The coastal zone of the Karachi metropolis represents a highly dynamic environmental-geological system where anthropogenic activities directly alter natural sedimentary, hydrogeological, and geomorphological processes. Untreated municipal solid waste and plastic debris discharged via the Lyari and Malir river basins act as anomalous, low-density clastic inputs, significantly modifying coastal sediment transport and depositional dynamics across key geologic environments, including the Clifton beach sands, Sandspit barrier dunes, and the low-energy mudflats of the Manora Channel. From a sedimentological perspective, microplastics behave as lightweight synthetic grains that integrate into the littoral drift, altering the natural porosity, permeability, and thermal conductivity of the intertidal granulometric matrix. Furthermore, the severe clogging of active fluvial channels and estuarine inlets by macro-plastics disrupts surface-water hydrology, which in turn alters tidal flushing and accelerates localized coastal erosion.
Recent empirical studies along the Pakistan coast reveal alarming statistical baselines, showing a mean microplastic abundance of 987.40 particles per kilogram of dry sediment, with the highest overall concentration of both marine and sedimentary microplastics recorded at the Manora Channel. Among these recovered synthetic particles, synthetic fibers represent the dominant morphotype, contributing up to 99% of the total microplastics identified across all sampling locations. As saline tide water and meteoric recharge percolate through these plastic-rich sedimentary strata, they facilitate the leaching and downward migration of toxic additives and adsorbed heavy metals into highly vulnerable, shallow coastal aquifers. Ultimately, the widespread preservation of these persistent plastic clasts within the modern marine depositional record is establishing a distinct anthropogenic stratigraphic marker along the Arabian Sea, demonstrating that managing plastic pollution is vital not only for ecological conservation, but for preserving Karachi’s coastal geomorphology, stabilizing sedimentary budgets, and safeguarding groundwater integrity.