Among one of the most interesting classes I took as an undergraduate student was one named The Environmental Application of Plants. Admittedly, the title is not *that* interesting (why do we need to “apply” plants to the environment, aren’t they already part of it?!), and I only picked it because I had already taken a class taught by the same professor.
Turns out, the class was about phytoremediation. If you hadn’t heard of this term before, no fear: a portmanteau of phyto- (plant-relating) and remediation, the course discussed using plants to restore sites contaminated by organic (carbon-based, e.g. gasoline) and inorganic (heavy metals, e.g. arsenic) pollutants.
How, you ask? Without getting too in the weeds, some plants – through symbioses with various microbes – have the ability to take up and to metabolize pollutants that are in the soil, in sediment, or in groundwater. Tree species that are usually planted to do this cleanup work often belong to the Populus (poplars) and Salix (willows) genera. Hybrid individuals are often deliberately bred and chosen for specific survival and/or remediation traits.
Figure from the EPA’s A Citizen’s Guide to Phytoremediation; generally how phytoremediation works!
Since we were in Seattle, our class discussed using phytoremediation as a tool to decontaminate Gas Works Park, a waterfront park just west of campus that used to be the site of an old gasification plant. The soil and groundwater are known to be contaminated with metals such as arsenic and organic pollutants such as benzene. There is no water access or boat launch off from the park due to this, and though mostly contained, leachates sometimes surface on the grass during heavy rainstorms. In fact, this work is continuously ongoing, and the project area is expanding into Lake Union, the water body that the park borders.
Bringing us slightly closer to home, O’Connor et al. from Tsinghua University examined a phytoremediation component of a brownfield redevelopment project in the Bay Area. Using modelling methods, the authors found phytoremediation to be a low-cost, low-footprint, attractive, and viable way to remediate their site.
Though promising, phytoremediation is unfortunately not a cure-all for site contamination; heavily polluted sites will still require heavy-handed techniques (Gas Works Park was capped with soil before because it was so polluted), and O’Connor’s study predicted that phytoremediation is likely not going to be too effective in a high sea level rise scenario. Overall, trees take their time, and contaminants are not going to go away overnight. In planning for cities that are more sustainable and likely denser in the future, we must think about both nature-based solutions as well as innovative technologies that can help us live and thrive better.
-Additional resources-
The EPA’s “A Citizen’s Guide to Phytoremediation” info sheet; graphic above is from this info sheet as well.
The Doty Lab at the University of Washington is doing a lot of phytoremediation work in the Northwest and the professor Sharon is who taught my class 🙂
Technical USDA document about phytoremediation of arsenic-contaminated soils and a proposed project in the Northwest
Sustainable Communities 
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