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Soil Geochemical Landscapes of the Conterminous United States

Project Objectives

Soil is a critical natural resource that plays a key role in determining human health and ecosystem integrity, supporting food production, and the natural recycling of carbon and essential nutrients in the environment. As stated by McNeill and Winiwarter (2004): "Soil ecosystems remain firmly, but uncharismatically, at the foundations of human life." On the other hand, many communities dispose of solid and liquid wastes from households and from agricultural and industrial processes by dumping them onto the soil. Either through ingestion, inhalation, or dermal absorption, soil can be a pathway for potentially toxic chemicals of natural or anthropogenic origin to enter the human body (Abrahams, 2002; Oliver, 1997; Plumlee and Ziegler, 2003). Although soil is so important in everyday lives, knowledge of the concentration and distribution of naturally occurring and man-made chemicals in soils is surprisingly limited. Such information on our Nation's soils is needed to better assess risk to both humans and ecosystems, guide decision making, and foster more integrated environmental regulation.

Beginning in 2002 in collaboration with the Geological Survey of Canada, the Mexican Geological Survey (Servicio Geológico Mexicano, or SGM), and other stakeholders in all three countries, the USGS Geochemical Landscapes Project established a sample design and developed sampling and analytical protocols for a soil geochemical survey of North America. This survey will provide (1) systematic data on background variations in soil geochemistry to support risk assessment and management; (2) a new understanding of the links between soil geochemical factors and environmental and human health; and (3) user-friendly data that support a wide range of applications, issues and disciplines. From 2004-2007, pilot studies were carried out in all three countries to test and refine these protocols. The results of the pilot studies were published as 21 papers in the journal Applied Geochemistry in 2009. In 2007, sampling was initiated by the USGS in the conterminous U.S. (4,871 sites). The sampling protocols represent both depth-based and horizon-based samples. One depth-based sample is taken from 0 to 5 cm regardless of what soil horizon this represents. A composite of the A horizon (the uppermost mineral soil) and a composite of the C horizon (usually the weathered parent material of the overlying soil) are also collected at each site. The 0-5-cm and A-horizon samples are more likely to represent anthropogenic influence on soil composition, and the C-horizon sample will be more representative of geologic influence. Each sample is sieved to <2 mm and then ground to <150 µm prior to chemical analysis for more than major and trace elements. The sampling was completed in 2010 with over 13,000 samples now ready for chemical analysis, quantitative mineralogical determinations for selected minerals, and determination of selected soil pathogens (USEPA providing partial funding for pathogens).

The problem addressed by the new project is to get all these samples analyzed (chemistry, mineralogy, soil pathogens), quality check all the generated data, compile the data into a database, publish the raw data in a user-friendly manner, and develop interpretive publications based on the new data. These products will directly benefit at least two of the strategic directions identified in the USGS science strategy for 2007-2017 (U.S. Geological Survey, 2007). These two strategic directions are "Understanding Ecosystems" and "The Role of Environment in Human Health." The soil geochemical database established will provide the basis for future monitoring of soil composition at a national scale and for recognizing and quantifying broad-scale changes in soil geochemistry that result from natural or anthropogenic processes.

The objectives of the project are as follows:

1. Analyze the more than 13,000 soil samples collected from 4,871 sites in the conterminous U.S. for total concentrations of the following chemical elements: Al, Ca, Fe, K, Mg, Na, S, Ti, Ag, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, Hg, In, La, Li, Mn, Mo, Nb, Ni, P, Pb, Rb, Sb, Sc, Se, Sn, Sr, Te, Th, Tl, U, V, W, Y, Zn, total carbon, carbonate carbon, and organic carbon (determined by difference between total carbon and carbonate carbon).
2. Using quantitative X-ray diffraction, determine the concentration of selected minerals (e.g., quartz, orthoclase, sanadine, microcline, albite, oligoclase, andesine, labradorite, bytownite, calcite, dolomite, gypsum, and various ferromagnesian and clay minerals) in the A- and C-horizon samples.
3. Complete the determination of selected soil pathogens in the 0-5-cm soils with the assistance of funding from USEPA. The soil pathogens determined include Bacillus anthacis (anthrax), Yersinia pestis (plague), and Francisella tularensis (Tularemia or rabbit fever).
4. Coordinate chemical analysis of Mexican samples with SGM. Train SGM chemists on USGS methodology. Develop adequate quality control protocol to ensure data comparability between USGS contract lab and SGM lab.
5. Compile and quality check data as received from both U.S. and Mexican laboratories.
6. Develop database format for releasing USGS data to public.
7. Generate publications based on existing data from the Geochemical Landscapes pilot studies or from newly generated data as it becomes available.
8. Organize one or more symposia, as appropriate, focusing on regional-, national- and international-scale geochemical mapping.
9. Develop publication plan for the national-scale soil geochemical data from the conterminous United States.

Relevance & Impact

The relevance and impact of establishing a consistent national-scale soil geochemical database for the United States is demonstrated by the stakeholders who participated in the development of the project and by the collaborators who have assisted in sampling or provided funding for sampling and analysis. In 2003, the Soil Geochemistry Workshop was held in Denver to develop recommendations for the design, sampling protocols, and analytical protocols for a continental-scale soil geochemical survey.

The project has received an official letter of support from the Centers for Disease Control and Prevention which stated "these data are enormously useful for risk assessment calculations and in explaining regional differences in the cleanup values used by the USEPA for Superfund sites in different states." The USEPA's National Homeland Security Research Center provided funding in 2010 to assist in soil sampling and in determining soil pathogens in the samples. This agency has a need to determine the background levels of naturally occurring high-priority biothreat agents in U.S. soils to establish appropriate cleanup levels if these agents should be used in an intentional contamination event. They recognized the usefulness of collaborating with the USGS in the ongoing sampling of the Nation under the Geochemical Landscapes Project. State Geological Surveys have recognized the importance of understanding the geochemistry of soils within their individual states. The state surveys of Minnesota, Pennsylvania, and Nebraska conducted sampling in these states at no cost to the project. The U.S. Department of Agriculture Natural Resources Conservation Service (NRCS) has actively participated in the project by collecting all the samples in the states of North Dakota and South Dakota. The National Academy of Sciences has recognized the importance of the project and requested a briefing to the U.S. National Committee for Soil Science in April 2009. The project was also the subject of a short article titled "From Alaska to Yucatan, a Long-Awaited Soil Survey Takes Shape" in the June 11, 2004, issue of Science.

In summary, the need for the type of soil background data being produced by the project includes many agencies and disciplines throughout the government and private sectors of the U.S.