Modelling plastic fluxes with INCA-macroplastics in the Imus catchment: impacts of long-term accumulation and extreme events

Abstract

Aquatic plastic pollution is of rapidly increasing concern threatening water resources, aquatic ecosystems and human wellbeing. Large uncertainties are still present on plastic debris environmental loadings at local to global scales. There is a lack of modelling tools operating at the catchment scale to challenge current knowledge, simulate the impact of waste management initiatives and complement global modelling and observation-based studies. Here we present the first spatiotemporally explicit model for mismanaged plastic mobilization and transport from land to sea from the INtegrated CAtchment (INCA) family. INCA-Macroplastics encompasses all components of the catchment, is driven by available data (weather, population counts, solid waste production rates and composition) and enables calibration and validation against a range of observations. The tool encompasses a litter source module driven by waste generation data as well as weather, land use and population data. The Imus River, Cavite, Philippines, is estimated to be among the top 50 most polluting rivers in the world. Given the large uncertainties on plastic retention on land and in the river system, two calibrations and two emission scenarios covering 1990-2020 are developed to describe plastic fluxes, residence time and stocks throughout the catchment. Model outputs, although not validated, highlight the importance of extreme flooding events in mobilizing and transporting plastic litter. Plastic fluxes to the sea are likely highly variable over seasons (55–75% of yearly exports occur during the wet season) and years and have increased by several orders of magnitude over the last three decades from 5–100 before 1995 to 2000–15000 tons/yr after 2015. INCA-Macroplastics is the first model to handle long-term accumulation of plastic litter on land and its potential remobilization during extreme events. Model outputs explicitly show that current plastic pollution on land can impact fluxes to the ocean for up to 30 years into the future.