Variability of heavy metals content in bottom sediments of the Wilga River, a tributary of the Vistula River (Kraków area, Poland)

The concentration of heavy metals in the sediments of the Wilga River has been studied. Comparison of a sample from the city border with that from the river outlet reveals increasing metal concentrations by factors of ca. 27 for Cr, 8 for Cu, 5 for Zn and Ni and 3 for Pb. Since 1989, for most metals, a decreasing trend in concentrations, at compared sampling sites, has been noted. Nevertheless, the amount of heavy metals is still rather high and ranges (in mg/kg) as follows: 185–715 Zn, 38–196 Pb, 27–195 Cu, 3–52 Cr, 3–98 Ni and 1–8 Cd. A strong variability of metal concentration along the river course has been found. Their elevated concentration results from some sources of pollution situated in the river valley, including: industrial plants, their settling ponds and busy roads. It is very probable that morphological features of the river, like river bends, cause the increase of metal contents. It has been found that the amount of heavy metals contributed by grain fractions <20 μm is most frequently comparable with or higher than the proportions of that fraction in the fine fraction of sediment (<63 μm).