Bernhard Westrich

Microbial stabilization of riverine sediments by extracellular polymeric substances

Sediment stability is a critical component for the understanding of cohesive sediment dynamics. Traditionally, physico-chemical sediment conditions have been regarded as most important drivers of sediment stability. However, over the last decade, the stabilization of sediment by biological activity, particularly the influence of highly hydrated matrices of extracellular polymeric substances (EPS) has been given increasing attention. However, most studies have focused on the sediment/water interface and, usually, of marine systems. The present study exploits current knowledge of EPS dynamics from marine systems and applies it to freshwater habitats, also considering a wide range of biological and physico-chemical variables. Natural sediments were taken from a freshwater site with high levels of heavy metal pollution (Lauffen reservoir, River Neckar, Germany). Vertical profiles from the flocculent surface layer to depth of 50 cm within the sediment were investigated, monthly, over the course of year. Tubificidae and Chironomidae larvae constituted the majority of the macrofauna. Despite the turbidity of the water column, a highly diverse and abundant microphytobenthic community of diatoms (11-82 microg g(-1) DW) was found at the sediment surface closely associated with high numbers of bacteria (10(9) cells g(-1) DW). The concentrations of all EPS moieties were remarkably high (0.1-0.5, 1.7-3.8, 0.9-5.2 mg g(-1) DW, for colloidal and bound carbohydrates and proteins, respectively) and levels were comparable to those determined in intertidal studies. The microalgal and bacterial biomass both showed strong correlations with the colloidal and bound EPS carbohydrate fractions. The data suggested that the present macrofauna as well as the metabolic activities of microalgae and bacteria interact with sedimentological factors to influence the properties of the sediment by binding fine-grained sediment, changing water content and enhancing the organic content through secretion products. The colloidal and bound EPS moieties showed strong correlation with the critical shear stress for erosion over sediment depth. It is suggested that the cohesive strength of the sediment was controlled by a high number of active adsorption sites and higher charge densities in fine grained sediments. The EPS network may significantly enhance this by embedding particles and permeating the void space but also in offering additional ionic binding sites and cross-linkages.

