United States
Environmental
Agency
Office of Res
Development
Washington, DC 20460
W600/F-01/014
s/larch 2001
http://www.epa.gov/ORD/NRMRL
'ational Risk Management Research Laboratory
Providing So'lutions for a
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Lead in Soil
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NOTE
Although this brochure contains widely-applicable information about the risks of lead exposure,
the hydroxyapatite treatment technique described within is primarily aimed at remediation of
Superfund sites; the use of the patented hydroxyapatite technique does not satisfy the goals of
residential soil remediation under the Toxic Substances Control Act. This brochure has been
peer and administratively reviewed and approved for publication as an EPA document.
US EPA Office of Research and Development
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"Nearly 1 million children living in the United States have lead levels
in their blood that are high enough to cause irreversible damage to
their health."
Centers for Disease Control and Prevention, 1998
Introduction
Flaking paint, decades of leaded gasoline use, mining
operations, smelter and industrial emissions, waste
incineration, and application of insecticides and fertilizers
have all contributed to elevated lead levels in soils. Urban
environments, with their higher concentrations of indus-
tries, aging buildings, and vehicular traffic, have corre-
spondingly higher levels of lead in soil. To complicate
matters, lead seems to remain near the surface of the soil
where it is deposited — increasing the chance of expo-
sure.
The quote highlighted at the top of this page brings
the risks associated with lead exposure into perspective.
The U.S. Environmental Protection Agency (EPA) has
acknowledged these significant risks and the need for a
better understanding of the associated issues. EPA's
Office of Research and Development, in cooperation with
other scientists and engineers, has developed risk assess-
ment tools that estimate the risks associated with lead
exposure and a cost-effective management technique to
reduce the risks associated specifically with exposure to
lead in soil. The following information is presented to
better inform citizens, decisionmakers, and remediation
engineers of these risks and give them some tools to help
solve the problem.
Effects Associated with Exposure to Lead
Whether lead enters the body through ingestion or
inhalation, the biological effects are the same: normal cell
function and a number of physiological processes can be
disrupted. In humans, lead primarily affects the nervous
system, blood cells, and processes for the metabolism of
Common Sources of Lead Exposure
vitamin D and calcium. Lead can cause reproductive
toxicity (i.e., it can be transferred easily to the developing
fetus via the placenta), impaired tooth and bone develop-
ment, kidney damage, and anemia. Other adverse effects
which may result from elevated blood lead levels include:
lower IQ scores; poor attention levels; hearing, speech
and language problems; reading disabilities; reduced
motor skills; and poor hand-eye coordination.
Although no safe lower threshold level for these
effects has been established, available evidence suggests
that lead toxicity may occur at levels as low as 10-15
micrograms of lead per deciliter of blood (|j,g/dL). The
current level of concern established by the Centers for
Disease Control and Prevention (CDC) and the American
Academy of Pediatrics is 10 |J,g/dL. According to recent
CDC estimates, 890,000 U.S. children under age 6 (i.e.,
72 months) have elevated blood lead levels. In light of
these statistics, lead poisoning is expected to cost billions
of dollars in medical and special education expenses and
decreased future earnings.
Why Children are at Greater Risk
Compared to adults, children proportionately eat
more food, drink more fluids, breathe more air, and play
outside more. Natural curiosity and the tendency to
explore leaves children open to health risks that adults can
more easily avoid. In crawling on the ground and playing
with and in soil, ingestion of contaminated soil and dust
(by chance and on purpose) can occur more often (Fig. 1).
Children's bodies are not yet fully developed, so
exposure to contaminants may affect their growth and
development. Children may be more or less sensitive than
Figure 1. Some of the sources of lead exposure are
shown at left. Since children are more often in
direct contact with potentially-contaminated soils,
they face greater risk of exposure to soil-related
contaminants. Children younger than 6 years are
most at risk for exposure to lead in soil.
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adults when confronted with an equivalent level of
exposure to pollutants. These age-related variations in
susceptibility are due to many factors, including body
composition, metabolism, state of the immune system,
and differences in the way the body absorbs, distributes,
transforms, and excretes a contaminant.
