Assessment Methods

Land to River, Lake, and Ocean Leakage

Borrelle et al. (2020) compiled the annual amount of mismanaged plastic waste entering aquatic ecosystems (covering oceans, rivers, and lakes) in 173 countries from 2016 to 2030. Applying a methodology similar to that of Jambeck et al. (2015), the estimation integrates expected population growth, annual waste generation per capita, and proportion of plastic waste and mismanaged waste. Those variables were integrated using a distance-based probability function, considering the spatially explicit waste generation and downhill flow accumulation.

The leakage in 2016 is the baseline estimation (Table 2), while the 2030 leakage is estimated for three scenarios: business as usual, where waste generation and plastic production follow current trajectories; ambitious, which draws upon existing global commitments in leakage reduction; and target (<8 million metric tonnes), estimated in 2010 by Jambeck et al. (2015). Russia tops the list, while two East Asia countries (China and Japan) and five ASEAN countries (Indonesia, Thailand, the Philippines, Myanmar, and Viet Nam) are included in the top 20. Under the business-as-usual scenario, global estimated leakage will reach up to 90 million metric tonnes/year by 2030.

River to Ocean Leakage

More than 1,500 rivers account for 80% of global plastic waste leakage from 31,904 rivers in 163 countries, Meijer et al. (2021) estimated. The estimated global leakage of 0.8 million–2.7 million metric tonnes/year by Meijer et al. (2021) is far below the estimate by Jambeck et al. (2015) in 2010. However, the lower estimate is due to the estimation methodologies, not the reduction of single-use plastics or the improvement of waste management systems. In addition to common variables, such as population, waste generation per capita, and proportion of mismanaged waste, Meijer et al. (2021) utilised a probabilistic model that considered additional variables, including land use, terrain slope, wind, and precipitation. The model was then calibrated and validated against field observations from 2017 to 2020. Despite the difference, the results show ASEAN countries remain the main contributors.

Table 3 lists five ASEAN countries in the top 10 contributors, ranking the Philippines as the largest with seven rivers in the top 10 plastic-emitting rivers (Table 4), followed by Malaysia (ranked 3), Indonesia (5), Myanmar (6), Viet Nam (8), and Thailand (10).

The results are consistent with Lebreton et al. (2017) and Schmidt et al. (2017), who found 1.15 million–2.41 million metric tonnes and 0.41 million–4 million metric tonnes, respectively, of plastic annually flowing from rivers to oceans. The top 20 polluting rivers were mostly in Asia (Table 5) and accounted for more than two-thirds (67%) of the global leakage (Lebreton et al., 2017). Seven rivers in the top 20 were in ASEAN countries: Brantas (ranked 6), Pasig (8), Irrawaddy (9), Solo (10), Mekong (11), Serayu (14), and Progo River (19).

Using underlying mismanaged plastic waste data similar to that used by Lebreton et al. (2017), Schmidt et al. (2017) Eight out of the 10 rivers are in Asia, including the Mekong (10), which flows through five ASEAN countries. The estimated leakage is higher because Schmidt et al. (2017) compiled a larger data set and treated microplastic and macroplastic separately. Reducing plastic leakage by 50% in the 10 top-ranked rivers would reduce total river-based leakage by 45%.

An estimation by Meijer et al. (2021) shows a significant leakage increase from several rivers in the Philippines, in comparison with the estimation by Lebreton et al. (2017).The amount of leakage from the Pasig River increased more than 60%, from 0.039 million metric tonnes/year in 2017 (Table 5). Rivers in ASEAN countries, especially in the Philippines, toppled rivers in China from the top spot as emitters of plastic to oceans, as Meijer et al. (2021) considered the spatial variability of the amount of mismanaged plastic waste within a river basin and utilised climate and terrain characteristics to differentiate the probability of leakage. With these assumptions, relatively small river basins, including those in ASEAN countries (eg the rivers in the Philippines), contribute proportionally more leakage than larger river basins, where the amount of mismanaged plastic waste is similar but located further upstream. Meijer et al. (2021) answer the limitation on Lebreton et al. (2017) and Schmidt et al. (2017), who overestimated the leakage from large rivers and underestimated the leakage from smaller rivers due to the exclusion of those important assumptions. The trend is a backstep and a warning the region must reduce marine plastic debris.

Harmonised Methodology to Support Effective Countermeasures

Although several estimations have been undertaken to capture plastic leakage into the marine environment, more data is needed more often. The estimations differ from one another depending on the scope and methodology applied. To avoid any underestimation, harmonising the methodologies is important. With a harmonised methodology, data can be compared and validated against each other. Using a larger set of data, as done by Schmidt et al. (2017) and Borrelle et al. (2020), will further increase the accuracy of estimations. However, some countries may not have country-specific data, so data estimated using a proxy value with assumptions and a level of uncertainties may lead to underestimation.

For instance, Schmidt et al. (2017) extended by 41 countries the estimation of Jambeck et al. (2015) of the mismanaged plastic waste generation rate from 192 coastal countries. The waste generation rate and plastic composition for these 41 countries were taken from Hoornweg and Bhada-Tata (2012) based on past regional estimations, while the mismanaged plastic waste was calculated based on average values for each World Bank economic classification (high income, upper middle income, lower middle income, or low income). In most developing countries, including India (Nandy et al., 2015), where plastic waste is mostly recovered by the informal sector, a significant amount is excluded from the estimation.

To address this issue, governments should support such research by monitoring leakage in rivers and regularly provide valid waste management data. The lack of actual waste management data, especially in ASEAN countries, might lead to lower or higher leakage estimations, depending on the proxy data. The lower estimation by Meijer et al. (2021) does not necessarily mean the reduction of leakage. By utilising the appropriate harmonised methodology, and supported by valid government data, policies can be formulated and/or evaluated to create effective countermeasures against marine plastic leakage.