engleski

Heavy Metals in the Alluvial Sediments of the River Drava (Croatia, Slovenia)

The geochemical investigation of soils, alluvial sediments and rocks in Croatia and Slovenia are carried out for the last two decades. This work resulted with number of scientific papers and several kinds of geochemical Atlases in both countries (Halamić & Galović, 1999 ; Šajn et al., 2000 ; Miko et al., 2001 ; Galović et al., 2002 ; Halamić et al., 2003 ; Šajn, Gosar, 2004 ; Gosar, 2004 ; Šajn, 2005 and others). The analyses of soil samples developed on the alluvial sediments of the Drava River have shown the higher values of heavy metal contents in the A0-25 horizons. The anomalies of the heavy metal are probably the consequence of the mining and smelting activities during the last century upstream in Austria (Bleiburg area) and Slovenia (Mežica). The aim of this paper is to determine the contents of heavy metal in the vertical profile (shallow drill-hole up to 2, 6m) and to show their spatial distribution in soils developed on recent alluvial sediments. The investigated profiles (8 in total - 4 in Slovenia and 4 in Croatia) are placed perpendicularly to the extent of the lithostratigraphic units (alluvium, terrace sediments and older sediments in the basis on the river flanks). A considerable dissimilarity can be observed between geochemical signatures of investigated alluvial sediments deposited on the terraces of the River Drava during the Pleistocene and Holocene epochs and recent alluvium, but distinction is still greater when both alluvia are compared to the older rocks whether of Tertiary or still older geologic age. In this respect the things are quite simple in the upper parts of the Drava valley in Croatia, from Ormož downstream, where relationship between the three naturally occurring groups (Alluvium, Terrace, Bedrock) points at the proportional enrichment in almost all elements contained in the river sediments (older terraces and recent alluvium) with respect to the bedrock built of the Upper Tertiary sedimentary rocks, which is supported by the partial results of the multiple discriminant analysis. This certainly results from the decrease in the river velocity due to abrupt widening of the Drava valley and rapid accumulation of fines downstream. Taking into account the whole river basin upstream from the furthermost investigated Legrad profile the chemical composition of silty material of the previously defined groups (Alluvium, Terrace, Bedrock) assumes a slightly different pattern. The change is partly caused by the presence of different lithologic units (igneous and metamorphic rocks of the Alpine metallogenic province as the parent material) incised mostly by the Drava and its tributaries prior to discharge of their waters into Drava and Ptuj depressions, and partly by geomorphological characteristics controlling the hydraulic parameters of the river Drava upstream of Maribor. The standard multiple discriminant analysis with three groups and 37 variables (major and trace elements) also determined the significant geochemical variation between bedrock (lithology) and young Quaternary sediments (Drava terraces and recent alluvium) but with clearly defined intermediary character of the terraces (Fig.1). The first discriminant function explains 70% of total (three-group) system variability and is typically bipolar in nature, discriminating between the groups Alluvium and Bedrock. Considering discriminant potential of the analysed variables (element loadings) one can infer that alluvial sediments along the entire investigated river area (profile sections) are highly enriched in Ca (indicating largely carbonate alluvium) as well as in heavy metals Sr, Pb, Zn, Cd and Mo, which represent anthropogenic signature characteristic for mining and smelter industry as main pollution sources in the upper courses of the river valley. Of secondary importance is the increased content of Na (more as a mirror image of its markedly low content in the bedrock material). In contrast to Alluvium, the Terrace group built of the Upper Tertiary sedimentary rocks and deluvium (close to Maribor, sampling point MB-7 and originated from the weathering of metamorphic and igneous rocks on the Pohorje Mt.) is more abundant in siliciclastic material owing to the higher contents of Al and K. Concerning the trace elements, Zr occurrences are typical, while elements of minor importance such as Hf and Rb are also present. However, a caution must also be issued to this geochemical signature showing to a greater extent an image of considerably lower contents of all chemical elements in the Bedrock group. Absolute incongruity is demonstrated by diagram cross-comparison involving the quadrants occupied by samples and variables (presence of samples – absence of variables, Fig.1). Transition from Alluvium to Bedrock is characterized first by decrease in content of Ca and associated elements, then by gradual increase in average content of all elements seen in their regular approaching to the sector of a diagram allocated to the Terrace group, and eventually by complete depletion in almost all elements in the sector belonging to the Bedrock group. The first discriminant function DF1 does not particularly distinguish the Terrace group from the other two by any of the above-mentioned elements as these mostly concentrate around the main centroid. Such a signature is indicative of average content of all elements among which typically occur Fe, Mg and P of major, and Ti, Mn, Ni, Ba, Co, Hg and As of trace elements. Also, some of these, such as Ni and Co tend to associate with the Bedrock group while, on the other side, Mg, Mn and Ba, and, characteristically, Mo and U tend to affiliate with the Alluvium group. With respect to the distribution of discriminant potential along DF1 it can be genetically properly labeled, the function of age, lying on the axis Ca (Alluvium) – Al, K (non-Bedrock). The second discriminant function DF2 clarifies only 30% of the total variability. Its residual discriminant potential is considerably lower though powerful enough to distinguish the Terrace group from both Aalluvium and Bedrock groups taken together. Discriminant loadings of particular elements specify the Terrace group as penetrating in quarter dominated by elements Al and Na. Contrary to the enrichment with aluminium and sodium indicating the presence of clayey material in the structure of the River Drava terraces, the occurrence of major elements Ca and Sr among major, and Pb, Zn, Cd and Mo among trace elements at the opposite pole of DF2 point clearly at the lowered content of carbonate material in the Terrace group but also at the pristine environmental conditions (relative to Alluvium). Owing to the described distribution of discriminant variables DF2 function can be properly labeled in a genetic sense as a function of anthropogenic impact. Its main axis is directed across Ca (Alluvium) – Al, Na (Terrace).

geochemistry; heavy metals; Alluvial sediments; pollution; Drava river; Croatia; Slovenia

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