Landscape Geochemistry on a Continental Scale
Laurel G. Woodruff, U.S. Geological Survey, Mounds View, MN

The Geochemical Landscapes California Pilot Study
Martin B. Goldhaber, U.S. Geological Survey, Denver, CO

Geochemical Modeling of Soil Parent Material Mineralogy
Rodney Klassen, Natural Resources Canada, Ottawa, ON, Canada

A Proposed Soil Geochemical Survey of North America

David B. Smith, U.S. Geological Survey, Denver Federal Center, MS 973, Denver, CO  80225, Tel: 303-236-1849, Fax: 303-236-3200, Email: dsmith@usgs.gov
Martin B. Goldhaber, U.S. Geological Survey, Denver Federal Center, MS 964, Denver, CO  80225, Tel:  303-236-1521, Fax:  303-236-3200, Email: mgold@usgs.gov
William F. Cannon, U.S. Geological Survey, 954 National Center, Reston, VA  20192, Tel: 703-648-6345, Fax: 703-648-6383, Email: wcannon@usgs.gov
Laurel G. Woodruff, U.S. Geological Survey, 2280 Woodale Drive, Mounds View, MN  55112, Tel: 763-783-3291, Fax: 763-783-3103. Email: woodruff@usgs.gov
Robert G. Garrett, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8 CANADA, Tel: 613-995-4517, Fax: 613-996-3726, Email: garrett@NRCan.gc.ca
Robert G. Eilers, Agriculture and Agri-Food Canada, 360 Ellis Building, University of Manitoba, Winnipeg, Manitoba R3T 2N2 CANADA, Tel: 204-474-6123, Fax: 204-474-7633, Email: eilersr@agr.gc.ca
Juan Carlos Salinas Prieto, Consejo de Recursos Minerales/Servicio Geológico Mexicano, 42080 Pachuca, Hidalgo, México, Tel: +52-771-711-39-30, Fax: +52-771-711-32-52, Email: jcsalinas@coremisgm.gob.mx

The U.S. Geological Survey, in collaboration with partners in Canada and Mexico, has initiated the Geochemical Landscapes Project with a long-term goal of a soil geochemical survey of North America.  The resulting database will enhance our ability to recognize and quantify changes in soil composition caused by urbanization, industrialization, agriculture, waste disposal, and other human activities.  The proposed sample design for the survey consists of a uniform grid of 10,000 sites across the continent.  At each site, up to five samples would be collected:  1) the upper five cm; 2) O-horizon (if present); 3) a composite of the uppermost mineral soil; 4) the most representative B horizon; and 5) C-horizon.  Analytical protocols include an extensive array of major and trace elements using ICP-AES and ICP-MS following a four-acid extraction to determine total elemental content.  This would be supplemented by single-element determinations (Hg, Se, Sb) as well as determinations for total carbon, carbonate carbon, and total sulfur.  An estimate of bioaccessibility will be made by a distilled-deionized water extraction and a simulated human gastric fluid extraction followed by ICP-MS.  A limited number of organic compounds will be analyzed to study long-range transport of organic pollutants and the distribution of pesticides, PAHs, and their breakdown products.  Selected samples will undergo microbiological characterization by a combination of phospholipid fatty acid analysis, BIOLOG analysis, agricultural and human pathogen screens, and enzyme assays.

The project is currently in a pilot phase to test and refine field and laboratory protocols.  Sampling has been completed along two transects across the continent.  One transect extends from northern Manitoba to the US-Mexico border.  The other extends from just north of San Francisco to the Maryland shore.  A regional-scale pilot study that is underway in an area of approximately 12,000 square miles in northern California will be completed in 2006.

Landscape Geochemistry on a Continental Scale

Laurel G. Woodruff, U.S. Geological Survey, 2280 Woodale Drive, Mounds View, MN, 55112, Tel: 763-783-3291, Fax: 763-783-3103, Email:  woodruff@usgs.gov
William F. Cannon, U.S. Geological Survey, MS 954 National Federal Center, Reston, VA, 20192, Tel: 703-648-6345, Fax: 703-648-6383, Email:  wcannon@usgs.gov
James E. Kilburn, U.S. Geological Survey, MS 973 Denver Federal Center, Denver, CO, 80225, Tel: 303-236-5514, Fax: 303-236-3200, Email:  kilburn@usgs.gov
David B. Smith, U.S. Geological Survey, MS 973 Denver Federal Center, Denver, CO, 80225, Tel: 303-236-1849, Fax: 303-236-3200, Email:  dsmith@usgs.gov
Robert G. Garrett, Natural Resources Canada, 601 Booth Street, Ottawa, ON, K1A 0E8, Tel: 613-995-4517, Fax: 613-996-3726, Email:  garrett@nrcan.gc.ca
Rodney Klassen, Natural Resources Canada, 601 Booth Street, Ottawa, ON, K1A 0E8, Tel: 613-992-6264, Fax: 703-648-6383, Email: klassen@nrcan-rncan.gc.ca
Robert G. Eilers, Agriculture and Agri-Food Canada, 360 Ellis Building, Winnipeg, MB R3T 2N2, Tel: 204-474-6123, Fax: 204-474-7633, Email:  eilersr@agr.gc.ca
John D. Horton, U.S. Geological Survey, 953 National Center, Reston, VA, 20192, Tel: 703-648-6399, Fax: 703-648-6383, Email:  jhorton@usgs.gov

