Spatial distribution and sediment-water fluxes of organochlorine pesticides in Daman Ganga and Tapi rivers, Gujarat, India.

The legacy and newly enlisted pesticidal persistent organic pollutants (POPs) are of serious concern in India due to their indiscriminate historical use and limited ongoing usage for vector control programs. Given the data gaps on monitoring and fate of organochlorine pesticides (OCPs) enlisted as POPs in Indian riverine systems, we investigated the levels, sources, exchange processes, and ecotoxicological risks of OCPs during COVID-19 pandemic, in Daman Ganga and Tapi rivers, ending in the Arabian Sea. The list included legacy POPs particularly dichlorodiphenyltrichloroethane (DDT), heptachlor, aldrin, dieldrin, and endrin and newly listed ones including methoxychlor, chlordecone, isomers of hexachlorocyclohexane (HCH), and endosulfan. Chlordecone was dominant in surface water, with concentrations significantly higher (p < 0.01) than other OCPs. Diagnostic ratios revealed that legacy contaminants primarily originated from aged historical applications, whereas newly listed OCPs reflected recent inputs at site-specific hotspots. Sediment-water partitioning showed that measured log KOC' values were generally lower than predicted equilibrium log KOC thereby confirming non-equilibrium conditions between sediment and water. Most OCPs were preferentially bound to sediments, except HCH and endosulfan isomers, which exhibited higher mobility. Dieldrin and endosulfan sulfate (fₛ/fw > 1.0) indicated net diffusion from sediments to water, while most sites showed deposition (fₛ/fw < 1.0). Ecotoxicological risk assessment revealed that DDTs posed the highest risk to lower trophic levels, particularly diatoms and dinoflagellates (HQ > 1). A majority of Daman Ganga and Tapi downstream sites showed moderate ecological concern (SQGQ, 0.1-1), thereby reflecting that these estuarine sediments are acting as sinks for both legacy and new OCPs.