Thesis
Reference
The fate of sediment-bound contaminants: a case study of Vidy Bay (Lake Geneva, Switzerland)
GRAHAM, Neil
Abstract
A multidisciplinary approach was used to investigate the transport and fate of sediment-bound contaminants in a polluted embayment of a mid-sized lake. This approach included the characterization of the colloidal suspension at the sediment-water interface, differentiation between vertical and lateral suspended sediment sources for the construction of a sedimentation component model, and the investigation of select hydrodynamic parameters.
The nepheloid layer colloidal suspension was found to be relatively unstable, leading to a decrease in long-range transport capacity. Sedimentation rates and lateral advections were noted to increase with proximity to the sediment surface and also with proximity to shore.
While, the hydrodynamic conditions in the nepheloid layer evidenced a decrease in velocity within the bay and the presence and persistence of a secondary gyre. Overall, it was found that contaminants located in Vidy Bay are likely to remain entrapped within the bay, leading to a continued accumulation of contaminants.
GRAHAM, Neil. The fate of sediment-bound contaminants: a case study of Vidy Bay (Lake Geneva, Switzerland). Thèse de doctorat : Univ. Genève, 2015, no. Sc. 4760
URN : urn:nbn:ch:unige-479852
DOI : 10.13097/archive-ouverte/unige:47985
Available at:
http://archive-ouverte.unige.ch/unige:47985
Disclaimer: layout of this document may differ from the published version.
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UNIVERSITÉ DE GENÈVE
Section des sciences de la Terre et de l’environnement Institut F.-A. Forel
FACULTÉ DES SCIENCES Docteur Jean-Luc Loizeau
The Fate of Sediment-bound
Contaminants: A Case Study of Vidy Bay (Lake Geneva, Switzerland)
Thèse
présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention sciences de l’environnement
par
Neil D. Graham
(Canada)
What odds my child.
Mary M. Sullivan (1919 – 2003)
A CKNOWLEDGEMENTS
I owe many thanks to family, colleagues and friends for their help and support during the long journey. I would like to extend my gratitude to my supervisor, Jean-Luc Loizeau, for his support, his time, and his guidance during this project. It was an honour to be your first Ph.D. student.
During my time at Forel I had help from many people, all of which I would not be able to thank individually here; however, a few cannot go without mention. Thanks to Alexandra for helping me through the yards of administrative red-tape and for a cheerful smile when things were going slowly. Katia, thanks to you for keeping the IT gears well oiled and for all the chats we had while sharing an office. And of course Philippe (aka Capitan, Mr.
Hollywood), for all those wonderful Bise-filled days on the lake. Everyone loves a boat ride in the middle of the summer but few are so willing to venture out for their samples when the cold sets-in. Thanks for all the help in every moment of sampling and with all the help in analyzing.
Thank you to the Swiss National Science Foundation and the University of Geneva for their financial support during my research.
Finally, for the unwavering support of my family, I am forever indebted. Mom and dad, thanks for all the support and for the foundation I had to build my life upon. Leena, kittos for all you did and continue to do. Rakastan sua mun muru. And last but not least, Mr.
Oliver Bradley Graham...thanks for a new and amazing chapter in life.
R EMERCIMENTS
Je tiens à remercier ma famille, mes collègues et mes amis pour leur aide et leur soutien pendant ce long parcours et sans qui je ne serais jamais parvenu à bout de cette thèse.
J’aimerais remercier tout particulièrement mon superviseur, Jean-Luc Loizeau, pour son soutien, son temps et ses conseils durant ce projet. Ce fut un honneur d’être votre premier étudiant en doctorat.
Pendant mon parcours à l’institut Forel, j’ai reçu de l’aide de la part de nombreuses personnes, et je n’arriverai malheureusement pas à tous les citer ici. Cependant quelques personnes en particulier se doivent d’être mentionnés. En premier lieu je tiens à remercier Alexandra sans qui je serai encore emmêlé dans des kilomètres de paperasse, merci aussi pour son sourire plein de bonne humeur lorsque la situation semblait difficile. Un grand merci également à Katia pour avoir gardé le système informatique en état de marche et pour toutes nos conversations lorsque nous partagions le même bureau. Et sans oublier Philippe (alias Capitan, Mr. Hollywood), pour toutes ces magnifiques journées passées sur le lac, lorsque la bise faisait fureur! Tout le monde aime faire des tours en bateau en plein été, mais peu de gens veulent s’aventurer à récupérer des échantillons un fois que le froid hivernal s’est installé. Merci pour toute l’aide apporté au moment de l’échantillonnage ainsi que lors des analyses.
J’aimerais également remercier la Fondation Nationale Scientifique Suisse et l’Université de Genève pour leur soutient financier pendant mon doctorat.
Finalement je tiens à remercier ma famille pour leur aide, leurs encouragements et leur
R ESUMÉ
L’objectif de cette recherche est de quantifier les processus de transfert des particules sédi- mentaires des zones côtières vers les zones profondes des lacs (focalisation des sédiments ou « sediment focusing ») et leur implication sur la dispersion et le devenir des contami- nants associés à ces particules, en prenant le site de la Baie de Vidy (Léman, Suisse) comme cas d’étude. La compréhension du devenir des contaminants liés aux particules est d’une grande importance pour l’évaluation et l’atténuation des impacts des polluants sur la santé, l’économie, et l’environnement en plus de la sécurité pour un approvisionnement durable en eau douce. Des approches complémentaires ont été utilisées pour étudier la dispersion des contaminants et les processus de sédimentation dans la colonne d’eau, l’influence des conditions hydrodynamiques sur le transport des particules sédimentaires, et la stabilité et la composition des particules et des colloïdes. Cette approche pluridisciplinaire mène à une compréhension plus globale du transport des contaminants en milieu lacustre.
Les sédiments de la Baie de Vidy (Le Léman, Suisse) sont fortement contaminés par l’effluent de la station d’épuration des eaux usées de la ville de Lausanne. L’étude des car- actéristiques de sédimentation et des conditions hydrodynamiques dans la baie a conduit à une meilleure compréhension du devenir des contaminants liés aux particules: accumula- tion dans la baie ou dispersion dans le bassin principal. Les recherches ont particulièrement porté sur la remise en suspension des sédiments et sur leur transport. Les taux de sédi- mentation et la composition des sédiments ont été déterminés, ainsi que l’hydrodynamisme local et les caractéristiques physico-chimiques des colloïdes et des particules en suspension
paramètres, notamment la charge de surface déterminée par mobilité électrophorétique; les distributions de taille et de concentrations en nombre par comptage de particules (« single particle counters »); la force ionique de la suspension colloïdale par chromatographie ion- ique, et leur structure et composition par microscopie électronique à balayage. Ces mesures montrent que les colloïdes et les petites particules forment une suspension instable et ne sont donc pas susceptibles de jouer un rôle important dans le transport à longue distance des contaminants. De plus, les caractéristiques mesurées ne varient pas significativement entre les différents sites d’échantillonnage, le type de sédiments, la présence d’un tapis bac- térien ou la présence de bioturbation ou d’ébullition de gaz dus à la décomposition anaéro- bie. La répartition et la composition des colloïdes en suspension et des particules étaient assez uniformes dans la baie. La distribution en masse et la concentration en nombre de colloïdes et de petites particules (A = 1.73×1012, β = 3.25) sont plus élevées que celles mesurées précédemment dans l’épilimnion (A = 1.9×1012, β = 3.95) et l’hypolimnion (A = 6.9×1012, β = 4.01), indiquant la présence d’une couche néphéloïde. Cette couche néphéloïde diminue la stabilité des colloïdes en augmentant la probabilité de collisions entre les particules colloïdales. Ainsi la présence de cette couche favorise leur élimination de la colonne d’eau par sédimentation. Les colloïdes à l’interface eau-sédiments présentent une charge globale de surface légèrement négative (−16.2±0.3 mV), ce qui implique une sta- bilité limite. Les observations au microscope électronique à balayage ont mis en évidence d’importantes quantités de biopolymères rigides. L’effet de ces biopolymères rigides est la déstabilisation et l’agrégation des colloïdes à travers des mécanismes de formation de pontage et de gel.
