Geochemical Properties of the Sediments of the Great War Island, Belgrade (Serbia)

Abstract

Rivers, which approximately deliver 20 billion metric tons of transported sediment to oceans every year, play a key role in Earth surface processes. Determining the content of potentially harmful heavy metals in rivers sediments is essential for estimating the environmental risk. The main sources of heavy metals in drainage basins are weathering of rocks and anthropogenic activities. The aim of this study is to determine the geochemical properties in river sediments in order to assess the intensity of possible anthropogenic influence. Sediments were collected from the Great War Island in Belgrade, located at the confluence of Sava and Danube River (Fig. 1). Number of authors analyzed Danube and Sava sediments [e.g. 1, 2], but so far no studies have been conducted on the confluence of these two rivers. Contents of Cr and Ni are of particularly interest since high concentrations of these two elements were reported in previous studies, but their origin is still not completely understood. For the purpose of this study sediment samples were collected on five locations: 3 locations along the shore of the island (Kalemegdan, Lido and Small port) and 2 inland locations (at the Big and Small Galijas channels) (Fig. 1). Mineralogical and geochemical properties were determined on 27 samples. Grain size composition was determined using the Laser Microsizer 2000, contents of major and minor elements using X-ray fluorescence diffraction (XRF). Content of organic carbon (Corg) was determined with CHNOS Elemental analyzer. Grain size analysis shows that analyzed sediments form four groups: sands, silty sands, sandy silts and clayey silts. Sediments from the shores show very high variation in dominant grain size, while sediments from the inner part of the island are clayey silts. This is in accordance with hydrological conditions since Island shores and the Big Galijas are under constant sediment input, although very low energy flows occur in the channel. Sediment deposition in the Small Galijas occurs only during extreme floods and consequently fine grain sediments are deposited. Clay content and Corg content have positive correlation. Sediments from the Small Galijas have highest Corg (~5%) and lowest carbonate content indicating calm reduce deposition environment. Future organic geochemical analyses will give important insight into distribution of organic compound at the molecular level. Contents of Cr and Ni in all samples have a positive correlation indicating the same geogenic origin. Content of Cr is grain size controlled in sediments that are under constant water flux, meaning higher Cr concentration in coarser sediments. Sediments in the Small Galijas do not obey this rule due to its specific depositional environment.