## 3 Results and Discussion
### Dissolved and Particulate Hg and MeHg: The Effect of the River Discharge
#### 3.3.2 Distribution Coefficients (K\({}_{\rm D}\))  
In estuarine systems, a chemical constituent can be partitioned in the solid or dissolved fraction reflecting the predominance of adsorption/precipitation or desorption/dissolution processes [80, 81, 82]. In this research, the partitioning behaviour of Hg and MeHg as well as that of Fe and Mn among different environmental conditions at the Nalon River estuary was evaluated by means of distribution coefficients (K\({}_{\rm D}\), L kg\({}^{-1}\)) which were calculated according to the following equation and expressed on a logarithmic scale [80] (Table 1):  
\[\rm LogK_{D}=[\rm Me_{SPM}]/[\rm Me_{DISS}]\]  
where \(\rm Me_{SPM}\) and \(\rm Me_{DISS}\) are the metal concentrations in the SPM and in the dissolved fraction, respectively. Although information on the precise chemical form is not provided by \(\rm logK_{D}\), this index can be useful in the evaluation of the behaviour of contaminants among different phases in aquatic systems [82,83].  
The \(\rm logK_{D}\) values were rather constant among different seasonal conditions of the river discharge and generally high suggesting that the investigated elements were preferentially partitioned in the suspended particles (Table 1).  
This is in agreement with the Eh values (always \(>\)400 mV) indicating oxidative conditions which can promote precipitation and/or adsorption processes. Mercury showed an average \(\rm logK_{D}\) value ranging between 5.66 \(\pm\) 0.16 and 6.65 \(\pm\) 0.21 (Table 1) which was comparable to that observed at the NY/NJ Harbor (5.3-6.5, [2]) as well as at the Isonzo River mouth (Italy, avg. 6.19) where the element was found to be almost completely partitioned in the SPM [57].  
Figure 6: Relationships between DMeHg and DOC (**A–C**), between PMeHg and POC (**D**), between PMn and PFe (**E**) and between PFe, PMn and SPM (**F,G**) along the water column at the Nalon River estuary under different seasonal conditions (spring, summer and autumn).  
In the case of MeHg, the logK\({}_{\rm D}\) values were found to be comparable with those observed for Hg and similar to those observed in the Adour River estuary (SW France, 4.8-6.9, [84]) and in the waters of the N7/NJ Harbor (4.5-5.6, [2]). Conversely, slightly lower values were reported by [PERSON] et al. [14] at the Tagus estuary (2.1-5.5) and by [PERSON] et al. [73] at the Delaware River estuary (3.4-5.1).  
The highest logK\({}_{\rm D}\) values were reached by Fe and Mn, especially in autumn when the river flow was highest, confirming their affinity for SPM which was often the main vehicle for trace elements from inland areas to estuaries and coastal environments [85,86]. Although significant disparities in the logK\({}_{\rm D}\) values were not observed among the different sampling campaigns, a clear relationship describes the partitioning behaviour of Fe and Mn which appeared to be governed by the partially mixed estuarine circulation at the Nalon River estuary and, as a result, by the increase in the SPM content from low to high river flow (Figure S3). Adsorption on the surface of the suspended particles as well as precipitation of Fe and Mn oxy-hydroxides promoted by the oxidative conditions along the water column (Eh always >400 mV) appeared to be responsible for the increase in PFe and PMn from low (summer) to high (autumn) river flow (Figure S3). This was consistent with the behaviour of Fe and Mn in the dissolved fraction since the lowest DFe and DMn concentrations were observed when the river flow was high most likely due to stronger dilution effects.