Impact of human activities on rare earth elements in coastal regions: Anthropogenic gadolinium sources, budget, and transport.

Rare earth elements (REEs) are emerging aquatic micropollutants, yet their anthropogenic budget and transport pathway from land sources to coastal regions remain poorly constrained. This study investigates anthropogenic impacts on REEs in Jiaozhou Bay (JZB), located on the west coast of the Yellow Sea in China, by analyzing bay water samples collected during spring 2021 (including inputs from the Dagu River and wastewater treatment plant (WWTP) effluent) and incorporating published winter 2021 bay water and groundwater data. Results revealed elevated total REE concentrations near aquaculture areas (samples collected only in spring, averaging 3578 ± 553 pmol/L), approximately fivefold higher than at other stations, likely linked to REEs-containing feeds and decomposed processes. Anthropogenic gadolinium (Gd) accounted for 12∼41 % of the dissolved Gd in JZB during spring, with concentrations ranging from 2.8 to 25.8 pmol/L. These concentrations were significantly higher than winter (t-test, p < 0.01), likely due to the higher Gd concentration in WWTP effluent during spring. Using a mass balance model, we quantitatively constrained the Gd budget in coastal region from various sources and sinks for the first time. Results showed that WWTP effluent was the dominant contributor to Gd in the bay during spring, accounting for 86 ± 18 %. Notably, WWTPs in Qingdao-the city surrounding JZB-discharged a high Gd flux (136.8 ± 28.4 mol/y), standing out in global rankings. The net flux of Gd​ from JZB to the North Yellow Sea is estimated at 158.3 ± 194.9 mol/y, which is comparable to the amount discharged into JZB by Qingdao's WWTPs. A two-end-member mixing model revealed that 21 ± 5 % (14∼31 %) of water masses significantly impacted by WWTP effluent in JZB were transported to the North Yellow Sea's coastal waters. These findings underscore the urgency to enhance Gd monitoring and regulate WWTP discharges. This study advances the understanding of anthropogenic REE dynamics in coastal regions and provides critical insights for mitigating emerging marine pollutants.