Analytical Methods

3.4.1 Particle Size Distribution

Particle size distribution analysis was conducted on a Coulter LS–100 grain size analyser (Coulter Electronics Ltd., USA), with a range of 0.4 µm to 900 µm. Samples were mixed

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3.4 Analytical Methods to ensure a homogeneous and representative sub-sample. No other pre-treatment was con-ducted on the samples. Samples were analysed within 24 h of being brought back to the lab.

The followed method can be found in Loizeau et al. (1994), with the noted exception that samples were humid and no hydration or sonication was needed. The mean sample error for this method is±0.18 µm (Loizeau et al., 1994).

3.4.2 Gamma Spectrometry

Specific activities (Bq g⁻¹) for⁷Be (477.60 keV),²¹⁰Pb (46.54 keV), and²¹⁴Pb (351.93 keV) were measured using HPGe well detectors (Ortec, GWL series, USA). Activities were cor-rected for detector efficiency, sample geometry, sample density and self-attenuation (Dulin-ski et al., 1992), background radiation, decay during the period of sample collection, and decay during measurement (specifics on activity corrections can be found in Annex B).

Prior to analysis, samples were freeze-dried and homogenized to ensure representative sub-samples.

Specific activities were multiplied by the total sample mass and then divided by the sampling surface area to attain the radionuclide inventory (Bq m⁻²). The inventories were summed for given sampling periods to attain the total inventory for longer seasonal or an-nual periods. This total inventory was subsequently divided by the total number of days for the elapsed seasonal or annual period to attain the radionuclide flux (Bq m⁻²d⁻¹). Sediment trap and atmospheric samples were decay corrected for the time during sample collection.

Sediment cores were corrected by taking the measured specific activity (Bq g⁻¹), decay-corrected to the date of sampling. The net flux (F_Net, Bq m⁻²d⁻¹, Equation 3.1) was cal-culated from the radionuclide inventory (Bq m⁻²) for a given sampling period (t) minus the decay-corrected inventory from the previous sampling period (t−1). Here, λ is the decay constant (0.013 d⁻¹for⁷Be and 8.51×10⁻⁵d⁻¹for²¹⁰Pb) and∆t is the period of time, in days, between sampling periodst andt−1. The first sediment core sample was corrected only for decay during measurement.

Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay

30 min to measure the CaCO₃ content (Dean, 1974). A certified reference material (CRM, CANMET LKSD–4, Canada) was used to verify the accuracy of this method. Analysis of the CRM by LOI produced accuracies of−3.94 % for OM content and−0.35 % for CaCO₃ content. CaCO₃content was estimated by taking the mass loss at 1000^◦C and multiplying this by 2.2742, which is the molar mass ratio of calcite to carbon dioxide. OM content was estimated using the mass loss of the sample after heating to 550^◦C and the initial dry mass of the sample.

3.4.4 Sediment Component Model

The vertical sedimentation model incorporated⁷Be/²¹⁰Pb_xsflux ratios to calculate seasonal and annual residence times. The sediment component model was constructed using three boxes or system compartments, one atop the other, each representing a homogeneous pro-cess within, including: adsorption and coagulation, particle settling through the water col-umn, and particle settling through the bottom boundary layer (Figure 3.2). The sum of the process-related residence times equalled the overall residence time (τ_R=τC+τP+τB). An understanding of which process-related residence times influence a measured flux can be found in table 3.1.

Table 3.1 A summary of the manner in which each process-related residence time was calcu-lated with the vertical sedimentation model. Accronyms are as follows: Atmo - atmospheric trap, STT - top sediment trap, STB - bottom sediment trap, SC - sediment core.

Process-related

Box Input/Output Residence Time R1 R2

1 + 2 Lake Surface – STB τ_C+τ_P Atmo STB

2 STT – STB τP STT STB

3 STB – Sediment Surface τ_B STB SC

The first box was considered to be of infinitesimal thickness. Here, the adsorption and coagulation process of sedimentation was represented by a steady-state mixed-reactor type model (equation 3.2 (Froehlich, 2009)).

R₂= R₁

(1+λ τ_C) (3.2)

Where R₂is the output flux ratio and R₁is the input flux ratio to a box or compartment.

λ is the⁷Be decay constant (d⁻¹) with the²¹⁰Pb decay constant being neglected since it is much smaller than that of⁷Be (Wieland et al., 1991). τ_C represents the residence time of aggregation and coagulation in box 1.

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3.4 Analytical Methods The second box extends from the lake surface (interface with box 1) down to the in-terface of the bottom-boundary layer represented by the bottom sediment trap. The third box extends from the interface with box 2 down to the sediment surface. Flux movement through boxes 2 and 3 were seen to follow a piston flow model (equation 3.3 (Froehlich, 2009)) with the output flux calculated as,

R₂=R₁e^{−λ τ} (3.3)

Whereτrepresents the residence time of a particle descending through the water column or the bottom boundary layer. The atmospheric trap provided the input flux ratio into box 1 and the system as a whole. The sediment traps were contained within box 2 with their measured fluxes being influenced by τ_C and τP since the fluxes measured in the sediment traps pass through both boxes. The flux ratio into the bottom sediment trap, at 5 m above the sediment surface, was at the interface between boxes 2 and 3, and in such, was the flux out of box 2 and the flux into box 3. The flux ratio out of box 3 was calculated from the sediment surface⁷Be flux divided by the²¹⁰Pb_xs flux into box 3 and the assumption of no lateral advections. This simplification was made since the measured²¹⁰Pb_xsof the sediment surface was in fact the total ²¹⁰Pb_xs for the top 1 cm of the sediment cores, as opposed to just the sediment surface flocs where the majority of ⁷Be would be found. In such, the

7Be/²¹⁰Pb_xs flux ratio would have been artificially decreased and would have lead to longer calculated residence times.

Since the sediment trap samples were influenced byτ_Candτ_P, calculation of the settling time between the top and bottom traps would give a time which is solely a factor ofτ_Psince this passage only occurs through box 2. By considering the flux into the top sediment trap (STT) as R₁and that into the bottom sediment trap (STB) as R₂of equation 3.3, the settling time (t_P) between the two trap levels can be calculated. This time was then extrapolated to the lake surface through the settling velocity to calculate τ_P between the lake surface

Discrimination Between Vertical and Lateral Sedimentation Pathways in a Contaminated Bay

Figure 3.2 A schematic overview of the various boxes and process-related residence times used for the sediment component model (not to scale). (i) and (ii) are the fluxes into STT and STB, respectively. (iii) is the flux through the bottom boundary layer, and (iv) represents the flux between STT and STB used to calculatet_Pandv, leading to the calculation ofτ_Pas per equations 3.4 and 3.5.

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