“For the record, there is no valid phytoremediation method for [lead].”(1) That’s how Rufus Chaney began a recent email regarding a contaminated community garden. Chaney would know — he was one of the original researchers in the promising field of phytoremediation, or the use of green plants to remove contaminants from soil. He believes that current phytoremediation strategies offer few solutions to people concerned about lead levels in yards and gardens. Questions lingered in my mind as well, after I documented activists uses of phytoremediation for Slingshot Issue 99. How long does it take to clean up lead with plants? Is this a practical strategy? Plants and mushrooms can remove or break down other contaminants, like arsenic and petroleum products, relatively easily. Getting plants to take up meaningful quantities of lead is tricky.
Successful lead remediation involves a multi-faceted approach, suggests Anita Malpani, the Research Coordinator at Worcester Roots Project, a community group in eastern Massachusetts with a great track record in neighborhood lead remediation. What makes the Worcester Roots Project stand out is their ability to combine activism and science to tangibly improve community soil health. With guidance from experts in the field of lead remediation, like Rufus Chaney and Sally Brown (Univ. of Washington), Worcester Roots conducts their own field experiments. They have strong connections with the UMASS soil testing laboratory. Youth comprise a large part of their organization and are the primary purveyors of the free soil test kits. In fact, neighborhood soil testing is specifically written into their budget and is a cornerstone of their mission “[t]o struggle for a world where everybody is able to access the necessary resources to live a healthy, dignified life, without prejudice, exploitation or toxic environments.”
Malpani took time out from moving offices — their old office in Worcester’s Stone Soup community center recently caught fire — to answer my questions and explain her organization’s new angle on lead remediation. After years of experimenting with lead phytoremediation using Pelargoniums (scented geraniums), the Roots Project is now looking into chemically immobilizing the lead right where it is.
Although Malpani says they’re still hoping to find practical phytoremediation techniques, and are running new experiments with Indian Mustard, she says their new strategy focuses on phytostabilization. They are experimenting with adding phosphate rock, ferrous materials, and compost to the soil, assessing each soil amendment’s ability to bind up and stabilize lead. After adding the amendments, they plant Pelargoniums, which act as a ground cover and keep down dust, in addition to possibly sucking up some lead.
Phosphorus is key to the problems associated with lead phytoremediation, and also to the potential success of their new tactic. A necessary plant nutrient, phosphorus (P) also binds with lead (Pb) in the soil to form the non-toxic mineral pyromorphite. EPA scientists and other researchers like Chaney find that lead joins with phosphorus to make pyromorphite rapidly, and that “pyromorphite will rarely be absorbed if ingested.”(2) The Worcester Roots strategy now depends not on taking the lead away, but on reducing it’s bioavailability. Bioavailable lead is the lead that can damage our bodies when we absorb it. Scientists estimate that only 30% of the total amout of lead in soil is bioavailable.(2) The rest is already sitting tight in complexes with phosphates, sulphates, and organic matter naturally in the soil. Instead of trying to divorce the lead that’s already bound up, why not try to immobilize the rest of it?
“No plant naturally accumulates really high levels of lead from soils,” Chaney points out. Plants physically can’t absorb lead when phosphates are around. He suggests scientists get good results — i.e. experiments showing that plants do absorb a lot of lead — by growing plants in a laboratory and feeding them nutrient solutions that don’t contain phosphates and sulfates. This loophole guarantees that the lead stays soluble instead of forming pyromorphite or other mineral complexes. But a plant growing in a phosphorus-deficient environment is unheathy and won’t give a good yield. Successful phytoremediation depends on a plant’s ability to remove a lot of lead, which is directly related to how large the plant grows. To negotiate this catch-22, Worcester Roots is experimenting with spraying the Pelargoniums with a foliar application of phosphorus. This helps insure the Pelargoniums grow densely and are an effective groundcover, regardless of the amount of lead they might be extracting from the soil.
Despite Chaney’s grim prognosis on the future of lead phytoremediation, other researchers are still trying to find the right plant–soil chemistry combination for significant lead removal. If scientists aren’t using the “no phosphorus in nutrient solution” trick, they’re probably using the chemical EDTA. EDTA is a chelating agent, meaning it surrounds the lead molecules and prevents them from absorbing onto the soil particles, or even prys the lead-mineral complexes apart. Malpani let me in on an interesting conundrum: it’s illegal in all 50 states to use EDTA in phytoremediation projects, because it’s essentially a recipe for groundwater contamination. Once the EDTA frees the lead, the lead can travel easily through the soil and into the groundwater. So why do scientists and remediation companies pursue research with EDTA and other, biodegradable additives like citric acid anyway? Because phytoremediation is potentially so much cheaper than traditional clean up methods, like excavating tons of contaminated top soil and dumping it elsewhere.
