Evaluation of an interdisciplinary method to assess the hydrotoxicological relevance of contaminated sediments after resuspension events

There is a consensus within the scientific community that sediments act as a long term sink for a variety of organic and inorganic pollutants which, however, can re-enter the water column upon resuspension of deposited material under certain hydraulic conditions such as flood events. In the last years, interdisciplinary research has become increasingly important in response to new challenges related to the implementation of the European Water Framework Directive (WFD). This directive commits European Union member states to achieve a good ecological and chemical status in European river catchments. Furthermore, the importance of sediments as a secondary long-term source for pollutants and, hence, as an important factor for water quality has been integrated into the WFD. Within the implementation of the WFD, it is important to understand the potential short and long term impact of suspended particulate matter (SPM)-associated contaminants on aquatic organisms as well as the related uptake mechanisms for a sound risk assessment. Increasingly frequent flood events usually cause remobilization of contaminated sediment layers in rivers and, thus, are of high relevance for a sound understanding of associated ecotoxicological impacts. The Floodsearch project framework, performed in close collaboration between the Institute for Environmental Research and the Institute of Hydraulic Engineering and Water Resources Management (both RWTH Aachen University), aimed to establish a novel approach combining methods of hydrodynamic engineering and ecotoxicological assessment to enable the investigation of the potential risks associated with such remobilization events. This new approach, developed by an interdisciplinary team was entitled the hydrotoxicological approach by Cofalla (2015). It can be used as a tool with practical relevance to investigate and assess contaminated sediments and their influence on aquatic organisms upon resuspension. The present PhD thesis consisting of three individual studies aims at the evaluation of the hydrotoxicological approach and has a focus on the ecotoxicological aspects without neglecting the engineering side. In the first proof-of-concept study, hydraulic engineering and ecotoxicology were combined in a new interdisciplinary approach to assess the toxicity of resuspended polluted sediments after a simulated flood event. Specifically, the objective of this study was to bridge the gap between the physical resuspension of pollutants and resulting toxicological impacts on aquatic organisms. Formulated sediment was prepared according to OECD guideline 218 and spiked with a mixture of four polycyclic aromatic hydrocarbons (PAHs). Rainbow trout (Oncorhynchus mykiss) were exposed to resuspended sediments. A suite of different molecular, biochemical and histological markers was used to verify the hypothesis that resuspension of sediments can lead to the remobilization of PAHs and subsequently to effects in aquatic organisms. The experiments were carried out in an annular flume designed to investigate the transport behavior of fine-grained sediments. Several physicochemical and sedimentological parameters as well as exposure concentrations of contaminants were measured to characterize environmental conditions and erosion behavior of sediments. During the elaboration of a biomarker battery comprising biomarkers with suitable endpoints, rainbow trout proved to be a suitable test species (a) being easy to rear, (b) being sufficiently robust for exposure experiments and (c) providing sufficient amounts of sample material for bioassays and chemical analysis. The micronucleus frequency was significantly 4.3-fold elevated after exposure and biliary metabolite concentration was shown to be the most sensitive marker of PAH exposure. The original setup of the annular flume was successfully modified including a flow-through cooling unit and an aeration system to maintain suitable and stable environmental conditions for the test animals. Flood events were successfully simulated, resulting in the resuspension of formulated sediment, and different erosion behaviors of sediments during the simulated flood events were observed and could be characterized. This proof-of-concept study clearly demonstrated the feasibility of a combined hydrotoxicological approach in support of the investigation of the potential ecotoxicological relevance of sediment resuspension events and showed that sediment remobilization during short simulated flood events in the annular flume can lead to uptake and effects of sediment-bound pollutants in rainbow trout. In the second study, preliminary tests were carried out in resuspension containers to provide a broader knowledge base for subsequent experiments in the annular flume. The main