binding site concentration was slightly less than 10
À6 mol L
À1 in Seine-Aval WWTP
effluent. Even if it was lower than those of L 1 and L 2 sites, it was environmentally
relevant given that trace metal concentrations in receiving waters were considerably
lower, ranging from 10
À10 to 10
À7 mol L
À1 .
For zinc, there was not a very-high-affinity site but, regardless of the sampling
site (receiving waters or EfDOM), the HPI fraction could systematically be
characterised by higher stability constants (K 1 and K 2 ) and a lower L 1 /L 2 ratio
compared to the other fractions [49].
Regarding mercury, stability constants for EfDOM fractions (10
25.2 M
À1 ) were
significantly higher than for the SRFA standard (10
22.8 M
À1 ) but roughly equal to
those observed for the Upstream I site DOM fractions (10
24.6 M
À1 ) [48]. In both
cases, no significant differences were observed depending on the different fractions.
Furthermore, it has been shown that stability constants are positively correlated with
total organic N and S. N- and S-enriched organic materials, such as EfDOM, had
stronger Hg-complexing ligands [48].
Overall, these results highlight the strong binding capacity of EfDOM. The high
binding ability of EfDOM and especially its HPI fraction could be explained by their
nitrogen and sulphur group contents [47–49]. It could be noted that EfDOM revealed
a high trace metal-binding ability despite its low aromaticity, as indicated by its low
SUVA [29].
3.4 Contribution of EfDOM from Seine-Aval WWTP
to the Copper-Binding Site Flux in the Seine River
Downstream of Paris
During the low-water period, the copper-binding site molar concentrations in the
Seine River evolved significantly from upstream to downstream of Paris, with an
increase of 173% for L 1 sites and 157% for L 2 sites [29]. This increase was probably
due to EfDOM discharges in the receiving waters. L 1 and L 2 site concentrations
in EfDOM were, respectively, three times and ten times higher than in upstream
waters [29]. Based on the experiments carried out in the Piren-Seine programme,
the contribution of EfDOM from the Seine-Aval WWTP to the binding site fluxes
in receiving water downstream of the Paris conurbation was assessed [29].
The ratio of binding sites originating from EfDOM to total binding sites measured
downstream of Paris was calculated as described in [29]. This instantaneous flux
ratio was calculated for all sampling dates (Fig. 5).
During high-flow periods, the contribution of EfDOM from the WWTP SeineAval to the total binding site concentration at the downstream site was around 5%
and 23%, respectively, for low-affinity (L 1 ) and high-affinity (L 2 + L 3 ) sites. Due to
the lower EfDOM dilution, during the low-flow period, these averaged values were
two to five times higher, i.e. reaching 19% and 58%, respectively. Downstream of
Paris, during the low-water period, the largest proportion of high-affinity binding
232
G. Varrault et al.
À6 mol L
À1 in Seine-Aval WWTP
effluent. Even if it was lower than those of L 1 and L 2 sites, it was environmentally
relevant given that trace metal concentrations in receiving waters were considerably
lower, ranging from 10
À10 to 10
À7 mol L
À1 .
For zinc, there was not a very-high-affinity site but, regardless of the sampling
site (receiving waters or EfDOM), the HPI fraction could systematically be
characterised by higher stability constants (K 1 and K 2 ) and a lower L 1 /L 2 ratio
compared to the other fractions [49].
Regarding mercury, stability constants for EfDOM fractions (10
25.2 M
À1 ) were
significantly higher than for the SRFA standard (10
22.8 M
À1 ) but roughly equal to
those observed for the Upstream I site DOM fractions (10
24.6 M
À1 ) [48]. In both
cases, no significant differences were observed depending on the different fractions.
Furthermore, it has been shown that stability constants are positively correlated with
total organic N and S. N- and S-enriched organic materials, such as EfDOM, had
stronger Hg-complexing ligands [48].
Overall, these results highlight the strong binding capacity of EfDOM. The high
binding ability of EfDOM and especially its HPI fraction could be explained by their
nitrogen and sulphur group contents [47–49]. It could be noted that EfDOM revealed
a high trace metal-binding ability despite its low aromaticity, as indicated by its low
SUVA [29].
3.4 Contribution of EfDOM from Seine-Aval WWTP
to the Copper-Binding Site Flux in the Seine River
Downstream of Paris
During the low-water period, the copper-binding site molar concentrations in the
Seine River evolved significantly from upstream to downstream of Paris, with an
increase of 173% for L 1 sites and 157% for L 2 sites [29]. This increase was probably
due to EfDOM discharges in the receiving waters. L 1 and L 2 site concentrations
in EfDOM were, respectively, three times and ten times higher than in upstream
waters [29]. Based on the experiments carried out in the Piren-Seine programme,
the contribution of EfDOM from the Seine-Aval WWTP to the binding site fluxes
in receiving water downstream of the Paris conurbation was assessed [29].
The ratio of binding sites originating from EfDOM to total binding sites measured
downstream of Paris was calculated as described in [29]. This instantaneous flux
ratio was calculated for all sampling dates (Fig. 5).
During high-flow periods, the contribution of EfDOM from the WWTP SeineAval to the total binding site concentration at the downstream site was around 5%
and 23%, respectively, for low-affinity (L 1 ) and high-affinity (L 2 + L 3 ) sites. Due to
the lower EfDOM dilution, during the low-flow period, these averaged values were
two to five times higher, i.e. reaching 19% and 58%, respectively. Downstream of
Paris, during the low-water period, the largest proportion of high-affinity binding
232
G. Varrault et al.