# Sediment Dynamics of the Neretva Channel (Croatia Coast) Inferred by Chemical and Physical Proxies
## 5 Discussions
### Metal Distributions
A preliminary evaluation of environmental risks due to contaminant loads within the channel has been carried out, comparing surficial concentration values with quality criteria for marine sediments (Table 2)--threshold effect level (TEL), effect range low (ERL), probable effect level (PEL) and effect range median (ERM) [60; 61; 62]--and with benchmarks (LCB and LCL) for Italian coastal areas [63]. These guidelines are screening tools to predict potential sediment toxicity, linking sediment metal concentrations to the adverse biological effects that may result from exposure to chemicals. The Pb and Cr levels in surficial samples are below the benchmarks of PEL, ERM, and the Italian LCL guidelines, except at sites 10 BC and 14 BC for Pb (ERL and LCB) and 08G for Cr (ERL). However, the incidence of the effect for these samples may be considered low or scarcely probable (8%-30% for Pb and 2.9%-21.1% for Cr; [62; 63; 64]). The Ni surficial concentration exceeds, at several sites, both PEL and ERM guidelines, whereas stations 06 BC and 07G and 10 BC comply under PEL; however, the Italian benchmark LCB is exceeded only at site 08G. In general, the effect incidence for exceeding ERM is common, although for Ni it is only 16.9% [62; 63; 64]. The Pb, Cr and Ni concentrations in surficial sediments of the Neretva Channel fall into an unpolluted class with exception of Pb content of sample 10 BC falling into a moderately polluted class. All this highlights a general unpolluted situation of the channel. The sediment flux (carried out where BCs were available; Figure 7) provides a quantitative estimation of the amount of material that deposits in the channel. Two areas of higher flux are identified: in front the river mouth and in the center of the NC, at site 04 BC. The latter location is not related to any particular morphologic setting of the channel and coincides with the area where we suggest the presence of an eddy in the water circulation, probably favoring a critical area of accumulation. TOC, following a similar distribution, tells the same story (Figure 2b), given that TOC input depends mostly from the river inflow because springs water may be assumed containing very few particles. These observations document that local hydrology plays a key role in controlling the sediment distribution in the area. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Cr** & **Ni** & **Pb** \\ \hline Mean surficial concentration & 56.3 & 58.2 & 25 \\ Minimum & 26.9 & 28.7 & 4.5 \\ Maximum & 84 & 86.8 & 67.9 \\ BG & 45 & 51.8 & 24 \\ WA & 100 & 40 & 10 \\ LCB * & 100 & 70 & 40 \\ LCL * & 360 & 75 & 70 \\ ERL ** & 81 & 20.9 & 46.7 \\ ERM ** & 370 & 51.6 & 218 \\ ISQGs ** & 52.3 & 15.9 & 30.2 \\ PEL ** & 160 & 42.8 & 112.2 \\ \hline \hline \end{tabular} Mean metal concentrations pre-seventies (BG); world average (WA) [64]; * Italian Sediment Quality benchmark: APAT and ICRAM [63]; ** International Sediment Quality Guidelines [60; 61; 62].  
\end{table} 
Table 2: Minimum, maximum and average concentrations (in \(\upmu\)g/g d.w.) of heavy metals in box-core sediments. Comparison with Italian and international sediment quality guidelines benchmarks.