Lead and Soil
The more you know the less it can hurt you. By Lee Reich
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| Old lead-based paint can leach
lead into the surrounding soil via improper removal methods
or exposure to the elements over time. The health risks can
be considerable—especially to children. Photo Courtesy
of Lee Reich.
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My friend Linda was understandably enthusiastic about moving. Her
"new" home was an older, country house in a bucolic setting
where she and her husband could raise homegrown vegetables, a few
chickens, and their new baby, Adam. Two years later, the family
was moving again. The reason for the dramatic turnaround was lead,
but the source of the problem wasn’t interior paint—a
common issue in older homes, and one that’s been widely publicized
for the dangers it poses to children. Linda's lead was in the soil
around her house.
Lead can damage almost every organ in the body, and children are
most susceptible to its adverse effects because they incorporate
the element into their growing bodies. As nearly every old-house
owner knows, lead poisoning in children can cause learning disabilities,
stunted growth, and even death. What they may not realize is that,
like lead in interior finishes, lead in soil may also pose health
problems—when kids eat it, for example, or don’t wash
up well enough after playing outdoors. Fortunately, applying some
straightforward methods for evaluating and mitigating lead-in-soil
conditions can go a long way in reducing the potential for problems.
Understanding the Problem
At Adam’s one-year checkup, Linda's pediatrician discovered
elevated lead levels in the boy’s blood, prompting him to
ask about the age and nature of Linda’s house. In testing
the building, the high readings near the entrance pointed to the
soil outside as the source, and when the soil was tested, it was
shown to harbor lead levels of more than 2,000 ppm (parts per million)—much
higher than the 10 to 50 ppm found naturally.
Elevated levels of lead in soil stem from decades of the element’s
use in paints, pesticides, and gasoline. Because lead pretty much
stays put in soil, it remains in the ground around some houses despite
the fact that lead-containing products have been banned for decades.
Certain situations warrant testing for lead. Any house built before
1978 (the date after which lead pigments were eliminated in common
paints) may hold lead in surrounding soil from exterior paint that
was scraped off, or that flaked or washed away over time. Houses
on or near old farms that once grew fruits, vegetables, or cotton
also are at increased risk because lead arsenate was commonly used
as an insecticide in the early 20th century. In addition, houses
close to busy roads can harbor elevated levels from automobile exhausts
that formerly spewed combustion by-products of leaded gasoline.
Even some new houses can be prone to soil lead if they were built
over a previous home site, like the McMansions sprouting nationwide
over tear-downs of more modest old houses.
Because lead in soil does not tend to migrate, it is important to
test each potential problem area separately. For example, if exterior
lead paint is one potential source of the problem, take soil samples
at varying distances from the house—in the first 3’,
between 3’ and 15’, and at 30’. Draw several samples
at each distance, combine and mix them (to equalize any variations),
then take from this composited sample the amount needed for testing.
If there’s evidence of an orchard too, then follow the same
procedure around the trees at varying distances as well. It’s
also a good idea to test the soil’s pH, because acidity can
influence how much ingested lead reaches the bloodstream. Many private
soil testing laboratories, as well as state laboratories (located
at state universities), test for lead in soil.
Taking Action
How do you deal with a home that has high levels of lead in surrounding
soil? Moving, as Linda did, is one way, but certainly not the only
way. The approach you take should factor in the level of contamination
and the individuals at risk. Because kids play in soil, homes with
children face the most stringent guidelines. HUD and EPA both allow
an uppermost limit of 400 ppm of lead in bare soil where children
play, 1,200 ppm otherwise. Emotions run high on the subject of lead
contamination, but remember:
There is no automatic relationship between environmental lead levels
and those found in children because several mitigating factors—like
good nutrition and hand washing—reduce lead’s absorption
or uptake.
There are a number of approaches you can take for dealing with lead
levels above the 400 ppm guideline. For levels between 400 and 1,200
ppm, keeping soil in place or putting some sort of barrier between
small, human hands (or clothes) and the soil is a viable safeguard.
Establishing a dense lawn, for example, keeps contaminated soil
from being tracked indoors on shoes and also acts as a barrier to
small hands. In areas where trees create too much shade and root
competition for a solid lawn, you can substitute a thick, shade-loving
groundcover like pachysandra. You can also lay landscape fabric,
topped with wood chips, on the ground to establish a barrier in
sun or shade. In flower and shrub beds, maintaining a permanent
mulch of leaves, straw, or other organic material is also effective.