La remise en suspension des sédiments est un processus lacustre important affectant le transport des particules et des contaminants associés, le cycle des nutriments et les taux de déposition des sédiments. En comprenant les voies de sédimentation et les conditions hy- drodynamiques, les zones d’érosion des sédiments, de dispersion et d’accumulation peuvent être identifiés. Deux systèmes de pièges à particules, équipés de courantomètres RCM9 ont été installés selon un profil longitudinal s’étendant de l’exutoire de la station d’épuration des eaux usées vers le bassin lacustre principal. Les pièges à particules se composaient de deux niveaux, l’un à la profondeur de 75 m et un autre à 5 m au-dessus de la surface des sédi- ments, afin de distinguer les sources de particules verticales et latérales. Cette différencia- tion a été effectuée par l’analyse des radionucléides naturels (7Be, 210Pb et214Pb) mesurés par spectrométrie gamma et par le développement d’un modèle de sédimentation incluant une composante latérale. Les données ont été complétées par la mesure de la granulométrie, des teneurs et des flux de matière organique et de carbonates. Les taux d’accumulation et
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les distributions de taille des particules ont montré des tendances saisonnières typiques des systèmes lacustres découlant de variations dans la production interne. L’accumulation de sédiments sur le site d’étude est plus élevée que celle mesurée au centre du lac. Cette aug- mentation est attribuable à la proximité de la zone littorale. La focalisation des sédiments est observée dans la baie de Vidy avec des flux de sédiments et des advections latérales croissantes, d’une part avec la proximité de la côte (48 % et 37 % d’augmentation entre les pièges supérieurs et inférieurs des deux sites, respectivement) et d’autre part avec la pro- fondeur (38 % et 47 % d’augmentation entre les pièges supérieurs et inférieurs dans la baie et dans le bassin principal, respectivement).
Le modèle de sédimentation donne des estimations du temps de résidence total des par- ticules dans la colonne d’eau, et des temps de résidence liés aux différents processus jouant un rôle dans les transferts vertical et latéral (adsorption/coalgulatio, sédimentation dans la colonne d’eau et sédimentation dans la couche néphéloïde ou chouche benthique limite).
Les temps de résidence totaux dans et autour de la Baie de Vidy étaient plus petits que ceux publiés précédemment sur des systèmes similaires. Il a été montré qu’en excluant la composante latérale, une estimation plus précise de la dynamique des particules dans la colonne d’eau est obtenue. Le modèle a montré un temps de résidence des particules lié à la sédimentation dans la couche limite de même ordre que ceux liés à l’adsorption et la coagulation à la surface du lac et à la sédimentation dans la colonne d’eau. Le modèle a également montré une sensibilité aux fluctuations des flux à la surface des sédiments sur des courts laps de temps, et qu’une estimation annuelle donne une meilleure représentation de la dynamique des particules. Le temps de résidence dans la couche limite inférieure (19 et 51 jours, à l’intérieur et à l’extérieur de la baie, respectivement) implique que les particules se trouvent dans cette couche pendant des durées prolongées, ce qui favoriserait le transport, l’adsorption/désorption de contaminants, les réactions chimiques, et de l’agrégation, tous processus qui influencent le devenir des contaminants liés aux particules.
La circulation dans le bassin principal et la topographie de la baie de Vidy influencent les conditions hydrodynamiques dans la baie. Grâce aux mesures des courantomètres RCM9,
de remise en suspension existe à des profondeurs inférieures à 60 m. Il en découle que les advections latérales calculées à partir du modèle de sédimentation ne sont pas dues à des événements locaux de remise en suspension. Les vitesses de courant diminuent du bassin principal (moyenne de 3.3 cm s−1) vers la baie (moyenne de 1.6 cm s−1). Cette diminution de vitesse contribue à la sédimentation des particules et participe aux advections latérales observées dans les pièges à particules. La source probable des advections latérales (fo- calisation des sédiments) dans la baie est les particules en suspension provenant du bassin principal, hypothèse appuyée par la corrélation entre les mesures de turbidité sur les deux sites (r = 0.36, p<0.001, n = 8070) et l’orientation préférentielles des vecteurs de courant.
A cette source s’ajoute la possibilité d’apports supplémentaires de particules en suspension provenant de la station d’épuration des eaux usées et la remise en suspension de particules de la zone littorale.
Les résultats globaux de ce projet de recherche montrent des conditions favorables à la sédimentation des particules et la probabilité de rétention des particules dans la baie. La présence d’une couche néphéloïde et les caractéristiques des colloïdes et des petites par- ticules dans cette couche, suggèrent une faible possibilité de transport à longue distance des contaminants liés aux colloïdes et particules de la baie. Les taux d’accumulation de sédiments et les advections latérales diminuent en s’éloignant de la côte et de la baie, mais également avec la proximité de la surface des sédiments. Les mesures hydrodynamiques ont montré un temps de rétention élevé au sein de la baie couplé avec une diminution de la vitesse du courant, menant à des conditions favorables pour une décantation des partic- ules en suspension dans la couche limite inférieure. Ces résultats, en combinaison avec l’absence de remise en suspension dans les parties profondes de la baie suggèrent que les contaminants liés aux particules restent probablement dans la baie où ils sont dilués par les apports latéraux provenant du bassin principal, plutôt que d’être transportés dans les régions plus profondes du bassin principal. Ainsi, la baie de Vidy semble être une zone de dépo- sition pour une grande partie de ses contaminants liés aux particules. Cette constatation souligne la préoccupation de la réintroduction des contaminants dans la colonne d’eau par l’intermédiaire des perturbations naturelles ou anthropiques. Elle met également en évi- dence la nécessité d’un plan d’assainissement et la mise en œuvre de mesures proactives pour atténuer les effets négatifs de rejets de contaminants de la baie de Vidy.
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A BSTRACT
The overall aim of this research was to investigate sediment focusing and its implications on the dispersal and fate of particle-bound contaminants in moving from the coastal zone to the deeper main basin of a mid-sized lake, using Vidy Bay (Lake Geneva, Switzerland) as a case study. Determining the fate of particle-bound contaminants is of great importance to miti- gating environmental, economical, and health impacts linked to contamination, in addition to the insurance of sustainable freshwater supplies. Complimentary research approaches were used to study the dispersal of contaminants and investigate sedimentation processes in the water column, the influence of bottom boundary layer hydrodynamic conditions on sediment transport, and colloid and particle composition and stability in the bottom bound- ary layer. This multidisciplinary approach lent to a more global understanding of long-term contaminant transport and water quality.
