3.3.1 Environmental Setting and Sampling Strategy
Lake Geneva (volume∼89 km3) is a peri-alpine lake and is classified as being warm mo-nomictic. Complete overturn of the water column occurs approximately once every seven years (Lazzarotto et al., 2006, 2013). Vidy Bay is an open and deep embayment and its cir-culation pattern is subject to the circir-culation patterns of the main basin and changes in wind direction (Bohle-Carbonell, 1986; Razmi et al., 2013). The main tributary to the lake is the Rhône River at its northeast end, and its main outlet, the continuation of the Rhône River, at its southwest end. The Rhône River provides approximately 70 % of the affluent waters to Lake Geneva (Loizeau et al., 2003). The particle load of the Rhône River varies between 20 mg L−1and 2000 mg L−1in the winter and summer, respectively (Dominik et al., 1987).
Vidy Bay also receives suspended particulate matter from the municipal WWTP and overflow from the area of Lausanne. Typically, the WWTP discharge rate varies between
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3.3 Sampling Sites, Materials and Methods 1 m3s−1 to 3 m3s−1 and can reach ∼7 m3s−1 during heavy precipitation (Razmi et al., 2013), with overflow discharges varying based on the amount of precipitation. The over-flow discharge from the Lausanne catchment area was around 6 million m3in 2008 which corresponds to approximately 16 % of the total collected sewer waters (Assainissement Lau-sanne, 2009). The Chamberonne River also flows into the bay; however, its additions are less, at∼0.2 and ∼4.8 m3s−1, for dry and wet periods, respectively. There is also the po-tential for lateral inputs from the Rhône River which have previously been shown to reach distal parts of the lake (Gandais et al., 1987). The differing particle sources and the local hydrodynamic conditions increase the complexity of sediment dynamics within the bay and surrounding water body. While the goal of the proposed model is not to identify individual sources of lateral advection, an understanding of the lateral component is needed prior to ascertaining the fate of an individual source of suspended sediments.
Extensive efforts were made to sample the inputs and outputs of the WWTP at Lausanne to support the findings of the sediment trap study. This type of sampling was sought to identify if the WWTP effluents had a marked influence on the sediment trap data and how this influence changed during wet and dry periods. Unfortunately, proposed restrictions on the use and types of data allowed for publishing resulted in this sampling being unfeasible.
The boundary of Vidy Bay used in this study has been adopted from Razmi et al. (2013) where the boundary was defined as the hydrodynamically modelled zero vortices line. This boundary marks the limit at which the waters in Vidy Bay no longer experience the largest extent of the secondary gyre in the bay. This limit is denoted by the 150 000 m Northing line in Figure 3.1. Two sediment trap systems were installed in an array along a transect ex-tending from the WWTP outlet (Figure 3.1). These sites were chosen due to their relatively flat sediment surface and the need that the sediment trap systems had to be submerged at depths greater than 70 m (restrictions implemented by the local fishing authority). The sed-iment trap at site NG2 (WGS84:46.501 69 N, 6.583 39 E, Swiss Coordinates (m): 150 400 N 534 350 E, 138 m depth) was in place inside the bay for a period of 22 months starting
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
Figure 3.1 Sampling site transect locations (NG2 and NG3) as related to the wastewater treatment plant (WWTP) outlet of Vidy Bay, to Lake Geneva, and to Switzerland. The topographic markings in the bottom map denote meters above sea level. Coordinates are given in the Swiss coordinate system. The atmospheric trap was located at Versoix and is denoted by the star (insert top-right showing lake bathymetry).
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3.3 Sampling Sites, Materials and Methods
3.3.2 Sample Collection
Sediment trap samples were collected on a near monthly basis, with a few exceptions result-ing from technical or meteorological factors. Each sediment trap tube contained a remov-able bottom portion, which allowed for easy and controlled removal of overlying water and sample recuperation. Each sampling campaign procured six samples from each trap level, ensuring sufficient quantities of sediment for all analyses once the six trap samples from the same level were combined. The samples were returned to the lab within 5 hours of sampling where the remaining overlying water was gently removed by syphoning. All samples were kept at 4◦C, in the dark, when not being processed.
Sediment cores were taken concurrently with the changing of the sediment traps. Sed-iment cores (SC) were taken using two Mortimer corers (fabricated at the University of Geneva), which were attached together to obtain two parallel sediment cores, 20 cm apart.
This allowed for the retrieval of sufficient surface sediments for subsequent analyses and decreased the potential influence of short-range spatial activity variability. The coring tubes had an internal diameter of 7 cm, for a combined surface area of 77 cm2. The top three cen-timetres of each core were subsampled at 1 cm intervals by extrusion with the corresponding subsamples from both cores being combined. The entire volume of sediments in each of the 1 cm layers was collected. The top three centimetres were sampled to ensure that the entire stock of7Be was collected since it is mainly found in the unconsolidated surface sediments and can easily be transferred to lower sections of the sediment core upon extrusion.
An atmospheric trap was installed at Versoix, Switzerland (see Fig. 3.1), to collect
7Be and210Pb brought to the Earth’s surface via wet and dry atmospheric deposition. The atmospheric trap was mounted on the roof of a building, clear of any obstructions that would affect its collection efficiency. The atmospheric trap was changed concurrently with the sediment trap and core samplings. The atmospheric trap was acidified with 300 mL of HNO3
and included Be and209Po standard spikes to calculate sample recovery. Radionuclides were precipitated with FeCl3and the precipitate was dissolved in 40 mL of 1 mol L−1HCl prior
Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay
sampling at Versoix is representative of inputs to the Vidy Bay area.
Due to unforeseeable technical and meteorological issues preventing the surfacing of the sediment trap systems, differences in sample date ranges were adjusted accordingly.
For these periods, inputs were adjusted by summing the inventories (i.e. radionuclide ac-tivity, sample mass accumulation, sediment composition, in units of measure m−2) for the necessary sampling periods and normalizing this by the total number of days of the sam-pling periods combined, resulting in a flux (unit of measure m−2d−1). Differences of a few days between samplings were neglected. For the purpose of mean lacustrine season values, henceforth referred to as seasonal values, thermal stratification was designated to occur between the beginning of April and the start of November of a given year, while the thermally unstratified season was designated as continuing from the start of November to the start of April of the following calendar year (Lazzarotto et al., 2006). Annual values were calculated from the beginning of April 2010 to the start of April 2011, allowing for a complete annual cycle in Lake Geneva, referring to one season of thermal stratification and one unstratified season. Both seasonal and annual means were calculated from the sum of the inventories from each sampling period during the seasonal or annual period and di-viding by the total number of days of the seasonal or annual period. The season transition of November 2010 occurred between two sampling periods at site NG3 (29-Oct-2010 to 10-Dec-2010). To adjust the inventory to agree with the NG2 sampling date of 5-Nov-2010, the inventory at NG3 for this period was weighted based on the number of days between 29-Oct to 5-Nov (35 d), to the total number of days of sediment trap exposure for the sam-pling period (72 d). Data from both sites were truncated in order to maximize the number of extractable seasonal data sets. This resulted in NG2 providing data from the beginning of November 2009 until the end of September 2011 (4 seasons) and NG3 providing data from the start of April 2010 to the end of September 2011 (3 seasons). Forecasting of the September – November sediment trap and sediment core data points completed the 2011 thermally stratified season. This forecast was achieved by taking the mean weighted value for September 2010 to November 2010 and multiplying this by the annual mean of 2011.