Ecological risks and recent inputs of banned and current-use pesticides in surface water and sediment from Malawi's tobacco-growing region

Pesticides play a key role in conventional agriculture and food security. However, their heavy use poses human and environmental risks. In this study, we employed the QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) method, followed by gas or liquid chromatography-tandem mass spectrometry, to quantify pesticide residues commonly associated with tobacco farming and to assess their ecological risks in river systems. We detected residues of banned and currently registered pesticides in water and sediment. Pyrethroid residues were higher, with concentrations of 7.62–15.2 μg/L in water and 0.51–8.71 μg/kg in sediment (α-cypermethrin). Organophosphates ranged from 0.94 to 13.3 μg/L in water and 0.08–5.37 μg/kg in sediment (chlorpyrifos-methyl); organochlorines from 0.11 μg/L (γ-HCH) to 10.8 μg/L (o,p'-DDT) in water and 0.08 μg/kg (p,p'-DDT) to 5.48 μg/kg (α-HCH) in sediment; and neonicotinoids from 0.02 to 4.19 μg/L in water and 0.03–4.52 μg/kg in sediment (imidacloprid). Seasonality and water quality had weak effects on most pesticide levels (95% credible intervals). Degradation and isomer profiles indicated recent inputs of DDT and HCH. Except for neonicotinoids, all detected pesticides in water posed considerable ecological risks (RQ > 1), and their mixtures could cause acute harm to algae, invertebrates, and fish (STU >1). In contrast, neonicotinoids in porewater primarily threatened sediment-dwelling organisms (RQ > 1), particularly Chironomus spp. This study provides critical evidence of ecological risks posed by tobacco pesticides in aquatic ecosystems. We recommend regular pesticide monitoring to safeguard river health and guide ecological management in tobacco-producing regions. Promoting safer pesticide use is strongly encouraged.