Contaminated sediments as a potential source of Zn, Pb, and Cd for a river system in the historical metalliferous ore mining and smelting industry area of South Poland

Background, aim, and scope Elevated levels of heavy metals in the aquatic and soil systems can be caused by the weathering of mineralized rocks. This enrichment is often considerably enlarged by historical and current mining and smelting activities. In Poland, the most contaminated river systems are those in the Silesia region. The metalliferous ore mining and smelting industries have been the main sources of heavy metal pollutions over the last 100-170 years. The previous and present studies have shown very high concentrations of heavy metals in the bottom sediments of the Mala Panew River, the most polluted tributary of the Oder River. The main objective of this work was to study temporary changes of selected metal (Zn, Pb, and Cd) concentrations in upper layer of bottom sediments at the measuring point near the outlet of the Mala Panew River into the Oder River, and to determine the vertical distribution of the metals in the sediment cores from the most polluted middle part of this river. The mobility of the metals and their potential bioavailability were assessed based on metal partitioning in the sediments and metal concentrations in pore waters. The presented data were compared with metal concentrations in aquatic sediments from similar historical mining and smelting sites in Poland and other countries. Methods The upper layer of bottom sediment samples from the same Mala Panew River measuring point were collected six times in the period 1997-2005, while five sediment cores were collected once from the middle course of Mala Panew River in 2006. Abiotic parameters such as pH and Eh have been determined in situ. Metal contents were determined in the <20 and <63 μm size fractions of sediments after digestion in a microwave oven with aqua regia or concentrated nitric acid. Metal mobility was assessed in the selected sediment cores by the chemical forms of metals (sequential extraction method) and their concentrations in pore waters were investigated. Results The concentrations of Cd, Pb, and Zn in the upper layer of sediments varied, depending on both the season and the year of sampling. Their mean concentrations (from six samplings) are [mg/kg]: Zn 1,846, Pb 229 and Cd 73. The metal concentrations in the sediment cores varied with the depth in the range of [mg/kg]: 0.18-559 for Cd, 26.2-3,309 for Pb and 126-11,153 for Zn, although the highest accumulations generally could be observed in the deeper layers. The most mobile metal fractions, i.e., exchangeable, carbonate and easily reducible fractions, are typical of Zn and Cd. Cadmium was found to be the most mobile metal and its relative contribution ranges from 84 to 96%, while in the case of Zn it ranged from 45 to 94%. Lead is mainly associated with the moderately reducible fraction (30-60%). Relative contributions of metal chemical forms slightly vary with the depth in the sediment profile. The results obtained for the pore water samples show very high concentrations of the metals studied, especially in the case of Cd (31-960 μg/dm³) and Zn (300-4,400 μg/dm³). Discussion Accumulation of Cd, Pb, and Zn in the upper layer of the bottom sediments and in the sediment core samples from the Mala Panew River is very high, considerably exceeding the local geochemical background. High contributions of mobile Cd and Zn and the toxicity of cadmium can cause environmental risk. Our measurements also suggest that mobile metals can migrate into groundwater, whereas the groundwater itself can leach some chemicals from river sediments, because of a relatively high water table in the study area, especially during rainfall periods. Comparison of the results obtained with the literature data from the last decade shows that the concentrations of Cd and Zn in the sediments from the Mala Panew River are the highest among other submersed sediments in Poland and other regions (e.g., the Mulde River, Germany). Conclusions The Mala Panew River is one of the most polluted rivers when compared with similar historical mining and smelting areas in Poland and elsewhere. The sediments studied are strongly polluted with the metals analyzed. In the upper layer of the bottom sediments there has been no reduction of Zn and Cd amounts over the last decade, which could suggests a long-term migration and a secondary contamination. Considerably higher accumulations of metals in overbank sediment cores and in the deeper core section could result from strong contamination in previous decades and translocation of Cd and Zn (secondary pollutants). The relatively high concentrations of the two metals in pore waters support these findings. Cadmium is crucial in the environmental risk assessment because of its high mobility and toxicity. These data are important for water/sediment management in the transboundary Oder River catchment, situated in Poland, Germany and the Czech Republic. Recommendations and perspectives It is important to assess mobility phase and pore water in the contaminated historical aquatic sediments. Such studies may help explain the changes, which take place in the sediment layers as well as at the water-sediment interface. Obtained results should be used for the risk assessment of the historical contaminated sediments at the local river-basin scale. The treatment of contaminated sediments, e.g., dragging activity, should be considered as very important in management strategies in order to avoid remobilization of metals.