Many existing clean-up methods have proven to be costly or limited. The US EPA’s Steven Rock, one of the fathers of phytoremediation, says the approach built on a natural mechanism in plants is competitive in terms of its effectiveness, cost and aesthetics. Capital and maintenance costs are low and planted sites are easier on the eye than most contaminated sites.
Rock, who is to tour Australia soon with Dr Ari Ferro of Phytokinetics Inc, says a demonstration program at the Magic Marker contaminated site in New Jersey cut lead levels in the top 15cm of soil by 20 per cent between May 1997 and November 1998.
US EPA researchers estimated the phytoextraction for soils covering a four-hectare site typically required 6-8 crops over three growing seasons. Harvesting this quantity of crop would ultimately result in the disposal of around 500 tonnes of biomass from 0.3 meters of top soil, a mere 0.25 per cent of the 20,000 tonnes of contaminated soil that otherwise would have required excavation and landfilling. The savings aren’t hard to work out.
The University of Western Sydney’s Dr Abdul Khan says in some cases the metals can be recovered from the plant. A few kilograms can be removed from several tones of vegetable matter.
Rock is a senior environmental engineer in the EPA’s Office of Research and Development. He co-ordinates the activities of the EPA-sponsored Alternative Covers Assessment Program (ACAP), which aims to find less costly and complex covers with the same degree of protection as prescriptive caps, such as clay.
“Phytotechnologies are an emerging set of techniques that use certain trees and other plants to remove organics and metals from soil and
groundwater”.
Landfills contaminate underlying groundwater and can cause adverse health effects through direct contact and by producing gases. A primary function of a landfill final cover system is to minimize deep percolation of water to prevent surface and groundwater contamination. Rock says evapotranspiration (ET) covers may do this. They are designed to achieve a water balance by cycling water from the soil to the atmosphere before it percolates downward through wastes.
ACAP is testing ET covers at 12 sites over five years, including municipal solid waste landfills, two hazardous waste landfills and a uranium mine tailings disposal site. Different communities of trees, shrubs and grasses are used, depending on local soil and climate conditions.
A casuarinas planted at the Nudgee landfill in Brisbane is doing well on 100 per cent leachate irrigation, reveals Richard Yeates.
Richard Yeates of PhytoLink Australia, who organized Rock’s tour, is running several trials in landfill applications in Queensland. Brisbane City Council’s Nudgee landfill is to close this year, prompting investigations of alternative leachate treatment systems in a bid to head off the need for a facility costing around $1 million. Some 15 species of trees have been planted over 4,000m2 of the landfill to test their ability to treat leachate and transpire rainfall. Five species in the plot being irrigated with 100 per cent leachate are still doing well after two years, results that Yeates describes as “interesting and encouraging”.