# Sediment Dynamics of the Neretva Channel (Croatia Coast) Inferred by Chemical and Physical Proxies
## 5 Discussions
### Metal Distributions  
Metal concentrations suggest that the river outflow plays only a marginal role in Pb, Cr and Ni surficial distributions (Figure 5). In detail, Pb shows the highest concentration at site 10 BC located in the center of the channel and far from possible direct sources. This may be due to an event of local discharge or to a pulse in the transport of metal enriched sediment, as suggested by the higher sand content at the top of the core and by the N/C values (Figure 2b,c and Figure 3). A second local maximum of Pb content is located in front of the river mouth following a distribution inversely correlated with the distance from the mouth (Figure 5a). Cr and Ni contents (Figure 5b,c) are affected by a source located along the coast to the NW of the river mouth (close to station 08G). Moreover, Cr content shows another local maximum at site 07G (70.4 \(\mu\)g/g), where an increase in the sand fraction is observed, suggesting an additional source. On the other hand, the minimum values of Ni and Cr contents at site 06 BC may indicate a scouring resuspension or not sedimentation, as suggested by the observed N/C values. In the southern part of the NC, where coarser sediments are located, an increase of Cr and Ni contents associated to a low organic carbon and to a low sediment accumulation rate of 2.3 mm/yr (15 BC; Figure 3) suggests that this zone acts as a trap for metal bearing particles. Alternatively, the Cr and Ni abundances in this area may be influenced by sediment inputs from Peljesac Peninsula. Some authors indicate the ratio Cr/Ni as a possible tracer of geo-genic versus anthropogenic influences [56; 57; 58] and suggest it as suitable to determining geochemical baselines in the case of high natural concentrations [34; 35; 59]. In particular, Cr concentrations in south Dalmatia show a mean value of 126 \(\mu\)g/g; that is the highest value found in karstic regions, probably due to the presence of chromite-bearing ultramafic rocks and/or of clastic deposits derived from older mafic magmatic rocks. Similarly, Ni levels in the same area show a mean value as high as 84 \(\mu\)g/g as a consequence of the presence of mafic and ultramafic rock outcroppings in the region. These rocks are probably a source of Ni in alluvial soils of the floodplains of rivers draining this region [57]. The expected Cr/Ni value reported in literature [56; 57; 58] is about 1.5, whereas in the study area Cr/Ni ranges from 0.7 (site 12G) to 1.9 (site 07G) with a mean value of 0.9. This discrepancy may be due to a natural variability but also due to the extraction efficiency of the leaching method. However, ratio values are generally uniform in the area supporting the hypothesis of Cr and Ni background concentrations. According to previous data [17], heavy metals (such as Pb, Cr and Ni) in surficial sediments are tightly linked to fine grain size fractions, and in particular, to clay minerals coated by iron oxides/oxy-hydroxides that may trap metal cations within their crystal-chemical structures. Univariate statistical analysis shown in Table 1 confirms the previous observations for Ni and Cr: that they correlate positively with each other, and with clay and porosity, while they correlate negatively with total and inorganic carbon. In addition, organic carbon shows a positive correlation with fine grain size, in particular, with the clay fraction, and negative ones with total carbon and inorganic carbon. These results may support the hypothesis of clay minerals and organic matter as driving factors of Cr and Ni areal distribution. Pb shows a different behavior, characterized by a negative significant correlation with porosity. Since porosity is correlated negatively to silt and positively to clay, this suggests that Pb is not immobilized in the clay fraction of the sediments having higher affinity with the silt fraction.