purpose of those experiments were (a) the assessment of the dissipation of PAHs from sediment/water systems and of the subsequent differences between desorption and bioavailability of spiked contaminants, and its implications for the following experiments in the annular flume, (b) kinetic monitoring of the chosen biomarkers to provide detailed insights in their dynamics as a function of extended exposure to suspended matter and (c) the assessment of a combination of chemical exposure and another environmental stressor, temperature, in aquatic biota. Due to recent changes in climate, temperatures of German rivers frequently exceed 25 °C during summer. Effects of resuspension of sediments on biota under elevated temperature regimes are likely to differ from those under lower temperature regimes. To elucidate this differential response of aquatic vertebrates, rainbow trout were exposed to suspensions of sediment from the Rhine River that was spiked with a mixture of PAH. The experiments were conducted under two different temperature regimes (24 °C or 12 °C) and physicochemical parameters, including concentration of PAHs in SPM, and biomarkers in fish such as biliary PAH metabolites were measured over the course of a 12 d study. Concentrations of pyrene and phenanthrene decreased over time, while no decrease was observed for chrysene and benzo[a]pyrene. The biomarker cascades, more specifically the temporal dynamics of biomarker reactions, did not only show quantitative differences (i.e. different induction intensity or rate of biomarker responses) at the two temperatures but also qualitative differences, i.e. different biomarker responses were observed. In bile of fish exposed to PAHs spiked sediment, concentrations of 1-hydroxypyrene and 1-hydroxyphenanthrene increased significantly during the first two days, and then decreased. At 12 °C, the uptake of PAHs was slower and maximum metabolite concentrations in bile were lower than in fish exposed at 24 °C. Following a latency of two days, the concentration of PAH metabolites in bile of fish exposed at 24 °C was followed by a peak in lipid peroxidation. PAHs spiked into sediments under laboratory conditions were significantly more bioavailable than the PAHs that were already present in un-spiked field-collected sediments. In the final study described in this thesis, findings from both preceding studies were joined to refine the hydrotoxicological approach and to re-evaluate the new interdisciplinary method. To elucidate the effects of sediment-bound organic pollutants, such as PAHs, rainbow trout were exposed to three resuspended natural sediments with different contamination levels. Physicochemical parameters including dissolved oxygen concentration, pH and temperature, total PAH concentration in sediments and SPM as well as different biomarkers of exposure in fish such as 7-ethoxyresorufin-O-deethylase (EROD) activity and biliary PAH metabolites were measured following 7 d exposure within an annular flume. Concentrations of PAHs in SPM remained constant and represented the different contamination level in the un-suspended sediments. Significant differences in bile metabolite concentrations as well as in EROD induction compared to control experiments (untreated animals and animals that were exposed in the annular flume without sediment) were observed for all exposure scenarios. The ratio between 1hydroxypyrene in bile from fish exposed to the three different contamination levels was 1.0 : 3.6 : 10.7 and correlated well with (1) the ratio of pyrene concentrations in corresponding sediments which was 1.0 : 3.1 : 12.7 and (2) with the ratio of particle-bound pyrene in SPM which was 1.0 : 2.7 : 11.7. In contrast, hepatic lipid peroxidation and micronuclei formation represented the different contamination levels less conclusively. The results of this study clearly demonstrated that firmly-bound PAH from aged sediments can become bioaccessible upon resuspension under flood-like conditions and are readily absorbed by aquatic organisms such as rainbow trout. Associated short term effects were clearly documented and possible adverse long-term impacts due to genotoxicity are likely to follow. The present thesis demonstrates that the interdisciplinary approach created an innovative research tool to assess the hydrotoxicological relevance of contaminated sediments after resuspension events. However, there are certain drawbacks and limitations of this innovative approach such as the influence of larger animals on erosion characteristics within the annular flume and the dependency on vertebrate test animals. In a next step and with future method optimizations, however, those shortcomings can be resolved and the hydrotoxicological approach can be used to the full extent as a tool with practical relevance to investigate and assess contaminated sediments and their influence on aquatic organisms upon resuspension