Historical Contaminated Sediments and Soils at the River Basin Scale

Background, Aims, and ScopeData from the Elbe River and its tributaries indicate, despite extensive improvement in water quality during the last 15 years, that the respective sediment situation of many priority pollutants has not reached an acceptable level. For the coming decades, risks for downstream sites and stakeholders will persist, mainly due to secondary sources originating from historical pollution of soils and sediments in the catchment area. In practice, a catchment-wide assessment of historical contaminated soil and sediment should apply a three-step approach: (i) Identification of substances of concern (s.o.c.) and their classification into ’hazard classes of compounds’; (ii) identification of areas of concern (a.o.c.) and their classification into ‘hazard classes of sites’; (iii) identification of areas of risk (a.o.r.) and their assessment relative to each other with regard to the probability of polluting the sediments in the downstream reaches. The conversion of this concept has to consider the underlying philosophy of the EU Water Framework Directive, particularly with respect to the analysis and monitoring of priority substances in solid matrices. However, major deficiencies are still in the assessment and prognosis of resuspension processes, and potential approaches to fill this gap are described both in theory and from examples of the Elbe River.MethodsThe sediment stability testing facilities consist of a unique triple set developed by innovative experimental laboratory and field research. The instrumental facilities consisting of a tube corer and a pressurized channel allow one to measure not only the onset of erosion (critical bed shear stress), but also the erosion rate for different sediment layers. Undisturbed sediment samples were taken from contaminated sites, e.g. in near-bank groyne fields and floodplains, using (i) core sampler (diameter 14 cm, length 150 cm) for sediment erodibility depth profiling and (ii) box sampler (30*70 cm2 top view area, 28 cm depth) for comparing and upscaling the results from the laboratory to the field. Sediment properties such as grain size spectrum (laser beam attenuation), water and gas content were analyzed by a non-intrusive, high frequency, capacity measurement method and bulk density by γ-ray.Results and DiscussionSediment core samples from flooded areas in the Middle Elbe indicate, that, except from the uppermost 5 cm and at a depth of from 47 to 48 cm, where the critical shear stress is very low (0.5 Pa), the critical bottom shear stress is between 1.2 Pa and 3.4 Pa, i.e. at a moderate level. Major reasons for the distinct heterogeneity of the erosion stability are differences in consolidation processes, grain size distribution and in the composition of stabilizing exudates in the individual sediment layers. Similar to the erosion stability depth profile, the metal data exhibit short-range heterogeneities; the variations in the individual layers can be explained by different proportions of fine grained components and by an improvement of suspended matter quality in the course of time. A comparison of the metal contents of embanked alluvial soils and unembanked alluvial areas suggests the following causal chain: Recent floodplain areas at low mean water levels exhibiting high concentrations of organic carbon represent the most highly contaminated sites. On the other hand, insignificant pollution has occurred on alluvial areas, which were embanked already at pre-industrial times. In the case of flood events, due to the combination of flooding probability and flow conditions, the most favorable conditions for the deposition of nutrient- and contaminant-rich suspended particulate matter are found in the low level depressions with low current. Within a typical river section of 1 km length in the lower middle Elbe, the groyne fields are recognized as dominant, slack-water zones containing the following nutrient and trace metal loads (reference year 2001, anthropogenic proportions): 287 t organic carbon, 17.6 t phosphorous, 17.4 t nitrogen, and 16.7 t sulfur; 8.6 t zinc, 1.1 t copper, 0.9 t lead, 0.4 t chromium, and 0.2 t nickel, respectively. The estimated nutrient and pollutant loads, deposited on the floodplains and in the river course, clearly demonstrate the specific sink function of both sites. At the same time, however, the results suggest, in contrast to the deposits in the floodplains, that sediments within the river course may partly be remobilized. This means that the longterm sink function can at least temporarily become a significant source character, involving the hazard of a substantial deterioration of the downstream sections of the river basin.Conclusions and OutlookIn view of the findings of relative low erosion stabilities of groyne field sediments and, in particular, after the extreme Elbe flood from August 2002, a prime question relates to the remobilization risks of these sediments typically enriched in contaminants and nutrients. The combined view on substances, areas and processes of concern in the Elbe catchment - with special emphasis on historical contamination of floodplain soils and sediments, as well as on groyne field sediments, as significant secondary sources of pollution - is a typical example for the holistic river basin approach of the European Water Framework Directive (WFD), both with respect to assessment of ecological risks and the development of remediation measures. In the latter respect, recent developments in ‘soft’ (geochemical and biological) techniques on contaminated soils and sediments, both with respect to policy aspects as well as to technical developments have led to a stimulation of in-situ remediation options, such as sub-aqueous depots, active capping, and application of natural attenuation processes. Limited financial resources require a direction of investments to those sites with the highest efficiencies in risk reduction. Establishing a rough sediment dynamic model, building on tributary/Elbe dilution factors, sedimentation data, suspended particulate matter monitoring data as well as calculations of long-term costs and benefits, based on risk management, could be essential steps in a basin wide river management.

Erosion investigation and sediment quality measurements for a comprehensive risk assessment of contaminated aquatic sediments.

In this paper, an assessment strategy is introduced which allows one to evaluate the ecological hazard of contaminated sediments in connection with the risk of in-stream erosion. Special techniques for sediment sampling, non-intrusive density profiling, and depth related measurement of erosion are presented, which, in combination with ecological aspects, lead to a comprehensive risk assessment of fluvial sediments. The strategy was applied to a lock-regulated reach of the River Neckar in Germany. The spatial pattern of contamination in the river reservoir was found to be remarkably heterogeneous. At some sites, very high heavy metal concentrations were detected at the sediment surface. A sudden increase in contamination with depth at other sites could be attributed to an erosional unconformity. The critical shear stress of erosion for old contaminated sediments is higher than for recently deposited material. Nevertheless, during major flood events, bottom shear stress in the river exceeds the critical shear stresses of erosion of all sediments. Accordingly, there is a substantial risk that old contaminated sediment can be mobilised from the reservoir and transported downstream.

