4.2.1. Levels and Spatial Variation

The Water Framework Directive 2000/60/EC [34] identified Cd and Pb as priority substances, posing a threat to, or via, the aquatic environment at the EU level. Environmental quality standards (EQS) have been set by its Daughter Directive 2013/39/EU [35] with annual average values for Cd: 0.2 <sup>μ</sup>g·L−<sup>1</sup> and 1.3 <sup>μ</sup>g·L−<sup>1</sup> for Pb. The concentrations of dissolved metals in the inner sector and the inlets were well below the EQS (Table 2). However, dissolved Cd in the stream (0.116 <sup>μ</sup>g·L−1) was marginally below the EQS, suggesting that this element is of environmental concern and should be regularly monitored.

The stream, of groundwater origin, was the major source of Cd into the lagoon. This is evidenced by the fact that dissolved, total, i.e., the sum of dissolved and particulate (w/v) shown in Figure 2, as well as particulate (w/w; Table 2) Cd concentrations were significantly higher in the stream than the inner sector. Furthermore, the stream constituted the primary source of dissolved Pb, dissolved and particulate (both in w/w and w/v) Mn, and particulate (w/w) Zn. In contrast, the concentrations of Fe and Pb in the particulate phase (w/v) were significantly higher in the inner sector than the stream, suggesting that other sources (e.g., runo ff) rather than the emanating groundwater are responsible for the transport of these elements into the lagoon.

**Figure 2.** Spatial variation of dissolved and particulate (w/v) concentrations for (**a**) Fe, (**b**) Mn, (**c**) Cd, (**d**) Cu, (**e**) Pb, and (**f**) Zn.

Figure 3 illustrates the detailed distribution of particulate elements between the two fractions of suspended particles (d < 8 μm and >8 μm). The concentrations of particulate metals are expressed in w/w in order to compensate for differences of the SPM concentrations among the stations. Particles with d > 8 μm contained larger amounts of Al and Fe than the finer ones, of d < 8 μm. This distribution pattern is in contrast to the general and well established trend that as the grain size of particles decreases, the surface area increases, as does the particulate metal concentrations [36,37].

**Figure 3.** Spatial variation of particulate metal contents (w/w) of the >8 μm and the <8 μm fraction of SPM: (**a**) Al, (**b**) Fe, (**c**) Mn, (**d**) Cd, (**e**) Cu, (**f**) Pb, and (**g**) Zn.

The most probable explanation of the rather unusual, relative enrichment of larger particles is that these are agglomerated grains, consisting of aggregates of smaller particles [38] that are produced *in situ* through flocculation processes at the fresh-saline water interface [27,39,40]. The elevated Al, Fe, and Mn contents of the larger particles indicate that these phases are either: (a) linked to the original, smaller individual particulates, as a result of coagulation and precipitation of colloids and/or as surface coatings on clays [41]; or (b) that the newly formed agglomerates are cemented together by Fe/Mn coatings [37,38]. Some of the cement coatings could be organic in nature as well [37,42], since bacterially mediated processes may promote flocculation of smaller particles [43].

The presence of Al and Fe/Mn onto the larger particles increases their ability to sorb other metals. This is supported by the significant correlations between the elements in the > 8 μm fraction, shown in Table 2. Significant correlations of Cd, Cu, Pb, and Zn with Al, Fe, and Mn in the >8 μm fraction of SPM (r = 0.505–0.853; < 0.05) are consistent with the scavenging of metals by Al-Fe-Mn oxyhydroxides, and explains the enrichment with metals of the suspended particles >8 μm in relation to the finer particles

(<8 μm) (Figure 3). The strong correlation of suspended Pb with Al, Fe and Mn (r = 0.619–0.779) explains its predominant partitioning in all samplings and stations in the > 8 μm fractions of particles. Furthermore, the higher correlation coe fficient of Pb and Fe over Al and Mn is compatible with its strong a ffinity (stability constant) for freshly precipitated Fe oxyhydroxides [32]. Cadmium and Zn correlated strongly with Mn (r = 0.949, and 0.710, respectively), suggesting the preferential association of these elements with Mn oxyhydroxides. According to Turner et al., [44] Cd binding onto Mn oxides is much stronger than on Fe oxyhydroxides or other phases.

The concentrations of particulate Mn, Cd, and Zn (in w/w) were significantly lower in the inlets than the inner part of the lagoon. This pattern suggests that these elements are entrapped within the lagoon. No statistical di fferences were determined for the other metals (Fe, Cu, Pb) between the inner sector and the inlets.

