Sediment-Bound Legacy and Current-Use Pesticides in a Tropical Ramsar Wetland: Spatio-Temporal Dynamics of Environmental Behavior and Ecotoxicological Risk

Vembanad Lake of Kerala, India, is a Ramsar-designated tropical estuarine system influenced by dynamic hydro-sedimentary processes that regulate the transport and persistence of sediment-bound pesticides. This study quantified 56 target pesticides in surface sediments during monsoon and non-monsoon seasons from 2019 to 2022, detecting 31 compounds. Diuron showed the highest detection frequency (68.75%) and mean concentration (3.59 ng g⁻¹). Organochlorine pesticides predominated (45.16%), followed by organophosphates (16.13%), indicating the combined influence of legacy contamination and ongoing agricultural inputs. Significant seasonal variation (p < 0.05) reflected monsoon-driven flushing and sediment resuspension, while non-monsoon conditions promoted pollutant accumulation. Multiple linear regression identified hydrodynamics and sediment characteristics as important determinants of pesticide distribution. Risk quotient and toxic unit assessments indicated greater toxicity potential in the central sector. The detection of DDT metabolites and endosulfan sulfate highlight persistent legacy pollution, emphasizing the need for strengthened pesticide governance, ecological surveillance, and ecosystem-based management.