Vidy Bay is highly contaminated through its reception of effluents from the wastewater treatment plant of the City of Lausanne. Understanding colloid and particle characteris- tics, sedimentation pathways, and hydrodynamic conditions in the bay provides an integra- tive understanding of particle-bound contaminant behaviour; whether it is an accumulation within the bay or dispersal into the main basin. To investigate sediment focusing in the bay, a series of studies were proposed. These studies tackle the issue of sediment resus- pension and transport on both spatial and temporal scales. Sedimentation rates, pathways, and composition were studied, along with the monitoring of local hydrodynamic conditions, and the characterization of suspended colloidal and particulate matter at the sediment-water
counting, ionic strength of the colloid suspension by ion chromatography, and structure and composition by scanning electron microscopy. Colloids and small particles were found to be unstable and, in such, are not likely to play a significant role in the long-range transport of contaminants. The measured characteristics did not vary significantly between sampling sites, sediment type, the presence of a bacterial mat, or presence of bioturbation (ebullition of anaerobic decomposition gases). In such, the distribution and composition of suspended colloids and particulate matter was fairly consistent over the area of the bay. The mass and number concentration distribution of colloids and small particles (A= 1.73×1012,β = 3.25) were found to be greater than previously measured in the epilimnion (A= 1.9×1013, β = 3.95) and hypolimnion (A= 6.9×1012,β = 4.01) of the lake indicating the presence of a nepheloid layer which decreases colloid stability through increased ionic strength of the colloid suspension and increased probability of collisions between colloids. The presence of a nepheloid layer predisposes colloids and small particles to destabilize and settle out of the water column. Colloids had a slight negative overall surface charge (−16.2±0.3 mV), im- plying a borderline stability. Scanning electron micrographs evidenced significant amounts of rigid biopolymers. The role of these rigid biopolymers is the destabilization and aggrega- tion colloids through a bridging mechanism and gel formation, as were shown in scanning electron micrographs. Overall, colloids and small particles in the nepheloid layer of Vidy Bay are unstable and are not likely to play a significant role in the long-range transport of contaminants.
Sediment resuspension is an important lacustrine process affecting sediment deposition rates, contaminant transport, and nutrient cycling. By understanding the sedimentation path- ways and hydrodynamic conditions of a lake, zones of sediment erosion (dispersion) and fo- cusing (accumulation) can be identified. Two sediment trap systems, equipped with RCM9 current meters were installed in a transect array extending from the outlet of the wastewater treatment plant into the main basin. The sediment traps consisted of two levels, one at 75 m depth and another at 5 m above the sediment surface, to differentiate between vertical and lateral sediment sources. This differentiation was enabled through the analysis of natural radionuclides (7Be, 210Pb, and214Pb) via gamma spectrometry and the development of a vertical sedimentation model with a lateral component. Results were supplemented with organic matter and calcium carbonate content results via loss on ignition, particle sizing, and distribution analysis.
Sediment accumulation rates and sediment particle size distributions showed typical sea- sonal lacustrine trends stemming from changes in internal production. Sediment accumu- lation was increased as compared to previous measurements in the centre of Lake Geneva,
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with this increase being attributed to proximity to the littoral zone. Sediment focusing was found to occur in Vidy Bay with sediment accumulation rates and lateral advections increas- ing with proximity to the shoreline (48 % and 37 % increases between the top and bottom traps, respectively) and with proximity to the sediment surface (38 % and 47 % increases between the top and bottom traps within the bay and in the main basin, respectively).
The sediment component model provided refined estimates of overall and process-related residence times through the segregation of the lateral component. Overall residence times in and around Vidy Bay were smaller than previously published for similar systems with the difference stemming from the removal of laterally advected sediments. It was shown that by removing the lateral component a more accurate estimation of particle dynamics and path- ways in the water column can be obtained. The sediment component model showed that the partical-residence time in the bottom boundary layer was of the same order of magnitude as those of settling through the water column and aggregation and coagulation. This infers that settling particles can spend a significant amount of time in the bottom boundary layer where conditions are favourable for particle aggregation and settling. The model showed to be sen- sitive to fluctuations at the sediment surface over shorter time frames inferring that overall annual estimates would be a better representation of particle dynamics. The significance of the process-related residence time in the bottom boundary layer (19 d to 51 d, inside and out- side the bay, respectively) implies that particles reside in this layer for extended times and allows for greater opportunity of transport, contaminant adsorption/desorption, chemical reaction, and aggregation, all of which influence the fate of particle-bound contaminants.
Circulation in the main basin and the topography of Vidy Bay influenced the hydrody- namic conditions in the bay. Through the use of RCM9 current meters the presence and persistence of a secondary gyre was shown to extend down to the bottom boundary layer of the bay. Previously, this gyre was only believed to exist in the upper layers of the wa- ter column. The gyre in Vidy Bay was found to increase particle retention time within the bay to around 10 d, longer than previously calculated. Application of the current velocity data and the overall mean sediment grain size (µm) in a sediment resuspension model in-
(sediment focusing) in the bay was suspended sediments from the main basin, as supported by the correlation between turbidity measurements at both sites (r = 0.36, p<0.001, n = 8070) and current vector displacement trends. There remains the possibility of additional inputs of suspended particles from the wastewater treatment plant and resuspension from the shallower littoral zone.
The overall findings of this research project demonstrate the favourable conditions for particle settling and the likelihood of particle retention within the bay. The presence of a nepheloid layer, and the characteristics of colloids and small particles in this layer, suggest the unlikelihood for the long-range transport of colloid- and particle-bound contaminants from the bay. Sediment accumulation rates, and lateral advections increased in moving from the main basin into the bay and also with proximity to the sediment surface indicating sediment focusing within the bay. Hydrodynamic measurements showed an increased reten- tion time within the bay coupled with a decrease in current velocity lending to favourable conditions for suspended sediment settling from the bottom boundary layer. These find- ings, in combination with the demonstrated lack of resuspension in deeper parts of the bay suggest that particle-bound contaminants likely remain in the bay where they are diluted by lateral advections from the main basin, as opposed to being transported to deeper parts of the main basin. In such, Vidy Bay appears to be the final resting place for much of its particle-bound contaminants. This finding highlights the concern of the reintroduction of contaminants back into the water column via natural or anthropogenic perturbations. This finding also highlights the need for a remediation plan and the implementation of proactive measures to mitigate the adverse effects of contaminant release from Vidy Bay.
