severely polluted floodplain.
Previous studies revealed that nearly all of the floodplain deposits of the Litavka River downstream from the ore district are polluted, indicating massive floodplain aggradation. Our aim was to decipher the role of aggradation in floodplain development by investigating the pollutant distribution in the floodplain fill.
Another important goal was to distinguish polluted (modern) sediments from secondarily polluted sediments (which were previously pristine) caused by extensive lateral and vertical post-depositional chemical migration of risk elements within the floodplain fill. The floodplain topography was characterized via a digital terrain model (DTM); the internal structure of the floodplain was determined by electrical resistivity tomography (ERT).
Sediment samples were collected from hand-drilled cores across the floodplain along the ERT line, and their elemental composition and magnetic susceptibility were analysed. Dating of sediments was performed via optically stimulated luminescence (OSL).
The concentrations of Zn, Pb and Cd, along with values of magnetic susceptibility decreased with increasing distance from the current river channel. However, the spatial patterns of risk elements were more complicated than expected in a fluvial sedimentary environment.
Inter-element relationships of risk elements exhibited a complex pattern, with abundant outliers that required the use of robust regression. The results helped distinguish the geochemical difference between proximal and distal floodplains, and to identify zones of anomalous inter-element ratios attributed to post-depositional element migration.
The observed migration was significant, and not only vertical but horizontal, spanning a distance of approximately half the floodplain width. Metal migration explains why unpolluted sediments are practically missing from the floodplain fill of the Litavka River.
Use of pollution chemostratigraphy, without considering the possibility of post-depositional migration of risk elements, could lead to overestimation of the volume of younger sediments deposited directly within the floodplain and to overestimation of the role of aggradation processes. (C) 2018 Elsevier B.V. All rights reserved.