[permaculture] Study about lead in gardens
dtv at mwt.net
Thu Dec 11 08:56:15 EST 2003
LEAD IN GARDENS
By Janet Raloff, Science News, Dec. 6, 2003
Soils in many cities of the United States carry a poisonous legacy:
heavy concentrations of lead. The metal was deposited for years as
fallout from flaking leaded house paint and the emissions of cars
burning leaded gasoline. Recognizing the threat posed by tainted soil,
environmental scientists have warned that growing edible plants in
soils near streets or within several feet of homes and other painted
structures risks extracting lead from the soil and bringing it to the
Doing so could have serious health implications, the experts warn,
because even low concentrations of ingested lead have been associated
in children with impaired brain development, balance problems,
heightened risk of tooth decay, hearing loss, and shortened stature. In
adults, such lead can boost the risk of high blood pressure.
The good news, engineers at Northwestern University now report, is
that even in heavily lead-tainted dirt, most of the metal usually
becomes trapped in the parts of plants that few people eat, such as
roots and stems. Indeed, the new data indicate that intentionally using
inedible plants to sop up lead could prove a low-cost strategy for
detoxifying backyards and school grounds, making them safer for
ROOT IS THE PROBLEM
For their new 2-year pilot study, Kimberly A. Gray and her colleagues
sampled edible plants that home gardeners had been growing in two
densely populated neighborhoods in Chicago. One community was dominated
by brownstone apartment buildings; the other, by painted frame homes.
Gray says that few measurements of plants' uptake of lead from soil
had ever been reported before this study. Her team collected whole
plants, so the researchers could compare lead deposition in roots,
stems, leaves, and fruit. Sampled plants included apples, cantaloupe,
corn, cucumbers, strawberries, tomatoes, zucchinis, cilantro, mint,
peppers, rhubarb, carrots, radishes, and basil. The researchers also
tested small quantities of the soil in which each plant was grown.
Since cooks wash garden produce before eating it, Gray's team measured
the metal's concentrations only after a plant had been washed-either in
a water rinse or a mild detergent solution. The idea was to remove any
lead-tainted dirt that had clung to the plant.
Even after washing, many of the plants exhibited notable
concentrations of lead-evidence that they had accumulated the toxic
metal in their tissues. "From a scientific standpoint, that's
interesting, because lead is molecularly sticky," Gray observes. "That
is, it's not very mobile. So, to get it from the soil into a plant
isn't easy." Yet, once it did enter a plant, her data indicate, it
could be transported upward into the shoots, leaves, and, occasionally,
Invariably, roots showed the highest lead concentrations, followed by
successively lower accumulations in stems, leaves, and the typically
edible portions. Root concentrations varied widely, even within a given
species. For instance, her team reports that one sample of soil with a
lead concentration of 1,600 parts per million (ppm) produced no
measurable lead anywhere in a grape plant. In another case, the roots
of grapes grown in soil with 944 ppm lead showed lead contamination of
480 micrograms per gram (?g/g)-though there was no measurable
accumulation higher in the plant.
Most sampled plants accumulated at least some lead in their roots. The
good news: In only a few species did the lead move higher into the
plant. In almost every case, these were leafy crops, such as cilantro,
collard greens, lemon balm, chard, and mint, or root crops such as
carrot, onion, and radish. Overall, lead concentrations in the edible
produce ranged from 12 to 60 ?g/g of tissue. Gray and her colleagues
report the findings in an upcoming issue of Science of the Total
The Environmental Protection Agency considers 400 ppm lead as the
upper concentration that might be safe for dirt in which children play.
The Northwestern engineers found that more than 75 percent of the
garden soils they sampled had contamination exceeding this
concentration. In many of the tested gardens, lead tainting ranged from
1,000 to 4,500 ppm.
"You'd have thought that lead levels that are so toxic to humans would
stunt the growth of plants," Gray says. "But they don't." Indeed, in
many of the most contaminated gardens, plant growth was lush. What this
means, she says, is that there's no way a homeowner will know whether
the heavy metal is present-short of sending the soil out for testing.
Moreover, the researchers had suspected that soil adjacent to painted,
wood-exterior homes would have higher lead concentrations than soils
around brick apartment buildings do. In fact, Gray's team found no
difference. She now suspects that this is because even brick buildings
have substantial painted trim and their yards are subject to lead
fallout from fossil fuel combustion.
Children are put at greater risk by lead ingestion than adults are,
the Northwestern researchers note. First, lead is neurotoxic and the
neural development of children continues for many years after birth.
Second, children absorb between 30 and 75 percent of the lead in what
they eat, whereas adults typically absorb only about 11 percent.
Gray's team calculated that eating as little as 1 tablespoon (1.75
gram) of cilantro tainted with the lead concentration found in the new
study-49 ?g/g-would result in an intake of 85.75 ?g of lead. Ingesting
that amount daily, a child could build up a blood-lead concentration of
roughly 10 ?g per deciliter of blood, which other studies have linked
to several neurological deficits, including reduced IQ.
Therefore, the researchers argue, "it is imperative that people be
equipped with the information and knowledge necessary to reduce or
eliminate potential risks associated with urban gardening." Toward that
end, the team recommends that city gardeners plant their crops in
containers or raised beds, using new topsoil. The researchers also
recommend that gardeners line the bottom of the new soil with a
semipermeable barrier that permits water drainage but prevents root
crops from penetrating to potentially tainted soil.
Any crops brought in from the garden should be washed carefully. For
leafy produce, outer leaves should be sacrificed, and root vegetables
However, Gray maintains, the best move would be to clean lead from
urban soils, perhaps by trapping it in ornamental grasses or other
inedible plants with substantial roots. Then the tissue could be
harvested and removed for hazardous-waste treatment as part of a
municipal lead-remediation program.
Currently, Gray's group is evaluating garden species for the best
candidates for such a lead-removal strategy.
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