Long-term dispersal of heavy metals in a catchment affected by historic lead and zinc mining

Abstract

The Matylda catchment, in southern Poland, was polluted by the discharge of mine waters from a lead and zinc mine that inundated parts of a valley floor and caused the accumulation of metal-polluted sediments. After a partial reclamation of the mine site in the early 1980s, polluted sediments continue to accumulate on downstream floodplains and in fishponds. The aim of this study was to reconstruct the changes in metal dispersal during 100 years of mining and during the 40-year post-mining period and to propose a strategy for pollution mitigation in the area. Analyses of Cu, Cd, Pb, Zn, Mn, Ca, Mg and Fe concentrations, speciation of heavy metals and mineralogical analyses were undertaken on overbank sediment cores and in stream sediments. Concentrations of the same elements and macro-ions soluble in stream waters were also determined. Concentrations of Zn, Cd and Pb in the sediment profiles vary between 40,000 and 55,000, 300 and 600 and 30,000 and 50,000 mg kg-1, respectively. Changes of metal concentrations and the stratigraphy of sediments from the floodplains, stream channels and fishponds suggest rapid changes of metal loads migrating downstream during both the mining and post-mining periods. Since the time of mine closure, fine-grained, mine-derived sediments (ca. 12 cm thick) have been the main source of pollution of post-mining sediments and surface waters. Closure of the mine was followed by a relatively short period of rapid redistribution of sediment-associated heavy metals in the stream channel. Since the 1980s, the floodplain and fishponds have received a constant supply of metals. It contrasts with the slow sediment accretion rate and a rapid decrease of metal concentrations in floodplain pools due to dilution by decomposed leaf litter. A fivefold increase of Cd content in waters over the 4.6 km reach of the Matylda stream indicates continuous leaching of this element from the contaminated valley floor. Unsuccessful mine site rehabilitation is due to leaching of mine-originated sediments dispersed over the valley bottom. However, the rate of metal remobilization over the last 40 years is low because of the small thickness and widespread anoxic conditions that prevail within both recent and mine-originated sediments and the alkaline pH of stream water, which reduces metal mobility. Distribution of the contaminated layer over a large area of the valley bottom precludes cost-efficient catchment rehabilitation.