Abundance and polymer composition of macroplastic debris along the Somber River, Balikpapan City, East Kalimantan

Abstract

Plastic debris accumulating along rivers reflects the intensity of human activities in their catchments. This study assessed the abundance, item types, and polymer composition of macroplastic debris on the riverbank of the Somber River, Balikpapan City, East Kalimantan, a river approximately 23.6 km long with a catchment of about 3,932 ha that discharges into Balikpapan Bay. Sampling was conducted at three stations selected by purposive sampling from upstream to downstream. At each station, macroplastic items larger than 0.5 cm were collected from three 1 × 1 m plots along a 15 m transect, classified by item type following the NOAA marine debris guidelines, and identified for polymer type using Fourier transform infrared spectroscopy (FT-IR). The number, weight, density, and polymer composition of the debris were calculated, and the relationship between upstream–downstream distance and macroplastic density was tested using simple linear regression. A total of 21 macroplastic items weighing 59.16 g were collected, giving a mean density of 2.33 items/m² and 6.57 g/m². Density increased from 0.67 items/m² at the upstream station to 5.33 items/m² at the downstream station. Food packaging (42.9%) and plastic bags (19.0%) were the dominant item types, while polypropylene was the dominant polymer (55.44% by weight), followed by polyethylene (21.25%), olefin fiber (20.50%), and polyester (2.81%). Distance along the river explained 77% of the variation in macroplastic density (R² = 0.77). These findings indicate that plastic pollution in the Somber River originates mainly from the household activities of riverside communities and highlight the need for integrated waste management to protect the river and the coastal ecosystem of Balikpapan Bay.