Segregation and retention of As, potentially toxic metals, and organic pollutants in a reservoir from the Ohře River (the Czech Republic)

Abstract

This work addresses the fates of elements from historical mining that peaked in the 16th (Ag, Cu, Pb, and Sn mining) and 20th (U) centuries, polycyclic aromatic hydrocarbons (PAHs), and polar pesticides in the Nechranice Dam Reservoir in the Ohře River. A steady fluvial input from secondary pollution (As, Cu, Pb, Sn, Zn) enabled the study of pollutant segregation during deposition and post-depositional migration in sediments. These elements are present in concentrations close to toxic levels for aquatic life, which will prevent the achievement of a favourable ecological status in the Ohře River, which is similar to many rivers in central and western Europe. The reservoir bottom morphology was acquired by sonar imaging. The sediments were then sampled by extracting 23 gravity cores to describe the distribution of pollution in sediments with sufficient reliability. The inflow delta was characterised by a sub-bottom profiler, aerial imaging, and photogrammetric modelling. X-ray fluorescence spectroscopy and chromatography were used for sediment analysis. The catchment-specific Hg and U pollution peaks provided local chemostratigraphic markers of the pre-dam/reservoir sediment boundary. The Sn-bearing pollutant particles were mostly retained immediately after entering the Nechranice Reservoir. Concentrations of Cu, Pb, and Zn are lower in the reservoir basin than in the inflow delta. The sediments of the reservoir basin with water depths > 20 m are enriched in the top layer with Mn species, which may be relevant for the fates of some pollutants. Arsenic shows a post-depositional enriched peak near the base of the Mn-enriched layer. The PAH concentrations normalised to the total organic carbon content (TOC) in the basin sediments are indirectly proportional to the Mn/Fe ratio, showing their oxidative degradation in the Mn-enriched strata. Our work reveals the important behaviour of pollutants in dam reservoirs, which has not yet been studied adequately to the relevance of the historical burdens in current fluvial systems, secondary fluvial pollution, and future reservoirs with historically polluted catchments. The inflow delta is a highly dynamic sedimentary system responding to changes in water level. The studied reservoir has behaved as a settling system with post-depositional chemical reactions relevant to pollutant fates. The retention of Mn(III,IV) oxides that is considerably enhanced by river damming might play a positive role in the degradation of modern organic pollutants. Arsenic in the sediments undergoes translocation to sharp subsurface peaks.