OSWER 9200.1-78
September 2007
SHORT SHEET:
Estimating the Soil Lead Concentration Term for the Integrated
Exposure Uptake Biokinetic (IEUBK) Model
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
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NOTICE
This document provides technical and policy guidance to the U.S. Environmental Protection Agency
(EPA) staff on making risk management decisions for contaminated sites. It also provides information to
the public and to the regulated community on how EPA intends to exercise its discretion in
implementing its regulations at contaminated sites. It is important to understand, however, that this
document does not substitute for statutes that EPA administers or their implementing regulations, nor is
it a regulation itself. Thus, this document does not impose legally-binding requirements on EPA, states,
or the regulated community, and may not apply to a particular situation based upon the specific
circumstances. Rather, the document suggests approaches that may be used at particular sites, as
appropriate, given site-specific circumstances.
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U.S. ENVIRONMENTAL PROTECTION AGENCY
LEAD COMMITTEE OF THE TECHNICAL REVIEW WORKGROUP FOR METALS AND ASBESTOS
The Lead Committee of the Technical Review Workgroup for Metals and Asbestos (TRW) is an
interoffice workgroup convened by the U.S. EPA Office of Superfund Remediation and Technology
Innovation
Region 1
Mary Ballew
Boston, MA
Region 7 (Co-chair)
Mike Beringer
Kansas City, KS
Region 2
Mark Maddaloni
Julie McPherson
New York, NY
Region 3
Dawn loven
Linda Watson
Philadelphia, PA
Region 4
Kevin Koporec
Atlanta, GA
Region 5
Andrew Podowski
Chicago, IL
Region 8 (Co-chair)
Jim Luey
Denver, CO
Region 10
Marc Stifelman
Seattle, WA
NCEA/Washington
Paul White
Karen Hogan
NCEA/Cincinnati
Harlal Choudhury
NCEA/Research Triangle Park
Andrew Rooney
Region 6
Ghassan Khoury
Dallas, TX
OSRTI
Aaron Yeow
Larry Zaragoza
Associate
Scott Everett
Department of Environmental Quality
Salt Lake City, UT
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Short Sheet: Estimating the Soil Lead Concentration
Term for the Integrated Exposure Uptake Biokinetic
(IEUBK) Model
Background
The soil lead concentration term (PbS) is the only input parameter of the Integrated Exposure Uptake
Biokinetic (IEUBK) Model for Lead in Children for which a site-specific value is always required
(although a site-specific value for soil and dust are preferred [EPA, 1994]). As stated in the Guidance
Manual for the IEUBK Model (EPA, 1994), the average, or arithmetic mean of soil lead concentration
from a representative area in the yard should be used for the PbS. The PbS may reflect the current
exposure scenario (i.e., to predict current risk) or future exposure scenarios (EPA, 1994).
This short sheet describes how to estimate the PbS for the residential exposure scenario and
recommends the use of the arithmetic mean or average concentration for the PbS. Specifically, this
document provides recommendations for under what circumstances the data should be weighted when
estimating the PbS, and how the data should be weighted for a residential yard, which includes common
play areas located within apartment complexes. The soil concentrations obtained by these averaging
procedures give appropriate central estimates for use in estimating blood lead levels for residential
children. However, these central estimates are subject to uncertainty, and if a risk assessor seeks to
provide a conservative estimate of the average concentration of lead present in yard soil, an upper bound
estimate on the mean may be appropriate for that purpose.
Children may also be exposed to lead at locations outside of the residence (e.g., day care center, park).
Exposure to such secondary sources of lead may be evaluated using the approach recommended by EPA
(2003b). Other EPA documents related to estimating the PbS address the soil particle size fraction that
should be analyzed (EPA, 2000), and the number, location and type of samples that should be collected
(EPA, 2003a).
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The Superfund Lead-Contaminated Residential Sites Handbook (EPA, 2003a) recommends dividing the
residential property into two to four quadrants, depending upon the size of the residential property, and
collecting a five-point composite from each quadrant (see Figures 4-la, 4-lb, and 4-2 of the Handbook).
