Central Mineral and Environmental Resources Science Center
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The U.S. Geological Survey, Geological Survey of Canada, Agriculture and Agri-Food Canada, Servicio Geológico Mexicano, and Instituto Nacional de Estadística Geografía e Informática have collaborated on a trinational project called Geochemical Landscapes that has as its long-term goal a soil geochemical survey of North America. Our understanding of the variability in chemical composition of the soils on the North American continent is very limited. Neither Canada nor Mexico has a national-scale soil geochemical database, and the most-often-quoted data set for estimating background concentrations of metals in soils of the conterminous United States (Shacklette and Boerngen, 1984; Gustavasson and others, 2001) contains only 1,323 samples (1 sample per 2,300 square miles). As a result, our ability to recognize and quantify changes to soil composition caused by urbanization, industrialization, agriculture, mining, waste disposal, and other human activities is severely impaired. USGS Fact Sheet 015-03 explains in more detail the rationale for such a survey in the United States.
Preliminary recommendations for sample design of the continental-scale survey, sample collection protocols, and analytical methods were developed at the 2003 Soil Geochemistry Workshop. The preliminary sample design consists of a uniform grid of approximately 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 horizon, or A horizon; 4) the most representative B horizon; and 5) C horizon. The analytical protocol includes an extensive array of major and trace elements using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) following a four-acid extraction to determine the near-total elemental content of the samples. This would be supplemented by single-element determinations for Hg and Se as well as analysis for total carbon, carbonate carbon, and total sulfur. Recommendations from the workshop also included 1) an estimate of bioaccessibility by a distilled-deionized water extraction and a simulated human gastric fluid extraction followed by ICP-MS; 2) analysis of a limited number of organic compounds to study long-range transport of organic pollutants and the distribution of pesticides, PAHs and their breakdown products; and 3) characterization of the microbial community in the A-horizon samples by a combination of phospholipid fatty acid analysis, enzyme assays, BIOLOG community profiling, and agricultural and human pathogen screening.
In 2004, the project entered a pilot phase to test and refine these protocols. The pilot studies were carried out at both a continental scale and a regional scale. Final protocols for the soil geochemical survey of North America were developed.
Marty Goldhaber and Dave Smith (co-chiefs of the Geochemical Landscapes Project) and W.A. (Bud) Norvell of the U.S. Department of Agriculture's Plant, Soil, and Nutrition Laboratory in Ithaca, NY convenined a symposium titled "Soil Geochemical Patterns at Regional, National, and International Scales" at the 18th World Congress of Soil Science (WCSS) (http://www.colostate.edu/programs/IUSS/18wcss/). The WCSS was held July 9-15, 2006 in Philadelphia, PA. The oral portion of the symposium consisted of three 35-minutes presentations: