3.5 Results
3.5.3 Radionuclide Ratio Fluxes
Figure 3.6 and 3.7 present the changes in flux ratios for the atmospheric trap (Atmo), STT and STB for NG2 and NG3. A summary of the radionuclide data used for the sed-imentation model and the lateral component calculations can be found in Table 3.4. The overall annual7Be/210Pbxsatmospheric flux ratio was 8.14 Bq m−2d−1and ranged between 8.08 Bq m−2d−1to 10.98 Bq m−2d−1on a seasonal time scale. This annual value is slightly lower than that previously determined for the Lake Geneva basin at 12.3 Bq m−2d−1 (Cail-let, 1999) with the difference likely stemming from several factors including decreased ra-dionuclide production and decreased precipitation. The atmospheric fluxes of both7Be and
210Pbxs and their ratio did not appear to follow any seasonal trend. The7Be/210Pbxs ratio trends noted for STT and STB at both sites are similar, with the ratio increasing during peri-ods of high internal production just as the SAR increased during these periperi-ods. However, the same cannot be said for the periods of low internal production. During periods of low inter-nal production, the ratio of7Be/210Pbxsreacts independently of both the atmospheric inputs and the SAR. Overall,7Be flux decreased in going from atmospheric inputs, to STT, to STB at both sites while210Pbxs evidenced the opposite trend of increasing flux with increasing depth in the system. Differences in radionuclidic fluxes at STT or STB were not significant between the two sites. The data for December 2010 does not indicate the presence of an event, as was seen in the SAR, OM, and CaCO3data.
68
3.5 Results
Figure 3.4 Sediment and component fluxes for the top and bottom sediment traps at NG2.
Error bars are too small to be visible on the scale of the graphs.
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
Figure 3.5 Sediment and component fluxes for the top and bottom sediment traps at NG3.
Data is presented on the same date scale as site NG2 for ease of comparison. Error bars are too small to be visible on the scale of the graphs.
70
3.5 Results
es(Bqm−2 d−1 ),sedimenttrapfluxratiosusedforresidencetimecalculations.Errorsaregivenas1σ. theperiod8-April-2010to6-April-2011.Theatmosphericinputswerethesameforbothsites. AtmoSTTSTBSC 210Pbxs7Be/210Pbxs7Be210Pbxs7Be/210Pbxs7Be210Pbxs7Be/210PbxsNet7Be7Be/210Pbxs ±0.60.69±0.019.82±0.872.41±0.851.09±0.083.50±1.232.37±0.241.50±0.133.44±0.341.69±1.792.45±2.06 0.390.72±0.018.08±0.534.21±0.440.94±0.115.82±0.614.08±1.411.21±0.125.65±1.953.11±0.814.31±1.12 0.160.35±0.018.30±0.462.08±0.450.69±0.075.86±1.271.69±0.390.91±0.084.75±1.101.44±0.864.05±2.24 0.210.56±0.0110.98±0.463.04±0.420.61±0.055.46±0.762.66±0.620.72±0.064.79±1.112.58±0.604.64±1.07 0.230.57±0.018.14±0.413.32±0.320.84±0.075.83±0.563.08±0.841.08±0.085.41±1.472.41±0.594.24±1.04 3.09±0.440.62±0.074.99±0.652.69±1.030.85±0.093.72±1.281.94±1.142.68±1.58 1.64±0.540.42±0.054.64±1.531.17±0.160.74±0.073.31±0.44-0.21±0.65-0.59±1.84 3.08±0.470.62±0.055.55±0.852.07±0.500.71±0.053.73±0.901.71±0.783.08±1.40 2.59±0.300.57±0.014.63±0.442.14±0.570.83±0.063.77±1.001.10±0.751.94±1.32
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
Figure 3.6 Changes in radionuclide flux ratios at site NG2. Error bars represent the error at
±1σ.
3.5.4 Sediment Component Model
Table 3.5 presents the calculated overall and process-related residence times and lateral advection components for both sites. Lateral advections ranged from 10 % to 61 % at NG2 and −17 % to 51 % at NG3 and were seen to increase during unstratified conditions and decrease during thermal stratification at both sites and both depths. Lateral advections were greater at NG2 than at NG3, showing a horizontal tendency for the lateral component to increase with proximity to shore. Lateral advections were also greater in STB than in STT, indicating a vertical tendency for increased lateral advection with proximity to the sediment surface. Overall annual lateral advections increased from−2 % and 31 % at NG3 STT and STB to 32 % and 48 % at NG2 STT and STB. The lateral component for NG3 STT was not significantly different than zero at a 2σ level. This indicates that lateral advections are likely to have a minimal influence at this site and depth and infer that sedimentation rates and sediment composition at this location are less likely to be influenced by boundary effects and are more representative of the lake’s background values.
