Rare earth element geochemistry in acid river water from the Numajiri drainage system, Fukushima Prefecture, Japan

Abstract

Acid river water samples collected from the Numajiri drainage system, Fukushima Prefecture, Japan, were analyzed for rare earth elements (REE) and major chemical components to elucidate the origin and behavior of REE in the system. The REE concentrations in the samples were observed to decrease downstream due to mixing with other river water and to co-precipitation with Fe and A1 hydroxide. Measured REE concentrations were in the range of ng L-1 to μ g L-1, and showed zigzag patterns consistent with the Oddo-Harkins law. Leaching experiments with dilute sulfuric acid solution were performed to study the origin and behavior of REE in the river water. We leached unaltered rock samples collected in the vicinity of an abandoned sulfur mine. The REE patterns of the leachate solutions revealed a striking negative Eu anomaly; this is in contrast to the slightly negative Eu anomaly found in the river water samples. The altered rock showed a positive Eu anomaly. These results indicate that most Eu is derived from acid-resistant minerals in the original rocks, and that some Eu in the water samples comes from a subsequent interaction between altered rocks and acid river waters.