This study was designed to investigate the vertical and lateral sedimentation dynamics in-fluencing Vidy Bay, Lake Geneva. The goal was to ascertain the sedimentation pathways of particle-bound contaminants and identify if sediment focusing was occurring in the bay.
The radionuclidic and sedimentological aspects of settling particles were used to describe the sedimentation process and discriminate between vertical and lateral fluxes. The vertical sedimentation model, with a lateral component, used 7Be/210Pbxs flux ratios to differen-tiate between and quantify vertical and lateral particle advections. The modelled results were in line with literature values while lending to more refined process-related and overall residence times. The model showed that process-related residence times τP and τC were the controlling factors in the overall residence time on a seasonal scale, while the process-related residence timeτBandτC were noted to be the controlling factors on an annual scale.
Sedimentological and radionuclidic aspects of settling particles evidenced that sediment fo-cusing was occurring in Vidy Bay. Sediment fofo-cusing increased with proximity to shore and was also most prominent during thermally unstratified conditions.
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3.8 Acknowledgements
3.8 Acknowledgements
The authors are thankful and acknowledge the financial support of the Swiss Science Foun-dation (Project: PDFMP 2-123 034). They would also like to thank Damien Bouffard for his help in improving the flow of the article and thank Philippe Arpagaus for his vital assis-tance in sampling. Thanks are also extended to the fishing authority of the Canton of Vaud, Switzerland for their helpfulness in addressing potential issues in sampling.
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C HAPTER 4
B OTTOM BOUNDARY LAYER HYDRODYNAMICS AND SEDI
-MENT FOCUSING : IMPLICATIONS FOR A CONTAMINATED BAY
This chapter investigates the hydrodynamics of the bottom boundary layer of Vidy Bay. This is necessary for a rounded, more complete, picture of sediment entrainment and transport dynamics and, in such, an understanding of particle-bound contaminant fate. This study was designed to investigate how the hydrodynamic conditions of the bottom boundary layer affect Vidy Bay in terms of sediment resuspension and transport dynamics and how these hydrodynamic conditions relate to the sedimentation pathways, lateral advections, and con-taminant transport discovered in the previous chapter.
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
Bottom boundary layer hydrodynamics and sediment focus-ing: implications for a contaminated bay
Neil D. Graham*1, Damien Bouffard2, Jean-Luc Loizeau1
1Institute F.-A. Forel, Earth and Environmental Sciences Section, University of Geneva Route de Suisse, 10, 1290 Versoix, GE, Switzerland
2Physics of Aquatic Systems Laboratory — Margaretha Kamprad Chair, ENAC École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
*Neil.Graham@unige.ch
An article of form similar to this chapter will be submitted for publishing.
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4.1 Abstract
4.1 Abstract
Understanding the hydrodynamics of the bottom boundary layer of a lake is necessary for a more complete picture of sediment entrainment and transport dynamics, and in such an understanding of particle-bound contaminant fate. Vidy Bay (Lake Geneva, Switzerland) is the most contaminated part of the lake due to its reception of treated wastewaters. This study was designed to investigate how the hydrodynamic conditions of the bottom boundary layer affect Vidy Bay in terms of sediment resuspension and transport dynamics. The aim of the study was to uncover how local hydrodynamic conditions influence sediment focusing and the potential spread of contamination from the bay to the main basin.
Hydrodynamic measurements showed the persistent influence of a secondary gyre ex-tending down to the bottom boundary layer while just outside of the bay circulation was influenced by the seasonal patterns of the main basin. Calculated mean displacement dis-tances in the bay indicated that suspended particles can travel∼3 km per month, which is 1.5 times the extent of the Vidy Bay gyre. This results in a residence time of approximately 10 d for suspended particles and is at the upper limit of, or longer than, previously mod-elled results. The calculated mobility Shield parameter never exceeded the threshold shear stress needed for resuspension in deeper parts of the bay. In such, increased lateral advec-tions to the bay are not likely due to local resuspension but rather external particle sources.
These external suspended sediment sources coupled with an increased residence time, and decreased current velocity are the precipitating factors in sediment focusing. Contaminant spread from the bay may occur through the transport of suspended sediments in shallower zones of the bay or fine suspended sediments of the WWTP effluent by longshore littoral currents.
4.2 Introduction
Sediment focusing is the accumulation of sediments in quiescent, less energetic, zones of
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
ecological, economic, and health impacts through the reintroduction of contaminants to the water column (Girardclos et al., 2007; Haller et al., 2009b; Herrero et al., 2013). Since sediments act as both a source and sink for many contaminants and nutrients, it is important to understand the nature of sediment transport dynamics to aid in planning and managing sustainable water reserves for the future.
Sedimentation can be studied over decadal and centurial time-scales through sediment core profiling; however, these methods provide little resolution for the relationships between sedimentation dynamics, hydrodynamics, and the physicochemical sediment characteristics.
Short-term spatial-temporal studies provide a more vivid picture of the sedimentation dy-namics at a given location. The use of sediment traps allows for the estimation of temporal and spatial variation in sedimentation rates and suspended particle composition. Analysis of sediment samples for radionuclides has proven to be useful in estimating particle residence times and identifying sedimentation pathways in various aquatic environments (Dominik et al., 1989; Muir et al., 2005; Wieland et al., 1991). However, distinguishing between sources of particles is difficult if no identifier or tag is present (Bloesch, 1994; Cornett et al., 1994).
For example, Wieland et al. (1991) were able to identify the vertical settling component to sediment traps using the pulse of137Cs recorded after the Chernobyl accident. Particles settling from the lake’s surface were labelled with137Cs, while sediments resuspended from the sediment surface were lacking this radioactive tag. A short review of other methods to identify resuspended sediments can be found in Bloesch (1994).
Ultimately, the combined use of sediment traps with current and turbidity meters is op-timal for in-situ studies on suspended particulate matter near the sediment surface (Bloesch, 1994). In this study RCM9 current meters were used to measure temporal and spatial vari-ations in hydrodynamic conditions to elucidate sediment transport dynamics in the bottom boundary layer of Vidy Bay. These results are used to help identify the source(s) of sed-iments previously shown to focus in the bay, see Chapter 3. The sediment trap study in Chapter 3 was conducted at the same sites as, and concurrent with, the use of the RCM9 current meters. In chapter 3, the use of7Be and 210Pbxs gave insight into residence times and differentiated between vertically and laterally advected sediments.
Vidy Bay is the most contaminated part of Lake Geneva due to its reception of efflu-ents from the wastewater treatment plant (WWTP) of the City of Lausanne. The spread of contamination in the sediments and in the water column of Vidy Bay have previously been investigated (Bonvin et al., 2011; Goldscheider et al., 2007; Loizeau et al., 2004; Pardos et al., 2004; Razmi et al., 2014); however, the effect of resuspension and sediment focusing have been neglected. An understanding of sediment transport in the bay is essential in
deter-86
4.3 Environmental Setting, Materials, and Methods