Seasonal Dynamics of Plastic Waste Generation and the South-West Monsoon’s Effect on the Transport of Plastic Debris in Riverine Systems: A Case Study at North Dikkowita Fishery Harbour, Sri Lanka

Abstract

Plastic debris contributes to marine pollution, with a portion transported from land via rivers. Understanding seasonal variations and tracking the movement of plastics are crucial for effective management. Sri Lanka lacks comprehensive studies on this issue due to limited resources. We conducted a year-long study (October 2022-September 2023) at North Dikkowita Fishery Harbour (NDFH). Macroplastic debris (>2.5 cm) was collected from twelve 10 × 1 m2 points at the land–water interface, following the weekly accumulation study method. To analyse seasonal dynamics, data were categorized into distinct periods: October–November (Second Inter-monsoon), December-February (North-East monsoon), March–April (First Inter-monsoon), and May–September (South-West monsoon; SWM). The One-way ANOVA test revealed a non-significant seasonal difference (P = 0.511). Despite the lack of notable seasonal changes by count, 76% (n = 23) of the fishermen were aware of such seasonal changes. Also, observations were made from the collection of identifiable plastic packaging waste. This was important as the packaging (ice packets a dairy production brand) was originated from an industry unit which exclusively distributed these products within a limited area in the Kegalle district. Two packets collected on June 14, 2023, were manufactured on December 20, 2021, and April 20, 2021, while one collected on June 2, 2023, was manufactured in 2022. This could be linked to the SWM activation. These packets travelled ~ 63 km along the Kelani River to NDFH within 1–2 years. The SWM is the primary weather source for the Kelani River basin and the primary ocean current through Dikkowita. During the dry season, the river’s flow decreases to 20–25 m3/s, but during monsoons, it reaches 800–1,500 m3/s, serving as a force for flushing plastic debris. Previous studies suggest the island deflects the eastward-flowing SWM surface current southward, creating an oppositely directed current at Dikkowita. Further recommended studies should utilize tagging with a larger sample size to precisely study debris movement patterns.