With exposure to lead, children absorb into their
blood and retain more of the contaminant in proportion to
their weight than adults. Nutritional deficiencies such as
inadequate iron and calcium may increase the chances for
lead absorption. Depending on a child's stage of develop-
ment at time of exposure, lead-damaged cells can hinder
development of important body systems.
Unavailability and Its Importance
The risk associated with a contaminant depends on
how easily an individual absorbs it; this measure of
absorption by the body defines contaminant "bioavailabil-
ity." Another important factor is how easily the contami-
nant is delivered to the part of a body (e.g., brain) where
its toxic action can occur.
Should a heavy metal like lead be bound to other
molecules when ingested, it might pass through the body
without being absorbed into the blood. While absorbed
lead has toxic effects, not all soil lead is bioavailable.
Three factors contributing to varying bioavailability are:
the size of the lead-containing particle (i.e., perhaps the
particles are too large to be dissolved and/or absorbed);
the chemical form of lead in the soil; and the geochemical
matrix incorporating that form of lead (i.e., lead may be
entrapped in a less soluble compound).
Contaminant bioavailability is important when
considering remediation needs. Historically, remediation
decisions have been based on an assumed bioavailability
level (30% is the accepted default level) for all forms of
lead in soil. Using the integrated exposure uptake
biokinetic (IEUBK) model developed by EPA's National
Center for Environmental Assessment, blood lead levels
can be predicted for a population given the amount of soil
ingested and its bioavailability.
Figure 2 demonstrates how the bioavailability of lead
in soil affects estimates of lead concentration in the body.
Using EPA's IEUBK model, if the lead bioavailability is
decreased (e.g., through soil treatment techniques), higher
exposures would result in the same blood-lead effect as
exposure to lead species with greater bioavailability. Such
a finding could translate into significant savings in
cleanup costs.
Determining Bioavailability
Even with the IEUBK model, direct bioavailability
estimates are not easily obtained. Clearly, children cannot
be used as test subjects to determine the amount of lead
absorbed through exposure to local soils; surrogate
measures must be used that best estimate lead uptake.
EPA Region VIII scientists have used immature swine to
examine oral lead bioavailability. They have completed
test studies using swine dosed with soils containing
varying levels and species of lead. Soil lead bioavailabil-
ity ranged from 6% to 86% relative to freely soluble
(aqueous) lead acetate.
Scientists from EPA's Region VIII have been
focusing on the use of site-specific risk-based information
in determining the level of cleanup needed. For instance,
by first determining that the bioavailability of lead
contaminants in soil at a Bingham Creek, Utah, site was
lower than the default level (19% instead of 30%), less
cleanup was necessary to manage the risk at the site. With
less disruption of the site, an estimated $4 million was
saved through this refinement of evaluation techniques.
Building on Region VHFs in vivo (live animal model)
studies, an in vitro (artificial, non-animal) chemical
extraction approach has been devised to imitate the
physiology of human and animal digestive tracts. Results
from this chemical method, using the same soil samples,
have compared well with the results of the swine study.
Such good correlation may make further animal bioavail-
ability testing unnecessary. A Solubility-Bioavailability
Research Consortium is currently verifying and standard-
izing this in vitro approach.
Figure 2. Information on the
bioavailability of lead in soil at
a specific site provides a more
accurate representation of the
risk at that site. The chart at
right (based on EPA's IEUBK
model) shows that if a treatment
method reduces the bioavail-
ability of lead in soil (e.g., from
the 30% assumed value to 5%),
the risk from exposure can be
reduced and less remediation
may be needed. This can result
in lower cleanup costs and less
disruption of the environment.
