Agricultural activity on the Mun River basin: insight from spatial distribution and sources of dissolved rare earth elements in northeast Thailand

Rare earth elements (REE) are emerging pollutants of concern, impacted by intensive fertilizer use and discharge of human and animal waste into agricultural watersheds. However, the natural values and potential anthropogenic enrichment of REE in aqueous systems of the agricultural basins remain poorly understood. This study investigated the spatial variation of dissolved REE in a predominantly agricultural river (Mun River) in northeast Thailand. Dissolved ΣREE concentrations in the Mun River ranged from 5.08 to 272.91 ng/L, with the highest concentrations observed in the middle reaches where agricultural fertilizers and wastewater increased dissolved REE concentrations. The PAAS-normalized patterns and dissolved Eu anomaly jointly reveal that the dissolved ΣREE mainly originated from local rocks and agricultural fertilizers. The dissolved REE in the Mun River is characteristic of a depleted light REE relative to heavy REE, slightly negative Ce anomaly, positive Eu anomaly, and positive Gd anomaly in a punctate distribution. The correlation analysis of (La/Yb)N with fluvial pH and HCO3− indicates that the water environment characteristics of the Mun River control dissolved REE fractionation. The Ce anomaly is associated with the oxidation environment, whereas the Eu anomaly is linked to the lithologic inheritance. Positive punctate Gd anomalies are influenced by human-caused wastewater discharge and applying fertilizers, raising Gd concentrations beyond natural background levels. This study has suggested that the geochemical characteristics of dissolved REE are affected by agricultural disturbances, and future environmental research on dissolved REE is essential to clarifying the impacts of REE on agriculture, the environment, and human health.