Urban Environmental Contaminants Migrate Quickly to Natural Ecosystems

City parks like this one just outside of Bangkok, Thailand, are thought by most to be free of many of the toxins common in nearby cities. A new study shows these are just as contaminated as their urban neighbors, and that natural areas even further removed from cities are heavily impacted by them as well. Image: Image by Igor Ovsyannykov from Pixabay

Although government officials may not wish to admit it, urban centers are considered a place where it is acceptable to allow higher amounts of not just airborne pollution from industrial and vehicle emissions, but also places where the presence of microplastic contamination from poor waste management practices, trash and litter, metals, tossed antibiotics and other medicines, petroleum leakage, runoff from heavy pesticides, and other toxic urban waste. This pollution has spread around the globe to even the most remote areas.

It is also widely believed that urban greenspaces put in place within these centers as well as nearby natural areas — even including natural wetlands and flowering regions — are somehow protected from the onslaught of the environmental contamination happening in the urban regions adjacent to them.

A new study points out what probably should have been obvious to all, that both the natural boundary areas and the urban centers are interconnected in a larger ecosystem, and that when a poison is dropped in one habitat it will quickly appear in the other.

It is also the first study to look at this problem systemically rather than by looking at single contaminants and how they propagate.

The study begins by summarizing some of the many dangers of urban pollution on its residents, including on physical health from what people breathe and what they ingest, including the drinking water sources which depend on keeping them pure but are often unavoidably affected by the toxins propagated within the cities.

Then it explains how some of our fundamental assumptions about how the urban and natural areas interact is all wrong.

“Urban greenspaces are expected to be more influenced by contaminants than natural ecosystems, which are geographically removed from anthropogenic activities,” the report explains. “However, studies have shown that contaminants of concern such as metal(loid)s, pesticides, microplastics, and antibiotic resistance genes (ARGs) can be dispersed through aerial transport, uncontrolled waste disposal/littering, and runoff^, and thus may have impacts on adjacent natural ecosystems.”

The scientists involved in the effort looked at this problem of contamination crossing urban/natural boundaries in and around 56 cities in 17 countries. As a means of comparison with what theoretically should be the most pristine regions in the world, the scientists used actual soil samples taken from three remote ecosystems in Antarctica as a comparison reference.

Their investigation examined the presence of contaminants already identified as of high concern to soil microbiome health and for all living things. Those included “eight heavy metals and metalloids (metal(loid)s, hereafter): arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni) and zinc (Zn), 46 pesticide residues, the shape and polymer type of microplastics, and 285 ARGs [antibiotics resistant genes]”.

At a high level, the scientists found similar distributions of contaminants in both the urban as well as natural areas nearby.

For materials such as metals which one would think would have the most difficult time migrating from one region to another, the data revealed many showed up in higher concentrations than expected.

“We found similar levels of metal(loid)s such as Pb, Ni, Cd, and As in soils of urban greenspaces and adjacent natural areas, but urban greenspaces had higher contents of soil Hg, Zn, Cu, and Cr than those in natural ecosystems,” the scientists wrote.

They did acknowledge the means of propagation of those metals to the natural areas needs to be understood better, and as of the date of this study were investigating further how they were transported there. Theories so far are focusing on transport potentially in the air as a primary means.

Pesticides, other chemicals, industrial and home waste, and toxins already transported easily via water or air, were found to be an even bigger concern in terms of “mirroring’ the urban habitat’s environmental problems in nearby natural communities. Yet even in that the scientists acknowledge that more work needs to be done because they did not test for many of the known widespread contaminants of this type, during this round of investigation.

“We detected the presence of pesticide residues, including fungicides, herbicides, and insecticides, in 63% of the surveyed natural ecosystems,” they explained, which suggests just about everything out there of these types is spreading farther into nearby natural environments than previously thought.

”The application of persistent pesticides in urban areas may influence adjacent natural areas, for example through surface runoff and atmospheric transport and deposition,” they continue. “In addition, agricultural pesticide application may contribute to the reported levels of pesticide residues in adjacent urban greenspaces and natural areas by deposition.”

That said, they note that, “Our analyses likely underestimate these levels as we did not measure several widely used pesticides, such as glyphosate, glufosinate, paraquat, and 2,4-Dichlorophenoxyacetic acid. We did not detect a difference in the occurrence of pesticides between urban and natural greenspaces.”

Regarding microplastics, the researchers found “similar contents of soil microplastics in the paired ecosystems worldwide”. Polyester and polypropylene were the most common types of plastics detected, with the form of those plastics appearing most often as fibers. The scientists hypothesized the source of these fibers was likely from “fabric, rope, and nets”.

Though not part of the study’s commentary, the microplastic contamination from fabrics is almost certainly correlated with the rise of the “fast fashion” industries, which encourage consumers to buy the latest looks and toss the others away. Clothes dryers create microplastics and blow them into the air.

One surprise regarding the microplastics part of the investigation was a more consistent average contamination of microplastics in natural areas throughout the world. As in the case of the metal contaminants which were somehow showing up at higher levels in those natural environments, that raised the question as to how they arrived at their destination and in such similar concentrations. That suggested to the scientists that airborne transport of the ultralight plastic fibers may be a part of this.

Once again, not discussed but now what should be a related high concern is that, if fibers are the dominant form of microplastic “migration” from urban to natural centers, and if those fibers are thought to be transported by air, would that mean all creatures which breathe air — humans included, of course — are inhaling increasingly high quantities of those fibers into our lungs. And even if all other pollutants were zero, the presence of those fibers is likely accountable for more health problems such as asthmas and lung cancers than we realize.

The presence of the antibiotic resistant genes (ARGs), which also propagate easily by air and water, is harder to track regarding global migration. But if it is, that also probably means that the presence of this kind of pollutant is already creating havoc worldwide as it helps accelerate the evolution of global “superbugs” which all species will have less ability to fight off.

The authors of the paper conclude their work with a tough message.

“Together, our work demonstrates that soils in nearby natural areas are as contaminated as our urban greenspaces at a large-spatial scale,” they write.

What this means to all of us, especially for urban planners hoping to make their metropolises more “sustainable”, is that it will be necessary to make radical changes in all aspects of how we live, work, and consume to slow the further degradation of the natural world around us.

There are several things that individuals can do to make a difference:

1. To reduce microplastics in the environment, avoid buying synthentic clothing and stick to cotton, hemp, linen and other natural fibers. If you have synthetics then don't put them in the clothes dryer and instead air dry them on a rack. Consider buying used clothing. Don't use plastic bags. Reduce your purchase of all plastics.

2. Buy organically grown food and switch to a healthy plant-based diet.

3. Lobby against trash incineration, a major cause of heavy metal pollution.

4. Consume far less. Recycle, re-use and re-purpose.

“Soil contamination in nearby natural areas mirrors that in urban greenspaces worldwide,” the paper which describes these results in more detail, by Yu-Rong Liu (of the College of Resources and Environment, in Huazhong Agricultural University, China), et. al., was published in Nature Communications on March 27, 2023.