Fate of radium in river and lake sediments impacted by coal mining sites in Silesia (Poland)

The Upper Silesian Coal Basin has been extensively mined since the beginning of the 20th century. Wastewaters released from Polish coal mines contain radium (Ra) in ionic form as well as in suspended matter. Although co-precipitation of Ra into radio-barite ((Ba, Ra) SO4) has been enhanced for reducing the impact of mining activities on water quality, sediments in rivers and lakes surrounding mining sites still show relevant activities of both 226Ra (1600 y half-life) and 228Ra (5.7 y half-life) isotopes. The management of this contamination currently relies on natural attenuation. This study focuses on assessing the long-term impact of Ra which has been trapped into river and lake sediments in the vicinity of coal mining sites in Poland. Two geochemical interfaces have been surveyed by sampling sediment, surface and pore waters downstream from coal mining sites: (1) the hyporheic zone of a river, where groundwater tends to mix with surface waters within the river bed; and (2) the water-sediment interface in a lake which was formerly used as a settling pond. Both represent redox interfaces where radium-bearing solid phases (barite, metal oxyhydroxides) can undergo various geochemical processes which have not been quantified yet. Our preliminary results show that pore water geochemistry within the hyporheic zone of the studied river is impacted by diffuse and direct wastewater releases from an operating settling pond. A geochemical model built with JCHESS r suggests that mixing of surface waters with wastewaters favors witherite (BaCO3) precipitation within the riverbed. Additional investigations are planned to assess the consequences of this suspected process on Ra behavior through the hyporheic zone. Moreover further investigations are performed for studying the fate of Ra trapped into BaSO4 within pond sediments where sulfate-reducing conditions could occur with depth due to organic matter degradation and where 210Po (226Ra decay product) is detected into surface waters.