During the first sampling period, at the eastern inlet an inflow of relatively dense saline water (S: 37.2) near the bottom and an outflow of brackish water (S: 22.0) at the surface was evident from salinity measurements. An additional sample for trace metals and SPM determinations was collected from the near-bottom, saline layer at this site, to ge<sup>t</sup> insights into the processes occurring at this interface. Figure 4 shows the detailed distribution of particulate SPM, Al, Fe, and Mn between the two fractions of suspended particles (<8 μm and >8 μm) in the two water layers. SPM concentrations in the bottom, saline layer were higher (sum of both fractions: 41.9 mg·L−1) than in the surface (13.4 mg·L−1), suggesting the re-suspension of bottom sediments. The coarser fraction of SPM was the predominant one. Despite the re-suspension, the metal contents of the larger particles of the bottom water layer were slightly higher than those of the surface layer. Aluminum, Fe and Mn contents of the smaller particles were higher at the surface water layer than the bottom layer. Apparently, the flocculation and the enrichment mechanisms described above continued under the high salinity regime.

**Figure 4.** Concentrations of (**a**) SPM, and (**b**) Al, (**c**) Fe, and (**d**) Mn (w/w) contents of particulate solids with diameter d < 8 μm and d > 8 μm in the brackish surface and the saline bottom water layer of station A1.

### 4.2.2. Partitioning and Interactions between the Dissolved and Particulate Phases

According to Figure 2, in the inner part of the lagoon, Fe and Pb were primarily particle-bound (on average 94% and 70% of the total concentrations, respectively), Cd and Zn were found to be predominantly in the dissolved phase (67% and 70% of the total concentrations, respectively), whereas Cu and Mn were equally associated with the solid and solution phases. In the stream water, all elements were predominantly found in the dissolved phase, except Fe. The predominance of the dissolved phase for Mn and the particulate phase for Fe in stream water sample is compatible with the fact that oxidation kinetics of Mn(II), emanating from groundwater, is slower than that of Fe(II) [8,45].

Solid-solution partitioning of metals in estuarine systems has been widely described by the partition coefficient KD, defined as [46]:

$$\text{KD} = \frac{\text{Particular concentration} \left(\frac{\text{w}}{\text{w}}\right)}{\text{Dissolved concentration} \left(\frac{\text{w}}{\text{v}}\right)} \tag{1}$$

The partition coefficient should be constant for a given composition of suspended particles and of solution; however, any change of the particle surface reactivity and/or solution properties may result in KD changes [47]. The partition coefficient is calculated in this study for all particles (KD-T), for the large particles with diameter > 8 μm (KD-L), and for the small particles with diameter d < 8 μm (KD-S); (Table 3). This distinction allows the investigation of the role of each fraction of particles on the solid-solution partitioning in detail.


**Table 3.** Partition coefficient (log10) KD-T for all the particles, KD-L for particles with diameter d > 8 μm, and KD-S for particles with 0.45 μm < d < 8 μm.

The average KD-T follows the order Fe < Pb < Mn < Cu < Zn, Cd (Table 3), and their values are similar to the ones reported for other transitional waters [48,49]. The elevated KD-T values for Fe and Pb indicate these metals are associated with and transported in the particulate phase, whereas the low KD-T values for Zn and Cd confirm their affinity to the dissolved phase.

The values of KD-L for the large particles were, in general, higher than the KD-S values for the smaller ones. This is consistent with the removal from solution through flocculation of Fe and Mn and co-precipitation processes for trace metals, resulting in elevated metal contents of the large particles (Figures 3 and 4). Figure 5 illustrates the variation of KD values for Cd, Zn, and Cu with salinity in the inner part of the lagoon (n = 10). These plots are advantageous to the widely used metals concentrations/salinity relationships because they allow for the exchange processes between the dissolved and particulate phases to be considered [49]. The KD-S values for Cd and Zn decreased with increasing salinity (r = −0.869; = 0.001 and r = −0.740; = 0.013, respectively), but not the KD-L. These results show that the exchange processes take place predominantly between the smaller fraction of SPM (<8 μm) and the dissolved phase, rather the coarser fraction of SPM (>8 μm). The lack of a clear relationship of the KD-L values for Cd and Zn with salinity could be attributed to their strong binding to Mn oxyhydroxides of the large particles (Table 2). Turner et.al. [44] showed that, when Cd is bound to Mn oxides, it is less prone to desorption across the salinity gradient. In contrast, the decrease of the KD-S values with increasing salinity could be attributed to desorption. In the case of Cd, this behavior is often attributed in the formation of highly stable and soluble chloro-complexes [1,50,51]. As far as Cu is concerned, both KD-S and the KD-L values decreased with increasing salinity, suggesting the removal of both SPM fractions to the solution. The non-conservative behavior of Cu has been ascribed in more detailed studies to the strong Cu-complexing ligands such as organic colloids and dissolved organic matter [52,53]. Dissolved Cu concentrations did not vary between the inner part and the inlets of the lagoon. Thus, it can be suggested that desorption from the solid phase enhances the dispersion of dissolved Cu beyond the boundaries of the lagoon.

**Figure 5.** Variation of partition coefficient (KD) with salinity the inner part of the lagoon: (**a**) KD-S for Cd of the small particles (<8 μm) (**b**) KD-L for Cd of the large particles (d < 8 μm), (**c**) KD-S for Zn, and (**d**) KD-L for Zn, (**e**) KD-S for Cu, and (**f**) KD-L for Cu.
