Mineral Resources Program
Geochemical landscapes of the United States
The geochemistry of the Earth's surface has evolved from the underlying rocks through natural processes such as weathering and erosion as well as from human activity such as industrialization, urbanization, mining, waste disposal, and agriculture. We have initiated a project to study the resulting "geochemical landscape." Mary Lou Zoback eloquently stated the rationale for such a project in her 2000 presidential address to the Geological Society of America:
"Documenting and understanding natural variability is a vexing topic in almost every environmental problem. How can we recognize and understand changes in natural systems if we don't understand the range of baseline levels?" (Zoback, 2001).
The goal of the Geochemical Landscapes Project is to define and understand the geochemical baseline levels for soils. Our current state of knowledge regarding the chemical composition of soils in the United States is at a remarkably low level. At the present time, the most frequently quoted data set for background concentrations of metals and other trace elements in soils of the conterminous United States consists of only 1,323 samples collected during the 1960s and 1970s by the U.S. Geological Survey (USGS) (Boerngen and Shacklette, 1981; Shacklette and Boerngen, 1984). A similar data set was later collected in Alaska (Gough and others, 1984, 1988). Samples for these studies were collected from a depth of about 1 foot, primarily from noncultivated fields having native vegetation, and analyzed for over 40 elements.
The only other national-scale soil geochemical data set for the United States was generated by the Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service (Holmgren and others, 1993). This data set consists of 3,045 samples of agricultural soil collected from major crop-producing areas of the conterminous United States. The primary purpose of this study was to assess the background levels of lead and cadmium in major food crops and in soils on which these crops grow. Thus, the samples were only analyzed for five metals: lead, cadmium, copper, zinc, and nickel.
The data from Boerngen and Shacklette (1981) allow us to produce geochemical maps for specific elements such as that shown in figure 1 for arsenic (Gustavsson and others, 2001). A map produced from such sparse data points obviously carries a large degree of uncertainty with it and does not have the resolution to answer many of the questions raised by land management and regulatory agencies, earth scientists, and soil scientists. The long-term objectives of the Geochemical Landscapes Project are to design and carry out a new, higher density national-scale soil geochemical survey in collaboration with NRCS and other Federal and state agencies as well as with the academic community. The first three years of the project will be primarily devoted to studying the feasibility of carrying out such a survey. We have initiated discussions with colleagues in Canada and Mexico in anticipation of expanding the survey to include all of North America.
We convened a Soil Geochemistry Workshop in Denver, Colorado, on March 4-6, 2003, to begin designing the new survey. The primary purpose of the workshop was to give potential customers for the data and maps generated from such a survey an opportunity to participate in the planning process from the earliest stages. We are seeking customer input to help with the actual sample design as well as with sampling protocols, analytical protocols, and end products/publications that would be most useful. If we are going to collect new soil samples across the United States, we feel it is imperative to extract all the information that we can from those samples. Thus, we will be discussing the possibility of including such things as selective extractions to estimate the "bioavailability" of elements in the soil; analysis of organic compounds such as pesticide residues, polycyclic aromatic hydrocarbons, etc.; and soil microbial characterization as part of the new survey.
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David B. Smith and Martin B. Goldhaber
Boerngen, J.G., and Shacklette, H.T., 1981, Chemical analysis of soils and other surficial materials of the conterminous United States: U.S. Geological Survey Open-File Report 81-197, 143 p.
Gough, L.P., Peard, J.L., Severson, R.C., Shacklette, H.T., Tompkins, M.L., Stewart, K.C., and Briggs, P.H., 1984, Chemical analyses of soils and other surficial materials, Alaska: U.S. Geological Survey Open-File Report 84-423, 77 p.
Gough, L.P., Severson, R.C., and Shacklette, H.T., 1988, Element concentrations in soils and other surficial materials of Alaska: U.S. Geological Survey Professional Paper 1458, 53 p.
Gustavsson, N., Bølviken, B., Smith, D.B., and Severson, R.C., 2001, Geochemical landscapes of the conterminous United States-New map presentations for 22 elements: U.S. Geological Survey Professional Paper 1648, 38 p. Available online at: http://pubs.usgs.gov/pp/2001/p1648/
Holmgren, G.G.S., Meyer, M.W., Chaney, R.L.,and Daniels, R.B., 1993, Cadmium, lead, zinc, copper, and nickel in agricultural soils of the United States of America: Journal of Environmental Quality, v. 22, p. 335-348.
Shacklette, H.T., and Boerngen, J.G., 1984, Element concentrations in soils and other surficial materials of the conterminous United States: U.S. Geological Survey Professional Paper 1270, 105 p.
Zoback, M.L., 2001, 2000 Presidential Address: Grand challenges in earth and environmental sciences: Science, stewardship, and service for the twenty-first century: GSA Today, December 2001, p. 41-47.
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