Toxicity and risk assessment of fluvial sediments and suspended matter: a short overview of past and recent developments in sediment toxicity assessment
© Springer-Verlag 2010
Received: 19 August 2010
Accepted: 8 September 2010
Published: 6 October 2010
Background In recent years, sediments have become a central topic of scientific and public discussion as an important factor for determining water quality. While the quality of surface waters in Germany has significantly improved during the past years, highly contaminated sediments still create a considerable threat to the quality of several European catchment areas.
Main features Here, we summarize different concepts and methods for the assessment of sediment quality and report on some novel integrative test methods for assessing sediment toxicity including contaminant re-mobilization during simulated re-suspension events.
Results and discussionCurrently, different approaches for assessment of sediment pollution exist. While instrumental chemical analyses are not suitable to accurately describe sediment toxicity, combinations of biological and chemical test procedures and integrated approaches, for example weight-of-evidence studies and effect-directed analysis (EDA), have the potential to identify key contaminants. Inter-disciplinary studies combining hydrodynamic and toxicological aspects coupled to real exposure of aquatic organisms to contaminants are currently being developed.
ConclusionsMonitoring and assessment of sediment quality are of increasing importance, not only for national legislation but also for the implementation of the European Water Framework Directive (WFD). Integrated approaches for the determination of sediment stability play a key role in the appropriate sediment-monitoring strategies.
Toxizität und Risk Assessment fluvialer Sedimente und Schwebstoffe: Eine kurze Übersicht bisheriger und neuerer Entwicklungen
Hintergrund Sedimente sind als wichtiger Faktor der Gewässerqualität in den vergangenen Jahren verstärkt in die wissenschaftliche und öffentliche Diskussion getreten. Während sich die Wasserqualität in jüngster Zeit deutlich verbessert hat, werden die zum Teil hoch kontaminierten Sedimente in vielen europäischen Einzugsgebieten die Gewässerqualität noch für viele Jahrzehnte nachhaltig beeinflussen.
SchwerpunkteDieser Artikel gibt einen kurzen Einblick in die aktuellen Entwicklungen und konzeptionellen Ansätze zur Bewertung der Toxizität und der Umweltrisiken von Sedimenten und Schwebstoffen. Des Weiteren wird ein neuer Ansatz zur Untersuchung der Sedimenttoxizität unter Berücksichtigung der Schadstoffremobilisierung während simulierter Resuspensionsereignisse vorgestellt.
Ergebnisse und DiskussionDerzeit existieren viele verschiedene Ansätze zur Bewertung der Sedimentbelastung: Während instrumentelle chemische Analytik für sich nicht in der Lage ist, die Toxizität von Sedimenten hinreichend genau zu beschreiben, können Kombinationen aus biologischen und chemischen Testverfahren und integrierte Ansätze, z. B. Weight-of-Evidence-Studien und effektdirigierte Analyse (EDA), zur Identifikation der Schlüsselkontaminanten beitragen. Methoden zur Umweltrisikobewertung kontaminierter Sedimente, z. B. die Simulation von Erosions- und Sedimentationsprozessen bei gekoppelter Exposition aquatischer Organismen, werden derzeit entwickelt.
SchlussfolgerungenÜberwachung und Bewertung der Sedimentqualität kommen nicht nur im Rahmen nationalen Rechts, sondern auch bei der Umsetzung der europäischen Wasserrahmenrichtlinie (WRRL) große Bedeutung zu. Um die daraus resultierenden Ansprüche an Sedimentuntersuchungen zu erfüllen, sind integrierte Ansätze und die Berücksichtigung der Sedimentstabilität unabdingbar.
1 Sediment quality assessment
Another example of a powerful blend of biological testing and instrumental chemical analyses is the concept of effect or bioassay-directed fractionation (Fig. 2b). This sequential procedure combines physico-chemical fractionation, biological testing, and instrumental chemical analyses (Brack 2003; Hecker and Hollert 2009) that ultimately leads to the identification of biologically active and, thus, problematic substance classes or even single substances. Bioassay-directed fractionation studies possess high potential for the causal analysis of complex environmental problems, especially those using acute as well as mechanism-specific biological endpoints in combination with in situ investigations in the context of weight-of-evidence studies. This will then provide important inputs and appropriate action plans for managing highly contaminated rivers and streams (Hollert et al. 2009).
2 Sediment mobility and flood risk assessment
Fluvial sediments mainly originate from the erosion of the terrestrial surface or the open channel itself and are transported in a coastal direction, ultimately leading to sedimentation in the estuary and the oceans. The sediment bed can be stable over remarkably long periods. However, increased discharge, for example after heavy rainfall, can lead to increased bed shear stress allowing the re-suspension of sediments and the consequent distribution into adjacent floodplains and flood retention areas. The various inorganic and organic constituents of sediments (e. g. clay minerals, humic substances, and extra-cellular polymeric substances, EPS) provide a large number of binding sites for contaminants (Calmano et al. 1993; Gerbersdorf et al. 2009). Consequently, suspended sediment particles can accumulate pollutants from the water column and, hence, they can constitute secondary sources of contamination during dredging or (extreme) flood events (Ahlf et al. 2002; Hollert et al. 2000, 2003a). Thus, the assessment of sediment stability and the associated uncertainty is now perceived to have an important and emerging role for further implementation of the European Water Framework Directive (WFD) (Hollert 2007; Hollert et al. 2007). The high relevance of sediment re-mobilization together with the increased bioavailability of particle-bound pollutants has also been viewed as an apparent result of global climate change. In general, globally intensified water cycling is expected, which will also cause a significantly greater flood risk (Hulme et al. 2002; Wilby et al. 2006). It is believed that the frequency and intensity of flood events will increase (Ikeda et al. 2005; Kay et al. 2006), and a higher number of extreme weather events and heavy rains is expected in Central Europe, leading to the occurrence of 500-year floods, such as the River Elbe flood in Germany in 2002 (Schüttrumpf and Bachmann 2008). In order to manage the threat of extreme flood events, many countries currently allocate additional retention areas, such as the “Integrated Rhine Program (IRP)” along the River Rhine in Baden-Württemberg (Disse and Engel 2001). Despite building and construction efforts to minimize damage by the flood wave itself, the transported pollutant load remains a great concern. A comprehensive and integrated flood risk assessment has to consider the consequences of such contaminant loads.