Sediment Dynamics and Pollutant Mobility in Rivers

This is the first interdisciplinary book on the mobilization of nutrients and pollutants in the water phase due to hydrodynamic processes. In its ten chapters, a broad set of practical process knowledge is presented, comprising simulation techniques, laboratory and in-situ studies on the interaction between biological, chemical and hydrodynamic factors as well as models, for solving combined quality and quantity problems of riverine sediments both on local and river basin scales. These papers result from an International Symposium on Sediment Dynamics and Pollutant Mobility in River Basins, held in Hamburg, March 26 to 29, 2006.

FLOW FIELD AND SCOURING EFFECTS OF STEADY AND PULSATING JETS IMPINGING ON A MOVABLE BED

The flow field and erosion pattern of continuous and pulsating impinging jets has been studied experimentally. The mean velocity field as well as turbulent and pulsation components, wall pressure and wall shear stress distributions were investigated in an axisymmetric air jet impinging upon a rough wall. The results show how harmonic velocity pulsations at the nozzle affect both the mean and the turbulent flow field. Corresponding erosion tests with a vertical water jet impinging upon a uniform sand bed exhibited under certain conditions a greatly enhanced erosion rate. The increase in erosion capacity due to pulsation seems to be due to both the changes in the mean flow field and to the greatly increased lateral correlation of the velocity field in the impinging region.

Untersuchungen zum ökotoxikologischen Schädigungspotenzial und Erosionsrisiko von kontaminierten Sedimenten in staugeregelten Flüssen

ZusammenfassungSchwebstoffgebundene Schadstoffe werden durch Sedimentation der Wasserphase und damit der Verfügbarkeit für viele aquatischen Organismen entzogen. Während bei durchschnittlichen hydrologischen. Verhältnissen die Freisetzung von Schadstoffen aus stabil gelagerten Sedimenten meist weitgehend unterbunden wird, besteht bei Hochwassereignissen die Gefahr einer Remobilisierung von kontaminierten Ablagerungen. In diesem Beitrag wird ein kombiniertes ökotoxikologisches und hydraulisches Untersuchungssystem zur Untersuchung der ökotoxikologischen Belastung und des Erosionsrisikos von Sedimenten vorgestellt. Der integrierte Ansatz wurde am staugeregelten Neckar angewendet, um das Schädigungspotenzial und die Gefahr einer Remobilisierung an Sedimentbohrkernen der Stauhaltung Lauffen sowie an Schwebstoffen zweier extremer Hochwasser zu überprüfen. Für die Bohrkernsegmente unterhalb einer Erosionsdiskordanz konnte eine sprunghafte Zunahme der ökotoxikologischen Belastung ermittelt werden. Bei Hochwasserereignissen mit einer 5-jährlichen Wiederkehrwahrscheinlichkeit (=HQ5) können prinzipiell alle Sedimente, auch die stärker kontaminierten und erosionsresistenteren Altsedimente, remobilisiert werden. Während der Hochwasserereignisse (HQ15 bis HQ20) kam es zu einer deutlichen Erhöhung des cytotoxischen und mutagenen Schädigungspotenzials der Schwebstoffe im Vergleich zu einem mittleren Hochwasser im Jahre 1995/96 (HQ1) Dies schien zumindest teilweise auf die Remobilisierung hochkontaminierter Altsedimente zurückzuführen zu sein.AbstractParticle-bound pollutants accumulate in river-bottom sediments, a process which results in a significant decrease in the ecotoxicological availability of toxicants for the majority of aquatic organisms. Under normal hydrologic conditions, the release of contaminants from bottom sediments is usually of minor importance. In contrast, flood events may remobilize highly contaminated sediments via in-stream erosion. The objective of this study was to develop a combined ecotoxicological and hydraulic approach to elucidate the ecotoxicological implications associated with the risk of erosion of contaminated sediments. This integrated strategy was applied to the lock-regulated Neckar river in Southern Germany. Both the bottom-sediment cores and suspended matter from two intensive flood events were investigated. Sediment samples below an erosional unconformity showed a sharp increase in the ecotoxicological load. Moreover, it was found that major flood events (HQ5 and higher) could possibly erode even very old, well-consolidated and highlycontaminated sediments. The suspended matter of the high discharge events investigated (return periods of 15 to 20 years) exerted significantly higher cytotoxicity and mutagenicity than a moderate flood with a 1-year return period. These findings support the conclusion that the observed ecotoxicological effects during major floods may at least in part be due to the in-stream erosion of highly contaminated bottom sediments.

