Legacy Pesticides in Aquatic Systems: Mechanistic Persistence, Regulatory Lag, and Catchment-Scale Remediation Imperatives

Legacy pesticides remain a persistent environmental challenge despite regulatory bans and reductions in agricultural pesticide use. Continued detections in surface water and groundwater indicate that historical residues stored in soils and sediments contribute to long-term contamination. This review evaluates the mechanistic processes governing legacy pesticide persistence, including adsorption–desorption dynamics, sediment storage, preferential flow pathways, and groundwater recharge behavior. A structured literature synthesis of peer-reviewed studies published between 2000 and 2025 was conducted to assess monitoring trends, physicochemical determinants of persistence, and remediation technologies applicable at the catchment scale. Evidence indicates that exceedances of the 0.1 µg L⁻¹ regulatory threshold remain frequent in vulnerable agricultural regions because of delayed release from environmental reservoirs rather than ongoing application alone. Adsorption-based technologies, particularly activated carbon systems, show potential for practical mitigation, while biochar amendments and advanced oxidation processes offer complementary treatment pathways. The analysis indicates that regulatory restrictions should be coupled with targeted remediation and soil risk assessment to achieve measurable improvements in water quality. An integrated catchment-level management framework is therefore proposed to address legacy contamination and support sustainable water resource protection.