This does not preclude the use of individual samples to estimate the PbS when this approach is preferred
over composite sampling. The Handbook also recommends sampling drip zones, gardens, play areas,
driveways and crawl spaces separately; i.e., these samples are not to be included as part of the 5-point
composites from each quadrant of the residential property.
The TRW recommends that the soil lead exposure point concentration should be based on the
appropriate sample depth for the exposure scenario being assessed. In general, exposure to surface soil
should be assessed through measurements of lead concentration determined from soil samples that are
collected from within the 0-1" depth interval (EPA, 1996, 2003a). However, in future use scenarios
for sites that anticipate excavation (e.g., basements), children might be exposed to soil contamination
below the 0-1" depth interval, and the appropriate sampling depth, therefore, might be deeper than the
0-1" interval (EPA, 1989, 2003a). In all cases, measurements of lead in the <250 |im soil fraction
should be used to estimate the exposure point concentration (EPA, 2000).
Recommendations
In general, the method that is used to estimate the PbS will depend upon the information that is available
for the site. This includes information on the geography of the site (e.g., size of play area), and the
behavior patterns of the child (i.e.., the time spent by the child in various areas of the property). The
unweighted average (i.e., Case 1) is appropriate for predicting risks for current exposure scenarios and
for future use scenarios. The area-weighted average and time-weighted average may not be appropriate
for future use scenarios where assumptions regarding site layout (e.g., location and size of play area) and
behavior patterns may be unknown or uncertain. Example calculations for these three approaches are
provided in Table 1.
Case 1: Information on geography of the site and behavior patterns is not available - unweighted
average. This is the simplest approach to estimating the EPC. This approach is consistent with the
random exposure assumption that is often made in Superfund risk assessment. Each composite sample
is given equal weight in the estimation of the EPC. An implicit assumption of this approach is that all
2
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areas of the yard (i.e., each quadrant, play area, etc.) contribute equally to exposure regardless of their
size. In this situation, the PbS should be estimated by a simple unweighted average of the 5-point
composites and the composites from other areas where the child may be exposed to lead in soil (e.g.,
play area, drip zone, driveway, etc.) (Equation 1). However, before including these other areas of the
yard in estimating the EPC, one should carefully consider whether children may be directly exposed to
surface soil in these areas (see EPA, 2003a for further detail).
Equation 1 - Case 1: unweighted average
n
V y. + p + other
n + 2
where,
yt = lead concentration measured in composite from yard quadrant /
n = number of quadrants (depends on size of property; see Section 4.2.2 of EPA, 2003a)
p = lead concentration measured in play area composite
other = unweighted average lead concentration measured in all other areas
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Case 2: Information on geography of the site is available but the behavior patterns are unknown -
area-weighted average. An area-weighted estimate of the PbS is also consistent with the random
exposure assumption that is often made in Superfund risk assessment (EPA, 1989, 1992). This approach
represents an improvement over Case 1 where it is assumed all areas of the yard contribute equally to
exposure regardless of their size. The area-weighted approach assumes that exposure is proportional to
the size of the yard area (i.e., quadrant, play area, etc.) sampled. For example, given the sampling
scenario shown in Figure 1, an unbiased estimate of the PbS (assuming random exposure) is provided by
Equation 2. If the site map(s), with sampling areas shown, are available in a geographic information
system (GIS), the area of each quadrant and play area (if present) can be readily obtained, and the
sample weights shown in Equation 2 are easily calculated (see Equation 2).
Equation 2 - Case 2: spatially-weighted average
where,
wt = spatial weight for lead concentration measured in composite from yard quadrant /' = (area of
quadrant /') / (total area of property)
Wj•= spatial weight for lead concentration measured in composite from other areaj = (area of other area
f) I (total area of property)
Otherp = lead concentration measured in other areaj composite
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Case 3: Information on behavior patterns is available - time-weighted average. When information
on the time spent by the child in various areas of the residential property is available, the PbS should be
estimated by a time-weighted average of the 5-point composites, and the composites from the play and
other relevant areas (Equation 3). Note that this approach may not be applicable to some future use
scenarios.
