Fipronil and its metabolites in a tropical estuary: multi-compartment distribution, trophic transfer, and risk...

Fipronil and its metabolites (FIPs) were systematically investigated in water, sediment, and aquatic organisms collected from the Nandu River Estuary, a typical tropical estuary in Southern China. Fipronil sulfone was the predominant metabolite across all matrices, exhibiting a 100% detection frequency in estuarine water, sediment, and biota, with mean concentrations of 0.445 ng/L, 0.149 ng/g dw, and 0.020-0.244 ng/g ww, respectively. Total FIPs concentrations (ΣFIPs) in aquatic organisms ranged from 0.020 to 0.470 ng/g ww, with taxonomic variation: fish (0.102 ± 0.107 ng/g ww) > mollusks (0.092 ± 0.064 ng/g ww) > shrimp (0.051 ± 0.042 ng/g ww). Biota-sediment accumulation factors (BSAFs) were <1 for all target compounds, confirming sediments as the primary sink. Trophic magnification factors (TMFs) for fipronil (1.85) and fipronil sulfone (0.91) were not statistically significant (p > 0.05), indicating weak or negligible biomagnification. Ecological risk assessment revealed phase-dependent patterns: moderate aqueous risks contrasted with widespread sedimentary hazards, with fipronil exceeding the high-risk threshold (RQ > 1) at 92.9% of sediment stations. Human health risks via seafood consumption can be negligible, with hazard quotients (HQs) ranging from 4.21 × 10^-5 to 4.76 × 10^-4, several orders of magnitude below safety thresholds (HQs <1). These findings demonstrate that while fipronil contamination in this estuary poses minimal direct human health concerns, sedimentary reservoirs represent an overlooked but significant ecological threat, highlighting the necessity of metabolite-specific, multi-compartment monitoring in estuarine risk management.