Evaluation of the content of trace elements in the soils of the river basin affected by flood events

Résumé

The trace elements content assessment in the river basin and the soils under agricultural use of floodplain can be still considered a valid research topic in terms of a growing risk of environmental pollution. Numerous samples of sediments were taken (0-20 cm) from the river banks on the stretch of a few dozen kilometers along the lower Vistula River (Poland) in order to study their trace elements content. For their part, floodplain soils were sampled (0-30 cm) in the vicinity of the riverbed. Each year the flood water deposits an enriched suspension in humic and mineral colloids on the floodplain. Basic properties in bulk soil samples and sediments (texture, pH, cation exchange capacity, electrical conductivity, total organic carbon (TOC) and the total content of trace elements) were determined. A three-step BCR sequential extraction procedure for the study of partitioning of Zn, Cu, Pb and Ni was used. Clay fraction (<0.002 mm) was separated for the X-ray diffraction analysis and chemical analysis. The river bank sediments contained relatively high concentration of trace elements, reflected in the values of the contamination factor, the contamination degree, the geo-accumulation index and the enrichment factor. The clay fraction of the soil samples contained a significantly higher total content of Zn (237-845 mg kg-1), Cu (160-351 mg kg-1), Pb (155-643 mg kg-1) and Ni (52.0-91.6 mg kg-1) in comparison with the concentration of trace elements in the clay fraction separated from the river bank sediments: Zn (97-392 mg kg-1), Cu (79.2-144 mg kg-1), Pb (35.9-108 mg kg-1) and Ni (31.4-64.4 mg kg-1). The statistical analysis of the results showed a significantly positive correlation between TOC and the total content of Zn and Pb in the river bank sediments. The total content of Ni, Cu and Pb was also significantly affected by the content of the clay fraction, whereas the total content of Zn, Cu and Pb was significantly correlated with the content of TOC in the soil samples. The Vistula River bank section under study undergoes river erosion and the deposited sediments during flood events on the floodplain are enriched in numerous elements and compounds. These flooding events are a source of contamination for arable soils and permanent pastures with trace elements. Consequently, these findings suggest that a more detailed river basin monitoring should be faced in order to get more accurate knowledge of the negative effects of flood events on the soil quality.