Good for groundwater - bad for crops? Plastic particles release
pollutants in upper soil layers

Date:
September 16, 2021
Source:
University of Vienna
Summary:
In agriculture, large quantities of nano- and microplastics end up
in the soil through compost, sewage sludge and the use of mulching
foils. The plastic particles always carry various pollutants with
them. However, they do not transport them into the groundwater,
as is often assumed.

Environmental geoscientists have now determined that the plastic
particles release the pollutants in the upper soil layers: they
do not generally contaminate the groundwater, but have a negative
effect on soil microbes and crops.



FULL STORY ==========================================================================
In agriculture, large quantities of nano- and microplastics end up
in the soil through compost, sewage sludge and the use of mulching
foils. The plastic particles always carry various pollutants with
them. However, they do not transport them into the groundwater, as is
often assumed. Environmental geoscientists have now determined that
the plastic particles release the pollutants in the upper soil layers:
they do not generally contaminate the groundwater, but have a negative
effect on soil microbes and crops.


========================================================================== Pollutants enter agricultural soils with plastic particles Wastewater and rivers carry microplastics into the oceans. Wind distributes the particles
to the remotest parts of the earth. However, agriculture itself plays a
far greater role in plastic pollution of agricultural land: fertilizers
such as compost manure or sewage sludge and the remains of agricultural mulching foils carry large quantities of plastic particles, so-called
macro-, micro-, and nanoplastics, onto agricultural land. According to
current estimates, for example, with every kilogram of sewage sludge, up
to 300,000 plastic particles end up on agricultural soils -- and with them pollutants. "Plastic always contains so-called additives. These additives ensure certain properties, durability or even the colour of a polymer. In addition, contaminants such as pesticides or pharmaceutical residues may
become adsorbed to the plastic particles," explains Stephanie Castan,
lead author of the study and PhD student at the Centre for Microbiology
and Environmental Systems Science (CMESS) at the University of Vienna.

Common assumption that microplastics carry pollutants into groundwater challenged "The plastic particles eventually release these pollutants
into the environment. We were interested in when exactly they do so,"
adds Castan. The research team reviewed the common assumption that the
plastic particles could transport the pollutants all the way to the
groundwater -- and they came to a clear conclusion: "Our calculations
show that they generally don't do that," says Thilo Hofmann, head of
the study and research group. "The pollutants remain in the upper layers
of the agricultural soil because they are already released there by the polymers." Calculation of transport and desorption time for different scenarios Whether pollutants can migrate to 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 focused on these two key figures -- the transport time and the desorption
time -- and calculated the so-called Damko"hler number: the Damko"hler
number expresses the ratio of the two key figures. "In order to be able to
make clear statements about the conditions under which plastic particles actually serve as transport facilitators for pollutants, we calculated
the Damko"hler number for two extreme settings -- the usual agricultural
soil and a more fractured rocky soil," reports Charlotte Henkel, co-first author of the study. "We also took into account different properties
of plastics and contaminants." Data show that plastic particles do not increase the mobility of pollutants Comparing the calculated scenarios
to measured data from literature, the researchers found no evidence that
nano- and microplastics are significant carriers for pollutants. "The
fact that plastic particles increase the mobility of pollutants in the
soil is only plausible for very specific polymers and specific soil
conditions, for example when soils are severely dried out and washed
out by heavy rain," explains Thorsten Hu"ffer, environmental chemist
and co-author of the study. Contamination of groundwater by this means
is therefore unlikely, he says. "However, we are by no means saying that
nano- and microplastics in agricultural soils are harmless," emphasises
Thilo Hofmann, who, as head of the PLENTY research platform and the Environmental Research Network at the University of Vienna, promotes interdisciplinary research on plastics in the environment. "Rather,
we are showing where the real problem of these pollutants bound to
plastic particles lies: they do not end up in groundwater, but in the
upper soil layers. Here, they can potentially be taken up by crops and microorganisms and subsequently also enter our food." Follow-up study to clarify whether plants absorb pollutants from soil The study thus provides
good news for groundwater, but rather bad news for agricultural crops:
The Environmental Geosciences team will conduct a follow-up study to
determine whether crops can actually absorb the pollutants through the
soil. In a laboratory beaker on her desk, Stephanie Castan is already
growing three lettuce seedlings for the upcoming experiments.

========================================================================== Story Source: Materials provided by University_of_Vienna. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Stephanie Castan, Charlotte Henkel, Thorsten Hu"ffer, Thilo Hofmann.

Microplastics and nanoplastics barely enhance contaminant mobility
in agricultural soils. Communications Earth & Environment, 2021;
2 (1) DOI: 10.1038/s43247-021-00267-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210916142800.htm

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