Publication details
Flood deposits in a modern oxbow lake (Czech Republic): Sedimentary archive of 25-year old river contamination
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| Year of publication | 2009 |
| Type | Conference abstract |
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| Description | Toxic compounds transported by rivers are usually bound to fine-grained solid particles, which are deposited as contaminated fluvial sediments. Much of this contamination is likely to be redistributed, constituting potential serious environmental hazards. Relatively complete sediment archives of river contamination can be found in floodplains and oxbow lakes. We analysed a high-resolution (month- to seasonal scale) stratigraphic record of heavy metal contamination in deposits of an artificial oxbow lake, a former meander of River Morava, a left hand tributary to River Danube. Since 1930s, the lake has been connected to the river channel via a water gate allowing contaminated fine-grained sediments to enter and settle in the lake. The accumulated sediment forms a distinct wedge, about 150 m long, 30 m wide and >2.5 m thick in the proximal part. 137Cs dating revealed a distinct (~300 Bq.kg-1) Chernobyl anomaly at ~130 cm depth, implying relatively high sedimentation rates of ~5.9 cm per year. Three cores were drilled in the proximal-distal direction of the wedge, and correlated using magnetic susceptibility logs and ground-penetration radar sections. Greyscale colour image analysis of X-ray images of two of the cores allowed further enhancement of the correlation on a cm-scale. The X-ray images revealed numerous sand-silt laminae 0.5 to 4 cm thick, which are correlatable over ~100m distance along the sediment wedge. Time distribution of the laminae was estimated from the average sedimentation rate and correlated with time series of monthly river discharge for the last 27 years. The thickest laminae roughly correlate with major flood events. Concentrations of most heavy metals (V, Cr, Co, Ni, Cu, Zn, As, Mo, Cd, Sb, Pb, Hg) show a long-term decreasing trends. These trends are, however, punctuated by high-amplitude, short-term variations, which is related to sediment grain size. While the sand-silt flood layers are depleted in heavy metals, the fine-grained laminae show the highest values of heavy-metal concentrations. We conclude, that the distribution of pollutants in sedimentary archives is dependent on delicate grain-size variations, which may obscure the long-term trends. |
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