The overall annual residence times were greater at NG3 than at NG2, calculated at 118 d and 53 d, respectively. τC andτB were the most influential process-related residence times to the overall residence time. τC ranged from 9 d to 134 d at NG2 and 28 d to 43 d at NG3
72
3.6 Discussion
Figure 3.7 Changes in radionuclide flux ratios at site NG3. Error bars represent the error at
±1σ.
whileτB ranged from 2 d to 26 d at NG2 and 15 d to 25 d at NG3. On a seasonal time scale, τC andτP values were seen to be the most significant contributors to the overall residence times. τC was the most influential at NG2 with a range of 3 d to 34 d, while τP was the most influential at NG3 ranging between 36 d to 49 d. No seasonal trends were noted for the predominance of any one process-related residence time over another at either site. The τBvalue for NG3 during November 2010 to April 2011 is omitted due to a negative activity ratio inR2in equation 3.3) implying a possible erosion of the sediment surface during this period. The negative ratio stems from a negative output inventory measured at the sediment surface than measured as input to box 3 of the model in the STB.
3.6 Discussion
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
Table 3.5 A summary of the results from the sediment component model. Residence times (d) are given for each process-related residence time at each site. Annual values are for the period 8-April-2010 to 6-April-2011. Errors are given as±1σ.
Lateral Component (%)
these fluxes being greater in STB than in STT. Although the flux was found to increase with depth, the percent OM and CaCO3 composition was found to decrease with depth. This infers that the increased SAR is due to lateral advections of detritus which have a lower OM and CaCO3composition. This would explain the increased fluxes with depth and lower percent compositions. The fact that the OM fluxes at NG2 do not follow the same trends as those found at NG3 could be due to the influence of lateral advections from the sediments surrounding the WWTP. These sediments are known to be extremely rich in organic matter with values reaching 21 % composition (Haller et al., 2009a). In such, sediments collected at NG2 could be influenced by resuspensions from the zone surrounding the WWTP out-let diluted with lateral advections from the main basin. In this case, a combination of the higher OM content of WWTP outlet sediments and lateral detrital main basin advections would increase the OM content and flux while masking seasonal trends seen at the other sites.
The measured annual and seasonal SAR values presented here are greater than those previously published for the centre of Lake Geneva. Dominik et al. (1993), found that the SAR at 59 m depth and 300 m depth were 2.02 g m−2d−1 and 2.35 g m−2d−1, respectively.
Their data compares well with another study at the same site, where the average SAR at 59 m depth and 300 m depth were found to be 2.40 g m−2d−1 and 3.36 g m−2d−1 in 1986 and 2.22 g m−2d−1and 3.44 g m−2d−1in 1987, respectively (Gandais, 1989). Both of these studies were conducted in the deepest part of the main basin, where conditions in the water column are relatively quiescent and there is less influence from boundary effects. The depths of 59 m and 300 m are used for comparison with the data presented in this study since these depths would best represent the conditions experienced at the sampling sites in and around
74
3.6 Discussion Vidy Bay (i.e. depth below the thermocline and height above the sediment surface).
Overall, the trends noted in and around Vidy Bay follow similar trends previously recorded in Lake Geneva. Seasonal trends in sedimentation are largely influenced by an-nual planktonic growth cycles (Dominik et al., 1993). The majority of the sediment flux was due to the input of plankton detritus with additional inputs of calcite, as evidenced by the particle size analysis spectra. Typically, an annual cycle in Lake Geneva consists of zooplankton and calcite inputs during periods of low primary production, with a shift to phytoplankton and zooplankton inputs during periods of high primary production (Dominik et al., 1993).
3.6.2 Spatial and Temporal Changes in Radionuclide Fluxes
Radionuclide flux ratios (Table 3.4) were applied to the sedimentation model in order to minimize input flux variations stemming from radionuclide production and atmospheric de-position. In turn, results indicated that the SAR had the greatest influence on radionuclide fluxes into sediment traps where previous studies noted that sediment trap fluxes were de-pendent on atmospheric inputs during periods of increased primary production and the sed-imentation rate during periods of lower primary production (Dominik et al., 1989; Wieland et al., 1991). Spearman rank correlations between the SAR and the radionuclide flux ratios at STT and STB of both sites indicate that correlations were significant for STT at NG2 (ρ = 0.461, p<0.05) and both STT and STB at NG3 (ρ = 0.675 and 0.537, p <0.05, re-spectively). NG2 STB was not significantly correlated (ρ = 0.158, p = 0.49). Correlations were stronger the further from shore, and the closer to the lake surface one measured. Both SAR and radionuclide flux ratios followed similar trends inferring the likelihood that the ad-vection and focusing of older sediments is responsible for the diminished correlation to the bottom sediment traps. This agrees well with the noted decrease in OM and CaCO3content between STT and STB. Focusing of older sediments subject to longer OM mineralization and CaCO3dissolution times would decrease their percent composition of the STB samples (Dominik et al., 1993).
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
the result of a mass resuspension of sediments. These resuspended sediments would have to be sufficiently young to still contain7Be, indicating proximity to the sediment traps, since the addition of sediments tagged only with210Pbxs would have significantly decreased the flux ratio. The data presented in table 3.4 indicate that this single event was not significant enough to skew the mean seasonal flux, meaning that it would not have had a major impact on the modelled results. A resuspension model, as presented in Chapter 4, indicated that sediment surface current velocities were not strong enough to incite resuspension at depths greater than 60 m while current direction data indicate the predominance of westerly cur-rents in this region during this period of time. While the exact source of this resuspension event is unknown, it is probable that the resuspension event was due to either a mechanical disturbance to sediments or the release of sediments from a gravity wave due to a land-slide on the sloping boundary of the surrounding basin. This resuspension would have had to occur east of both traps so that the westerly currents would advect these resuspended sediments to both traps.
