In the present study, chemical water quality changes downstream a pool-riffle type stream located at a former Pb-Zn mining site are discussed. The watershed of the Toka stream (Mátra mountain, Hungary) being under rehabilitation is still continuously affected by a neutralised acidic mine drainage, and temporarily, by an abandoned mine tailing. Upstream the AMD confluence which gives the major part of the water flux in dry season, indeed, the stream is of high water quality, while, directly after that, the electric conductivity is app. quintupled, and the pH as well as the trace element concentrations increase, as expected. However, within the range of 10 kilometres, Fe, Zn and Pb concentrations are all remarkably decreased in relation to the distance from the contamination source, and they almost reach the actual background levels. The electric conductivity does not show significant change along the investigated section of the stream, which would indicate dilution. At the same time, presence of amorphous Fe(OH)3 in the sediment is obvious, based on the pH-Eh relations and geochemical analysis, thus the adsorption processes have considerable role in the decrease of trace element concentrations in solution.
Key words: abandoned mining site, heavy metal, surface water