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C ONTENTS
Resumé ix
Abstract xiii
Contents xvii
List of Figures xxi
List of Tables xxiii
Context of the Study xxv
1 Setting the Stage 1
1.1 Particle Sources and Sedimentation . . . 2
1.1.1 Sources: Where Do Sediments Come From? . . . 2
1.1.2 Sedimentation: Colloids and Aggregation . . . 4
1.1.3 Sediment Advection and Influencing Factors . . . 5
1.1.4 Resuspension and Sediment Focussing . . . 9
1.2 Environmental Settings . . . 13
1.2.1 Lake Geneva . . . 13
1.2.2 Vidy Bay . . . 13
1.3 About This Thesis . . . 16
Contents
2.3.2 Single Particle Counter . . . 25
2.3.3 Treatment of SPC data . . . 27
2.3.4 Zeta Potential . . . 28
2.3.5 Ion Chromatography . . . 28
2.3.6 Scanning Electron Microscopy X-ray Energy Dispersive Spectrometry 29 2.4 Results . . . 29
2.4.1 Colloid Concentrations and Surface Charge . . . 29
2.4.2 Colloid Suspension Ionic Strength . . . 31
2.4.3 Colloid and Aggregate Composition . . . 33
2.5 Discussion . . . 33
2.5.1 Colloid and Small Particle Concentrations . . . 33
2.5.2 Surface Charge Character . . . 40
2.5.3 Ionic Strength and Colloid Stability . . . 41
2.5.4 Colloid and Aggregate Composition . . . 42
2.6 Conclusions . . . 43
2.7 Acknowledgements . . . 43
References . . . 44
3 Discrimination Between Vertical and Lateral Sedimentation Pathways in a Con- taminated Bay 49 3.1 Abstract . . . 51
3.2 Introduction . . . 51
3.3 Sampling Sites, Materials and Methods . . . 54
3.3.1 Environmental Setting and Sampling Strategy . . . 54
3.3.2 Sample Collection . . . 57
3.4 Analytical Methods . . . 58
3.4.1 Particle Size Distribution . . . 58
3.4.2 Gamma Spectrometry . . . 59
3.4.3 Loss on Ignition . . . 59
3.4.4 Sediment Component Model . . . 60
3.5 Results . . . 63
3.5.1 Particle Size Distribution . . . 63
3.5.2 Sediment Accumulation and Composition . . . 64
3.5.3 Radionuclide Ratio Fluxes . . . 68
3.5.4 Sediment Component Model . . . 72 xviii
Contents
3.6 Discussion . . . 73
3.6.1 Sedimentation and Sediment Composition . . . 73
3.6.2 Spatial and Temporal Changes in Radionuclide Fluxes . . . 75
3.6.3 Sediment Component Modelling . . . 76
3.7 Conclusion . . . 78
3.8 Acknowledgements . . . 79
References . . . 79
4 Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay 83 4.1 Abstract . . . 85
4.2 Introduction . . . 85
4.3 Environmental Setting, Materials, and Methods . . . 87
4.3.1 Wind, Currents, and Sampling Sites . . . 87
4.3.2 Sediment Surface Sampling and Mean Particle Size . . . 89
4.3.3 RCM9 Current Meters and Data Treatment . . . 89
4.3.4 Sediment Resuspension Model . . . 91
4.4 Results . . . 93
4.4.1 Mean Particle Diameter . . . 93
4.4.2 RCM9 Current Meters . . . 93
4.4.3 Sediment Resuspension and Residence Times . . . 100
4.5 Discussion . . . 101
4.6 Conclusions . . . 103
4.7 Acknowledgements . . . 104
References . . . 104
5 Discussion 109
6 Conclusions 115
Contents
B.3 Sample Geometry Correction . . . 130
Appendix C Tables of Raw Data 133
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L IST OF F IGURES
1.1 Phytoplankton biomass variation in Lake Geneva . . . 3 1.2 Bernoulli’s Principle . . . 6 1.3 Entrainment Velocity Curves . . . 7 1.4 Internal seiche dynamics about the thermocline . . . 10 1.5 The situation and topography of Vidy Bay . . . 15 2.1 Colloid sampling sites within Vidy Bay . . . 26 2.2 Rigid biopolymer strand and gel with embedded iron oxides . . . 34 2.3 A section of an X-EDS spectrum showing the presence of Fe from iron
hydroxide spheres . . . 35 2.4 A rigid biopolymer in pearl necklace formation . . . 35 2.5 An rigid biopolymer mesh and gel adsorbed to a quartz inorganic colloid . . 36 2.6 Rigid biopolymer coverage and bridging between an inorganic colloid and
a diatom frustule . . . 37 2.7 A rigid biopolymer gel adsorbed to diatom shells . . . 38 3.1 Sediment trap sampling sites . . . 56 3.2 Sediment component model diagram . . . 62 3.3 Sediment particle size distributions . . . 65 3.4 Sedimentological fluxes at NG2 . . . 69
List of Figures
4.5 Progressive vector plot for NG2 . . . 99 4.6 Progressive vector plot for NG3 . . . 100 4.7 Sediment resuspension model for sites NG2 and NG3 . . . 101 5.1 An aerial view of Vidy Bay showing the current state of knowledge of its
contaminated sediments and contaminant transport . . . 112 5.2 A cross-sectional view of Vidy Bay showing the current state of knowledge
of its contaminated sediments and contaminant transport . . . 113 A.1 238U Decay Chain . . . 126 B.1 Probability distribution for Type I and Type II errors . . . 128
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L IST OF T ABLES
1.1 Characteristics of Lake Geneva . . . 13 2.1 MIR submersible sample description . . . 25 2.2 Results from single particle counting and the mean zeta potential analyses . 30 2.3 Ion chromatographic results . . . 32 3.1 Sample and residence times with the sedimentation component model . . . 60 3.2 Overall mean particle size diameters for NG2 and NG3 . . . 64 3.3 Sediment accumulation and composition fluxes . . . 67 3.4 Radionuclide fluxes and ratios at NG2 and NG3 . . . 71 3.5 Percent lateral advections and sediment component model results . . . 74 4.1 Intrasite RCM9 Pearson correlation coefficients . . . 94 4.2 RCM9 seasonal decomposition Pearson correlation coefficients . . . 94 4.3 Intersite correlation coefficients between NG2 and NG3 . . . 96 B.1 Detector blank counts used to calculate the LD . . . 129 B.2 Relevant efficiencies and error of the efficiencies for the detectors used to
measure the sediment samples . . . 130 B.3 Sample geometry fitting coefficients . . . 130 C.1 Atmospheric trap sampling data . . . 134
C ONTEXT OF THE S TUDY
The present research constitutes part of a larger interdisciplinary research project entitled
“Léman 21 - Scientific concepts for the sustainable management of mid-sized lakes in the 21st century”. Léman 21 is a ProDoc class of project funded by the the Swiss National Science Foundation (SNF) and consists of four research modules with objectives seeded in different facets of environmental science. The four modules provide the tools and under- standing to quantify the terrestrial to deeper basin transfer processes of a mid-sized lake, as well as shoreline transfer of water, sediments and aqueous phase constituents. They ad- dress key issues for the question of long-term contaminant transport and water quality in mid-sized lakes, using Lake Geneva as a case study.
The modules of Léman 21 are entitled as follows;
Module1 Water dynamics and contaminant transfers from the shallow to the deep waters of a mid-sized lake
Module2 The origin of micropollutants in the catchment and their transport to a mid-sized lake
Module3 Micropollutant degradation in a mid-sized lake: Coupling degradation and dilu- tion processes with lake dynamics
Module4 Microbial resistance, ecotoxicological impact, and risk assessment of micropol- lutants in a mid-sized lake
C HAPTER 1
S ETTING THE S TAGE
Sediment resuspension affects nutrient cycling, sediment deposition, and contaminant trans- port, and has long been recognized as an important internal process in large lakes (Bloesch, 1994; Wieland et al., 2001). Resuspension (or entrainment) occurs in more energetic parts of an aquatic system, such as in the littoral zones of a lake with the suspended particles settling or focussing in less energetic, more quiescent, parts (Blais et al., 1995; Håkanson, 1977). Resuspension may have substantial implications on water quality by increasing the suspended particle load in the water column, attenuating light transmission through the wa- ter column, releasing contaminants back to the water column, and also introducing anoxic interstitial waters into the system.
The accumulation of resuspended sediments in less energetic parts of the system is re- ferred to as sediment focussing. Sediment focussing has serious implications for the envi- ronment and economy. Hydrophobic contaminants introduced into an aquatic system are known to readily and strongly adsorb onto suspended particulate matter. These particle- bound contaminants are then transported with their host through the water column. So, by investigating and understanding the sediment pathways and dynamics of a system, we in-turn understand the transport dynamics and potential fate of the adsorbed hydrophobic contaminants. Focussing (or accumulation) of this contaminant-laden particulate matter
Setting the Stage (Forsythe et al., 2013).
Even without the involvement of human perturbation, natural processes allow for the entrainment and advection of particle-bound contaminants. In an aquatic system, sediments can be entrained through several means; currents and wave action being the most predom- inant. A review of the driving forces and basic processes of sediment resuspension and transport in lakes is discussed in Håkanson et al. (1983). The following sections provide an overview of some of these processes found in large- to mid-sized lakes and also some of the methods used to identify resuspension. In such, this overview provides the basis for un- derstanding particle-bound contaminant transport which helps elucidate to the fate of these particle-bound contaminants.
