Sites of historical application act as long-term sinks for legacy persistent organic pollutants (POPs), yet assessing their environmental stability is confounded by physical sediment transport. To evaluate historical POP persistence, spatial mobility, thermodynamic bioaccessibility, and ecological risks, we utilized a model terrestrial-aquatic system (Hamilton Harbour wetlands, Canada) with a history of dichlorodiphenyltrichloroethane (DDT) applications (1945-1969). Geochemical analysis of DDT and its metabolites, dichlorodiphenyldichloroethane (DDD) and dichlorodiphenyldichloroethylene (DDE), collectively termed DDx, in terrestrial soils and aquatic sediments indicated localized zones currently retaining up to 630 ng DDx g-1 dry weight, exhibiting spatial heterogeneity aligning with likely historical applications. While empirical mass-dissipation models suggested apparent half-lives of 6.2-20 years, continuous-input kinetic modeling (DDT/∑DDx)-utilized to bypass unquantifiable historical inputs-indicates physical redistribution masks true chemical persistence, with degradation half-life estimates spanning 24 to >195 years. Furthermore, aerobic weathering signatures (DDE/DDD >1.0) in surface sediments mirrored adjacent terrestrial catchments, indicating terrestrial soils act as eroding, ongoing secondary sources. This physical mobilization may drive substantial ecotoxicological threats. While buried aquatic residues pose a latent hazard (Hazard Index (HI)≤91) relevant to burrowing overwintering organisms, the ongoing erosion of terrestrial soils (HI ≤ 72) poses a more immediate surface risk. Complementing these HI, equilibrium partitioning modeling suggests dissolved porewater concentrations exceed aquatic safety guidelines by factors up to >100. Ultimately, the study suggests that mitigating the secondary fluxes of legacy residues at dynamic historical application sites requires management strategies that extend beyond monitoring the receiving basins to address the physical stability of adjacent source sites.
