Regrettable Substitution for Pesticides: Critical Insights into the Comparative Distribution and Prioritization of Regulated and Unregulated Pesticides in River Sediments

The transition from regulated to unregulated pesticides poses emerging challenges for aquatic ecosystem management. This study combined field monitoring in the Taipu River network with a global literature review to evaluate the contamination status of regulated and unregulated pesticides in river sediments. In the Taipu River network, 56 pesticides and 7 degradation products were detected, with unregulated compounds dominating the contamination profile (45%-85%). Despite regulatory actions, certain regulated pesticides (e.g., parathion) and specific degradation products (e.g., phorate sulfone) still exhibited elevated levels and season-specific variations, indicating potential illegal use. Spatial and buffer zone analyses showed that nonpoint source inputs from croplands and built-up lands were the main sources of pesticides locally. A multicriteria prioritization framework identified 21 priority pollutants, including 13 unregulated pesticides, 5 regulated pesticides, and 3 degradation products. Comparative analysis with global data sets reveals that unregulated pesticides are an emerging worldwide concern, yet they remain insufficiently monitored. These findings underscore the phenomenon of "regrettable substitution" and emphasize the urgent need to establish adaptive, risk-based management strategies for high-risk unregulated substances and their environmental transformation products.