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 1438-4639/01/203/5-6-473 $ 15.00/0 474 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).