Microplastics in the Himalayan environment: a review of sources, atmospheric inputs, and subsurface pathways.

Microplastic (MP) pollution has emerged as a growing concern in remote mountainous ecosystems, yet its presence, transport, and ecological implications in the Himalaya remain significantly underexplored. This review synthesizes current research on MP contamination across multiple environmental compartments including atmosphere, snow and ice, surface water, and emerging subsurface domains in the high-altitude Himalayan region. Key pathways of MP input include long-range atmospheric deposition, cryospheric meltwater, and surface runoff, while shallow groundwater systems are identified as a critical but poorly understood sink. Anthropogenic activities such as tourism, urban expansion, and inadequate waste management further exacerbate MP load in these fragile environments. The unique cryo-hydrogeological setting characterized by steep terrain, dynamic glacier systems, and fractured aquifers may influence MP retention, transformation, and mobility in distinct ways compared to lowland systems. Climate change amplifies this vulnerability by accelerating glacial retreat and permafrost thaw, facilitating the release of legacy pollutants, including MPs, into downstream ecosystems. The interaction of MPs with co-contaminants such as heavy metals, black carbon, and pharmaceuticals may further compound ecological risks, particularly in alpine watersheds. This review also proposes a conceptual model of MP cycling in the Himalaya and introduces a preliminary MP Vulnerability Index to identify high-risk zones for monitoring and intervention. It calls for harmonized sampling protocols, long-term cryosphere-hydrosphere surveillance, and transdisciplinary research to address critical data gaps. By integrating scientific evidence with policy and community engagement, the study aims to inform future strategies for mitigating MP pollution in ecologically sensitive regions.