3.6.3 Sediment Component Modelling
The overall annual residence times modelled in the present study were 53±10 d and 118
± 30 d, for NG2 and NG3, respectively (Table 3.5). These values are lower than those previously published for Lake Zurich and Lake Geneva. Previous studies have determined the residence time of particles in the water column using individual radionuclides and ra-dionuclide ratios. However, these studies acknowledge limitations to their proposed models since lateral sources were not segregated (Dominik et al., 1993, 1989; Wieland et al., 1991).
Annual steady-state7Be residence times have been estimated as 176±62 d for Lake Zurich (130 m depth (Wieland et al., 1991)), and 329 ±23 d for Lake Geneva (308 m depth (Do-minik et al., 1993)). In the study by Do(Do-minik et al. (1993), the residence time of particles in the water column were also calculated using the7Be/210Pbxs ratio. A mean annual value of 380±34 d was attained; however, lateral and vertical sources were again not segregated. By not removing the lateral component, the flux of210Pbxsis elevated, resulting in a decreased
7Be/210Pbxs flux ratio and a longer residence time as calculated from equation 3.3.
τC values calculated in this study ranged between 6 ±6 d and 134 ±1 d at NG2 and NG3. These compare well with τC values estimated for Lake Zurich by Wieland et al.
(1991). Wieland et al. (1991) calculated a τC in the range of 60 d to 185 d (mean = 120 d) for7Be and 3 d to 31 d (mean = 17 d) for 210Pb. They determined that the coagulation and aggregation step was the controlling factor of the overall annual residence time in Lake Zurich. This is in partial agreement with the results of the current study where the overall
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3.6 Discussion residence times were also strongly influenced by τB at NG2 and NG3 on an annual scale inferring the strong influence of the bottom boundary layer on slowing particle settling.
τP values, having been calculated with the vertical component alone, enabled a more re-fined estimate of the settling velocity in the water column of Lake Geneva. Seasonal settling velocities, as calculated from the depth divided bytP (equation 3.5), varied between 3 and 41 m d−1, for both sites with the overall annual settling velocities of 10 and 8 m d−1for NG2 and NG3, respectively. Previous settling velocity estimates for Lake Geneva were around 10 m d−1for the centre of the lake (Dominik et al., 1989) and were estimated from sediment traps in the upper water column where they were not influenced by lateral advections. These published results for Lake Geneva are in agreement with these settling velocities, and ergo the τP values, presented here. Settling velocities on a seasonal basis were fairly consis-tent, with the exception of the first two seasonal periods at NG2, November 2009 to April 2010 and April 2010 to November 2010. Here, the settling velocities were faster (41 and 25 m d−1, respectively), resulting in smallerτP values. These increased settling velocities are likely a result of lowered inputs or elevated outputs to STT and STB respectively, used to estimateτP for box 2 of the model. Either of these instances could occur through lateral advections of7Be to the bottom sediment traps or a decreased flux of7Be to the top traps and would imply that the assumption that7Be is only transported in the vertical component may not be necessarily true under all circumstances.
τCandτBvalues were found to be the dominant factors in controlling the overall annual residence time at both sites. τB was less influential to the overall residence time on a sea-sonal time scale. The fact that theτBvalues were one of the controlling factors in the overall residence time implies that there is greater opportunity for transport, contaminant adsorp-tion/desorption, and chemical reaction within the BBL. The omission of the NG3 τB value for November 2010 to April 2011 demonstrates the sensitivity of the model to sediment surface erosion and focusing events. During this period there was most likely a removal of
7Be from the sediment surface at NG3 which in turn resulted in a negative R2 and negative R1 to R2 ratio in equation 3.3. In actuality, it would not be possible to have a “negative
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
location. This being said, the model is sensitive enough to detect seasonal fluctuations on the scale of∼6-month time-intervals. The sedimentation model assumed that the BBL ex-tends down from STB to the sediment surface. In actual fact, the BBL in Lake Geneva has been estimated to extend up to∼11 m above the sediment surface (Bouffard et al., 2013). In such, the bottom sediment trap could in fact be influenced by the BBL which in turn could be the reasoning for the additional lateral advections of7Be which were shown to influence the estimation ofτPat NG2.
The overall results of the sediment component model showed agreement with previously published values, while enabling a differentiation between lateral and vertical sources and demonstration of its sensitivity to changes at the sediment surface. The model provides reasonable ranges of estimated residence times for particles in the water column and pro-vides a manner to quantify lateral advections. With the overall lateral component now being quantifiable, the effect of resuspension by localized hydrodynamic conditions needs to be investigated to understand its influence on contaminant resuspension and advection into and from Vidy Bay and to help elucidate the source(s) of these advections to the sediment trap samples.
3.7 Conclusion
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.
78
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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