Nature Knows More Than We Do
Capitalism values its money over earth’s biodiversity, even though biodiversity is crucial for the planet’s ecosystems to thrive and bounce back from disasters. We’re increasingly hearing news of dangerous pollutants in our soil and water: newly discovered impacts of a former factory, for example, or critical levels of phosphorus in surrounding streams, elevated levels of toxic metals in surrounding areas’ drinking water, or recently discovered levels of carcinogens in nearby rivers.
The root systems of plants move more water around the continent of North America than all of the continent’s rivers and streams combined. Complex and dynamic ecosystems live under our feet, with a diverse array of lifeforms. As the root systems and underground ecosystems are the first impacted by soil contamination, they are the first responders to the crisis. Plants have been mitigating pollutant impacts on soil for as long as pollution has been a factor, as many are structurally capable of doing so. This process is loosely called phytoremediation.
There are two types of remediating (cleaning) processes for soil: ex situ and in situ. “Ex situ” remediation involves removing the soil from the site, cleaning it elsewhere, and putting the soil back afterwards. This is often expensive and relatively disruptive to the surrounding organisms (and those that live in the soil) compared to “in situ” methods, which involve cleaning the soil where it is.
Phytoremediation, an “in situ” method of remediation, is the process by which plants absorb, volatilize, or contain heavy metals and other contaminants from the soil through their natural structural capabilities. How the plants stabilize or clean the soil they grow in depends on the type of plant, its growth, the soil pH, the type of soil, their proximity to groundwater, and, crucially, what pollutants the plant is dealing with. Organic contaminants are contaminants that can be broken down. Pesticides, personal care products, chlorines, and oil are all examples. These can optimally be broken down by phytoremediators that, throughout their lives, are able to continuously clean the soil through their photosynthesis processes. Inorganic contaminants are a little more problematic. “Inorganic” in phytotechnology refers to any contaminant which is a single element—those that can be found on the periodic table. Inorganic compounds cannot be broken down any further than their current state: lead, arsenic, and cadmium are examples. Case studies indicate that inorganic contaminants can be extracted from soil using certain plants that effectively absorb that contaminant, but often it’s more practical to use plants which will contain the contaminant where it is, not allowing it to leach into the groundwater or spread.
This makes phytoextraction a less than perfect solution to the Water Street property’s pollution: after so many years of manufacturing in that area, the contaminants have leached deep into the soil.
Extracting inorganic contaminants from the soil using plants, phytoextraction, poses the most challenges out of all methods of phytoremediation, and often yields the least effective results. Once the contaminant has been absorbed by the plants’ roots, they are dispersed into the fibers of the whole plant. These plants have to be harvested and landfilled, as the whole plant body then contains high amounts of the contaminant. These plants also ideally should not reproduce; when using certain mustard plants to extract arsenic out of soil, for example, they should be pulled up and disposed of before they flower, as spreading the seeds of these plants also spreads the pollution. Since phytoextractors have to be replaced so frequently, the process is more expensive than most other types of “in situ” phytoremediation.
Plants only impact the soil as far as their roots can reach. This complicates phytoextraction. Since the plants need to be harvested before reproducing, their root systems never get the chance to go too deep. Thinking locally, this makes phytoextraction a less than perfect solution to the Water Street property’s pollution: after so many years of manufacturing in that area, the contaminants have leached deep into the soil.
Phytostabilization, a different method of handling inorganic contaminants, involves planting phytoexcluders: plants which will not absorb contaminants, but will instead hold them in place and prevent their spread. These plants do not have to be disposed of since they aren’t taking the contaminants into their fibers and spreading it, and thus can establish deeper root systems and treat more of the pollution.
There are much more promising results in the treatment of organic contaminants. Through their natural photosynthetic processes, some plants are able to degrade pollutants and neutralize their threat to the environment. While plants release oxygen, they also put sugars and other photosynthetic byproducts into the soil for microbes to eat. This creates an ecosystem around plants’ root systems far denser in life than the soil around it.
As they pull water from and release sugars into the soil, phytoremediators break down organic contaminants in two main ways. During rhizodegradation, the microbes surrounding their roots break down the contaminants. During phytovolatilization, the plants pull the contaminants out of the soil along with the water, break them down in the leaves, and release them into the air where they are less potent.
Phytovolatilization could have a promising future in the Huron Valley area, where we’re having issues with PFAS, PCBs, and 1,4 dioxane— all organic compounds which could very effectively be remediated by poplar trees.
Plant life has as diverse an array of skills and proclivities as human life does. Different plants have a knack for affecting different contaminants, and different plants will tackle contaminants differently. Depending on the site, soil, and other factors, different methods of remediation are suitable for a given area. Because of this, it’s hard to tell from a phytotechnology standpoint which plants would be most useful for your backyard contamination without testing the soil first. That said, while remediation efforts ideally happen in large-scale projects, if you have an idea of your yard’s contamination makeup and are interested in looking into what plants could be helpful, planting leafy friends to improve your nearby soil quality can’t hurt.
Plants have always been acting on pollution: phytotechnology is concerned with how to strategically place the best suited plants at sites to promote the work they’re already doing. As more case studies come forth in favor of phytotechnology, we’re learning more about the wisdom of the life surrounding us. As though instinctually, nature knows how to respond to capitalism’s unfortunate disregard for life––human and nonhuman. Nature knows a lot more about the world than we do.
Phytoremediation: Right Plants for Right Pollutants
Editorial: Journal of Bioremediation & Biodegradation, 2011: 102e
International Phytosociety Website: phytosociety.org
A lecture / documentary on YouTube: Phytoremediation Demystified
MLive: Water Street contamination prompts Ypsilanti to close Border To Border Trail