Earth Science Frontiers ›› 2022, Vol. 29 ›› Issue (3): 129-144.

Distribution and fractionation of rare earth elements in high fluoride groundwater from the North China Plain

LIU Haiyan1,2(), LIU Maohan1,2, ZHANG Weimin1,2, SUN Zhanxue1,2, WANG Zhen1,2, WU Tonghang1,2, GUO Huaming3,*()

1. 1. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330032, China
2. School of Water Resources and Environmental Engineering, East China University of Technology, Nanchang 330032, China
3. School of Water Resources and Environment, China University of Geosciences(Beijing), Beijing 100083, China
High-fluoride groundwater is a major environmental problem that has caused global concerns. Although numerous studies have been done on the chemical characteristics, formation and migration mechanisms of high-fluoride groundwater, little is known about the concentrations and fractionation characteristics of rare earth elements (REE) in high-fluoride groundwater or if REE can be used as a tracer for studying the formation and distribution of high-fluoride groundwater. These uncertainties have limited REE application in high-fluoride groundwater research. In this paper, we investigated the fluoride and REE concentrations and distributions in groundwater collected along a flow path in the North China Plain (NCP) where strong fluoride anomalies in groundwater were observed. We found the groundwater fluoride concentrations ranged from 0.28 to 9.33 mg/L, with 55% groundwater exceeding the China drinking water standard (1 mg/L). According to PHREEQC calculation, fluorine occurs in groundwater as NaF, CaF+, MgF+ and, predominantly, F-(85.42%-99.39%). High-fluoride groundwater is mainly distributed in the central alluvial lacustrine plain and the eastern alluvial marine plain, with 60% at depths below 180 m. Hydrochemical analysis indicated the formation of shallow high-fluoride groundwater is mainly controlled by evaporation and concentration, while deep high-fluoride groundwater is a result of mineral dissolution and competitive ion adsorption through water-rock interactions. The groundwater REE concentration is at pico to nanomolar level, and, according to PHREEQC calculation, REE species are mainly carbonate complexes ( $REECO 3 +$ and $REE(CO_{3})_{2}^{-})$, with 0-1.18% REE in complex with F-(REEF2+ and $REEF 2 +$). The upper continental crust (UCC)-normalized REE patterns are characterized by enrichments of heavy REE (HREE) and middle REE (MREE) over light REE (LREE) and have significant negative Ce anomalies (0.11<Ce/Ce*=CeUCC/(LaUCC×PrUCC)0.5<2.29). The HREE enrichment in groundwater is mainly attributed to the preferential complexation of HREE over LREE or HREE with carbonate in forming more stable carbonate complexes. Along a groundwater flow path, both REE and fluoride concentrations in deep aquifers generally increase following similar trends. Besides, high-fluoride groundwater is more prone to HREE enrichment. These findings suggest that REE can potentially be used as a fluoride indicator for studying fluoride enrichment in natural aquifer systems.