Appraisal of geochemical composition and hydrodynamic sorting of the river suspended material: Application of time-integrated suspended sediment sampler in a medium-sized river (the Sava River catchment)

Abstract

The suspended particulate matter (SPM) carried by the rivers shows a wide range of particle size classes, mineralogical and chemical compositions and is mainly influenced by hydrodynamic sorting and provenance during the transport. Here, we have investigated the composition of the SPM in the Sava River and its tributaries (Ljubljanica, Savinja and Krapina) using a time-integrated suspended sediment sampler (TIMS). The representativeness of material collected by TIMS was evaluated comparing fine-grained sediments, single-point SPM and SPM collected using a shallow and deep-positioned sampler. The main results have revealed that the mineralogical and geochemical composition of the material is largely dependent on hydrological conditions. The differentiation of element composition is especially emphasized at low water stage when most of the SPM consists of slow-settling mineral phases (clay minerals and metal oxyhydroxides) which can be trapped in the sampler. During periods of high discharges, differentiation is less prominent, and homogenization of the SPM occurs, mainly as a part of bed load is also taken into suspension. These conditions have proven unfavorable for sampler efficiency, as at least part of the finest particles could not be retained. Additional issues that may occur during TIMS employment relate to biologically driven carbonate precipitation, which is triggered by changes in physico-chemical conditions at low water table in the summer period. Increased concentration of Ca, related to that process, influences the elemental composition of the SPM, which is particularly important when anthropogenic impact or sediment source is assessed. Hence, in order to interpret the geochemical and mineralogical data collected by TIMS, these factors should be taken into account. Our findings emphasize the need for detailed studies of chemical composition of the SPM (time-integrated) in medium-sized rivers and point out the significance of evaluating sampling representativeness during different hydrological conditions.