Occurrence and ecological risk assessment of contrast agents in a highly polluted riverine-reservoir system in Central Mexico

The Atoyac River Basin is the largest riverine-reservoir system in Central Mexico. Along its course, the Atoyac River is joined with the Zahuapan River (major tributary) and receives several inputs of untreated or poorly treated urban and industrial wastewaters before flowing to the Valsequillo reservoir in the lower basin. In addition, its contamination is a cause of concern, given the presence of several hospitals and health centers along the main watercourse in Puebla and Tlaxcala States. Water from the Atoyac River Basin serves as a source for irrigation in surrounding areas. Therefore, this study aimed to evaluate the occurrence and ecological risk of five contrast agents—amidotrizoate, iomeprol, iopamidol, iopromide, and gadolinium—through the Atoyac River Basin. For this purpose, a total of 29 surface water samples from this system were collected and analyzed by HPLC coupled to tandem mass spectrometry and ICP-MS (in the case of Gd) in April 2022.

Among the contrast agents, amidotrizoate was not detected in any of the water samples, and iomeprol was only found in the Zahuapan River, with concentrations ranging from 0.023 to 0.091 µg L^-1. In addition, iopamidol and iopramide were detected in the Zahuapan River (up to 0.2 and 0.86 µg L^-1), Atoyac River before the Zahuapan-Atoyac confluence (up to 0.15 and 0.21 µg L^-1), Atoyac River after Zahuapan-Atoyac confluence (up to 1.3 and 3.7 µg L^-1), and Valsequillo Reservoir (0.39 and 0.49 µg L^-1). Although the Valsequillo Reservoir acts as an oxidation lagoon, both compounds were found in the dam curtain with concentrations between 0.2 and 0.35 µg L^-1, respectively. These values were not far from their average concentration in nine water samples analyzed in this reservoir (0.26 and 0.41 µg L^-1). Therefore, this demonstrates their recalcitrant nature, as well as their high persistence in the environment. On the other hand, anthropogenic gadolinium was only detected along the Atoyac River, with concentrations ranging between 9.97 × 10^-3 and 0.35 µg L^-1. The highest concentrations of most contrast agents were found in the urban area of Puebla, corresponding with the Atoyac River after the Zahuapan-Atoyac confluence.

The ecological risk assessment revealed that the analyzed aquatic organisms —fish, daphnids, and green algae— exposed to these contrast agents resulted below the safety limit. Although these contaminants are considered safe, it is important to highlight that this study does not consider temporal concentration changes; therefore, significant anomalous events related to wastewater discharges may occur. In addition, the presence of contrast agents in this basin is highly concerning because its waters are used for crop irrigation. Therefore, the accumulation of these contaminants in soils and plants may have negative impacts on the health of consumers, particularly causing nephropathies and heart diseases.