Microplastics have been increasingly documented globally in numerous environmental compartments. However, little information exists in the Philippines despite the fact that the country is considered to be one of the largest contributors of plastics in oceans. This study, considered as one of the pioneering microplastic research, evaluated the abundance, distribution, and composition of microplastic pollution in the mouths of five rivers, namely Cañas, Meycauayan, Parañaque, Pasig and Tullahan, draining to Manila Bay. Surface water and sediments samples were collected, then passed through a stack of sieves with sizes from 2.36 mm at the top to 0.075 mm at the bottom. These samples were digested to remove organic matter, and salt solutions were added to allow the microplastics to float. Extracted particles were examined under a stereo microscope, and quantified and categorized into shape, size, color, and type. Results show that microplastics were present ubiquitously at all river mouths but with concentrations varying from 1,580 to 57,665 particles/m3 (surface water) and 386 to 1,357 particles/kg (dry sediment). Fragment was the most abundant shape, while white, blue, and transparent were the most prevalent colors. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed that polypropylene (PP), high and low-density polyethylene (high-density polyethylene and low-density polyethylene) and polystyrene were the main types of microplastics present in the river mouths.

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<p>This study reports on the amount of plastic wastes in five river mouths discharging to Manila Bay, a natural harbor which drains approximately 17,000 km<sup>3</sup> of watershed area. Of the 17 rivers discharging to the Bay, five rivers which run through the densely populated and highly urbanized Metropolitan Manila are included in the study namely, Pasig, Cañas, Tullahan, Meycauyan and Parañaque. A Waste Analysis and Characterization Study (WACS) was conducted to investigate the composition of the wastes that were on the river banks. Samples were taken from the wastes that were found lying on the banks. The wastes collected in each study site varied from each other, although plastic wastes and yard wastes were gathered from all areas. Based on their % wet weight, plastics alone comprised 28% of wastes in Cañas, 46% of wastes in Meycauayan, 42% of wastes in Parañaque, 37% of wastes in Pasig and 27% of wastes in Tullahan. The disposed plastics collected were also characterized and categorized into different types: hard plastic (PP, HDPE), film plastic (PP, PE), foam (PS, PUR) and other type (PVC, PET). In Cañas River, film plastics (79%) were the most ubiquitous type of plastic waste which primarily consist of different sachets of household products and single-use plastic bags. Few hard plastics and other types of plastic such as PVC and PET were collected. Meycauayan River and Parañaque River had almost the same plastic type distribution wherein the most dominant plastic type were hard plastics. These hard plastics collected were mostly composed of bottles of detergents and toiletries. Meycauayan River has relatively fewer establishments near its river mouth, indicating that the sources of the accumulated plastic wastes came from the mid and upstream of the river where the urbanized and industrialized areas were located. Furthermore, even though hard plastics represented 38% of wastes in Paranaque, numerous plastic straw ropes were collected as fishermen use these straws to tie up their boats to the docking area. Significant amount of foams  and PET bottles were also amassed in these rivers. Plastic wastes from the Pasig River were mostly comprised of both film plastics (39%) and hard plastics (30%). The plastic wastes taken were all household products directly dumped by the those residing by the Pasig River mouth. Notable quantity of foams and other types of plastic were fetched from the sampling area. Tullahan River has abundant amount of film plastics (35%) and foams (33%) in its river mouth. Some of these plastic wastes are stuck to the rafts tied up along the bank of the river. Sachets of household products were dominantly present. Few hard plastics and other type of plastic were extracted from the site. Substantial amount of plastic wastes in each of the river mouths signifies poor waste management infrastructure, lack of materials recovery facilities, and lack of discipline of people as these plastics are found to be directly dumped into the water bodies.  </p>