First environmental surveillance, source-sink fate modelling, and risk assessment of polybrominated diphenyl ethers in the Tapi and Daman Ganga riverine catchments draining into the Arabian Sea.

Tapi (TP) and Daman Ganga (DG) are the perennial rivers that flow through major plastic recycling and chemical industrial hubs of Surat and Vapi, before meeting the Arabian Sea. These cities are located in southern Gujarat, a state that accounts for over 50% of India's chemical production. We have investigated organic brominated flame retardants, specifically polybrominated diphenyl ethers (PBDEs), in the TP and DG riverine catchments, covering freshwater reservoirs up to the point of discharge into the Arabian Sea. Fingerprinting PBDE sources, fate modelling, and risk assessment provide critical insights for mitigation strategies. Dissolved phase mean total PBDE concentrations in ng/L were higher in midstream (196.17) than upstream (137.07) of TP, while in DG, both upstream (105.78) and midstream (103.49) were comparable. Downstream of these industry-marked catchments, mean PBDE levels were high (TP: 177.38 ng/L; DG: 123.91 ng/L), with a prevalence of heavier congeners in surface water (dissolved and suspended particulate) and in sediment. Conversely, the mean open ocean PBDE level was <20 ng/L. Soil from the industrial corridor engaged in plastic recycling along the DG catchment was found to have the highest BDE-209 (486 ng/g dw). A greater propensity for partitioning of heavier BDEs into sediment, with elevated levels of BDE-183 and BDE-209 in hotspots and estuarine regions, suggests that sediment is a sink for these PBDEs. Source-receptor positive matrix factorization (PMF) model reflects that burning of mixed plastic waste in open dumps is a source for a cocktail of tetra-octa BDE congeners. Recycled plastic debris, including discarded fishing gear, is a possible source for higher BDE-209 load in fishing areas. The lack of inclusion of such local sources resulted in substantially lower predicted levels from the PERSiST hydrological model. For most sites, estimated ecotoxicological risk of PBDEs was high for fish and BDE-209 was the major contributor.