Is urban micro-watershed driven organophosphate flame retardants threatening major tropical river basin? First sediment-water exchange evidence from India.

The first baseline assessment of organophosphate flame retardants (OPFRs) in an Indian riverine micro-watershed revealed Σ12OPFR concentrations ranging from 366.0-107,958.6 ng/L (mean: 9,434.0 ng/L) in surface water and from 11.2-4,002.5 ng/g (mean: 869.7 ng/g) in surface sediments. Surface water exhibited significantly higher Σ12OPFRs (p < 0.05) in the urban upstream of Nag and Pili rivers. Alkyl OPFRs were the dominant class in both surface water and sediments. However, a few outlier TPhP concentrations at the upstream of the Nag river suggested localized inputs associated with recreational activities. Principal component analysis, compositional profiles and field observations identified possible source signatures in surface water and sediments, and identified TPhP, TDCIPP, TCIEP, TiPrP, TPrP, TCIPP, TEP and TnBP as potential indicators of municipal wastewater, industrial discharges, polymer-processing activities, urban runoff, and plastic waste inputs. Sediment-water partitioning revealed higher aqueous mobility for TMP and TEP, suggesting potential urban activities and wastewater-derived inputs. Field-derived partitioning indicators (logKOC vs. logKOC') and fugacity fraction (ff) suggested that relatively hydrophilic TMP and TEP preferentially remained in the aqueous phase. In contrast, relatively hydrophobic TDCIPP, TnBP, and TPhP exhibited potential for sediment accumulation. High ecological risk (RQ > 1) for TPhP to fish, invertebrates, and algae at specific sites, while moderate risk for TDCIPP, ToCP, TpCP, and TnBP suggested potential bioaccumulation and trophic transfer. The estimated annual riverine input of Σ12OPFRs was 1.2 t/y, suggesting that urban micro-watersheds are important contributors of emerging contaminants, such as OPFRs, to larger tropical river basins.