Microplastics in River and Mangrove Sediments: A Case Study of a River in Peninsular Malaysia

Mangrove systems that receive water from inland rivers and streams are one of the most important sinks for pollutants. Among the most threatening emerging pollutants is microplastics, which has been found to contaminate the marine food chain. The accumulations of microplastic begin at the lower trophic level, especially via sediment -dwelling organisms. Approximately 75% of these plastics sourced from the inland water. Thus, it is necessary to understand the quality of river sediment and its delta in monitoring the abundance of microplastics This study investigated the spatial abundance, distribution, and composition of microplastics in sediments along a river system and its delta in Peninsular Malaysia. The selected site namely Serkam River and Serkam Mangrove, serves several purposes including tourism, commercial, and recreational activities, while accommodate numerous industrial facilities including petrochemical, rubber, paper and packaging industries. For the purpose of this study, sediments from 11 sampling locations along the river and mangrove area were sampled using a stainless-steel shovel. A density separation method using concentrated NaCl solution was used to extract the microplastics from the sediments, after which they were sieved through a set of Tyler sieves. Extracted microplastics were then observed under a binocular dissection microscope and were grouped according to their types, color and sizes. The concentration of microplastics was found to increase as it goes downstream of the river (average 31.88 ± 10.95 particles/kg). The mangrove sediments contained higher number of microplastics (average 43.26 ± 3.92 particles/kg) which highlight the fact that the mangrove system enables the ‘sink’ of this contaminant. However, the trajectory and speed of microplastics transported are controlled by their physical characteristics (i.e. density, size, and type) and water dynamic conditions (i.e. wind, waves, tides, thermohaline gradients, and the influence of benthic sediments). Among all size and types of the microplastics identified, 1.0 to 5.0 mm white fragments are the most prevalent. The findings revealed that transportation of land-based microplastics occurs along the river system, which will contribute to the total outflow flux of microplastics into the river-mouth and mangrove and finally to the oceans.