4.3 Environmental Setting, Materials, and Methods
4.4.3 Sediment Resuspension and Residence Times
Using equations 4.4 and 4.5, we estimated a critical mobility value,θcr= 0.15, for sediment resuspension. Calculations based on equations 4.1 and 4.3 indicated that the bottom current at NG2 (138 m depth) and NG3 (192 m depth) were always too weak to produce a shear stress strong enough to exceed the critical mobility value (Fig. 4.7). Therefore, sediment surface currents were too weak to incite resuspension. Incorporation of the Delft3D mod-elled field flow over the surface of the bay with the mean particle diameter of NG1 and the resuspension model (equations 4.1–4.3) indicated that resuspension could be possible no deeper than 60 m in the bay.
The mean monthly displacement estimated from figures 4.3 and 4.4 for NG2 and NG3 were 3 km and 27 km, respectively. Based on a mean monthly displacement of 3 km for NG2 (0.1 km d−1), a gyre volume of 0.178 km3, and a discharge rate of 0.017 km3d−1, a residence time of∼10 d is calculated.
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4.5 Discussion
Figure 4.7 Sediment resuspension model results showing the critical mobility value needed for resuspension (dotted red line) and the mobility values for sites NG2 and NG3.
4.5 Discussion
The RCM9 current meter data showed the influence of the main circulation pattern of Lake Geneva on the bottom boundary layer and the persistent effect of the secondary gyre known to exist in Vidy Bay. This study evidenced the secondary gyre’s descent down to the sed-iment surface and its influence on particle trajectories during all lacustrine seasons. The circulation pattern at NG3 showed very clear evidence that the displacement of suspended particulate matter follows specific trajectories during the different lacustrine seasons. Based on this, we calculated the mean monthly distance travelled by a particle at NG2 and NG3.
This monthly interval was chosen to match the monthly basis of sediment trap collection in
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
in Chapter 3. Results in Chapter 3 showed that the sediment accumulation rate and quantity of lateral advections increased vertically in the water column with proximity to the sedi-ment surface, and horizontally with proximity to the shoreline. While NG3 is not affected by the lateral transport from Vidy Bay, but rather suspended particles from the main basin, NG2 is likely to experience increased lateral advections from multiple sources (main basin, Chamberonne River, overflows, resuspension, WWTP). These increased lateral advections are likely due to the increased retention time in the bay and the decreased current velocity providing more suitable conditions for particle settling. Since the WWTP overflows and Chamberonne River enter the bay at the shoreline, it is likely that they could be transported out of the bay by longshore currents. Their contributions to the bay are significantly less than those of the WWTP and should they enter the gyre of the bay, they would be diluted by suspended sediments from the WWTP and from the main basin. In such, shoreline inputs are less likely to affect NG2. This being said, the likely sources of the lateral advections measured at NG2 are the WWTP, resuspension from around the WWTP outlet (<60 m), and suspended sediments carried in from the main basin.
In Chapter 3, note was made of increased sediment accumulation rates during periods of high internal productivity. Statistical analysis of the current meter data indicated the pres-ence of a seasonal trend in the turbidity data and a correlation between the turbidity at both sites (r = 0.360, p <0.001). The turbidity data shown in figure 4.2 showed the presence of small peaks (i.e. June/July 2010 and December 2011); however, seasonal increases in turbidity readings were not apparent. Therefore, the increased sediment accumulation rates shown in Chapter 3 must be obscured by, and are not significant in comparison to, the high turbidity measured in the BBL of Lake Geneva. Turbidity of the BBL has previously been shown to be significantly greater than levels measured in the water column. In Lake Lugano, another peri-alpine lake, turbidity levels in the BBL were found to be approximately three times that measured in the water column (Hofmann et al., 1999). This infers that the sus-pended sediment load in the water column would have to increase by a significant amount in order to influence turbidity measurements in the BBL.
