Trace elements and rare earth elements in post-mining pit lakes of the Muskau Arch (Poland): AMD-related enrichment and toxicity assessment

This study presents results for trace elements (TEs) and rare earth elements (REEs) in five pit lakes located within the Muskau Arch, one of the largest regions in Central and Eastern Europe affected by acid mine drainage (AMD). Concentrations of TEs (Ag, Al, As, Ba, Be, Bi, Cd, Co, Cr, Cu, Fe, Li, Mn, Mo, Ni, Pb, Rb, Sb, Sc, Se, Th, Tl, U, V, Zn) and REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were determined using inductively coupled plasma triple quadrupole mass spectrometry (ICP-QQQ-MS). The highest concentrations were recorded for Fe (0.14–156.9 mg/L), which was the dominant TE in all pit lakes except MA1, where Al was dominant. PCA indicated that TEs such as Al, Be, Co, Fe, Li, Mn, Ni, Rb, Sc, Th, and Zn were strongly associated with pit lakes affected by AMD. Two subgroups were identified: (1) Be, Co, Ni, and Zn, which correlated with Al and low pH, and (2) Fe, Mn, Li, Rb, and Th, which correlated with slightly higher pH and anoxic and more reducing conditions. The toxicity analysis of TEs revealed substantial variation among the pit lakes (from extreme to low toxicity) and indicated that the most important TEs contributing to water toxicity were Al, Mn, Zn, and Ba. Total REE concentrations ranged from 0.15 μg/L to 149.3 μg/L, with by far the highest values recorded in MA2, and their concentrations were strongly influenced by pH. The pit lakes generally exhibited LREE (including La to Eu) enrichment, as well as a weaker MREE (including Sm to Dy) enrichment. Positive Gd anomalies were identified at all sampling points. Additionally, positive Eu anomalies were observed in all pit lakes except MA2, which was the most strongly affected by AMD, and positive Tb anomalies were recorded primarily in samples influenced by AMD.