Sources and effects of microplastics in water still not sufficiently known

Because of their suspected harmful impact on the environment and on water quality, microplastics attract a great deal of societal concern. Within the Water Quality Knowledge Impulse (KIWK), which is aimed at gaining a better understanding of the quality of groundwater and surface water, and of the factors that influence this quality, microplastics constitute one of the key focus areas. The Chain Explorer project, which is part of the KIWK, attempts to answer questions related to microplastics, with the ultimate goal of reducing their emissions into the aquatic environment. Where do the microplastics come from? And how can we measure their effects? Two research questions about which there is still a great deal of uncertainty, as Erwin Roex of Deltares, co-author of the Deltafact Microplastics, fully appreciates.

Still no standardised monitoring and analysis

‘Several methods are currently used to sample and analyse microplastics in water,’ says Roex. ‘Microplastics are a relatively new field of research, which is why we don’t have any standardised methods yet. Moreover, their chemical composition and physical form – microplastics are particles – are varied. Since each method uses its own parameters, there is no overview of which microplastics are present in the water system nor of what their sources are. There are of course estimates, for example, for microplastics from car tyres. If you know how many cars are driven in the Netherlands, how many kilometres they are driven here every year, and how many microplastics are released by tyre wear, then you can get a reasonable picture of how many microplastics could end up in the water. But you are making all kinds of assumptions involving significant uncertainties.’

Second knowledge gap: effects of microplastics unknown

There are certainly numerous uncertainties when it comes to the detection of nanoplastics. ‘For this reason they have been left out of the Deltafact Microplastics,’ says Joep van den Broeke, project leader of the Chain Explorer and KWR researcher. ‘We have focused on microplastics measuring between 100 nanometres and 5 millimetres.’ Besides the tracing of the source of the microplastics and the analytical methods, the Deltafact Microplastics points to a second knowledge gap: What are the effects of this substance on the environment and the water quality? Roex: ‘Many toxicity studies have been done to determine what consequences microplastics can have. But the lab work often uses much higher concentrations of microplastics than you encounter in the natural environment. And standardised particles of a single form, size and composition are used. While in the environment you come across a whole range of microplastics. What we need are long-term tests with low concentrations of environmentally-relevant particles. This would resemble the real situation much more.’

Still more Deltafacts

Apart from microplastics, the Chain Explorer also charts the state of research for biocides and consumer products, like shampoo and scrubs. Van den Broeke: ‘The four-year programme of the Water Quality Knowledge Impulse runs until the end of 2021. Following the publication of this first Deltafact within the Chain Explorer, we still expect this year to bring out a number of fact sheets for each substance group. With regard to microplastics, a new Deltafact on the sources and the toxicity will also be presented. And we are also looking at technologies to remove microplastics from water. We are collecting all the existing knowledge about this question, including the effectiveness and applicability of these methods.’

Focus for water managers

What does the Deltafact Microplastics offer those for whom this compact and accessibly-written status report is intended, namely: the Water Authorities, drinking water utilities, Rijkswaterstaat and the provinces? ‘What we have noticed in our discussions with water managers, is that they have trouble forming a clear picture of what is, and what is not, known about microplastics,’ says Roex. ‘Knowledge institutes keep close track of these developments. But there is nothing self-evident about this knowledge reaching the end-user. I think that, with the Deltafact Microplastics, we provide water managers with a good guide to help them bring focus to how they should address microplastics.’ A conclusion with which Van den Broeke completely agrees, while adding a valuable point: ‘As long as too much uncertainty exists about the effects of microplastics, one can’t discuss whether controlling these substances in water should be a top priority of water managers.’