Composition and temporal variability in macrolitter intercepted by a passive litter trap in the Aker River in the capital city of Norway, Oslo

Sammendrag

With the launch of Oslo (Norway) municipality’s action plan against plastic pollution in 2019, the municipality deployed a Portbin TrashTrawl in the city’s main river, Akerselva where it still remains operational. The trawl is specifically designed to passively collect litter at the surface of streams and rivers. Litter has since been collected 2-3 times each spring and fall, and analyzed annually by a local NGO funded by the municipality of Oslo. The study showcases how a low-tech litter trapping installation can be used to produce data informing urban waste management and litter prevention. The main aim of the project is to use data from litter collected by the TrashTrawl as a basis for evaluating sources of marine litter in the waterway, and to capture acute incidents and seasonal variations. In addition, the project aims to evaluate the functioning and impacts of the TrashTrawl technology for litter catchment. The technology is based on principles for collection of leaked oil, where all floating materials in the water flow is lead into the TrashTrawl and a collection net. This is an unselective method, meaning that floating organic material is collected along with anthropogenic litter (the former constitutes >80% of trapped material by wet weight, corresponding to an estimate of 60-75% dry weight). The anthropogenic litter was counted, weighted, categorised, and allocated a likely source based on area of usage when possible. Of the items that could be allocated to a source, tobacco related products were the most common, followed by drug-related products and “on-the-go”-products such as food wrappings. In other words, a large part of the litter was user-based and related to activity in the vicinity of the river, including drug-related activities. With cigarette butts identified as one of the most commonly found items, information campaigns and dedicated waste bins have been launched and installed by the city council to incentivise smokers to stop littering. In 2021, the dominating item was Leca aggregates, suggesting a larger spill and illustrating how the setup can identify specific pollution events. The analysis also showed that many of the litter items found in the river will be banned as part of the EU plastic directive, which was implemented in Norway in 2021. Thus, the litter collection and analysis will provide a method for monitoring effects of the single use plastic ban in the future. The study also indicates seasonal and small-scale temporal variations in litter occurrence, an important factor for planning preventive actions and cleanups. The setup has provided valuable information to the local government on waste management and litter prevention, by identifying common litter items in the area.