Spatial patterns of rare earth elements and trace metals in an acid mine drainage-impacted wetland ecosystem

Climate trends in the Colorado Mineral Belt have intensified acid rock drainage (ARD) and acid mine drainage (AMD), increasing the need to understand trace metal and rare earth element (REE) cycling in affected watersheds. This study investigated hydrologic and biogeochemical controls on metal and REE concentrations across an AMD-impacted wetland located below an abandoned mine. Wetland surface waters had higher conductivity, sulfate, trace metal, and REE concentrations than underlying groundwater. REE concentrations varied spatially, with highest levels in wetland surface waters where Ce, Nd, and Y concentrations reached ~100 to 200 μg/L, exceeding those of traditional AMD contaminants (e.g. Cd and Pb). Flow patterns and residence times influenced trace metal distributions, with Zn and Cu concentrations of 16.4 and 0.8 mg/L in wetland surface waters compared to 2.7 and 0.01 mg/L in groundwater. Cerium anomaly patterns revealed spatial gradients in oxidative processing, with values ranging from 0.96 in an adjacent stream, 0.51 to 0.97 in wetland surface water, and 0.70 to 0.85 in groundwater, reflecting the influence of extended groundwater–sediment contact allowing progressive REE transformation. These findings demonstrate that AMD-impacted wetlands are multizone ecosystems where hydrologic flow paths control biogeochemical processing of trace metals and REEs.