Deposition of trace metals in sediments of the deltaic plain and adjacent coastal area (the Neretva River, Adriatic Sea)

• The Neretva River sediment load comprises mostly fine-grained materials.
• The terrestrial SOM influences the surface properties and reactivity of sediments.
• The distribution of trace metals is governed by clayey particles and SOM deposition.

The deposition and distribution of trace metals in surface sediments of the karstic, microtidal and low-wave energy environment of the Neretva River delta and the adjacent coastal region were investigated. These processes were studied in relation to the sedimentological characteristics of the deposits, their surface physico-chemical properties, i.e., specific surface area (SSA) and cation exchange capacity (CEC), and the content, characteristics and origin of sedimentary organic matter (SOM). The results indicate two principal mechanisms governing the spatial distribution of trace metals in the delta plain and adjacent marine surface sediments. Firstly, the presence of SOM in the delta plain freshwater sediments exerts a significant effect on their surface properties, thus influencing the distribution of trace metals, particularly Cd. In the Neretva Channel, the highest share of the terrestrial component of SOM (estimated from δ13C values of organic carbon) was found in the proximity of the river mouth. This coincides with the markedly elevated concentrations of Pb, Cu and Cd, indicating their association with organic substances of terrestrial origin. Secondly, the spatial distributions of Ni, Co and Zn in the surface marine sediments of the Neretva Channel are closely related to the sedimentation dynamics and deposition pattern of the river-borne fine-grained particles. The strong correlation between the concentrations of Ni, Co and Zn and the content of Al, Fe and Mn indicates their binding on Mn and Fe oxide and oxyhydroxide coatings at the clay mineral surfaces. The results indicate a low level of anthropogenic metal contamination in the investigated area.