1.1 Particle Sources and Sedimentation
The source and composition of sediments are important to understanding their pathways and focussing potential. They also help in understanding the fate of contaminants that are adsorbed onto these particles.
1.1.1 Sources: Where Do Sediments Come From?
Sources of particles in an aquatic system can be divided into two general categories; al- lochthonous (stemming from outside), and autochthonous (produced within). Primary bio- logical production and chemical precipitation reactions (i.e. phytoplankton and zooplankton production, calcite precipitation) are processes that introduce autochthonous particles to an aquatic system. Allochthonous particles are typically introduced by wind transport, wet and dry atmospheric deposition, terrestrial run-off, and erosion of surrounding soils and bedrock (Håkanson et al., 1983). Both autochthonous and allochthonous particles can be of organic or inorganic forms.
Autochthonous inputs have a seasonal tendency in warm monomictic lakes whereby they increase during the spring and decrease again in late fall. This seasonal trend is linked to the planktonic cycle where phyto- and zooplankton increase during this period of in- creased solar radiation (Håkanson et al., 1983; Wetzel, 2001). The planktonic cycle follows a fairly classical predator/prey relationship. The onset of increased solar radiation induces increased temperatures in the littoral zone, which spreads over the lake surface and estab- lishes thermal stratification (Wetzel, 2001). This increase in temperature and amount of solar radiation initiates increased planktonic growth. As the population of phytoplankton
2
1.1 Particle Sources and Sedimentation thrives, the population of zooplankton increases with a slight time lag. The increases in phyto/zooplankton populations can be marked by a significant increase in light attenuation through the epilimnion. A clearing of the epilimnion in Lake Geneva is typically noted in mid-to-late June, after which, plankton bloom again (CIPEL, 2012). This secondary plank- tonic bloom lasts until late fall, when decreasing solar radiation is unable to sustain large populations of phytoplankton. This trend is exemplified in Fig. 1.1.
During the process of photosynthesis, algae (a type of phytoplankton) in the epilimnion uptake dissolved CO2. A decrease in dissolved CO2 ensues and shifts equilibria between dissolved CO2, CO2−3 , and dissolved Ca2+, which results in the precipitation of calcite (CaCO3) (Håkanson et al., 1983). This calcite can dissolve in the deeper parts of the water column when the level of dissolved CO2is increased via cellular respiration and the degra- dation of organic matter (Håkanson et al., 1983). Dissolution is further exacerbated in lakes containing an oxycline whereby bottom waters are void, or nearly void, of dissolved oxygen and elevated levels of dissolved CO2.
����
Figure 1.1 Variations in the phytoplankton biomass by algal class in the main basin of Lake Geneva for 2011 (Rimet, 2012)
Particulate silica is also noted to increase in the water column during this time of the
Setting the Stage
1998; Subramanian et al., 2010; Wilkinson et al., 1999).
Natural organic matter can also stem from allochthonous sources such as run-off from the surrounding watershed and agricultural areas (pedogenic organic matter (Daouk et al., 2013)). The organic forms typically consist of refractory organic matter, and humic and fulvic acids. A further description of the types of colloidal and particulate forms of organic matter can be found in section 1.1.2.
Inorganic sources of allochthonous particles usually reflect the geology of the surround- ing watershed and stem from the erosion of the soils and surrounding bedrock. Their in- troduction into a lake can be from aeolian (wind) transport or, for the most part, from the inflow of tributaries in the watershed.
1.1.2 Sedimentation: Colloids and Aggregation
Colloids bridge the dissolved and solid phases. In environmental sciences they are defined as molecules or polymolecular “particles” dispersed in a medium, which have at least one dimension between 1 nm to 1 µm, or that discontinuities are found at distances of that order (Everett et al., 1972). Colloids are of interest for several reasons; primarily, they are small enough to remain suspended in a given medium, allowing for their transport. Secondarily, they also have a large specific surface area, over which charge is dispersed, allowing for the adsorption of various trace metals (Benoit et al., 1994) and other pollutants (Landrum et al., 1984; Rogers, 1993).
The number distribution of colloids can be described using a power law. It tells us that the size class of a colloid is inversely related to the concentration of that colloid size class.
In other words, over a given ’population’ of suspended particles in an aquatic system, the frequency of colloids and particles decreases logarithmically with increasing size.
Colloids vary in composition depending on their source. Inorganic colloids typically re- flect the mineralogy of the surrounding watershed, and can include forms of aquagenic com- pounds such as calcite, iron hydroxides, and silica in the form of diatom frustule fragments.
Organic colloids also take many forms, which includes fulvic- and humic-like substances, biopolymer chains, proteins, and carbohydrates (Buffle et al., 1998).
Each type of colloid has its own composition and surface charge distribution. Colloid stability is described by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory with surface charge being a key parameter for predicting colloid stability (Elimelech et al., 2000).
Typically, inorganic colloids are known to have a fulvic- or humic-like colloid coating on their surface (Lead et al., 2006). This coating results in an overall negative charge on the colloid surface and increases both electric and steric repulsion, thereby leading to increased
4
1.1 Particle Sources and Sedimentation stability. Increasing the ionic strength of the surrounding medium acts to compress the colloids electric double layer and reduced steric repulsion for a given distance from the colloid (Tipping et al., 1982). Additionally, the added cations in solution act to partially neutralize the surface charge character of the colloids (Kim et al., 2005). Overall, this leads to destabilization of the colloids in solution and promotes aggregation. It has also been shown that divalent cations, such as Ca2+, act to destabilize particles (enhance coagulation) through ionic bridging mechanisms and gel formation between rigid biopolymers (Chin et al., 1998; Wells, 1998), while biopolymers have also been shown to self-aggregate via hydrophobic moieties found within their chain structure (Ding et al., 2008).
With the onset of destabilization, colloids aggregate to form progressively larger parti- cles. These particles eventually achieve a large enough volume and density to allow gravity to influence its motion. In such, the small particles start their descent through the water column towards the sediment surface. These particles will continue to be influenced by dif- ferent factors throughout their descent, some of which have previously been noted. Some of these factors include; further aggregation in the water column, dissolution/mineralization of the inorganic/organic constituents, and the adsorption dissolved ions (i.e. nutrients, trace metals, radionuclides, and other hydrophobic contaminants) (Dominik et al., 1993; Gardner et al., 1985; Warren et al., 2001).
1.1.3 Sediment Advection and Influencing Factors
Sediment advection can incorporate large and small amounts of sediment, and can occur at both fast and slow rates. The type and amount of sediment advected is a factor of the energy input to the system. Advection involving mass sediment movements can be rapid and energetic, as in the case of sediment avalanches induced by earthquakes (Hilbe et al., 2011; Schnellmann et al., 2004) and rock falls (Kremer et al., 2012), and in the case of
Setting the Stage
Figure 1.2 Bernoulli’s Principle and its influence on Entrainment. Adapted from Sloss et al.
(2012)
Sediment Characteristics
Resuspension depends not only on the energy of the system, but also on sediment character- istics including the surface charge, particle density, and particle diameter. Should a particle be too large, too dense, or be cohesive in nature (electrostatic attraction between particles), the shear stress applied by sediment surface currents needs to be much greater to surpass the critical stress required to achieve sufficient lift on the particle for it to be entrained. It has previously been shown that clay and silt can be resuspended with current velocities as low as 2 to 3 cm s−1, while sand could be resuspended at velocities around 20 cm s−1(Lam et al., 1976).
