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In 2007, the U.S. Geological Survey began a low-density (1 site per 1,600 sq. km., 4857 sites) geochemical and mineralogical survey of soils of the conterminous United States. Sampling and analytical protocols were developed at a workshop in 2003, and pilot studies were conducted from 2004-2007 to test and refine these recommended protocols. The final sampling protocol for the national-scale survey included, at each site, a composite sample from a depth of 0 to 5 centimeters, a composite of the soil A horizon, and a deeper sample from the soil C horizon or, if the top of the C horizon was at a depth greater than 1 meter, from a depth of approximately 80 to 100 centimeters. The <2-millimeter fraction of each sample was analyzed for a suite of 45 major and trace elements by methods that yield the total, or near-total, elemental content. The major mineralogical components in the samples from the soil A and C horizons were determined by a quantitative X-ray diffraction method using Rietveld refinement. Sampling in the conterminous U.S. was completed in 2010 with chemical and mineralogical analyses completed in May, 2013. The resulting data set provides an estimate of the abundance and spatial distribution of chemical elements and minerals in soils of the conterminous U.S. and represents a baseline for soil geochemistry and mineralogy against which future changes may be recognized and quantified. This report (1) describes the sampling, sample preparation, and analytical methods used; (2) gives details of the quality control protocols used to monitor the quality of chemical and mineralogical analyses over approximately six years, and (3) makes available the soil geochemical and mineralogical data in downloadable tables. |