Rare earth element abundances in high phosphorus and low iron groundwaters from the Nishinomiya district, Japan: Variations in Ce anomaly, redox state and heavy rare earth enrichment

抄録

High phosphorus and low iron-content groundwaters from the southern Nishinomya district (Miyamizu groundwater), Japan, were analyzed for rare earth element (REE) and some major element concentrations. The chondrite-normalized REE patterns of these groundwaters are characterized by strong heavy REE enrichments relative to light REE, and slight-to-moderate negative Ce anomalies. The magnitude of Ce anomaly increases with decreasing Fe and increasing NO₃^- contents and pH. Such correlations are explained by the redox changes with depth in the area. The heavy REE inclinations of the chondrite-normalized REE patterns correlate positively with Ca and Mg contents and negatively with Fe contents. Heavy REE enrichment is explained by successive water-rock interactions supplying REE to the water in which differential scavenging of light REE by particulate matters occurs. The high P characteristic is achieved by the decomposition of biogenic materials under oxidizing conditions. The low Fe concentration of these groundwaters is ascribed to precipitation of Fe-hydroxides and their subsequent filtration as groundwater flows through the aquifer. The low P and high Fe groundwaters are thought to originate from a deeper aquifer experiencing reducing conditions. The unique geochemical characteristics of the groundwaters of the study area may be closely related to a peculiar geological situation in which an organic-rich layer is seated at relatively shallow depths within an aquifer of relatively large permeability.