Quantification of erosion rates for undisturbed contaminated cohesive sediment cores by image analysis

In order to model the transport of cohesive sediments and associated contaminants accurately, knowledge of erosion rates is indispensable; at present, these are mostly obtained from the literature and usually extended by field data. In contrast to these theoretical approaches we have developed a method to determine erosion rates directly from experiments conducted with undisturbed sediment cores in a laboratory flume. Regularly spaced laser lines are projected onto the sediment surface during the erosion process. To compute the volume loss over a certain period of time, snapshots of the projected lines are taken and the relation between the deformed projected lines and the sediment surface is derived. With this relation and bilinear interpolation, the sediment surface is reconstructed. The volume difference between the reconstructed sediment surfaces corresponding to the snapshots is then calculated. Consequently, volume loss can be determined in mm3 per time with a maximum error of 7.2%. The average error is less than 1%. Combination with the bulk density results in erosion rates in kg m−1 s−1. The high sensitivity of the laser lines to sediment surface changes allows the determination of even small erosion rates. Thus, it is possible to determine erosion rates directly as a function of shear stress. Since it is possible to vary the area of calculated sediment-volume loss, scaling effects can be investigated as well.

The Yangtze-Hydro Project: a Chinese–German environmental program

Water of good quality is one of the basic needs of human life. Worldwide, great efforts are being undertaken for an assured water supply. In this respect, one of the largest water technology projects worldwide is the Yangtze Three Gorges Dam in China. There is a need for extensive scientific and technical understanding of the challenges arising from this large hydrological engineering project. German and Chinese groups from various scientific fields are collaborating to provide knowledge for the sustainable management of the reservoir. In this project description, the Yangtze Three Gorges Dam Project, its goals and challenges, are described in brief, and the contributions of the German research projects are presented.

PAH distribution and mass fluxes in the Three Gorges Reservoir after impoundment of the Three Gorges Dam

Mass fluxes of polycyclic aromatic hydrocarbons (PAHs) were calculated for the Three Gorges Reservoir (TGR) in China, based on concentration and discharge data from the Yangtze River. Virtual Organisms (VOs) have been applied during four campaigns in 2008, 2009 (twice) and 2011 at sampling sites distributed from Chongqing to Maoping. The total PAH mass fluxes ranged from 110 to 2,160 mg s(-1). Highest loads were determined at Chongqing with a decreasing trend towards Maoping in all four sampling campaigns. PAH remediation capacity of the TGR was found to be high as the mass flux reduced by more than half from upstream to downstream. Responsible processes are thought to be adsorption of PAH to suspended particles, dilution and degradation. Furthermore, the dependence of PAH concentration upon water depth was investigated at Maoping in front of the Three Gorges Dam. Although considerable differences could be revealed, there was no trend observable. Sampling of water with self-packed filter cartridges confirmed more homogenous PAH depth distribution. Moreover, PAH content of suspended particles was estimated from water concentrations gathered by VOs based on a water-particle separation model and subsequently compared to PAH concentration measured in water and in filter cartridges. It could be shown that the modeled data predicts the concentration caused by particle-bound PAHs to be about 6 times lower than PAHs dissolved in water. Besides, the model estimates the proportions of 5- and 6-ring PAHs being higher than in water phase.

Synthesis of the sednet work package 2 outcomes

1 National Soil Resources Institute, Cranfield University, UK 2 SEA Environmental Decisions, UK 3 Universitat de Lleida, Spain 4 Cranfield University, Silsoe, UK 5 Rotterdam Municipal Port Authority, The Netherlands 6 Port of Hamburg / CEDA-ESC, Germany 7 Netherlands Ministry of Transport, Public Works and Water Management, The Netherlands 8 Federal Institute of Hydrology, Germany 9 Lancaster University, UK 10 Manchester Metropolitan University, UK 11 University of Stuttgart, Germany 12 Cranfield University, Silsoe, UK 13 Environment Agency, UK