Sediment Properties for Assessing the Erosion Risk of Contaminated Riverine Sites. An approach to evaluate sediment properties and their covariance patterns over depth in relation to erosion resistance. First investigations in natural sediments (11 pp) |
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Authors: | Sabine U Gerbersdorf Thomas Jancke Bernhard Westrich |
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Institution: | (1) Institute of Hydraulic Engineering, Hydraulic Laboratory, Pfaffenwaldring 61, University of Stuttgart, 70550 Stuttgart, Germany |
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Abstract: | Background, Aim and Scope
Riverine sediments store large quantities of hazardous contaminants, remaining a 'legacy of the past' world-wide. Natural
events such as floods may cause the resuspension of polluted sediments and accordingly, the former immobilized contaminants
might become bioavailable and toxic again. Hence, a comprehensive erosion risk assessment of contaminated sites is of crucial
importance. The present study aimed to implement 'master-variables' for a reliable, easy-to-manage and economically more viable
determination of stability in cohesive sediments.
Materials and Methods:
A wide range of physico-chemical (bulk density, water content, particle size, mineral composition, cation exchange capacity
/ CEC, total organic matter / TOC, liquid and plastic limits of a soil) and biological (macrofauna abundances, microalgal
biomass and species composition, bacterial cell numbers, EPS fractions such as carbohydrates and proteins) properties were
determined simultaneously over depth spanning the zone between 0-35 cm. The data were related to sediment stability, determined
as the 'critical shear stress for mass erosion' in the SETEG (Stroemungskanal zur Ermittlung der tiefenabhaengigen Erosionsstabilitaet
von Gewaessersedimenten) - flume. The investigations were done on natural sediments, thereby covering vertical (over depth),
spatial (different study sites) and temporal (different seasons) aspects to ensure the transferability of the data. Here,
first data originating from three contaminated reservoirs in the lock-regulated River Neckar / Germany are presented.
Results:
Comparison of the rather low critical shear stress values (resisting force of sediment, determined in SETEG) with the possibly
occurring natural bottom shear stresses (attacking force, calculated for different hydraulic scenarios) at the three reservoirs
indicated a severe risk of sediment erosion even under moderate hydraulic conditions and was not restricted to the surface.
Critical shear stress was characterised by the following sediment properties of depth, grain size, CEC (Cation Exchange Capacity)
and concentrations of TOC (Total Organic Carbon), proteins as well as carbohydrates (water- and resin-extractable). Firstly,
biological stabilisation by extracellular polymeric substances (EPS) could be shown for riverine sediments, even over depth.
Secondly, erosion resistance was determined by the inter-particles forces, an interplay of the biologically produced compounds
constituting active surfaces and the binding capacity as well as charge densities of the sediments. The combined influence
of sedimentological and biological properties on sediment stability over depth was assessed by PCA (Principal Component Analysis).
Discussion:
Hence, a better correlation coefficient between sediment stability and the master variables could be achieved (Main component
II: Polymeric substances, R = 0.7, Main component III: Grain size, TOC, CEC, R = 0.9) compared to single correlations.
Conclusions:
The present paper revealed the combined influence of physico-chemical and biological properties on sediment stability over
depth by simultaneous investigation and statistical evaluation. It can be shown, that inter-particle forces, determined by
particles size classes, CEC, TOC and polymeric substances such as proteins and carbohydrates, affected sediment stability
most. Thereby, the impact of biogenic sediment mediation on riverine sediment stabilisation became evident, even over depth,
where mostly sedimentological parameters were considered as important before.
Recommendations and Perspectives:
The importance of a comprehensive risk assessment of contaminated riverine sites was again highlighted in the present study
by the comparison of natural occurring bottom shear stresses with the determined sediment erosion resistance. If a realistic
risk assessment is to be derived, the stabilizing potential of micro-organisms needs to be taken into account and the covariance
patterns of biological and physico-chemical sediment properties have to be addressed. |
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Keywords: | sediment stability carbohydrates critical shear stress extracellular polymeric substances (EPS) bulk density Benthic microalgae |
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