Microplastic pollution in riparian soils of the rapidly growing city of Thimphu, Bhutan

Abstract

Plastic has become a vitally important material for humans; however, the large amount of plastic waste generated annually pollutes the environment. Plastic decomposition generates microplastics (MPs), which have emerged as a concerning global environmental pollutant because of their potential to be more harmful to the environment than their larger counterparts. This study investigated the effects of land use, distance to impervious cover (IC), and plot IC% on the abundance of MPs, as well as the relationship between MPs and urban riparian soil (RS) properties, surface plastic, textile waste, and distance to upstream drainage outlet in the riparian zone (RZ) of the Wang River in Thimphu City, Bhutan. A density separation technique was used to extract MPs from the soil. The results showed that RS in urban sites had higher concentrations of MPs (93.31%) than in other land use (7.69%, p = 0.04). Blue MPs (46.15%) were the most common plastic colour. MPs in urban RS sites were distributed unevenly, ranging from 0 to 500 particles/kg. MP concentration was positively correlated with RS available phosphorus (p = 0.01). Our findings highlight the nature and pathways of MP pollution in developing cities, where the emphasis on economic growth often leads to environmental management being deprioritized. We also identify opportunities for developing cities to reduce MP pollution by recovering surface plastic and textile waste, recycling, and disposing of them in low-impact landfills.