The U.S. Geological Survey, Geological Survey of Canada, and Agriculture and Agri-Food Canada have completed a pilot study to test and refine sampling and analytical protocols for the proposed soil geochemical survey of North America. In 2004, soil samples were collected from 266 sites along two continental transects, one from northern Manitoba, Canada to El Paso, Texas, and a second along the 38^th parallel from the Atlantic Ocean to the Pacific Ocean. The transects crossed multiple geologic, climatic, physiographic, land use, soil order, and ecological boundaries. This imposed rigorous field testing of sampling protocols across a broad range of conditions. Each transect was divided into approximately 40 km segments. For each segment, a 1 km wide latitudinal strip was randomly selected; within each strip, the most representative landscape and soil type was chosen as a potential sample site. At one in four sites duplicate samples were collected 10 meters apart to estimate local spatial variability. Samples from each sample site includes:  1) soils collected by horizon (O-, A-, C-horizons, where present) for multi-element four-acid and weak soluble extraction analyses and determination of soil texture, 2) A-horizon samples collected for soil moisture and microbiological characterization, and 3) topsoils collected from 0-5 cm for multi-element chemistry and determination of selected pesticides and other organic compounds. The in situ volumes of O- and A-horizon samples were measured so that elements loadings can be calculated. Geochemical results from soil analyses will be integrated in a site-specific descriptive database to identify relations between trace soil constituents and landscape and soil processes across North America.

The Geochemical Landscapes California Pilot Study

Martin B. Goldhaber, U.S. Geological Survey MS 964 Denver Federal Center, Denver CO 80225, Tel: 303-236-1521, Email: mgold@usgs.gov
Jean M. Morrison, U.S. Geological Survey MS 973 Denver Federal Center, Denver CO 80225, Tel: 303-236-6366, Fax: 303-236-3200, Email: jmorrison@usgs.gov
David B. Smith, U.S. Geological Survey, MS 973 Denver Federal Center, Denver CO 80225, Tel: 303-236-1849, Fax: 303-236-3200, Email: dsmith@usgs.gov

The USGS Geochemical Landscapes project, which has as its long-term goal a soil geochemical survey of North America, is presently in a pilot study phase.  The goals are to evaluate sampling and analytical methodologies for a continental-scale study and to explore the utility of regional soil baseline data. One pilot study is a latitudinal transect from Marin County north of San Francisco to the Nevada border in California.  We have chemically analyzed nearly 2000 soil samples including 1300 shallow (upper 30 cm) soils collected in 1980 during the National Uranium Resource Evaluation (NURE) Program from El Dorado, Placer, Sutter, Sacramento, Yolo, and Solano Counties.  The NURE samples are supplemented by 100 soil profiles from across the entire transect. Comparison of the NURE soil geochemical results with aeroradiometric K, U, and Th data show close agreement, giving us confidence that the NURE soil sampling is representative of surface soil chemistry.  Several elements of potential environmental concern, including Cr, Ni, As, and Pb, occur at elevated concentration in the samples.  Soils with high contents of Cr (up to 2700 ppm) and Ni (up to 2000 ppm) form both in the foothills of the Sierra Nevada Mountains above Cr and Ni-rich rocks such as serpentinite, and from transported materials in Sacramento Valley alluvial fill.  Mineralogical composition and human impacts in soils from these two geologic settings may be important in determining mobilization and toxicity of the metals.  Arsenic associated with the Mother Lode gold belt in the Sierra Nevada resulted in the highest soil As concentrations (>80 ppm) in the study area.  Elevated Pb concentrations (up to nearly 2000 ppm) are recognized largely in proximity to highways and cities including both Sacramento and Stockton. We are presently conducting studies to determine the mineralogical residence and bioaccessibility of elements in these soil samples.

Geochemical Modeling of Soil Parent Material Mineralogy

Rodney Klassen, Ph.D., Natural Resources Canada, 601 Booth Street, Ottawa, ON, K1A 0E8, Phone: 613.992.6264, Fax: 703.648.6383, Email: Klassen@nrcan-rncan.gc.ca
Robert Garrett, Ph. D., Natural Resources Canada, 601 Booth Street, Ottawa, ON, K1A 0E8, Phone: 613.995.4517, Fax: 613.992.6264, Email:  Robert.Garrett@nrcan-rncan.gc.ca

As a contribution to the pilot study for the proposed soil geochemical survey of North America, C-horizon soil samples were collected to investigate the potential for geochemical modeling of soil mineralogy. The samples were collected from 35 sites along the North Dakota - northern Manitoba segment of the continental transect. The samples represent geologically diverse soil parent materials, including glacial lake sediment and till derived from Canadian Shield and Phanerozoic sedimentary bedrock. The silt and clay-sized (<0.063 mm) and the clay-sized (<0.002 mm) fractions were separated by seiving and by wet centrifuge methods, respectively, and analyzed by XRF (fused pellet) and by ICP-OES after aqua regia digestion. Geochemical modeling of soil mineralogy is based on approaches developed by the Geological Survey of Finland.  The procedure apportions the chemical elements to mineral groups predicted to be digested by aqua regia digestion and to those decomposed by fusion. The quantitative estimates of mineralogy are based on a simultaneous solution of multiple linear equations using a computer program (MODAN).  The purpose of the study is to indicate geochemical differences among grain size fractions, including the <2 mm fraction analyzed for the pilot study, that may be due to mineral partitioning resulting from geological processes. The modeling will provide a basis to interpret soil geochemistry in terms of its mineralogy and potential environmental reactivity.

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