A Proposed Soil Geochemical and Microbiological Survey of North America: Continental-Scale Pilot Study in Canada and the United States.
David B. Smith1, Laurel G. Woodruff1, William F. Cannon1, James E. Kilburn1, Robert G. Garrett2, Rodney Klassen2, Robert G. Eilers3, Martin B. Goldhaber1, John D. Horton1, and Jean M. Morrison1. (1) U.S. Geological Survey, Denver Federal Center, MS 973, Denver, CO 80225, (2) Geological Survey of Canada, 601 Booth St, Ottawa, ON K1A 0E8, Canada, (3) Land Resource Unit, Agriculture and Agri-Food Canada, University of Manitoba, 360 Ellis Bldg, Winnipeg, MB R3T 2N2, Canada
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 265 sites along two continental transects: one from northern Manitoba, Canada to El Paso, Texas, and a second along the 38th 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. A 1-km-wide strip was randomly selected for each segment; within each strip, the most representative landscape and soil type was chosen as a potential sample site. Duplicate samples were collected 10 meters apart to estimate local spatial variability at one in four sites. Samples from each site include: 1) soils collected by horizon (O-, A-, and C-horizons, where present) for multi-element analysis following a near-total four-acid digestion and for bioaccessibility determinations by a deionized water leach and a simulated human gastric fluid extraction; 2) A-horizon samples collected for soil moisture and microbiological characterization; and 3) topsoil 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 element 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 geochemical processes across North America.