A better understanding of entrainment can be found in the Bernoulli Principle. A particle on the lakebed experiences a drag force that is perpendicular to the gravitational force.
According to Bernoulli’s principle, higher fluid velocities on one side of an object lowers the pressure on this side and subsequently induces a lift component on the object. This is quite the same principle for the lift component experienced by the wings of an aircraft.
Depending on the density of the particle and the velocity of the water flow above it, the particle at rest can be resuspended and transported should the lift component be sufficiently large. A depiction of Bernoulli’s Principle and the forces acting upon a particle are depicted in Fig. 1.2.
Current velocity, particle size, and the water content of the sediment surface govern the processes of erosion, transport, and deposition. Velocity curves, or Hjulström curves, are presented in Figure 1.3 and show the relationship between the particle size and water content as it relates to threshold velocities. The upper curves show the critical erosion
6
1.1 Particle Sources and Sedimentation
Figure 1.3 Velocity curves showing the relationship between current speed, sediment poros- ity, and particle size on the entrainment of a particle of a given density. Taken from Chapman (1996).
velocity (cm s−1) for different sediment surface water contents while the lowest curve shows the critical deposition velocity. The plots show several concepts regarding the relationships between erosion, transportation, and deposition. For larger diameter particles, friction is the dominant force preventing erosion or entrainment; both the erosion and deposition curves follow each other closely with increases in velocity with increasing particle size. However, for finer, more cohesive sediments (i.e. clay and silt) the erosion velocity is seen to increase with decreasing grain size. This relationship stems from electrostatic forces being of greater significance between these smaller grain sizes (Gargani, 2004).
An example of how increasing current speeds influence entrainment and erosion can be found from the Flix reservoir in Tarragona, Spain. Here, a mass release of pollutants, most notably mercury, affected the local drinking water supply and killed thousands of fish (Her- rero et al., 2013). It was determined that the most probable cause for this acute contaminant
∼500 m3
Setting the Stage
organisms may not actually be the suspension of sediments itself, they do alter the sediment micro-surface structure and allow for facilitated entrainment by water currents.
Bottom dwelling fish are known to be one of the main sources of sediment entrainment, where they are found. Ground-dwelling fish in a fjord in British Columbia, Canada, were found responsible for the resuspension of surface sediments from the coastal shelf. Resus- pension by these fish was deemed to be the reason for a 1.7 factor increase in sediment accumulation rate (SAR) with depth between 50 m and 115 m depth (Katz et al., 2012).
These fish,∼10 cm long) were estimated to resuspend approximately 1.3 L m−2d−1of bulk sediment (Yahel et al., 2008).
The degradation of organic matter in sediments, both oxic and anoxic, produces various gases, which from time to time escape the sediment and travel to the water surface. This gas ebullition reworks the surface sediments and incites suspension and entrainment of finer particles. The production of these gases is well known to Vidy Bay and is more pronounced upon the extraction of sediments from this area of the lake. Gas ebullition has also been shown to rework sediments in other lakes. One study investigated pockmarks centralized near the Rhine tributary of Lake Constance (Germany). Biogenic gases were found to be released from the edges of sand waves or the sides of old channels carved into the Rhine delta region. Most pockmarks were between 5 m to 8 m in diameter with 0.5 m to 1.5 m depth and were responsible for the movement of large quantities of sediment (Wessels et al., 2010).
Hydrodynamics
The hydrodynamics of a body of water has a significant impact on sediment entrainment and advection. Wind is the main force driving circulation within a lake (Wüest et al., 2003).
Should the lake be large enough, the Coriolis force influences the lake’s circulation. The Coriolis force results in the rightward deflection of a water mass in the northern hemisphere, with leftward deflection in the southern hemisphere.
Internal waves form at density gradients in the water column (typically the thermocline) and are incited by the input of wind energy to the lake. Internal waves take two forms in Lake Geneva, Kelvin-type, and Poincaré. Poincaré waves occur in open water of larger lakes with their reflections generating Kelvin waves when encountering shore boundaries.
The important difference between Poincaré and Kelvin waves is that Poincaré waves extend, with undiminished amplitude, across a basin, whereas Kelvin waves decrease in amplitude away from shore and are thereby “trapped” or constrained to travel along the shore (Wüest et al., 2003).
8
1.1 Particle Sources and Sedimentation In the case of strong, sustained winds on a stratified system an internal seiche can be produced. This occurs through the epilimnetic build-up at the lee end of a lake due to wind forcing. While the epilimnion is increased in depth, the thermocline is tilted in response.
As the wind subsides, the wind stress is removed and the tilted water mass attempts to achieve a state of equilibrium. However, due to its momentum, the equilibrium state is surpassed and results in a rocking about one or more nodal points (Lemmin et al., 2005).
Addition of the geostrophic effect to the water circulation results in a circular oscillation pattern (or long internal wave), as depicted in Fig. 1.4. The combination of thermocline tilting and the geostrophic effect has previously been demonstrated in Lake Geneva (Bohle- Carbonell, 1986). The importance of these long waves becomes apparent when considering their interaction with water masses and the shoreline boundary.
Internal waves on the thermocline are roughly an order of magnitude or more larger in amplitude than waves found on the surface of large lakes. They propagate and break much the same as surface waves are seen to. The turbulence associated with internal waves is analogous to that at the surface; however, it occurs on a much larger scale. Since these large internal waves break at the sides of the basin, their effects coupled with the vertical movement of the seiche on which they move, are particularly significant. Their vertical displacement has been recorded to attain velocities of 20 cm s−1, which is sufficient for the resuspension and movement of surface sediments (Thorpe et al., 1996).
Ekman transport, in the form of up- and downwelling, can have significant implica- tions for water behaviour near shorelines, particle transport, and sediment resuspension.
Upwelling or downwelling can produce current speeds, which can be sufficient to incite sediment entrainment and transport in the water column (Fer et al., 2002). With this type of transport, should the wind be blowing along a shoreline, water can either be pushed to- wards or away from the shoreline, depending on the direction of the wind. In the case of upwelling, Ekman transport pushes the water body away from the shoreline and hence water from deeper parts of the lake is brought up towards the shore. This typically brings with it nutrient-rich waters and is typical of fishing banks along continental oceanic coastlines. In
Setting the Stage
Figure 1.4 Internal seiche dynamics about the thermocline (Wetzel, 2001). A rotating two- layered internal seiche lake model. Key diagram shows, (i) the oscillating lake surface (heavy line); (ii) the equilibrium lake surface position (thin line); (iii) the equilibrium inter- face position (broken line); and (iv) the oscillating interface (shaded surface).
The two diagrams in the upper left show a hypothetical layer distribution during wind stress, setting a seiche in motion. B and A respectively indicate the wind-driven surface and return currents in the upper layer and C indicates the lower layer.
Eight phases of one oscillation cycle of the first mode internal seiche are shown. Directions of flow in the upper and lower layers are shown by heavy arrows. P is the nodal point, the point of zero elevation change.
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1.1 Particle Sources and Sedimentation mann et al., 1995; Palanques et al., 2002) to the development of simple to complex models (Merritt et al., 2003). Resuspension is of great concern in oceanic settings (Baker et al., 1978; Baskaran et al., 2002; Giffin et al., 2003), as well as in large (Eadie, 1997; Lou et al., 2000) and small lakes (Hongve et al., 1995; Kelderman et al., 2011), and in rivers (Herrero et al., 2013; Navarro et al., 2010; Terrado et al., 2006). Resuspended and advected sedi- ments have been shown to account for a significant amount of the total SAR. For example, resuspended sediments were estimated to account for ∼34 % of the SAR in Lake Honda, The Netherlands (Kelderman et al., 2011), and ranged from 10 % to 30 % in offshore traps in Lake Superior (Urban et al., 2004).
