Using sedimentary archives to reconstruct pollution history and sediment provenance: The Ohře River, Czech Republic

Abstract

In industrialised areas rivers and their floodplains are often highly polluted. The historic contamination of floodplains offers an important archive of past polluting activities and fluvial sediment dynamics. It can also present a significant threat to contemporary aquatic ecosystems and water quality resulting from the remobilisation of sediment-associated pollutants from floodplain secondary sources. Therefore, knowledge of the type and level of pollutants present and their spatial distribution in floodplains is essential for supporting measures to mitigate future impacts. Here, we outline an approach for reconstructing pollution history and extent by using floodplains as archives of historic chemical contamination coupled with geochemical and geomorphological analyses to account for variations in sediment provenance and depositional processes. The study is situated in the Ohře River (total catchment area of 5606km2), which provides an excellent test case given its complex pollution history, heterogeneous geology and variable deposition patterns.Most severe pollution in this river system has originated from a chemical factory in Marktredwitz, Germany (mainly Hg) and uranium processing in Nejdek, the Czech Republic (mainly U); historical lead mining and modern coal combustion have also played a role. The geological/geochemical variability of the Ohře catchment required extensive sampling along the river course. Variations in several major lithogenic elements, in particular Al, K, Rb, Si and Ti allowed the river course to be subdivided to geochemical river reaches using simple scatter plots (Ti against Al/Si ratio and Rb against K) or PCA; there was no need to sieve the sediment samples before analyses or perform other discrimination of sediment lithology. The geochemical river reaches reflect catchment geology and hence they exhibit distinct background functions for Cu, Pb, and Zn and varying background concentrations of U. Titanium was the best performing reference element. The Ohře River has deposited most historical pollution in former (abandoned or laterally shifting) channels rather than in overbank fine-grained materials. The former channel sediments can be recognised by a specific surface topography, vegetation cover and sediment lithology; the use of geographic information systems (evaluating historical maps, aerial photographs and DTM by LIDAR) is indispensable in the study of the depositional patterns. The narrow floodplain limited floodplain deposition, and has probably allowed particulates to travel through the channel for tens of kilometres, in the case of U pollution in <10years. Dams constructed in the 1960s in the upper and middle river reaches have suppressed downstream transport of pollution and combined with certain channel engineering projects to prevent bank erosion are likely to have further limited the deposition of overbank fines. The study clearly shows that in the Ohře floodplain the fills of former channels are more suitable as pollution archives than the distal floodplain sediments (overbank fines outside the channel belt). The approach outlined here is applicable to reconstructions of pollution histories for any river characterised by varied catchment geochemistry and deposition in their channel belts.