<p>In these decades, several studies reported that rivers running through populated area show apparent positive anomalies of Gd in the REE pattern [1,2]. These studies suggested that the increasing of Gd content in the river water is related to the Gd-based contrast agents for magnetic resonance imaging (MRI). The Gd ion could pose harmful effects on human body because it can compete with Ca ion in cellular and biochemical processes, thus, monitoring of Gd-based contrast agents in river water is required. Recently, our group has developed an HPLC-ICP-MS technique for the separation of Gd-based contrast agents [3]. In this study, the presented technique was applied for river water to identify and quantify Gd-based contrast agents present in the river water.</p><p>River water samples were collected from the Muko River (Hyogo). These samples were collected from close by the outlet of the wastewater treatment plant (WWTP), and downstream and upstream from the WWTP. The large positive anomaly of Gd in rare earth element pattern was observed from the outlet of WWTP. As the result of HPLC-ICP-MS analysis, 3 kind of Gd-based contrast agents were recognized from the outlet of WWTP. The concentration of these Gd-species makes up 70% of the anthropogenic Gd. The contents of Gd-based contrast agents were significantly decreased in the downstream, while they could not be detected in the upstream. These results clearly indicate that at least a part of Gd-based contrast agents pass through the treatment in WWTP.</p><p></p><p>[1] Bau and Dulski, Earth Planet. Sci. Lett., (1996). </p><p>[2] Inoue et al., Mar. Pollut. Bull. (2020). </p><p>[3] Okabayashi et al., Talanta, (2021).</p>