3 The joint research project SEDYMO
As a reaction to these requirements, the joint research project SEDYMO (“Feinsedimentdynamik und Schadstoffmobilität in Fließgewässern”; Westrich and Foerstner 2007), funded by the German Federal Ministry of Education and Research (BMBF) from May 2002 to July 2006, aimed at a profound understanding of sediment dynamics and pollutant mobility. It comprised 13 research projects geared towards an inter-disciplinary approach for investigating the redistribution of contaminated sediments and the re-mobilization of nutrients and pollutants into the water column as a consequence of hydrodynamic processes in rivers and estuaries. The gained data revealed that it is inevitable to consider erosion stability of the sediment bed in the implementation of the WFD and other comprehensive management concepts of other catchment areas. An important contribution was the development of experimental procedures and models for the transfer of laboratory data to in situ conditions. In a recently published book, the uncertainty of chemical, hydraulic and biological sediment data was highlighted (Ahlf and Heise 2007; Westrich et al. 2007). Furthermore, the urgent need for studies combining sediment mobility and toxicology was highlighted (Hollert et al. 2007).
4 The projects RIMAX-HoT and FLOODSEARCH
However, questions concerning the implications of flood events to particle-bound contaminants have not yet been sufficiently addressed by the scientific community. To bridge this gap, a recent project assessed the risk of (extreme) discharge events for drinking water supply at the site of the future flood retention area in Bellenkopf-Rappenwört, near Karlsruhe, Germany (Maier et al. 2005). Furthermore, within the framework of the German Excellence Initiative, the “Pathfinder” project Floodsearch at RWTH Aachen University, Germany, has made the first attempt to experimentally link hydrodynamic questions with ecotoxicology to assess the ecological relevance of re-suspension events to aquatic ecosystems (Wölz et al. 2009). In this new inter-disciplinary approach, rainbow trout (Oncorhynchus mykiss) were exposed to unspiked artificial sediment and to sediment that was spiked with polycyclic aromatic hydrocarbons (PAH) under simulated 5-d flood-like conditions in an annular flume which is a device developed for the investigation of erosion and deposition processes (Schweim et al. 2001; Wölz et al. 2009). Following exposure, a set of different biomarkers in the test organisms was investigated (Brinkmann et al. 2010). The set included measurements on the transcript level (quantitative reverse-transcription real-time PCR), on the protein level (Western immunoblot analysis of Cytochrome P450 1A concentrations, 7-ethoxyresorufin-O-deethylase activity, glutathione-S-transferase activity, catalase activity), as well as biochemical (lipid peroxidation) and cellular alterations (micronucleus formation). Furthermore, concentrations of the PAH metabolites 1-hydroxypyrene, 1-hydroxyphenanthrene, and 3-hydroxybenzo[a]pyrene in the bile fluid of exposed animals were measured. Experimental results supported the assumption that re-mobilization of sediments during simulated flood events in the annular flume can lead to uptake and biological effects of sediment-bound contaminants. Especially the average micronucleus frequency was significantly elevated after exposure to spiked sediments compared to exposure to unspiked artificial sediment and positively correlated with the biliary concentrations of 3-hydroxybenzo[a]pyrene, which is a metabolite of the indirectly genotoxic PAH benzo[a]pyrene.
Thus, this new approach has been shown to be promising for successfully conducting hydro-toxicological studies with rainbow trout. However, technical modifications of the annular flume (e. g., monitoring of more environmental variables) and an increase of the dimensions to (1) better simulate environmental conditions, and (2) reduce the influence of the exposed organisms on the physico-chemical processes, would be desirable. Currently, rainbow trout have a significant influence on the re-suspension dynamics of the sediment bed. Furthermore, it appears necessary to systematically control environmental variables such as pH and temperature in future studies, especially with respect to investigations of climate change effects.
The results of the study have shown that relatively short exposure to re-suspended sediments during simulated flood events can lead to – potentially adverse – alterations of biological functions in rainbow trout. Thus, the ecological and toxicological impacts of contaminant re-mobilization during flood events have to be considered highly relevant. Integrated approaches for risk assessment of regularly flooded rivers are urgently required.
5 Conclusions and outlook
Various approaches for assessing the toxicity of fluvial sediments and suspended matter were introduced and proposed in the past. The application of well-designed bioassay-directed fractionation techniques using acute, as well as mechanism-specific biological endpoints in combination with in situ investigations in the context of weight-of-evidence studies possess high potential for the causal analysis of complex environmental problems. In addition, such an approach can also lead to better sustainable action plans for the management of highly contaminated rivers and streams. However, there is also an urgent need for the development of other reliable tools for flood risk assessment integrating sediment stability and hydrodynamics in management strategies for the catchment areas in focus.
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