Sediment advection has been well studied, albeit predominantly along oceanic coast- lines and in the great lakes (Buesseler et al., 2009; Dibb et al., 1989; Mercier et al., 2007;
Rao et al., 2012; Schmidt, 2002; Urban et al., 2004; Wilson et al., 2007). These studies typi- cally monitor the movement of suspended sediments from shallow coastal zones into deeper waters. Although the study settings differ, their findings are fairly consistent whereby sus- pended sediments are typically found to be transferred in a long-shore direction, as opposed to a cross-shore direction. These findings evidence an an increased sediment accumulation rate in near-shore zones, as opposed to deeper, main basin or profundal zones.
Few studies have investigated water-movement or contaminant transport in bays. Those which have, typically investigate the influence of tidal pumping on contaminant spread since these systems are influenced by oceanic tides (Wilson et al., 2007). This tidal flow occurs in and out of the bay, perpendicular to the mouth of the bay. One interesting publication investigated the formation of a gyre in Blackpool Sands embayment (Devon, U.K.) (Elwell, 2004). This gyre forms from the detachment and recirculation of a main coastal current which runs parallel to the mouth of the bay; much the same case as that found in Vidy Bay. In essence, it has been shown that the formation of a gyre under these hydrodynamic conditions segregates the water packet of the main coastal current from that of the gyre in the bay where the two are seperated by a shear-layer. This shear layer does not readily allow for the trasfer of waters from the bay into the main basin, nor vice versa (Elwell, 2004).
Setting the Stage
Nepheloid, or bottom boundary layers, are well known in oceanic settings (Bell et al., 1983;
Bourgault et al., 2014; Gardner et al., 1985) and also in large lakes (Bell et al., 1983; Gard- ner et al., 1985; Urban et al., 2004). This bottom boundary layer has also been detected in mid-sized lakes and has been estimated to have a mean thickness of∼11 m in Lake Geneva (Bouffard et al., 2013) and to reach up to 15 m in thickness in Lake Lugano (Hofmann et al., 1999) and 10 m in Lake Hallwill (Bloesch et al., 1986). A summary of resuspension and the methods used to measure it can be found in Bloesch (1994).
The identification of resuspended sediments has been attempted through various means.
Sediment characteristics, such as grain size, have been used to identify resuspended sedi- ments, zones of transport and zones of deposition in lakes (Blais et al., 1995; Håkanson, 1977). The composition of trapped sediments has also been used to distinguish between the vertical and lateral (or resuspended) sediment components (Wieland et al., 2001). Natural and anthropogenic radionuclides are commonly used as tracers for sediment and particle transport. The use of long-lived and short-lived radionuclides, most typically137Cs, 7Be,
234Th, and210Pb, have proven useful in the estimation of resuspended sediments and particle pathways (Amiel et al., 2002; Cornett et al., 1994).
The use of single radionuclides can provide good estimates of parameters such as settling velocity and residence times; however, they contain inherent errors based on assumptions of their use. The first being that, with respect to fallout radionuclides (7Be and210Pb), their in- put into a given aquatic system is mainly driven by wet precipitation events and also on their production in the atmosphere. This introduces seasonal variation which if not accounted for would alter calculation-derived conclusions. The second error is that the use of a single ra- dionuclide cannot necessarily help differentiate between particles descending from the lake surface (vertical component) and those that are resuspended (lateral component) (Dominik et al., 1989; Wieland et al., 1991). However, in using the ratio of a short-lived to a long-lived radionuclide, seasonal production and input variations can be minimized and the influence of lateral sedimentation to vertical sedimentation pathways can be estimated. In order for this to work, the two radionuclides need to be introduced to the aquatic system in the same manner, and they need to behave in a similar manner (i.e. adsorption onto, and transport with, the particles) within the aquatic system.
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1.2 Environmental Settings
1.2 Environmental Settings
1.2.1 Lake Geneva
Lake Geneva, also referred to as Le Léman, is the largest freshwater basin in Western Europe in terms of volume and the second largest lake in terms of surface area (surpassed only by Lake Constance, Germany/Switzerland/Austria). Lake Geneva is considered a mid-sized lake and is classified in the warm monomictic category. Due to its size, it is large enough to include all the physical-limnological processes known to lakes (i.e. Coriolis Force, Poincaré waves). Lake Geneva is greatly influenced by the local wind regime. The predominant wind is northerly and is funnelled by the central Alps on the eastern side of the lake and the Jura mountains on the western side. This northerly wind (regionally known as “la Bise”) is the driving source of metalimnetic breakdown and oxygenation of deeper waters. Complete overturn in the lake is typically seen on a six to seven year time scale (Lazzarotto et al., 2006, 2013). Table 1.1 provides some of the characteristics of the lake (CIPEL, 2010).
Table 1.1 Characteristics of Lake Geneva Average Volume 89 billion m3or 89 km3
Surface Area 580.1 km2
Maximum Length 72.3 km
Maximum Depth 309.7 m
Average Depth 152.7 m
Residence Time 11.3 year
The lake is fed from its alpine watershed in Switzerland running into the main tributary, the Rhône River. The Rhône tributary feeding Lake Geneva has an average input flow of 182 m3s−1, while the outflow from the lake has an average flow of 250 m3s−1 (CIPEL, 2010). The Rhône River provides approximately 73% of the affluent waters to Lake Geneva (Loizeau et al., 2003). The particle load of the Rhône River varies between 20 mg L−1 to
−1
Setting the Stage
the main basin and changes in wind direction (Bohle-Carbonell, 1986; Razmi et al., 2013).
The fact that the bay is the most contaminated part of the lake (Loizeau et al., 2004) results in social, economic and health implications for the City of Lausanne and the Canton of Vaud. Vidy Bay is a recreational area and its beach has been closed on occasion due to the health risk associated with high levels of bacteria in the water. These bacteria stem from the local wastewater treatment plant (WWTP), which dumps its treated effluents into the bay. In addition to the WWTP effluents are the effluents from combined sewer overflows, which relieve the wastewater load to the WWTP during periods of heavy precipitation. The overflow effluents bypass the WWTP and the untreated wastewater is diverted directly to the bay. Typical WWTP discharge rates vary between 1 m3s−1to 3 m3s−1, reaching as high as ∼7 m3s−1 during heavy rain events (Razmi et al., 2013). The overflow discharge rate from the Lausanne catchment area varies based on the amount of precipitation received and was around 6 million m3in 2008 which corresponds to approximately 16 % of the collected sewer waters. The majority of this overflow (∼80 %) occurred at WWTPs in the Lausanne catchment area with ∼20 % occurring at combined sewer overflows in the network before arrival at the WWTPs (Assainissement Lausanne, 2009). The only other source of water to the bay is the Chamberonne River, and it has input rates of∼0.2 m3s−1 and∼4.8 m3s−1, for periods of low and high precipitation, respectively (Razmi et al., 2013).
The issue of effluent surfacing on the Vidy beaches was improved in 2001 when the end of the outlet pipe from the WWTP was moved from 300 m offshore (15 m depth), to 700 m offshore (35 m depth). The displacement of the outlet meant that the WWTP effluents were introduced to the bay below the thermocline during the stratified season and reduced the transport of effluents to the shoreline by surface currents.
The WWTP was originally constructed in 1964. It initially consisted of a two-stage treatment plant (mechanical and biological treatments), which was equipped with a chemical stage treatment in 1971. This stage adds ferric chloride (FeCl3) to precipitate dissolved phosphate. In 1976, the WWTP was expanded and the efficiency of wastewater treatment improved.