Avoid any tillage. If elevated lead levels are confined to certain
areas—near house walls, for example, from past use of lead
paint—you can exclude children from those places with fencing
or by planting prickly shrubs.
Lead is not a plant nutrient, so roots take up very little of the
metal. The hazard from growing edible plants in lead-rich soil comes
from earth that clings to root vegetables or adheres to the leaves
of lettuce and leafy greens. Eating vegetables, fruits, or herbs
grown in soils with lead levels as high as 1,000 ppm shouldn’t
present a hazard as long as root vegetables are well-scrubbed or
peeled, and leafy vegetables are washed well. Even soil lead levels
above 3,000 ppm shouldn’t pose a risk for fruits and vegetables
that grow tall or that have smooth skins, which limit the chances
of holding airborne particles. (Nonetheless, bringing this soil
indoors on clothes, shoes, and hands still presents a hazard.) An
alternate gardening approach appropriate for any level of lead in
soil is to construct raised vegetable beds and fill them with new,
uncontaminated soil. Avoid deep tillage, which would bring lead-rich
soil to the surface.
Lead can exist in more than one form in the soil, and all forms
aren’t equally able to find their way into our bloodstreams
when ingested. Dr. Rufus Chaney, along with co-workers at the U.S.
Department of Agriculture, has developed a method of "deactivating"
soil lead. This bioremediation entails tilling large amounts of
compost rich in both iron and phosphorus into the soil. You need
about five pounds of compost per square foot, or an 8” depth,
plus you must add limestone or wood ashes to make the soil more
alkaline. These treatments have reduced lead bioavailability by
more than 50 percent and work in soil with lead levels up to 2,000
ppm. Granted, this bioremediation requires a large amount of compost,
but the process is a one-time deal. In addition to deactivating
the lead, it also creates a very rich soil that, in turn, nourishes
dense grass growth that helps keep the soil in place.
Dr. Chaney is exploring the possibility of specially formulating
composts for lead remediation. Municipal composts from sewage treatment
plants work well because iron is added during processing to separate
phosphorus and prevent it from entering waterways; your backyard
compost isn’t rich enough in iron and phosphorus to effect
this level of lead remediation.
One final way to deal with lead in soil is to remove all the surface
soil and replace it with a clean layer—what I call the Herculean
Approach because it’s so labor-intensive. Thankfully, there
are many other options to consider before this act of last resort—or
moving like Linda did.
Lee Reich, PhD, gardens around an old house in New Paltz, New
York, and is the author of Uncommon Fruits for Every Garden (Timber
Press, 2004) and Weedless Gardening (Workman Publishing, 2001).
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Related Stories
see:
Looking
Out for Lead Paint.
Best Ways to Test Getting accurate
soil samples can be a challenge because, even for a
modest-sized problem area of 100 square feet, the amount
of soil used for a test will only be about two cups’
worth—or representative of about .002 percent
of the top 6” of the ground. When you think of
it this way, it’s easy to understand why it’s
so important to composite the samples you take in each
area in order to get accurate results.
A test area should be relatively uniform. Where obvious
differences in soil or topography exist, you should
subdivide these spots into separate testing areas. To
avoid non-representative samples, take a half-dozen
specimens from random spots within the designated area.
Remove surface debris like compost, sod, or plant residues,
then make a hole to sampling depth—for lead you
should sample the top inch, then the top 6” of
soil, separately—but don’t use this first
bit of soil for your specimen. Instead, take a slice,
uniformly thick from top to bottom, from along the edge
of the hole you just made. You could also use a soil
probe.
Gather specimens from each test area and depth into
a separate clean plastic bucket. Thoroughly mix them
together, crumbling the soil while discarding stones,
sticks, insects, and other debris. Spread the soil out
on a clean baking pan to air dry for a day, then remove
two cups for testing. Throughout sample preparation,
it’s important to avoid contamination from either
your hands or dirty utensils. Follow any packing instructions
supplied by the testing laboratory, and label the 1”
and 6” depth samples from each area—making
a note for yourself of their location on your property.
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