Lead at Superfund Sites: United States Geological Survey (USGS) Background Soil-Lead Survey
USGS data sets for soil provide a baseline for the amount and distribution of chemical elements and minerals against which scientists can measure future changes from natural processes or human activities
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EPA’s Office of Land and Emergency Management (OLEM) recommends using the Integrated Exposure Uptake Biokinetic Model for Lead in Children (IEUBK model) as a risk assessment tool to support environmental cleanup decisions at residential sites.1 The IEUBK model uses empirical data from scientific studies of lead uptake and biokinetics, contact and intake rates of children with contaminated media, and data on the presence and behavior of environmental lead to predict a plausible distribution or geometric mean (GM) of blood lead (PbB) for a hypothetical child or population of children.
Representative site-specific data are essential for developing a risk assessment (as well as cleanup goals) that reflect current or potential future conditions. The IEUBK model uses more than 100 input parameters that are initially set to default values. Of these, there are 46 parameters that may be input, or modified, by the user; the remainder are locked.1 Default values represent national averages or other central tendency values derived from: a) empirical data in the open literature -- these include lead concentrations in exposure media, including diet representative of national food sources; b) contact and intake rates such as the soil/dust ingestion; and c) exposure durations.2 The representativeness of IEUBK model output is dependent on the representativeness of the data (often assessed in terms of completeness, comparability, precision and accuracy).1
The default soil-lead concentration in the IEUBK model (i.e., 200 parts per million), is a reasonable, nationally representative soil lead concentration for the continental United States.1,2 However, this value or the values identified by the U.S. Geological Survey (USGS) are not intended to represent soil lead concentrations at a specific site. Instead, these values provide a glimpse at the geogenic background soil-lead concentrations throughout the United States. The IEUBK model requires representative, site-specific soil lead concentration data to achieve representative results for the population being assessed.
USGS Background Soil-Lead Survey
From 2007 to 2010, the U.S. Geological Survey (USGS) collected soil samples at more than 4,800 sites (one site per 1,600 square kilometers) throughout the conterminous United States. Target sites were selected using a Generalized Random Tessellation Stratified design.3 However, the field crews were given considerable flexibility to move the site if access to the target site was not possible or if the target site represented an area of obvious contamination. Ideally, three samples were collected at each site, resulting in a total of more than 14,400 samples. One sample was collected from a depth of 0 to 5 centimeters. A second sample represented a composite of the soil A horizon. A deeper sample was collected from the soil C horizon or, if the top of the C horizon was at a depth greater than 1 meter, from a depth of approximately 80 to 100 centimeters.
The <2-millimeter fraction of each sample was analyzed for a suite of 45 major and trace elements by methods that yield the total, or near-total, elemental content. The major mineralogical components in the samples from the soil A and C horizons were determined by a quantitative x-ray diffraction method using Rietveld refinement.4,5 The resulting data set is a snapshot in time (2007 to 2010) of the abundance and spatial distribution of chemical elements and minerals in soils of the conterminous United States. It represents a baseline for soil geochemistry and mineralogy against which future changes may be recognized and quantified.
- Geochemical and Mineralogical Data for Soils of the Conterminous United States: U.S. Geological Survey Data Series 801 Exit
Available information suggests that actual soil-lead concentrations can vary widely. Published and anecdotal reports of urban soil lead concentrations can vary greatly from city to city and be as high as 2,500 parts per million.6
These values, therefore, represent geogenic background soil lead concentrations with some impact from widespread anthropogenic sources. In general, the background soil lead information from USGS are not applicable to populated areas and they may not be relevant for characterizing background for most Superfund site assessments. There may, however, be instances where the USGS data are appropriate for characterizing background lead levels, such as for remote wilderness western areas and for ecological risk assessment. For more information on characterizing background at Superfund sites, see EPA’s 2002 Guidance for Comparing Background and Chemical Concentrations in Soil for CERCLA Sites.7
1 U.S. Environmental Protection Agency (U.S. EPA). 1994a. Guidance Manual for the Integrated Exposure Uptake Biokinetic Model for Lead in Children. United States Environmental Protection Agency, Office of Emergency and Remedial Response. Publication Number 9285.7-15-1. EPA/540/R-93/081. Available here.
2 White, P. D.; Van Leeuwen, P.; Davis, B. D.; Maddaloni, M.; Hogan, K.A., Marcus, A.H.; Elias, R.W. 1998. The conceptual structure of the integrated exposure uptake biokinetic model for lead in children. Environ Health Perspect 106 Suppl 6: 1513-1530. Available online at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1533456/.
3 For more information on Generalized Random Tessellation design, see:
4 For more information on soil horizons, see:
http://education.usgs.gov/lessons/soil.pdf (PDF)(12 pp, 675 K)
5 For more information on Quantitative Random X-Ray diffraction and the Rietveld Refinement Method, see:
http://rruff.info/doclib/cm/vol39/CM39_1617.pdf (PDF) Exit(17 pp, 950 K)
http://www.minsocam.org/ammin/AM78/AM78_932.pdf (PDF) Exit(9 pp, 1 MB)
http://xray.tamu.edu/pdf/getting%20started/langord_louerz.pdf (PDF) Exit(104 pp, 926 K)
6 U.S. Environmental Protection Agency (U.S. EPA). 1996. Urban Soil Lead Abatement Demonstration Project. Volume I: EPA Integrated Report. United States Environmental Protection Agency, Office of Research and Development. EPA 600P-93-001AF.
7 U.S. Environmental Protection Agency (U.S. EPA). 2002. Guidance for Comparing Background and Chemical Concentrations in Soil for CERCLA Sites. United States Environmental Protection Agency. Office of Solid Waste and Emergency Response. EPA 540-R-01-003.
A Laboratory Manual for X-Ray Powder Diffraction: X-Ray Diffraction Primer. USGS Open-File Report 01-041. January 2013.
Pb-concentrations and Pb-isotope ratios in soils collected along an east-west transect across the United States. Riemann C., Smith D.B., Woodruff L.G., Flem B. 2011. Applied Geochemistry 26:1623-1231.
Spatially Balanced Sampling of Natural Resources (PDF)(17 pp, 1.4 MB) Stevens DL and Olsen AR. 2004. Journal of American Statistical Association 99(465): 262-278. March.
What's in My Soil USGS Science Education Handout. March 2011.