At the crossroads of different fine sediment fingerprinting methods in the Jiu River Basin (SW Romania)

Fine sediments in rivers hold the imprint of the lithological and geochemical features of their origin areas and sometimes intermediate storage, as well as of the influence of human activities. This research addresses the issue of the heterogeneous sources and transfer paths of fine sediments in a medium-sized (10,080 sq.km), complex hydrographic basin, by combining several fingerprinting methods. The study basin belongs to the Jiu River (340 km length), which originates from the Meridional Carpathians and drains the pre-Carpathian hills and plains in SW Romania, before flowing into the Danube, to which it contributes with a considerable volume of suspended sediments (up to 20-25% during floods). A part of this fine sediment load is due to the coal industry in the upper and in the western half of the middle sectors, but also to socialist-time coal mining legacies from the alluvial deposits remobilized during floods, hence the particularity of the sediment chemical composition, which we explore in this research.

Given the geological, geomorphological and anthropic complexity of Jiu River Basin and the different spatial and temporal scales involved in the production and transfer of fine sediments, their fingerprinting was attempted investigated through both conventional (heavy metals and lanthanides geochemistry) and alternative (colorimetry, image analysis and organic petrology) laboratory methods. In order to try to corroborate the different fingerprinting methods, alluvial samples were collected from: a) the Jiu riverbed and alluvial deposits on its banks, and b) the riverbeds of the major tributaries of the Jiu River (intermediate alluvial accumulations from both natural and man-disturbed geochemical sources).

Different number of samples (from the total of 88) were used for each of the fingerprinting method. For the geochemical analyses, coal particles were separated by species (lignite and bituminous coal) by their density, while elemental analyses (for both heavy metals and rare earths) were performed by X-ray fluorescence (XRF) spectrometry (SR EN 15309: 2007) on the subsequent >2.8 g/l fraction. Based on their abundance, concentrations of the most relevant elements were retained for descriptive statistics. The main indicators (Zr/Si, Ti/Fe, Cu/Fe, Cu/S, Ca/Mg, Na/K, different Lanthanides/P ratios) were further correlated with the underlying lithology by means of nonparametric statistical tests. The color-based approach was conducted using a Minolta colorimeter and was further corroborated with the image analysis (performed by supervised classification and segmentation algorithms), to better distinguish the river sediments and coal samples in terms of the color shades and, thus, highlight the presence of coal. Finally, yet importantly, the organic petrology complemented the research by indicating the maceral composition of the coal-bearing bulk and alluvial samples and by improving our knowledge of the proportion of the two coal species present in the fine sediments.

The laboratory analyses of the sediment samples combining several fingerprinting methods contributed to a better understanding of the hydro-sedimentary dynamics, providing new insight into fine sediment sources, their composition and transfer paths within Jiu River Basin.

Key words: coal, fine sediments dynamics, fingerprinting, laboratory analysis, Jiu River Basin