Equation 3 - Case 3: time-weighted average
where,
tj = proportion of time child spends in yard quadrant /' = (time child spends in yard quadrant /' per day ) /
(total time child spends in yard per day)
tj = proportion of time child spends in other areaj = (time child spends in other area per day) / (total
time child spends in yard per day)
Notes:
1) The spatial weights (wi;2,....n, wi;2,....m,) should sum to 1; similarly, the time weights (ti;2,....n, ti,2,....m,)
should sum to 1.
2) The other term drops out of Equation 3 if the child is not exposed to lead in other areas (e.g., garden,
play areas) (i.e., when ti,2,...m = 0).
3) Equations 2 and 3 will produce the same estimate for PbS if the time spent within each area of the
yard is proportional to the size of the area (i.e., if one assumes random exposure).
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References
U.S. Environmental Protection Agency (EPA). 1989. Human Health Evaluation Manual, Part A,
RiskAssessment Guidance for Superfund, Volume I, Interim Final. Office of Emergency and
RemedialResponse, Washington, DC. EPA/540/1-89/002. (December 1989).
U.S. Environmental Protection Agency (EPA). 1994. Guidance Manual for the Integrated Exposure
Uptake Biokinetic Model for Lead in Children. EPA/540/R-93/081 PB93-963510, (February 1994).
U.S. Environmental Protection Agency (EPA). Soil Screening Guidance. 1996. Pub 9355.4-23.
Available online: http://www.epa.gov/superfund/resources/soil/ssg496.pdf. (July 1996).
U.S. Environmental Protection Agency (EPA). 2000. Short Sheet: TRW Recommendations for
Sampling and Analysis of Soil at Lead (Pb) Sites. EPA/540/F-00/010 OSWER #9285.7-38. (April
2000). Available online: http://www.epa.gov/superfund/programs/lead/products/sssiev.pdf.
U.S. Environmental Protection Agency (EPA). 2003a. Superfund Lead-Contaminated Residential Sites
Handbook. OSWER #9285.7-50. (August 2003). Available online:
http://www.epa.gov/superfund/programs/lead/products/handbook.pdf
U.S. Environmental Protection Agency (EPA). 2003b. Assessing Intermittent or Variable Exposures at
Lead Sites. OSWER #9285.7-76. EPA-540-R-03-008. Available online:
http://www.epa.gov/superfund/lead/products/twa-final-nov2003.pdf
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sample aliquots
five-point
yard
composite
sample
garden
(with
three-point
composite
sample)
quadrant
number
play area
(with
three-point
composite
sample)
Figure 1. Example residential property site plan. The figure shows a typical residential property that
has been divided into four quadrants, with garden and play areas present. Five-point composites (i.e..,
composites made up of five aliquots) are shown for each quadrant, and three-point composites are
shown for the garden and play areas. The number of aliquots for the garden and play area depend upon
the size of these areas; a minimum of three is recommended. Note that the composite samples for each
quadrant do not include aliquots from the garden or play area; the latter two are sampled separately.
Estimation of the soil concentration term (PbS) is illustrated in Table 1.
Figure adapted from Figure 4-2 of the Superfund Lead-Contaminated Residential Sites Handbook (EPA,
2003a).
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Table 1. Estimation of the soil concentration term (PbS) for the IEUBK model.
unweighted estimate
(Equation 1)
yard area
quadrant 1
quadrant 2
quadrant 3
quadrant 4
garden
play area
PbS:
concentration
measured in
composite (y;)
100
50
75
150
600
500
246
spatially-weighted estimate
(Equation 2)
yard area
quadrant 1
quadrant 2
quadrant 3
quadrant 4
garden
play area
PbS:
concentration
measured in
composite (y;)
100
50
75
150
600
500
185
sum of weights:
spatial weights
0.20
0.20
0.20
0.20
0.10
0.10
1
time-weighted estimate
(Equation 3)
yard area
quadrant 1
quadrant 2
quadrant 3
quadrant 4
garden
play area
PbS:
concentration
measured in
composite (y;)
100
50
75
150
600
500
286
sum of weights:
time weights
0.10
0.10
0.15
0.20
0.05
0.40
1
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