From 1998 to 2002 investigations were carried out to estimate both the quantity and quality (heavy metal contents) of suspended matter loads discharged into a floodplain area measuring approx. 5 km 2 of the Central German river Weiße Elster, a tributary of the river Saale in the Elbe river basin. Flood sediments, suspended particulate matter, and floodplain soils were investigated especially for the main pollutants Cd, Zn, Cu, Pb, Cr, and Hg. Supplementary gamma spectroscopy examinations were performed to help to identify the age of deposits in order to estimate sedimentation rates for last decades. The recent flood sediments are contaminated by up to 33 times the geogenic background (Cd). Up to 55% of the total annual SPM load of the river Weiße Elster is relocated into the investigated retention area in the flood-stricken year 2002. The sedimentation rates for the last 50 years vary between 0.5 mm/a and 1 mm/a in far away and rarely flooded parts and between 1 mm/a and 2 mm/a in the frequently flooded parts of the inflow of the retention area.
Based on the hydrographical and geological conditions of the river Saale catchment area, the regional geogenic background values are given for heavy metals in fluviatile sediments. They serve as a yardstick for evaluating the current sediment load of the Saale. Compared with the data from the 1990s, level reductions in the Saale can only be proven for the main contaminants Hg and Cr. By contrast, heavy metal levels in the formerly highly contaminated Weiße Elster river basin have clearly decreased, in particular Cd, Zn, Ag, and U. At the same time, increased manganese levels throughout the river system indicate a reduction in organic loads and an increase in oxygen levels.
Schwermetalle in den Sedimenten der Saale -Veränderungen in der Belastungssituation seit 1991Ausgehend von den hydrographischen und geologischen Gegebenheiten des Einzugsgebietes der Saale werden die regionalen geogenen Hintergrundwerte für Schwermetalle in Flusssedimenten vorgestellt. Sie dienen als Maßstab für die Beurteilung der aktuellen Sedimentbelastung 2der Saale. Im Vergleich mit Daten aus den 90er Jahren sind im Saalestrom nur bei den Hauptkontaminanten Hg und Cr Rückgänge in den Gehalten nachweisbar. Demgegenüber haben sich die Schwermetallgehalte im ehemals hoch belasteten Teilflussgebiet der Weißen Elster insbesondere bei Cd, Zn, Ag und U deutlich verringert. Gleichzeitig deuten erhöhte Mangangehalte im gesamten Flusssystem auf reduzierte organische Belastung und höhere mittlere Sauerstoffgehalte hin.
Geochemical studies of carbon, nitrogen, delta13C, delta15N as well as Fe, Mn, Cd, Zn, Cr and Hg in suspended matter taken from the river Weisse Elster (central Germany) between 1997 and 2001 reveal significant changes to the composition of the organic sediment load, which correlate with the hydrological period and flow rate. Using C/N ratios and the isotope values of carbon and nitrogen as source indicators, it was found that the organic suspended matter fractions in hydrological winter periods comprise both resuspended mortal plankton material from the riverbed and terrigenous C3 plant material from the clastic input. During the 6 month summer periods, increased bioproductivity results in more dissolved carbon and mineral nitrogen compounds being taken up by the freshly formed aquatic organic substance (freshwater plankton). These compounds stem from bacterial breakdown processes affecting organic components of the river sediment and/or the peripheral soil zone. Increasing fractions of freshwater plankton during the summer period are accompanied by an increase in the nitrogen content and by isotope signatures shifting (delta13C to lower but delta15N to higher values) in the suspended matter. Seasonally opposite correlations between metal contents (e.g. Cd, Zn, Cr, Hg and Fe) and the carbon and nitrogen levels of suspended matter (significantly positive in winter and significantly negative in summer) show that in suspended matter these elements mostly bind to resuspended mortal (rather than the freshly formed living aquatic) organic substance. According to long-term measuring series, between 1993 and 2002 the levels of heavy metals (especially cadmium) in the suspended matter of the river Weisse Elster decreased. Similarly, between 1997 and 2001 the oxygen level in the river Weisse Elster improved. This caused the faster breakdown of organic substance on the riverbed, resulting in the increased uptake of 15N-rich nitrogen compounds into the fresh aquatic organic substance formed every year, and an increase in the conversion of dissolved manganese in the water into insoluble manganese compounds in the river sediment.
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