ReviewHow do long-term development and periodical changes of river–floodplain systems affect the fate of contaminants? Results from European rivers
Introduction
Floodplains are characterized by a shifting mosaic of interconnected aquatic, semi-aquatic and terrestrial habitats and are among the most productive and heterogeneous ecosystems on continents. On a global scale, they occupy a substantial area (more than 2 × 106 km2 or 1.3% of the earth's land surface) and show strong alteration and degradation with increasing human population density (Tockner and Stanford, 2002). In Europe and in the densely populated areas in Asia, for instance, between 60 and 99% of the riparian land is intensively cultivated or urbanized (Ravenga et al., 2000), which forces increased fluxes of nutrients, metals and other chemicals into associated water systems. In recent years, it has become evident that floodplain pollution severely impairs the integrity of freshwater and coastal ecosystems and poses serious threats to food security (e.g. Förstner, 2004, WWAP, 2006, Atapattu and Kodituwakku, 2009). Several international efforts have been launched to better understand hydrological, biogeochemical and ecological processes and dynamics in polluted floodplains. One such effort was the EU-funded project “AquaTerra” (2004–2009) with the full title “integrated modeling of the river–sediment–soil–groundwater system; advanced tools for the management of catchment areas and river basins in the context of global change” (Barth et al., 2007). One aim of this project was to investigate trends in river–floodplain systems and their impacts on pollutant behaviour and ecotoxicology in different river catchments in Europe.
Based on results of the “AquaTerra” project, this paper explores the question how long-term geological and soil development, anthropogenically induced alterations such as river channelization and pollutant emissions, as well as short-term hydrological fluctuations affect the fate and bioavailability of contaminants in different compartments of river–floodplain systems. We examine how natural and anthropogenic processes have affected floodplain evolution and soil development, and how this in turn has influenced the filter and storage capacity for pollutants. We show different pathways of organic and inorganic pollutants into floodplains and discuss retention mechanisms operating in floodplain soils and sediments. We further analyze the effects of seasonality and flooding on the behaviour and toxicity of pollutants in the river–floodplain system. The presentation of these various aspects shall enhance the understanding of pollutant dynamics in floodplains and aid decision makers in devising sustainable strategies for floodplain management in the future.
Section snippets
Alpine terrace formation and fluvial (re)sedimentation during the Quaternary
River flow regimes and sediment supplies are often strongly influenced by climatic conditions and tectonic processes (Miall, 1996). In glacial times, coarse fluvial sediments accumulated at the borders of the uplifted Alpine mountain massifs due to frost action and glacial meltwater erosion. The glacial facies model of Penck and Brückner (1909) postulated riverbed incision during interglacial warm periods, which resulted in the formation of terrace staircases (Fig. 1). The thickness of
Contaminant release and pathways into river–floodplain systems
Chemical contaminants undergo multiple changes in their manufacture, usage patterns and release into the environment. A typical situation is that a new chemical product is introduced to the market, becomes popular so that its use increases, and then eventually may be replaced by other new chemicals and products. River catchments are the recipients of these changes, in complex ways. One common scenario is that chemicals are emitted to the atmosphere, deliberately or accidentally, and the
Temporal changes of soil properties in riverine floodplains
Soil studies along substrate age gradients (so-called chronosequences) have enhanced our understanding of directions and rates of pedogenic processes (Huggett, 1998). In fluvial environments, most chronosequence studies have covered thousands to millions of years and so provided information on long-term pedogenic trends. These include mineralogical changes and specific surface area increase (White et al., 1996) as well as pH decrease (Vidic and Lobnik, 1997). Under moist conditions, increasing
Fate of nutrients in floodplain soils and sediments
The nutrient conditions in floodplains are strongly affected by surface water exchange with the river channel and by the redox state of the sediment/soil compartment (e.g. Baldwin and Mitchell, 2000, Hein et al., 2004). For example, in the floodplain downstream of Vienna (see Fig. 4A), increasing surface water connectivity generally led to increased nutrient concentrations in the water column of floodplain water bodies, followed by intensified phytoplankton productivity (Hein et al., 2005). In
Ecotoxicity of contaminants in polluted river–floodplain systems
The distribution of contaminants over different environmental compartments determines exposure of organisms and potential risk to ecosystem functioning. Bioassays enable assessment of bioaccumulation (body or tissue concentration) and thereby integrate the effects of the mixture of bioavailable contaminants that usually is present at polluted sites (e.g. McMillen et al., 2003). In the following, we present case studies from the rivers Dommel (The Netherlands) and Elbe (Germany). The Dommel
Conclusions
Since the Middle Ages, humans have increasingly altered the size, structure and dynamics of many European rivers and floodplains, which act as buffers for flood waters and as filter and storage sites for pollutants. Especially in the 19th and 20th century, river channelization and the construction of flood protection dikes and hydropower dams have severely changed river hydrology, sediment fluxes and floodplain erosion/sedimentation patterns. Dikes along river channels physically limit the area
Acknowledgements
This work was supported by the European Union FP6 Integrated Project AquaTerra (Project no. GOCE 505428) under the thematic priority “sustainable development, global change and ecosystems”. We are grateful to P. Gratwohl, E. Frank, H. Pfalz-Schwingenschlögl, W. Ahlf, E.A.J. Bleeker, A.S. Sneekes, C. van der Wielen and M. Hémart for their additional support and fruitful cooperation. This work was also supported by a grant from the Ministry of Science, Research and the Arts of Baden Wuerttemberg
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