Abandoned tire lying on the seabed in the lake of Bolsena, in central Italy. More than 90% of the fragments retrieved by the study were either fibres or fragments, originated from the degradation of larger plastic objects. Credit: Marco Scataglini/UCG/Universal Images Group via Getty Images
The density of plastic waste in lakes can be higher than that of the largest oceanic garbage patches, and even lakes located in pristine areas contain significant debris, a large international study found, led by Barbara Leoni and Veronica Nava from the University of Milan Bicocca1.
Starting in 2019, Nava and Leoni coordinated an international group of researchers who gathered data on plastic concentration in the surface waters of 38 lakes and reservoirs in 23 countries. “We wanted a realistic picture of the importance of plastic pollution in lakes and to shed light on the pathways that plastics follow on Earth,” Leoni explains. So far, most studies focused on specific lakes and were hard to compare to each other because they used different methods.
“We selected 38 lakes which covered a wide variety of features”, Nava explains. The group considered lakes with different surface areas, depths, and retention time (the average time water spends in the lake), and also different surrounding population density, urban land cover and presence of wastewater treatment facilities.
For plastic fragments larger than 250 micrometers, the three most polluted lakes in the sample were Lugano, Maggiore (both between Italy and Switzerland) and Tahoe in California, with on average 11.5, 8.2 and 5.4 particles per cubic metre, respectively.
Of the 38 lakes surveyed, 21 had between 0.1 and 1 particle per cubic metre, 14 between 1 and 5 particles per cubic metre and 3 contained more than 5 particles per cubic metre. Using a similar approach, a 2010 study2 estimated that the area of highest plastic accumulation in the North Atlantic subtropical gyre contains 1.62 particles per cubic metre.
“We limited our analysis to fragments larger than 250 micrometres to obtain more reliable and conservative estimates, since the density of smaller particles is much more subject to laboratory contamination”, Nava explains.
Scientists used plankton trawls to sample on average 140 cubic metres of water per lake, probing three different areas in each of them.
Of the 9400 fragments retrieved, more than 90% were either fibres or fragments, originated from the degradation of larger plastic objects. Nearly a quarter of the fragments were analyzed with spectroscopy to determine their chemical composition. “We performed all the spectroscopic analyses in our laboratories, to ensure more consistency in the results”, Nava says. Polyester, polypropylene and polyethylene constituted most of the polymers identified.
Looking at the correlations between plastic concentration and environmental variables, researchers found that population density and surface area are the most relevant. Lakes with surface larger than 230 square kilometres have on average 4 fragments per cubic metre. For smaller lakes, plastic concentration increases with population density.
“Larger lakes have longer shorelines and therefore a greater number of pollution sources from land” Nava says, adding that they could also be more subject to atmospheric deposition.
“Our study indicates that worrying levels of pollution can be reached in lakes, that are a valuable resource for human life especially in a changing climate,” Leoni concludes.