0 500 1000 1500 2000 2500 3000
Lead Concentration (mg/kg) in the Ingested Soil
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Reducing the Risks Associated with
Lead Contaminants in Soil
In 1992, the Remediation Technologies Development
Forum (RTDF) was organized by EPA to foster collabora-
tion between the public and private sectors in developing
innovative solutions to mutual problems of contaminated
materials. EPA's National Risk Management Research
Laboratory (NRMRL) and the DuPont Corporation
formed the "In-place Inactivation & Natural Ecological
Restoration Technologies" (IINERT) Soil-Metals Action
Team in November 1995, under the RTDF. The 'IINERT'
Soil-Metals Action Team includes representatives from
industry and government who share a common interest in
developing and validating surrogate measures of bioavail-
ability. Through this effort, the Team supports the
Solubility-Bioavailability Research Consortium men-
tioned earlier.
The IINERT team also has as objectives developing
and validating in situ (in-place) techniques as viable
technologies for eliminating the hazards of metals in soils
and surface materials. Their purpose is to develop and
demonstrate in-place inactivation and natural ecological
restoration technologies that reduce or eliminate the
human health and ecological risks of heavy metals in soil
and to achieve regulatory and public acceptance of these
technologies.
The 'IINERT' Soil-Metals Action Team concluded
that if soil-lead species can be converted to less bioavail-
able forms, the overall lead risk of the soil can be re-
duced. Therefore, three facets of the issue are being
studied: 1) innovative approaches to reduce lead bioavail-
ability; 2) techniques to measure the conversion of lead to
less bioavailable forms; and 3) identification and quantifi-
cation of soil lead forms in intact soil systems and their
relationship to bioavailability.
Developing Innovative Alternative
Remediation Methods
Efficient remediation treatments for soils attempt to
capitalize on the differences in physical and chemical
properties between a contaminant and soil constituents.
For example, remediation efforts for metal-contaminated
sites use properties such as solubility, density, particle size
distribution, surface chemistry, boiling point or magnetic
susceptibility to allow separation and recovery.
Metals found as relatively soluble species or weakly
bound to other soil components might be dissolved by
applying mild acids. If the metals are present as separate
mineral particulates, their typically higher density might
permit the physical separation of these species from the
less dense soil constituents. If the metal species are
volatile, then a soil heating method might allow recovery.
Separation methods relying on the magnetic susceptibility
of ferromagnetic or strongly paramagnetic metal species
have also been attempted. If separation techniques are not
feasible, the metals can be bound in a solid cement or
vitreous (glass) matrix.
Soil-lead remediation has typically been accom-
plished by soil removal for off-site disposal, covering, or
diluting by mixing with uncontaminated soil. Cost
(approximately $1.6 million per acre foot), logistical
concerns, and regulatory requirements associated with
excavation, ex situ (off-site) treatment and disposal can
make in situ treatment an attractive option. Current
understanding of lead exposure, associated bioavailability
factors, and environmental chemistry may allow develop-
ment of less costly and environmentally less disruptive
methods of remediation.
Innovative Approach
NRMRL, as part of its effort within IINERT, has
demonstrated that by adding phosphorus (P) to lead-
contaminated soil, lead can rapidly and effectively be
bound into a stable compound called pyromorphite (see
Fig. 3) that will rarely be absorbed if ingested. Also,
hydroxyapatite has proven to be an effective source of
phosphorus for binding with and reducing bioavailable
lead in contaminated soil to below the U.S. EPA action
level of 15 micrograms of dissolved lead per liter of
drinking water.
NRMRL scientists have demonstrated that hydroxya-
patite effectively immobilizes lead in soil even in the
presence of interfering elements and compounds (e.g.,
anions, cations, and metals such as zinc, cadmium,
nickel, copper, iron, and aluminum). The completeness
and rate of this transformation have been found to depend
upon the lead species, amount of hydroxyapatite added,
and the pH (acidity/alkalinity) of the soil system. The
speed at which soluble lead and phosphorus from
hydroxyapatite react to become pyromorphite illustrates
that the reaction can potentially occur during transit
through the gastrointestinal tract — further reducing the
potential exposure to bioavailable lead.
Figure 3. Pyromorphite crystals. Phosphorus from a
hydroxyapatite additive can immobilize soil-based lead
into this stable compound and make it less bioavailable.
