Characterization of microplastic pollution in Koggala lagoon and developing pollution risk indices

Abstract

Microplastic pollution is an emerging concern in both aquatic and terrestrial environments. The present study aimed to assess the abundance, diversity and environmental risk of microplastics (MPs) in the Koggala lagoon, Sri Lanka. Two sites were selected from the lagoon (Site A: lagoon river mouth; Site B: The mid region of the lagoon), and three replicates of water (100 mL for each replicate) and sediment samples (100 g for each replicate) were collected from each site over a six-month period, from August 2023 to January 2024. The number, colour, polymer and shape of MPs were analysed using standard analytical methods. The polymer type of microplastics were determined using FTIR spectrophotometry. The pollution risk indices (PRF - Potential Ecological Risk Factor, PLI - Pollution Load Index, and PHI - Polymer Hazard Index) were calculated for the two sites. The abundance of MPs between two sites was compared using Student’s t test. The mean microplastic abundance was significantly high, in both water and sediment, at the lagoon river mouth (water:0.17±0.04 MPs/cm3, sediment: 0.25±0.02 MPs/g; p<0.05) compared to the mid region of the lagoon (water: 0.12±0.01 MPs/cm3, sediment: 0.10±0.05 MPs/g; p<0.05). Microplastics of different colours (white, green, yellow, blue, red, black, pink and translucent), shapes (fibre, filament, fragment, and film), and polymer types (polyethylene, polypropylene, polystyrene, and ethylene propylene diene rubber) were identified from both study sites. Site A recorded significantly higher mean percentage of fibre (16.0±0.50) and filament-shaped microplastics (11.00±0.00), higher mean percentage of translucent (6.00±0.01) and yellow (7.50±0.02) coloured microplastics, higher mean percentage of polyethylene polymer type (63.00±0.01) and polystyrene polymer type (8.00±0.00) compared to the site B. Site B recorded significantly higher mean percentage fragment (52.00±0.00) and film- shaped microplastics (30.00±0.50), a higher mean percentage of green (39.50±0.01) coloured microplastics and higher mean percentage of polypropylene (39.00±0.02) and ethylene propylene diene rubber polymer types (12.00±0.00) compared to the site A. All three pollution risk indices considered in this study were highest in the site A (PRF= 2.155, PLI= 1.465, PHI= 927). The study area (Koggala lagoon) was classified as low risk (Category I) for microplastic pollution based on the Pollution Load Index (PLI) values. According to the PLI, Koggala lagoon can be categorized as Hazard Level 1. However, according to the PHI, Koggala lagoon shows significant danger for microplastic pollution based on polymer types.