Physico-chemical and biological sediment properties determining erosion resistance of contaminated riverine sediments – Temporal and vertical pattern at the Lauffen reservoir/River Neckar, Germany

Abstract

In order to contribute to a reliable, easy-to-handle and economically viable erosion risk assessment of contaminated riverine sites, the present study aims to implement master-variables best characterising the sediment stability. Thus, a wide range of sediment properties was related to the critical shear stress for mass erosion, determined in the SETEG (Stroemungskanal zur Ermittlung der tiefenabhaengigen Erosionsstabilitaet von Gewaessersedimenten) pressurised channel, with special emphasis on vertical and temporal gradients in the Lauffen reservoir on the River Neckar. Over the course of 1 year, positive impacts of some macrofauna species and benthic diatoms on the sediment stability were detected for the sediment surface (0.5 cm). However, a high seasonal variability of biological parameters caused varying relations with erosion resistance in the upper sediment layers as shown for the colloidal carbohydrates. Considering only deeper sediment layers (5-35 cm), a more general pattern could be revealed with correlations between the critical shear stress and single sediment properties such as depth, grain size, total organic carbon (TOC), cation exchange coefficient (CEC), carbohydrates and proteins. Firstly, the influence of physico-chemical and biological properties on erosion resistance became evident, even over depths at 0-35 cm. Secondly, inter-particle forces are most important for erosion resistance. These are enhanced in fine-grained sediment layers, offering high binding capacities but also strengthened by polymeric substances permeating the void space and coating particles. These covariance patterns of sedimentological and biological parameters are addressed by multivariate statistical tests (principal component analysis), resulting in a higher magnitude of the correlation coefficient between critical shear stress and the master-variables in main component II (polymeric substances, grain size, TOC, CEC; R ¼ 0:77) compared to single correlations.

Microbial assemblages as ecosystem engineers of sediment stability

“…Firstly, EPS proteins have been measured in higher concentrations in the nutrient-enriched assemblages compared to nutrientdepleted treatments, but the stability of the substratum was higher in the latter. This suggests that the EPS quantity alone is not decisive for binding forces between particles as stated earlier (Gerbersdorf et al 2005). Presumably, the proteins originating from microalgal cell lysis show very different features than bacterial EPS secreted as extracellular enzymes or for attachment.…”

Anthropogenic pollutants affect ecosystem services of freshwater sediments: the need for a “triad plus x” approach

“…There is now a consensus that biology, and in particular organic material such as EPS, plays a crucial role in sediment stability since it acts like a "glue" by permeating the pore space between the sediment grains and enhancing their interparticle forces (Underwood and Paterson 2003;Gerbersdorf et al 2008bGerbersdorf et al , 2009a. Today, we have a much better understanding of the importance of the interactions between physics, chemistry and biology in influencing the stability of cohesive sediments (Gerbersdorf et al 2005(Gerbersdorf et al , 2008a(Gerbersdorf et al , 2009b. Still, whilst the number of publications on biostabilisation is steadily increasing, academic sediment transport models, as well as commercially available software, consider the process of incipient motion as primarily being governed via equations describing the physical processes; if biostabilisation is considered at all, it is included by empirical relations.…”

Combining Field and Laboratory Measurements to Determine the Erosion Risk of Cohesive Sediments Best

“…For instance, Haag et al [34] measured in a backwater area of the River Neckar (Germany) a range of 0.6 N/m 2 to 8.5 N/m 2 with a mean value of 4.5 N/m 2 including surface and depth-dependent analyses. Gerbersdorf et al [14] measured in the same river a range of 2.5 to 4.5 N/m 2 in deeper layers and values lower than 2.0 N/m 2 in upper horizons. Consequently, these erosion thresholds seem consistently higher than in the present samples from Rivers Elbe and Saale which might be reasoned by the higher clay contents (20%-46%) in the River Neckar samples [14,33].…”

“…Gerbersdorf et al [14] measured in the same river a range of 2.5 to 4.5 N/m 2 in deeper layers and values lower than 2.0 N/m 2 in upper horizons. Consequently, these erosion thresholds seem consistently higher than in the present samples from Rivers Elbe and Saale which might be reasoned by the higher clay contents (20%-46%) in the River Neckar samples [14,33].…”

“…For the large majority of these erosion devices, determination of fine sediment stability is restricted to the surface layer. However, once the surface layer has been eroded, the underlying sediment layers vary extremely in stability and consolidation depending on a magnitude of influencing parameters such as particle sizes, water content, cation exchange capacity, organic substances (dead or alive), and microbially secreted polymeric substances [14]. The erosional behavior of these deeper sediments is of particular interest since many hydrophobic anthropogenic pollutants, which preferentially bind to small particles and originate from the peak-inputs around 30 years ago, are immobilized in these long-deposited layers ("legacy of the past" [15]).…”