Distribution and ecological risk of Iodinated and Gadolinium-based contrast agents in an impacted basin of central Mexico

Abstract

Iodinated X-ray contrast media (ICM) and Gadolinium-based contrast agents (Gd-CAs) are extensively used in medical imaging and are excreted unchanged by patients. However, conventional wastewater treatment plants (WWTPs) are ineffective at removing these compounds, leading to their discharge into surface waters, where they persist due to their recalcitrant nature. Although the presence of both groups of contrast agents in the aquatic environment has been documented, data on their co-occurrence, distribution, and ecological risk in freshwater systems remain scarce. This study addresses this knowledge gap by providing the first comprehensive assessment of the co-occurrence, spatial distribution, and ecological risks of ICM, specifically amidotrizoic acid (DIA), iomeprol (IOM), iopamidol (IOD), and iopromide (IOP), as well as anthropogenic Gd (Gdanth, encompassing all Gd-CAs), in the surface waters of the Atoyac River basin in Mexico. Concentrations of these contaminants ranged from 6.38 ng L-1 for Gdanth to 3,700 ng L-1 for IOP, with the highest levels measured near the WWTP discharge sites and areas with a high density of medical facilities. Elevated concentrations were also observed in river sections with fewer upstream medical facilities, suggesting additional sources or transport mechanisms. In the reservoir, the concentrations of IOD and IOP were high across nine sites, ranging from 261.10 ± 61.20 ng L-1 to 412.20 ± 48.50 ng L-1. This finding highlights their persistence and resistance to biodegradation in aquatic environments. Interestingly, while ICM and Gd-CAs were released into the river from medical facilities, they did not co-occur in the river waters, indicating distinct environmental behavior and/or sources. Finally, deterministic ecological risk assessments revealed that contrast agents posed no ecological risk to aquatic species. This study is the first to document the co-contamination of ICM and Gd-CAs in river waters and to evaluate their associated ecological risks, providing critical insights into the environmental presence of these medical imaging agents.