Due to the combined treated and untreated wastewaters, Vidy Bay is a significant source of metals, organic micropollutants, and faecal indicator bacteria (Haller et al., 2009; Loizeau et al., 2003; Poté et al., 2008). Vidy Bay has previously been studied for its circulation patterns (Goldscheider et al., 2007; Le Thi et al., 2012; Razmi et al., 2013), water-soluble contaminant and pathogen fate (Bonvin et al., 2011; Czekalski et al., 2012), and the spread of various contaminants in the surface sediments (Gascon Diez et al., 2013; Loizeau et al., 2004; Poté et al., 2008). Determining the pathways and dynamics of suspended sediments
14
1.2 Environmental Settings in Vidy Bay is of great importance to understanding the fate of contaminat-laden particles and is the basis for the following studies.
A
near Rhône-Alpes — France
1 of 1 0
0 55 10 km10 km
B
C
Figure 1.5 A topographic representation of Vidy Bay (map C). The red dot shows the loca- tion of the WWTP outlet, while the red circle denotes the location of the outlet prior to 2001.
Maps A and B show the situation of Vidy Bay with respect to Lake Geneva and Switzerland as denoted by the blue arrows.
Setting the Stage
1.3 About This Thesis
The studies presented herein form part of an overall study which focuses on the under- standing of sedimentation processes, the influence of hydrodynamic conditions on sedi- ment transport, and colloid and particle stability as key issues for the question of long-term hydrophobic contaminant transport and water quality in mid-sized lakes, using Vidy Bay (Lake Geneva, Switzerland) as a case study. This manuscript is written as an amalgama- tion of manuscript and article-based formats. Each chapter is self-containing and represents the form of a published article or article in the process of being submitted. In such, minor repetition of common subject matter is inevitable. Each chapter addresses one of the main objectives of the overall study. The first study characterized suspended colloids and particles at the sediment-water interface. The aim of this study was to understand the composition and characteristics of colloids and small particles and how these characteristics lend to sta- bility and long-range transport. The second study investigated sedimentation pathways in the water column in an attempt to identify and segregate vertical and lateral sedimentation pathways and to quantify sediment focusing. The third study investigated the hydrodynamic conditions affecting the bottom boundary layer of the bay and to identify sources of later- ally advected sediments. This in turn led to determining long-term suspended sediment dis- placement trends in the bottom boundary layer of the bay. This introduction was written to provide a background to each chapter, with each chapter containing a brief, more specific, introduction to its research. A methods and discussion section are uniquely found in the chapter to which they pertain. This allows for a more concise description and understanding of its aims, goals, and findings without the need to refer to distal parts of the manuscript. In such, these sections are not repeated in the front and back matter of the thesis manuscript.
16
1.3 About This Thesis
List of Publications
The following is a list of the publications written during the time span of this thesis or those in preparation.
Graham, N. D., Stoll, S., Loizeau, J.-L., 2014. Colloid characterization at the sediment- water interface of Vidy Bay, Lake Geneva. Fundam. Appl. Limnol. 184, 87−100.
doi:10.1127/1863−9135/2014/0591
Gascon Diez, E., Garcia Bravo, A., à Porta, N., Masson, M., Graham, N. D., Stoll, S., Akhtman, Y., Amouroux, D., Loizeau, J.-L., 2013. Influence of a wastewater treatment plant on mercury contamination and sediment characteristics in Vidy Bay (Lake Geneva, Switzerland). Aquat. Sci. 76, 21−32. doi:10.1007/s00027−013−0325−4
Thevenon, F.,Graham, N. D., Chiaradia, M., Arpagaus, P., Wildi, W., Poté, J., 2011.
Local to regional scale industrial heavy metal pollution recorded in sediments of large fresh- water lakes in central Europe (lakes Geneva and Lucerne) over the last centuries. Sci. Total Environ. 412−413, 239−247. doi:10.1016/j.scitotenv.2011.09.025
Thevenon, F.,Graham, N. D., Herbez, A., Wildi, W., Poté, J., 2011. Spatio-temporal distribution of organic and inorganic pollutants from Lake Geneva (Switzerland) reveals strong interacting effects of sewage treatment plant and eutrophication on microbial abun- dance. Chemosphere 84, 609˘617. doi:10.1016/j.chemosphere.2011.03.051
Gascon Diez, E., Garcia Bravo, A.,Graham, N. D., Bouchet, S., Cosio, C., Amouroux, D., Loizeau, J.-L., Understanding the origin of high methylmercury concentrations on set- tling particles in Lake Geneva (Switzerland). (in preparation)
Graham, N. D., Arlaud, F., Argagaus, P., Loizeau, J.-L. The development of a hydro- static sample for sampling the sediment water interface. (in preparation)
C HAPTER 2
C OLLOID C HARACTERIZATION AT THE S EDIMENT - WATER I N -
TERFACE OF V IDY B AY , L AKE G ENEVA
This chapter investigates the composition and characteristics of colloids found at the sediment- water interface of Vidy Bay, Lake Geneva. Colloids play a critical role in the transport of particle-bound contaminants due to their charged surface character and their ability to re- main suspended in the water column. By characterizing colloids and small particles at the sediment-water interface of Vidy Bay, a greater understanding of contaminant dispersion due to colloid and small particle transport can be had.
Colloid Characterization at the Sediment-water Interface of Vidy Bay, Lake Geneva
Colloid Characterization at the Sediment-water Interface of Vidy Bay, Lake Geneva
Neil D. Graham*, Serge Stoll, Jean-Luc Loizeau
Institute F.-A. Forel, Earth and Environmental Sciences Section, University of Geneva Route de Suisse, 10
1290 Versoix, GE, Switzerland
*Neil.Graham@unige.ch
An article of form similar to this chapter has been published. Refer to,
Graham, N.D., Stoll, S., Loizeau, J.-L., 2014. Colloid characterization at the sediment-
water interface of Vidy Bay, Lake Geneva. Fundam. Appl. Limnol. 184, 87100. doi:10.1127/1863- 9135/2014/0591
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2.1 Abstract
2.1 Abstract
Colloids play a critical role in the transport of particle-bound contaminants. Knowledge of colloids and their aggregates provides insight into contaminant transport and fate within a given aquatic environment. Here, colloids and aggregates at the sediment-water interface of Vidy Bay, Lake Geneva, Switzerland, were characterized with a combination of analytical techniques to understand their structure, size distribution, concentration, and stability (the potential for aggregation). Vidy Bay is known to be the most contaminated part of Lake Geneva, being influenced by the effluents of a municipal wastewater treatment plant. Col- loids were a heterogeneous mix of inorganic constituents (diatom fragments, quartz, clay, endogenic calcite, iron oxy-hydroxides) bridged together by rigid biopolymer strands or gels. The presence of rigid biopolymers was quite significant and they were typically found to have iron oxy-hydroxides embedded within their structure. Ion chromatographic data were comparable to previous values attained for the water column of Lake Geneva; how- ever, single particle counting results indicated the presence of a nepheloid layer in Vidy Bay.
In such, the stability of colloids was likely influenced by their proximity to the sediment- water interface. Zeta potential results inferred charge neutralization and destabilisation of colloids and aggregates. Self-assembly of rigid biopolymers, along with cationic bridging between rigid biopolymers and inorganic constituents readily aggregated colloids. Taken together, colloids at the sediment water interface of Vidy Bay appeared to be unstable and play only a minor role in the transport of contaminants over long distances.
Acronyms:
CRM certified reference material EDL electric double layer
FeOx iron oxy-hydroxide