Observed bottom currents counter intuitively showed that the mean current was stronger at NG3, the deepest station, as compared to NG2 (Figure 4.2). Consequently, this result is confirmed by using a sediment resuspension model where both direction and intensity were found to be consistent with this observation (Figure 4.5). A near inertial motion, rotating clockwise, with an oscillation period of around 14 h constantly energises both sites. A lower frequency oscillation around 300 h was also observed; however, only at NG2. The exact role of the near inertial waves energizing the BBL remains to be analysed. Regardless, neither
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4.6 Conclusions current was strong enough to resuspend particles at these two sites. The mobility Shield parameter that was calculated never exceeded the threshold bed shear stress, above which sediment grains are expected to move or to be resuspended (Quaresma et al., 2007; Soulsby et al., 1997). However, in using the same sediment size distribution over the entire bay, the sediment resuspension model, combined with the modelled Delft3D-FLOW field flow, indicated that currents may be strong enough to resuspend sediments at a water depth no greater than∼60 m, approximately 25 m deeper than the WWTP outlet and 18 m shallower than NG1; however, this finding is rather preliminary and provides only an order of magni-tude for the maximum depth of resuspension in the bay since it is based on a modelled field flow and there was no hydrodynamic data measured for this depth. Hence, it is possible that contaminated surface sediments in the bay are resuspended and transported. Based on this study it is therefore not possible to determine the exact origins of the lateral component of NG2. Sediment focusing is likely due to suspended particles from the main basin combined with WWTP inputs with the possibility of resuspension from shallower parts of the bay.
Regardless of the source, particles resuspended in the bay would not specifically be found at NG3 since they would be retained in the bay by the gyre or advected into the main basin via longshore littoral currents. Extension of this research to include physicochemical and mineralogical analyses would lend to a more precise identification of suspended sediment sources.
4.6 Conclusions
This study aimed to investigate the hydrodynamic conditions of the bottom boundary layer influencing Vidy Bay to better understand the transport and fate of contaminated sediments and to help elucidate the source of lateral advections known to accumulate within the bay.
The measurement of the hydrodynamic conditions of the bottom boundary layer at Vidy Bay is novel. Hydrodynamic data allowed for the determination of seasonal trends in the main basin close to Vidy Bay and validation that the clockwise gyre is persistent and extends
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
residence time in the bay provides an ideal place for suspended particles to destabilize and settle out of the bottom boundary layer.
While Vidy Bay could release some contaminated sediments to the deeper main basin, it can be viewed mainly as a repository or sink for contaminated sediments. Any deposition in the bay and any advections out of the bay are likely diluted with suspended sediments from the main basin. While this means that the contamination from the bay is less likely to spread to the deeper basin, it also implies that Vidy Bay is likely to remain a “hot spot”
of contamination. This poses a potential problem with respect to energetic resuspension and entrainment events which could reintroduce mass amounts of contaminants to the water column.
4.7 Acknowledgements
The authors are thankful and acknowledge the financial support of the Swiss Science Foun-dation (Project: PDFMP2-123 034). They would also like to thank Ulrich Lemmin and Ahn-Dao Le Thi for their help and guidance with treatment and interpretation of hydro-dynamic data. Thanks are also extended to Philippe Arpagaus for his vital assistance in sampling.
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C HAPTER 5
D ISCUSSION
The research presented herein investigated sedimentation processes, the influence of hy-drodynamic conditions on sediment-bound contaminant transport, and colloid and particle stability as key issues for the question of long-term contaminant transport and water quality in mid-sized lakes, using Vidy Bay as a case study.
Overall, colloids and small particles at the sediment-water interface of Vidy Bay were found to be unstable and, in such, are not likely to play a significant role in the long-range transport of contaminants. Their composition, size distribution, number and mass concen-trations, and mean surface charge were characterized. The measured colloid characteristics did not vary significantly over the area of the bay. The mass and number concentrations of colloids and small particles were found to be greater than previously documented for shal-lower layers of the water column suggesting the presence of a nepheloid layer. The presence of colloids in the nepheloid layer predisposes them to destabilize and settle out of the water column. This nepheloid layer decreases colloid stability through an increased probability of collisions, leading to an increased chance of collisions between colloids. Colloids at the sediment-water interface showed a slight overall negative surface charge∼16 mV, implying a borderline stability. The nepheloid layer contains significant amounts of rigid biopolymers as evidenced in SEM micrographs. These rigid biopolymers were shown to destabilize and
Discussion
culations. In turn, process-related residence times were refined and more representative of the actual process within the water column. Overall, the process-related residence time in the bottom boundary layer (τB) was on the same order of magnitude as the other process-realted residence times. The importance of this finding is such that a longerτBlends to the destabilization of particles in this layer and in-turn facilitates particle settling.
