Rare earth element patterns as process indicators at the water–solid interface of a post–mining area

Abstract Rare earth element (REE, La – Lu) patterns in groundwater are the result of complex solid-water interactions and can therefore be applied as process indicators in this system. At a remediated basement area of a heap in a post–U–mining area (Ronneburg, Germany), REE patterns in near surface groundwater were studied for 10 years (2005–2014) along a groundwater flow path. The groundwater passed through Quaternary sand, silt, and again sand, resulting in increased element and ion concentrations (e.g., Al, Co, Mg, Mn, REE, SO 4 2− ) along the flow path, most of which decreased steadily over the 10 years. The decline in concentrations was more pronounced for observation points in the downstream area. It is likely that, during mining operations, elements were retained by sorption or buffering reactions in the silty/loamy material and have been gradually mobilized with fading subsequent delivery. The REE patterns in groundwater were enriched in middle (Sm–Dy) and heavy REE (Ho–Lu) and featured a positive Ce anomaly. The basic pattern (without Ce anomaly) in groundwater was attributed to mobilization from the Silurian black shale that had been dumped. This signature was also retained in the silty central part currently being leached, resulting in increasing concentrations of heavy REE along the flow path. Efflorescent salts at Gessenhalde indicated capillary rise because they reflected the major groundwater characteristics in the REE patterns. Moreover, especially in the downstream area, Mn oxide coatings occurred in the zone of capillary rise above the groundwater table. These Mn phases (birnessite and todorokite) preferentially accumulated Ce over other REE. Positive Ce anomalies in groundwater indicate at least partial dissolution of these Mn phases. Ferric phases like schwertmannite and goethite were found to slightly enrich light REE. However, based on this dataset, it was possible to relate specific features of the REE patterns to the input pattern of the heap and local geology as well as the formation/dissolution of secondary phases.