As architects delve further into sustainable design methods, the use of plants to remediate contaminated urban landscapes is growing.
Phytoremediation is used to rehabilitate polluted landscapes that must be cleaned up before future development can occur on the land. The methodology is simple in that plants are used to remediate and purify contaminated soil by extracting the pollutants. The difficulty is knowing which plants have the capability to absorb which chemicals and exactly how long the process will take.
The technology is most effective on land where the use of alternative clean-up techniques would cost more than the land’s value. In cases such as these, the importance of the longer time needed for plants to heal the landscape loses relevance.
There are four processes in phytoremediation: phytodegradation, phytostabilisation, phytovolatilisation and rhizofiltration.
This method of land remediation is getting a lot of interest lately as it proves to be a low-cost, sustainable method of landscape clean-up. Though it’s a slow process of amelioration, phytoremediation is inexpensive, environmentally friendly, easy to integrate, requires little labour and equipment, and is an in situ method allowing the landscape to be cleaned without removing the polluted soil.
“Phytoremediation is basically a solar-powered pollutant removal system,” said Sharon Doty, a plant biologist at the University of Washington. “It is more than 10 times cheaper than other technologies, it’s less intrusive and more aesthetically pleasing.”
Use of phytoremediation is set to increase as Australia aims to uphold its ‘green’ and ‘sustainable’ image abroad, it part because of increased pressure from regulators such as the Environmental Protection Agency (EPA).
Australia, like every country in the world, has several contaminated land sites from former waste disposal, petrol stations or industrial activities that have harmed the soil and groundwater. Rather than letting valuable land sit vacant, developers often want to ameliorate the land to bring it back to a usable status.
The state EPA deems the severity of the contamination and oversees its mitigation.
Professor R. R. Brooks was a pioneer in the field of phytoremediation in Australia and New Zealand. His work originally focused on the extraction of heavy metals from contaminated sites by plants that hyper-accumulate such metals. Phytoremediation now uses plants in an in situ fashion, using them as biopumps to reduce contaminant mobility.
Phytoremediation technology is mostly used in New Zealand to improve land degraded by agricultural production whereas Australia’s greatest potential is the clean-up of land affected by the mining industry.
In Wollongbar, New South Wales, a two-year field study beginning in 2009 examined the phytoremediation potential of two arsenic-accumulating fern species at an old cattle-dip site contaminated by arsenic.
With regular soil testing, the uptake of arsenic by the fern species was recorded. It was estimated that using the first species of fern that it would take between 55 to 125 years to remediate the soil whereas the second variety of fern would take 143 to 412 years.
Research from 2009 out of Amsterdam shows how phytoremediation can be used in sustainable landscape design in the former harbour areas of northern Amsterdam’s Buiksloterham port. Covering approximately 100 hectares, the area included abandoned industrial buildings and heavily polluted landscapes.
The project aimed to transform the industrial landscape into an area suitable for a mixed residential and commercial site through the use of plants.
The landscape was categorised into levels of pollution found: heavily polluted soils which take up to 200 years to clean, medium polluted soils which take approximately 60 years to clean, and clean soil. Areas with heavily polluted soils were closed to access during the remediation process but remained to offer a visible history of the site to the general public .
The port project detailed which areas will remain public green spaces and those that will be built on after the process of remediation.
In the US, researchers are attempting to take phytoremediation to the next level by experimenting with the use of genetically engineered grass and trees to be used in the process.
“[Unfortunately], phytoremediation is often viewed as being too slow to be of practical use,” said Doty. “Many sites are abandoned rather than cleaned up because of the cost of effective cleanup. It is for this reason that we became interested in enhancing phytoremediation using transgenic technologies.”
She noted that there are 12,000 highly contaminated sites in the US (called SuperFund sites) and upwards of 500,000 former commercial properties that have been abandoned and are too polluted to be built on without remediation.
“So many of us have been impacted by cancer that I am strongly motivated to find ways to reduce the amount of carcinogens from the environment in a fast and economical way,” she said.
The American Society of Landscape Architects (ASLA) has been advocating for increased use of phytoremediation on New York’s urban brownfields for the past few years.
Kaja Kuhl, urban designer and Associate Professor at Columbia University told ASLA that more than 11 per cent of New York City consists of abandoned lots.
“If applied across these small sites, low-cost and highly-effective phytoremediation techniques could have a powerful impact and help ensure future urban development is really just redevelopment,” said Kuhl.
Though its greatest downfall is the length of time it takes, phytoremediation integrated into a good landscape design strategy can be extremely beneficial in improving urban spaces and making polluted landscapes safe for future use.