Flood-related changes in heavy metal concentrations within sediments of the Biala Przemsza River

Flood-related changes of heavy metal concentrations in channel sediments were investigated on the Biala Przemsza River, southern Poland, which receives effluent from lead and zinc mining. Concentrations of Zn, Pb and Cd in samples taken in August 1993 were compared with those in samples collected after a 100-year flood of July 1997 and after a 2-3-year flood in March 1998. The first flood caused a three-fold decrease in heavy metal concentrations in the < 1-mm size fraction over the 40-km reach. The following period (August 1997-March 1998) was characterised by a progressive "recovery" of concentrations to the level observed in 1993. After the second, lower flood, approximating the bankfull discharge, the heavy metal content was decreased in the lower course of the Biala Przemsza but increased near the location where mine waters are discharged into the river. The decrease in heavy metal concentrations was accompanied by a coarsening of near-bank channel sediments caused by winnowing of the fine particles. In the future, pollution levels will decrease less rapidly owing to the mobilisation of secondary sources of pollution such as the overbank deposits. (C) 2001 Elsevier Science B.V. All rights reserved.