International Journal
of Hygiene and
Environmental Health
Int. J. Hyg. Environ. Health 203, 473-474 (2001)
© Urban & Fischer Verlag
http://www.urbanfischer.de/journals/intjhyg
Short Communication
Refining the risk assessment of metal-contaminated soils
Stan Casteel1, Tim Evans1, Jim Turk1, Nick Basta2, Chris Weis3, Gerry Henningsen3, Eva Hoffman3
1
2
3
College of Veterinary Medicine, University of Missouri, Columbia, Missouri, USA.
Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, Oklahoma, USA.
United States, EPA Region VIII (8HWM-SM), 999 18th Street, Denver, Colorado, USA.
Received October 10, 2000 · Accepted March 19, 2001
Abstract
Determining the bioavailability of toxic metals (Pb, As, and Cd) in a diverse range of soils, allows
scientifically derived data to dictate site-specific remedies to reduce the risk for sensitive human populations. Based on a series of dosing trials in a juvenile swine model, site-specific estimates of relative bioavailability of Pb in soil ranged from 3 % to 86 % compared to soluble lead acetate. Another experiment using a pregnant swine model revealed: 1) Pb accumulation in fetal tissues was
50 % or more of maternal and; 2) pregnant females accumulated 2-to-4 times more lead in tissues
than unbred females. Relative bioavailability results for arsenic- and cadmium-contaminated soils
further support the view that soil metals are not always as well absorbed as soluble forms, therefore use of default toxicity factors for assessing human health risk may overestimate the hazard.
Key words: Lead, arsenic – cadmium – bioavailability – soil – fetus
Introduction
Sites contaminated with arsenic, lead, and cadmium
present ongoing problems in terms of exposure assessment for various segments of the population. From behavioral and physiological perspectives, children and
pregnant women represent two uniquely vulnerable
populations. Childhood behaviors such as hand-tomouth activity increase the likelihood of ingestion and
inhalation of contaminated materials. Physiological
changes associated with pregnancy alter the enteric absorption fraction, mobilization of maternal stores and
distribution pattern of xenobiotics.
Bioavailability of lead impacts exposure assessment
and decision criteria for establishing cleanup levels for
contaminated sites (Casteel et al., 1997). Likewise,
when data are available on the bioavailability of arsenic (Casteel et al., 1999) and cadmium in contaminated
material, this information improves the accuracy of exposure and risk calculations for such sites.
Materials and methods
Test soil composites were collected and prepared by sieving
to capture particles 150 µm in diameter. Characterization
was by inductively coupled plasma emission spectroscopy,
scanning electron microscopy, and electron microprobe analysis.
Semi-fasted weanling swine (N = 4 or 5) were dosed twice
daily for 12–15 days with soil (e. g. oral dose groups of 75,
225, 675 µg Pb per kg body weight and day) or a soluble reference salt (e. g. lead acetate = PbAc2 3H2O) (dose groups
Corresponding author: Stan W. Casteel, VMDL, 1600 E. Rollins, Columbia, Missouri, 65211, USA, Phone: 011 573-882-6811,
Fax: 011 573-882-1411, E-mail: CasteelS@missouri.edu
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474
S. Casteel et al.
of 0, 75, 225 µg Pb per kg body weight and day). Lead and
cadmium relative bioavailabilities (RBA) were based on areaunder-the-curve (blood concentration versus time) versus
dose and/or terminal tissue concentration versus dose. Specific RBAs were calculated as the ratio of doses of Pb or Cd from
their soluble reference compounds (PbAc2 3H2O or CdCl2)
to test soil doses that produced equivalent responses. The
RBA of arsenic was estimated by the urinary excretion fraction (UEF) for both test (soils) and reference material (sodium arsenate = NaAs. Dose groups of 50, 100, 200 µg As per
kg body weight and day). RBA was calculated as UEF(test soil)/
UEF(NaAs).
Groups of 4 primigravida swine received daily doses of
lead acetate (0, 10, 100 or 1000 µg Pb/kg d) during days
80 to 108 of gestation. The 10 and 100 doses were spiked
with 30 % 206Pb to distinguish between endogenous (maternal background) and exogenous (oral dose) lead sources in
fetal tissues. Fetuses were harvested by terminal C-section on
day 108 of gestation. A single unbred group of adult females
was given 1000 µg of Pb from lead acetate/kg day for 28
days for comparison to pregnant females.
Results
The RBA of Pb from 20 soils ranged from 3 % to 86 %
of Pb from lead acetate. With respect to the Environmental Protection Agency’s default assumption of
60 %; higher RBAs ( 75 %) are associated with
PbCO3 and PbMn(Metal)O, average RBAs (25–75 %)
with PbO, PbFe(Metal)O, PbPO4, and Pb-slags, while
lower RBAs ( 25 %) are associated with PbS, PbSO4,
Pb(Metal)O, PbFe(Metal)SO4, and elemental Pb.
Principal findings from the pregnant swine study
were: 1) Pb concentrations in fetal tissues were about
50 % of maternal; 2) calculations based on 207Pb/206Pb
ratios in the 10 µg dose-group revealed endogenous Pb
contributions to fetal tissues of 28 % in femur, 56 % in
liver, 87 % in kidney, and 97 % in blood; contrasted
with endogenous contributions in the 100 µg dosegroup of 1 %, 5 %, 41 %, and 50 % in femur, liver, kidney, and blood, respectively, and 3) pregnant females
accumulated 2-to-4 times more Pb in tissues than unbred females.
Best estimates of arsenic RBA for the 5 soils tested
ranged from approximately 20-to-50 %. Arsenic trioxide was the primary arsenic-bearing phase in the soils
tested.
Discussion
Pregnant swine clearly absorb and retain more Pb than
non-pregnant females. Relatively small differences occur in blood Pb concentrations between open and pregnant females while large differences occur in soft tissues and bone. Fetal femur Pb concentration was about
50 % of maternal, however, differences in the relative
size of maternal and fetal bone reservoirs (e. g. as percent of body weight, skeleton in adults is 13 % vs. 24 %
in newborn pigs) reduce apparent discrepancies in skeletal lead burdens. Changing bone/blood Pb concentration ratios in juvenile swine, open mature females,
pregnant females, and fetuses demonstrate the futility
of attempting to draw conclusions about the body burden of Pb based on blood concentrations alone.
Such large variations in bioavailability challenge the
public health relevance and scientific defensibility of
generalized action levels for metals in soil. Variation in
metals bioavailability can be attributed to geophysicalchemical properties of the contaminated material and
the physiologic state of the dosed animals. Bioavailability of Pb, As, and Cd in soils may be more or less
than default assumptions, therefore measuring sitespecific RBAs provides a basis for improving exposure
and risk assessment.
References
Casteel, S., Cowart, R., Weis, C., et al.: Bioavailability of lead
to juvenile swine dosed with soil from the Smuggler
Mountain NPL Site of Aspen, Colorado. Fundam. Appl.
Toxicol. 36, 177–187 (1997).
Casteel, S., Brown, L., Cowart, R., et al.: Bioavailability of
arsenic in contaminated media. Tox. Lett., Supplement
1/99, Abs P128, pg 55, (1999).