Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
this understanding, naturally occurring radionuclide tracers can be used to help differenti-ate between vertical and ldifferenti-ateral sediment components and also calculdifferenti-ate particle residence times.
Hydrodynamic processes have already been well studied in Lake Geneva. For example, internal wave dynamics during thermal stratification have been characterized (Lemmin et al., 2005). These large-scale internal waves are mostly dissipated at the bottom boundary layer (Bouffard et al., 2013) and in the lake interior (Ozen et al., 2006; Thorpe et al., 1996).
In winter the cooling and downwelling of dense cold water from near shore is also associ-ated with enhanced mixing and sediment resuspension (Fer et al., 2002a,b). Temperature and circulation patterns of Lake Geneva have been modelled (Le Thi et al., 2012) and the sedimentation dynamics analysed (Dominik et al., 1993; Gandais et al., 1987).
Recent works have been focused on Vidy Bay as an important source of anthropogenic contaminants. Vidy Bay is located near Lausanne on the northern Lake Geneva shoreline.
It is of great interest due to the treated outfall and untreated overflows it receives from the municipal wastewater treatment plant (WWTP). Due to these combined wastewaters, Vidy Bay is a significant source of metals, organic micropollutants, and faecal indicator bacteria (Haller et al., 2009b; Loizeau et al., 2003; Pardos et al., 2004; Poté et al., 2008a). The hydrodynamics of the bay suggest that circulation patterns do not always follow a westerly current along the northern shoreline (Goldscheider et al., 2007; Le Thi et al., 2012; Razmi et al., 2013). Certain wind conditions can lead to the formation of a gyre with an increased retention time in the bay (Razmi et al., 2013). Beside the hydrodynamics of the bay, other studies have investigated the fate of water-soluble contaminants and pathogens (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; Pardos et al., 2004; Poté et al., 2008b). A study on the suspended sediment transport dynamics in Vidy Bay is of great importance to understanding of the potential spread and fate of contaminated particles from the bay.
Sediment transport models can incorporate radionuclide tracers as surrogates for con-taminants, while estimating particle residence times based on the decay of the tracer from its input into the system and its output to the sediments (Dominik et al., 1989; Savoye et al., 2006; Wieland et al., 1991). While this type of one-dimension steady-state model pro-vides some insight into sedimentation dynamics and residence times, they fail to account for lateral inputs, which skew calculated residence times.
In the current study a vertical sedimentation model, with a lateral component, is pro-posed to differentiate between vertical and lateral components of collected sediments and
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3.2 Introduction refine calculated residence times. To achieve this, the model incorporates 7Be/210Pbxs flux ratios to alleviate temporal input fluctuations due to changing rates of production and input to the system. 7Be has a half-life of 53.29 d and is formed through cosmic-ray spallation reactions on nitrogen and oxygen in the upper atmosphere (Arnold, 1998). It is brought to the Earth’s surface by wet and dry deposition and readily adsorbs onto organic and inorganic particles (Dominik et al., 1993; Mabit et al., 2008; Wieland et al., 1991). 210Pb can also be formed in the atmosphere; however, through different means. 210Pb is part of the238U de-cay chain (see Annex A), has a half-life of 22.3 y, and is found in much of the earth’s crust.
210Pb is one of the daughters of222Rn and has the longest half-life in the bottom portion of this decay chain. 222Rn is present in gaseous form and is capable of escaping its surround-ing solid matrix if it is sufficiently close to the earth’s surface (Olley et al., 1996). Once the 222Rn escapes, it mixes into the atmosphere where 210Pb is eventually produced after a succession of short-lived decay products. 210Pb is returned to the earth’s surface through wet and dry deposition and readily adsorbs onto organic and inorganic matter (Mabit et al., 2008; Noller, 2000; Wan et al., 2005; Wieland et al., 1991). Since238U is naturally found in the environment and sediments, it is also necessary to differentiate between the excess and supported fractions (210Pbxs and210Pbsup, respectively). The supported fraction being that which is found in sediments that was initially part of the solid matrix of allochthonous par-ticles entering the lake (from222Rn which did not escape to the atmosphere) and the excess fraction (from222Rn that did escape to the atmosphere) that was returned to the lake via wet and dry deposition (Drexler et al., 2008; He et al., 1996; Olley et al., 1996). These fractions were calculated by concurrently measuring the 214Pb and 210Pb activities of the samples.
222Rn is one of the parents of214Pb in the238U decay chain and hence214Pb remaining part of the solid matrix can be used to determine the supported fraction of210Pb (He et al., 1996).
These radionuclides can be used as proxies for hydrophobic contaminants since they exhibit similar behaviours in an aquatic system (Fitzgerald et al., 2001; Mabit et al., 2008; Wieland et al., 1991). Also, due to the relatively short half-life of7Be, as compared to210Pb, their ratio makes them prime candidates for studies on the near monthly time-scale (Fitzgerald
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
shortest realistic particle residence times by attributing the largest possible flux of7Be to the vertical component without an artificial flux ratio decrease resulting from the sole use of the210Pbxs atmospheric flux.
The sedimentation of particles in the water column consists of several steps, each of which has an associated residence time. In considering a hydrophobic contaminant, the steps and process-related residence times are: its adsorption onto a colloid or particle and subsequent coagulation/aggregation between colloids and/or particles (τC, d), particle set-tling through the water column (τP, d), particle transport through the bottom boundary layer to the sediment surface (τB, d), and the resuspension and lateral transport of a particle from the sediment surface (τL, d). The overall residence time of a particle in the water column is the sum of each of the individual residence times (τR=τC+τP+τB+τL). In the proposed sediment component model, the lateral transport residence time (τL) is not considered in the overall residence time calculation since it is accounted for in the surplus 210Pbxs flux calculations, prior to residence time calculations.
This study was designed to investigate the vertical and lateral sedimentation dynamics affecting Vidy Bay. The goal was to ascertain the sedimentation pathways of particle-bound contaminants and identify the process of sediment focusing within the bay. The radionu-clidic and sedimentological aspects of settling particles are used to describe differences in vertical and lateral fluxes in and around the bay.