Good For The Groundwater - Bad For Crops? Plastic Particles Release Pollutants In The Upper Soil Layers
Study shows that microplastics do not contribute to the mobility of organic pollutants in arable soils
In agriculture, with compost, sewage sludge and the use of foils, large amounts of nano- and microplastics get on the arable soils. The plastic particles always have various pollutants in their luggage. However, they usually do not transport these to the groundwater, as is often assumed. Environmental geoscientists working with Thilo Hofmann have now determined that the plastic particles release the pollutants in the upper soil layers: They therefore usually do not contaminate the groundwater, but have a negative effect on soil microbes and crops. The study by the University of Vienna appears in Nature Communications Earth & Environment.
With plastic particles pollutants get into arable soils
Wastewater and rivers carry microplastics into the oceans. Wind distributes the particles to the remotest parts of the world. Agriculture itself, however, plays a far greater role in the plastic pollution of agricultural land: compost fertilizers, sewage sludge and residues of agricultural films carry large amounts of plastic particles, so-called macro-, micro- and nanoplastics, onto arable land. According to current estimates, every kilogram of sewage sludge releases up to 300,000 plastic particles on the arable land - and with them pollutants. "Plastic always contains so-called additives. These additives ensure certain properties, durability or even the color of a polymer.
The common assumption that microplastics carry pollutants into the groundwater refutes
"The plastic particles release these pollutants into the environment at some point. We were interested in when exactly they do this," adds Castan. The research team checked the common assumption that the plastic particles could transport the pollutants into the groundwater - and came to a clear result: "Our calculations show that they usually don't do that," says Thilo Hofmann, head of the study and research group . "The pollutants remain in the upper layers of the arable soil because they are already released there by the plastics."
Calculation of transport and desorption times for different scenarios
Whether pollutants can migrate to the groundwater by means of micro- and nanoplastics depends on whether the transport of the plastic particles through the soil layers is faster than the release (desorption) of the pollutants from these particles. For the study, the researchers therefore looked at these two key figures - the transport time and the desorption time - and calculated the so-called Damköhler number: The Damköhler number expresses the relationship between the two key figures. "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 have calculated the Damköhler number for two extreme settings - the usual arable soil and a rather rugged rock soil", reports Charlotte Henkel, co-first author of the Study. "
Data shows that plastic particles do not increase the mobility of pollutants
When comparing the calculated scenarios with measurement data from the literature, the researchers found little evidence that pollutants in the luggage of nano and microparticles could migrate into the groundwater. "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 severely dried out and washed out by heavy rain," explains Thorsten Hüffer, environmental chemist and co-author of the study. Contamination of the groundwater is very unlikely in this way. "By no means do we mean to say that nano- and microplastics in arable soils are harmless," emphasizes Thilo Hofmann, who, as head of the research platform PLENTY and the research network environment of the University of Vienna, promotes interdisciplinary research on the subject of plastics in the environment. "Rather, we show where the real problem lies with these pollutants bound to plastic particles: 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 end up in our food."
Follow-up study to clarify whether plants absorb pollutants from the soil
The study thus provides good news for groundwater, but rather bad news for agricultural crops: In a follow-up study, the environmental geosciences team will investigate whether crops can actually absorb the pollutants through the soil. In a laboratory beaker on her desk, Stephanie Castan is already growing three head of lettuce seedlings for the upcoming experiments.
Source: University of Vienna