The reconstruction of river system pollution changes with metals in shallow floodplain water reservoirs during the onset of the post-industrialisation period

Towards the end of the 20th century, Poland’s economy experienced a transformation in terms of its industry and increasingly stringent environmental requirements. Together, these resulted in the general improvement of the quality of the aquatic environment. The Chechło River catchment is an example of such changes, where the closure of many industrial plants (including a zinc-lead [Zn-Pb] mine), the reclamation of several hot spots and the extension of sewage treatment contributed to a general decline in sediment pollution with Zn, Pb and Cd. The aim of the investigations was to evaluate the rate of these changes in order to assess the river recovery rate to the pre-industrial pollution level. The research involved the comparison of sediment pollution in subsidence reservoirs from two different eras: the peak of pollution and the post-industrial era. We observed a decrease in sediment pollution, mostly influenced by the closure of the Zn-Pb mine in 2010, despite many sources being closed or mitigated at that time. The change in sediment pollution was very well observed in the younger reservoir of the post-industrialisation period which is an efficient trap for sediments transported since ca. 2007. Considering that the sampling took place less than 10 years after the closure of the mine, we could regard the 3–4-fold drop as a rapid change. In older reservoirs, which were active already during the period of peak river pollution but connected with the river only by narrow ditches, changes in sediment pollution were minor. Pollution changes expected in the future will be much slower because the supply of pollutants from diffuse sources has dispersed in the prevailing part of the catchment.