Iron-Controlled Attenuation of Rare Earth Elements in an Acid Mine Drainage–Impacted Watershed, North-Central Pennsylvania

Acid mine drainage (AMD) systems mobilize a wide range of trace metals; however, controls on rare earth element (REE) behavior under field conditions remain incompletely constrained. This study evaluates dissolved lanthanide concentrations across five sites within an AMD-impacted watershed in north-central Pennsylvania, including a control, an AMD source, mixing zones, and a downstream reach characterized by extensive iron precipitation. Dissolved REE concentrations increased under acidic conditions, with the AMD-impacted Coal Creek site exhibiting both the lowest pH and highest total REE concentrations. Intermediate concentrations at Fall Brook reflect mixing between background waters and AMD inputs. Despite elevated iron and sulfate concentrations, the downstream Island Park site exhibited lower dissolved REE concentrations. Field observations of extensive iron oxide precipitation indicate that iron oxyhydroxides act as effective scavengers of dissolved REEs. This work provides field-based evidence that a source–transport–sink framework in which acidic conditions promote REE mobilization, while downstream pH increases and iron precipitation drive REE removal from solution. This work demonstrates that iron precipitation appears to exert a primary control on REE partitioning in AMD systems and highlights AMD-impacted watersheds as both natural attenuation environments and potential targets for REE recovery.