In agriculture, large amounts of nano and microplastics get into the soil with compost, sewage sludge and foils. Researchers have now investigated what happens to them and the pollutants they contain. The good news: The plastic particles usually release their pollutants in the upper soil layers and therefore do not transport them into the groundwater. The bad news: our crops could absorb more pollutants as a result.
Plastic particles have long been everywhere: Wastewater and rivers carry microplastics into the oceans, while wind spreads it to the remotest parts of the world. In agriculture, compost fertilizer, sewage sludge and the remains of agricultural films ensure that large amounts of macro, micro and nanoplastics get into the soil. According to current estimates, up to 300,000 plastic particles can get onto the arable soils with just one kilogram of sewage sludge.
When does microplastic release its pollutants?
But the plastic is not the only thing that gets into the soil with the plastic particles – they also contain pollutants: “Plastic always contains so-called additives. These additives ensure certain properties, durability or the color of a polymer, ”explains Stephanie Castan from the University of Vienna. “In addition, it is possible that contaminants such as pesticides or drug residues cling to the plastic particles.” Once they have reached the ground, the plastic particles release these pollutants into the environment at some point. “We were interested in when exactly they did that,” says Castan. The speed of this release depends, among other things, on whether the pollutants contained in the plastic or accumulated on it can get into the groundwater.
For their study, the research team therefore used a model to investigate whether the plastic particles are transported through the soil layers faster than the pollutants are released. The ratio of these two key figures – the transport time and the desorption time – is also known as the Damköhler number. “In order to be able to make clear statements about the conditions under which plastic particles actually serve as a transport aid for pollutants, we calculated the Damköhler number for two extreme settings – the usual arable soil and a rather rugged rock soil”, reports Castan’s colleague Charlotte Henkel.
Contamination of the upper soil layers
The evaluations showed: Most plastic particles give off their pollutants relatively quickly – long before they sink into the groundwater. “The pollutants remain in the upper layers of the arable soil because they are already released there by the plastics,” reports working group leader Thilo Hofmann. Contamination of the groundwater in this way is therefore very unlikely. “The fact that the plastic particles increase the mobility of the pollutants in the soil is really only conceivable for very specific polymers and specific soil conditions, for example when the soil is very dry and washed out by heavy rain,” explains Hofmann’s colleague Thorsten Hüffer.
“But we don’t mean to say that nano- and microplastics in arable soils are harmless,” emphasizes Hofmann. “Rather, we show where the real problem with these pollutants bound to plastic particles lies: They do not end up in the groundwater, but in the upper soil layers and can possibly be ingested by crops and microorganisms and then get into our food.” The scientists now want to clarify in a follow-up study that they can actually absorb pollutants through the soil.
Source: University of Vienna; Technical article: Communications Earth & Environment, doi: 10.1038 / s43247-021-00267-8