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The hydroxyapatite in situ inactivation technique (for
which EPA was awarded a patent) has captured the
interest of industry and the remediation research commu-
nity. As tested in field studies by EPA's Region VII, the
cost for this innovative, less disruptive method is in the
thousands of dollars per acre foot of treated soil (in
contrast to traditional treatment costs of more than a
million dollars per acre foot). The technique, employing
common agricultural application and tilling approaches
for the addition of hydroxyapatite, leaves the treated soil
capable of sustaining plant growth, which can in turn hold
the soil in place and act as a barrier against contact with
soil contaminants.
Conclusion
Efforts to reduce exposure to lead contamination
have been very effective in the last two decades. EPA's
ban on lead in gasoline and the Consumer Product Safety
Commission's ban on lead in paint have reduced potential
exposure considerably. Since lead solder is no longer used
in the canning of foods, another avenue of exposure has
been closed. Declining reliance on lead solder and lead
pipes for the distribution of drinking water has further
reduced exposure potential.
Some simple remedies can reduce the risk of expo-
sure to lead in soil and dust. Careful handwashing should
become a practice after contact with soil and before
eating. Toys, bottles, and pacifiers should be cleaned —
especially after use outside. Floors, window sills, and
other surfaces should be cleaned regularly to reduce the
risk of exposure to lead-contaminated dust. Deliberate
ingestion of soil and paint chips should be strictly
discouraged or prevented. Prior to commencing renova-
tion in homes built before 1978, the surfaces to be
disturbed should be tested for the presence of lead-based
paint. If lead-based paint is found above the regulatory
limit, contact the National Lead Information Center at
1-800-424-LEAD for guidance on how to proceed.
Additional information on evaluation and control of lead-
based paint in homes can be found in EPA publications at
the following website:
http ://www.epa. gov/lead/leadpbed. htm
In spite of lead bans and behavior precautions, lead
poisoning remains a significant threat to human health —
especially for children. Although the symptoms of
moderate exposure may be subtle, the damage can be
irreversible. Local health agencies can provide useful
information on the need for blood lead testing. Should
unsafe lead levels be discovered, medical treatment
should be sought at once and the sources of exposure
must be identified and dealt with carefully.
EPA's National Risk Management Research Labora-
tory, in cooperation with many other scientists and
engineers, is working to better understand the scientific
issues surrounding exposure to lead in soil. By continuing
to develop and refine assessment and remediation tools,
EPA will be better poised to offer real solutions — like the
patented hydroxyapatite treatment process — to high
priority risks faced by our society. Through increased
awareness of the dangers associated with lead exposure,
and continued innovative risk management research, a
safer environment can be provided.
EPA's National Risk Management Research
Laboratory Lead Remediation Contact:
Jim Ryan, Ph.D.
513/569-7653 (Telephone)
513/569-7879 (Fax)
Email: ryan.jim@epa.gov
For additional information about U.S. EPA's risk
management research, visit the following internet
web site:
http://www.epa.gov/ORD/NRMRL
To learn more about lead bioavailability and
related topics, visit the following U.S. EPA
Technical Workgroup for Lead internet web site:
http://www.epa.gov/superfund/programs/lead/
For more information on the risks and control of
lead hazards, visit the U.S. Dept. of Housing and
Urban Development's Office of Lead Hazard
Control internet web site:
http://www.hud.gov/offices/lead/index.cfm
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This publication was written and produced by Patrick Burke and Jim
Ryan of USEPA's National Risk Management Research Laboratory
(NRMRL) within the Office of Research and Development. Comments
were provided by Rufus Chaney, U.S. Department of Agriculture's
Agricultural Research Services, and Bill Berti, DuPont Company and
IINERTRTDF.
An electronic version of this publication can be viewed and
downloaded from the Office of Research and Development's Internet
website at http://www.epa.gov/ORD/ (in the "Publications" section).
Printed copies can be requested from USEPA's Publications Center by
calling 1-800-490-9198.
Printed on Recycled Paper
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