Dissolved rare earth elements distribution and fractionation in a subtropical coastal river: a case study from Jiulong River, Southeast China

Dissolved rare earth elements (REE) can reflect the environmental changes in river systems and trace the mixing processes between rivers and the ocean at the estuary. In this study, the dissolved REE concentrations were measured in the river and estuary of the Jiulong River, which supplies drinking water to about one-third of the population of southern Fujian Province. The results showed the total dissolved REE varied within a wide range, from 19.94 to 2041.58 ng/L. (La/Yb)_N varied between 0.15 and 1.03 in the Jiulong River, and apparent fractionation between LREE (depleted) and HREE (enriched) was noticed, especially at the estuary. The fluctuations of dissolved REE concentrations were mainly related to the riverine pH and the complexation with colloids and ions. Higher dissolved REE concentrations in the Xixi River were attributed to the application of chemical fertilizer. The relatively lower concentrations in the estuary may result from salt-induced flocculation, and the preferential removal of LREE resulted in lower (La/Yb)_N and more pronounced HREE enrichment in the estuary. The Ce anomalies observed in the Jiulong River were associated with redox conditions, while positive Eu anomalies were linked to the dissolution of the rocks within the basin. The distinct Gd anomalies were observed in N4 and W10 flowing through the urban area, suggesting the point source inputs from the wastewater associated with anthropogenic activities. This study highlights the effect of natural processes and human activities on dissolved REE in fluvial-estuarine systems, which should raise concerns regarding anthropogenic inputs of REE to rivers and coastal regions.