Microplastics in hydrologically connected himalayan urban lakes: Occurrence, potential sources, wastewater emissions and human exposure

This study provides a baseline integrated assessment of microplastic occurrence, abundance, spatial distribution, wastewater emission loads, ecological and human exposure risks in the hydrologically connected lakes of the northwestern Himalayas. Surface water and wastewater samples were analyzed using microscopy, FT-IR spectroscopy and SEM/EDS to characterize microplastic abundance, size distribution, polymer composition and surface morphology. Wastewater discharges from multiple potential point and diffuse sources revealed higher microplastic abundance in the surface water of Dal Lake with (median = 7600 MPs/m³, and IQR = 6250 MPs/m³), compared to Nigeen Lake with (median = 2525 MPs/m³ and IQR = 3450 MPs/m³). This results in elevated ecological risk associated with hazardous polymers, including PVC, PAN, PBD and PA, with a pollution hazard index (PHI) (median = 473.16 and IQR= 1407.73), and a potential ecological risk index (PERI) with (median = 103.12 and IQR = 299.98), placing Dal Lake within the minor to high-risk categories. Whereas, the assessment for Nigeen Lake indicated minor to moderate ecological risk. A process evaluation of wastewater treatment plants (WWTPs) showed higher microplastic removal efficiency in MBBR systems (92.3%) compared to SBR systems (85.5%) with (t = 2.663, df = 12.81, p = 0.020). Despite higher removal efficiency, three MBBR plants and a single SBR plant released approximately 107 and 78 million particles/day, respectively, in to the receiving lakes. These findings provide the first integrated quantitative evaluation of potential source-specific microplastic pathways, wastewater emissions and associated ecological risks in hydrologically connected Himalayan lakes.