Estimating plastic discharges from rivers and coastlines in Indonesia

Inland communities can generate large amounts of mismanaged plastic, which can be mobilised and transported into the sea through runoff and rivers. Quantifying plastic inputs from land-based sources and identifying those hotspots that are contributing the most to these leakages is key to target appropriate measures and monitor the effectiveness of efforts to prevent plastic pollution. Using integrated approaches that combine state-of-the-art, high-resolution modelling, Deltares is supporting governments and financing institutions, by providing reliable and realistic baseline estimates of marine litter and plastic discharges into the sea.

Plastic marine pollution is one of the major environmental challenges of our times. It is estimated that every year, 8 million tonnes of plastic enter our oceans, creating severe impacts on marine life but also on human livelihoods. In a scenario of no significant changes, it is estimated that in 2040, 2.6 more plastic will enter aquatic environments than in 2016. Globally, most of plastic waste originates from land, as a result of massive amounts of single-use plastics our societies produce and, the littering behaviour of people and, particularly in less developed countries, very poor waste management. Rivers are known to be key pathways of transport of plastics from land into the oceans. However, it is not well understood nor quantified how plastic enters waterways and how much plastic is discharged by rivers.

Deltares has developed a suite of tools to model the transport of plastic from land into the sea, via rivers, integrating data on plastic waste leakages with hydrology. In 2020, Deltares applied this approach to Indonesia, in order to estimate a national baseline of plastic discharged from rivers and coastlines. Using Deltares suite of models, we simulated the mobilisation and transport of plastic in all major catchments in the country, considering the topography, elevation, soil-type, land-use and spatially and variable weather data, such as rainfall. This was done by first modelling the amounts of mismanaged plastic waste generated and the waste handling practices that originate it, for each of the 514 Indonesian regencies and cities, using the best available local data.

As a result, we constructed a spatially and temporally variable representation of the transport and fate of plastic waste from source into the sea, as a reflection of certain performances of waste generation and management from the communities that live within a catchment area. These results were then aggregated to determine the contribution of each major river to marine plastic pollution, the regencies/cities that were most problematic, as well as the total national figure of plastic discharges from Indonesia into the marine environment.

Such a detailed type of information can help the Indonesian government and other countries to tackle plastic pollution in several ways. For example, by establishing a baseline of total input of plastic into the sea for any given year, progress can be tracked as waste management improves. This approach can also help to pinpoint those communities where interventions are most needed and possibly in which areas of the river clean-up operations might be optimal.