The threat of groundwater contamination is real. Minnesota sued a Superfund lead cleanup project at a Twin Cities ammunition plant when the state discovered lead migrating into the groundwater. The EDTA applied to the experimental phytoremediation plot of corn and mustard plants, to increase the plants’ ability to access and remove the soil lead, was instead helping the lead move deeper into the soil and reach the groundwater. The remediation contractors had failed to obtain a permit for using EDTA in the first place.(3)
Back east amidst a sea of old clapboard houses covered in layers of lead paint, the Worcester Roots Project began their foray into lead clean-up with a series of experiments in 2003 and 2004. They set up 8 test plots, tested for lead content, and planted mostly Pelargoniums, with some corn and pumpkins thrown in for variety. They also looked at how the simple addition of compost to the contaminated soil affected the lead content. Malpani told me the experiments were basic, mimicking the lack of control experienced in peoples’ yards. But the concept was proven: lead content was reduced by about 30% in three of the Pelargoniums test plots, which started out with between about 1,000-6,000 parts per million lead. Adding compost also seemed effective. In 17 community gardens and 1 residence with relatively low lead levels (ranging from 48-323 parts per million), no lead was present after annual additions of compost. And in the one compost-only test plot, lead was reduced 41%, down to about 3,500 ppm, after Roots Project volunteers removed the sod and added 1 inch of compost.
These results are great — they make home-scale lead remediation look easy! I wondered, though, if you could really pull out all the lead after planting Pelargoniums for 3 years, or if less and less lead would be taken up each year. Perhaps that 30% was the fraction of the soil lead that was unabsorbed and easy for roots to access.
I found cautious optimism in the scientific literature regarding phytoremediation with Pelargoniums. I also found the time estimate for lead phytoremediation that I had been longing for: a whopping 150 years to remove all the lead from a contaminated field
in northern France!(4) With 1830 mg/kg lead, the field was not that different than some of the backyards tested by Worcester Roots. Even if you were only trying to reduce the lead to below 400 mg/kg, the EPA limit for yards (not playgrounds or vegetable gardens), it would still take upwards of 110 years. That’s a long time to harvest yearly crops contaminated with lead. To make Worcester soils safe for vegetable cultivation and children’s games, Roots Project activists would have to plant, harvest, and dispose of Pelargoniums for more than a century!
Hence the fundamental change in the Roots Project approach. However, a quick search of the scientific literature revealed that there are, of course, still questions about remediating lead with phosphorus and other soil amendments. For instance, what happens when you add phosphorus to the lead-rich soil of a firing range? After 32 months – almost 3 years – only 45% of that soil lead was bound into pyromorphite.(5) Immobilizing more lead would probably require understanding and changing the soil pH. Plus, it’s important to use a less soluble source of phosphate, like bone meal or rock phosphate, to avoid turning any ponds or streams in the area into bright green algae blooms as inorganic phosphate fertilizers leach into the water.
While they’re researching new methods of lead cleanup, what does the Roots Project recommend in the mean time? “If the lead levels are really high (more than 2,000 parts per million), you have bare soil and you have children who play in the yard, we advise you to use barrier methods like building patios, landscaping fabric with mulch, raised beds, and maybe other lead-safe landscaping methods we use,” suggested Malpani. “If it is between 400 to 1200 ppm you could add layers of compost and phosphorus to bind lead and grow Pelargoniums and dispose of them safely.”
For more information, contact Worcester Roots Project at info@WorcesterRoots.org or visit www.WorcesterRoots.org.
1. Email from Rufus Chaney, USDA-Agricultural Research Service, to Barbara Emeneau, regarding lead remediation in a community garden in Ottawa, Ontario, Canada http://mailman.cloudnet.com/ pipermail/compost/2009-May/015737.html
3. EPA (2001). Providing Solutions for a Better Tomorrow : Reducing the Risks Associated with Lead in Soil. EPA/600/F-01/014 www.epa.gov/ORD/NRMRL
5. MPCA Settles Alleged Violations at Twin Cities Army Ammunition Plant, 06/09/2004. http://www.pca.state.mn.us/news/data/newsRelease.cfm?NR=263167&type=2
7. Arshad, M. et al. (2008) A field study of lead phytoextraction by various scented Pelargonium cultivars. Chemosphere, Vol 71, Issue 11, pp. 2187-2192.
9. Chrysochoou, M. et al. (2007) Phosphate application to firing range soils for Pb immobilization: The unclear role of phosphate. Jour. Haz. Matls. Vol 144 Issue ½, p. 1-14.