Freely Dissolved Organochlorine Pesticides (OCPs) and Polychlorinated Biphenyls (PCBs) along the Indus River Pakistan: Spatial pattern, Air-water Exchange and Risk Assessment

Freely dissolved OCPs and PCBs were measured by using polyethylene passive samplers, at 15 sites during 2014 throughout the stretch of Indus River to investigate spatial pattern, air−water exchange gradients and risk assessment. Concentrations (pg/L) of dissolved ∑OCPs and ∑PCBs ranged from 34–1600 and 3–230. Among the detected OCPs, dissolved DDTs (p,p′-DDE, followed by p,p′-DDT) predominated with levels of 0.48 to 220 pg/L. The order of occurrence for other studied OCPs was as follows; HCB, Endosulfans, Chlordanes, and HCHs. Spatially, dissolved (pg/L) ∑OCPs varied ( p < 0.05 ) as: surface water of Alluvial Riverine Zone (ARZ) showed highest levels (114) followed by Frozen Mountain Zone (FMZ) (52.9), Low Lying Zone (LLZ) (28.73) and Wet Mountain Zone (WMZ) (14.43) respectively. However, our zone wise PCBs data did not exhibit significant differences ( p > 0.05 ). The PCA/MLR results showed that pesticides usage in crop/orchard fields and health sector, electric & electronic materials, and widespread industrial activities as the main source of OCPs and PCBs along Indus River. Air-water exchange of OCPs at FMZ, o,p′-DDE, p,p′-DDE and o,p′-DDD exhibited net volatilization while p,p′-DDD/o,p′-DDT showed net deposition, while all other studied zones (i.e., WMZ, ARZ and LLZ) showed net deposition of DDTs . Nevertheless, air-water exchange of PCBs showed that, there was net volatilization at the FMZ, WMZ and ARZ and net deposition at LLZ. Our results showed that OCPs and PCBs contaminated water intake, played an important role towards the considerable cancer/non-cancer risk (HI and CR values) along the Indus River Flood-Plain.