The hydrodynamic conditions of the bottom boundary layer of Vidy Bay were investi-gated to better understand the transport and fate of contaminat-laden sediments and to help elucidate the source(s) of the lateral advections found to accumulate in the bay. Hydrody-namic data verified the presence of the secondary clockwise gyre in the bay and showed that its influence persisted throughout the annual lacustrine cycle and extended down to the sediment surface. The persistence of the gyre increases particle retention time within the bay to around 10 d, greater than previously calculated.
Calculated mobility Shields parameters calculated using a field-flow model for the sur-face of Vidy Bay found that resuspension was possible no deeper than ∼60 m. In such, lateral advections to the bay are likely due to inputs from the main basin with additional inputs from WWTP effluents, and possible resuspensions from shallow littoral zones. The source of inputs from the main basin is supported by correlations found between turbidity readings at NG2 and NG3 alongside the current patterns recorded at NG3. These lateral ad-vections could influence sites within the bay but would bypass deeper sites of the main basin since these deeper sites are not influenced by the Vidy Bay gyre and are not in the trajectory of shallow longshore currents. The noted decrease in velocity and extended residence time in the bay provides ideal conditions for suspended particles to destabilize and settle out of the nepheloid layer.
The findings, overall, demonstrate the accumulation, the favourable conditions for par-ticle settling, and the likelihood of parpar-ticle retention within the bay. The presence of a nepheloid layer, and the characteristics of colloids and small particles in this layer, sug-gest that long-range transport of colloid- and particle-bound contaminants from the bay is unlikely. Sediment accumulation rates, and lateral advections increased with proximity to the sediment surface and proximity to the shoreline. Hydrodynamic conditions showed an increased retention time within the bay. Coupling the increased retention in the bottom boundary layer with a decrease in current velocity produces favourable conditions for the localized settling of suspended sediments out of the bottom boundary layer. These findings, atop the demonstrated lack of resuspension in deeper parts of the bay and the retention of particles by the secondary gyre suggest that particle-bound contaminants are likely to re-main within the bay where they are diluted by lateral advections from the re-main basin, as
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opposed to being dispersed to deeper parts of the lake. While there remains the possibil-ity of contaminant dispersal via shallow longshore currents which are known to occur in around 20 % of the regional wind patterns, Vidy Bay can be seen to act primarily as a sink for contaminated sediments. Deposition in the bay and any advections out of the bay would be mixed with, and diluted by, suspended sediments from the main basin. Regardless of this dilution, Vidy Bay is likely to remain a “hot spot” of contamination which, in itself, poses potential concerns with respect to the economic and ecologic implications of the release of contaminants through energetic mass resuspension events.
To aid in synthesizing the findings, two diagrams are presented (Figures 5.1 and 5.2).
These figures incorporate the current state of knowledge of Vidy Bay, including the findings from this thesis manuscript and data previously published. Figure 5.1 shows an aerial view of Vidy Bay with the outlet of the WWTP depicted by a yellow dot. The dark red oval (A) shows the modelled environmental impact area and direction of spread of the WWTP effluent plume in the upper water column as published in Bonvin et al. (2013). The grey oval (B) depicts the surface area of Hg contaminated sediments surrounding the WWTP outlet as published by Poté et al. (2008). This is the affected surface area as of 2005 and occupies
∼0.8 km2 and is reduced from ∼1.3 km2 before the extension of the WWTP outlet pipe.
∼0.8 km2 and is reduced from ∼1.3 km2 before the extension of the WWTP outlet pipe.