NEW SOIL GEOCHEMISTRY AND MINERALOGY FOR THE CONTERMINOUS UNITED STATES

The U.S. Geological Survey has completed a low-density (about 1 site per 1600 km²) geochemical and mineralogical survey of soils of the conterminous United States. Soil samples were collected from 2007 to 2010 at 4857 randomly selected sites in a spatially balanced array using consistent sampling protocols. At each site, three samples were collected: 1) a sample from a depth of 0 to 5 cm, regardless of soil horizon, 2) a composite of the soil A horizon (uppermost mineral soil), and 3) a sample of deeper subsoil typically from 80 to 100 cm depth (soil B or C horizon). The < 2 mm fraction of each sample was analyzed for more than 45 major and trace elements using a near-total digestion method. Major mineralogical components were quantified in soil A and B/C horizons by X-ray diffractometry. A split of each sample is archived for future research.

The data array provides a three-dimensional framework of soil geochemistry and mineralogy. Spatial differences in geochemistry and mineralogy at continental and regional scales can be tied to distinctive soil parent materials modified by climate-related processes such as weathering (driven by hemisphere-wide gradients of temperature and precipitation) and glaciation. Element distributions among the three soil samples from each site reveal human influences superimposed on natural soil background concentrations. Anthropogenic influences, such as atmospheric deposition from industry and mining, agricultural practices, and changes in land use, have resulted in input and accumulation of a number of elements in surface soils, some (for example, As, Cd, P, Pb, and Hg) of environmental concern to ecosystem and human health. Soil mineralogy exerts control on both soil chemical and physical properties. Quantitative mineralogy allied with element concentrations allows inference of mineralogical hosts for many major and trace elements.

All geochemical and mineralogical data with accompanying land use/land type information will be publically released in 2013 as downloadable files. This new data set for the conterminous U.S. represents a major step forward from prior national-scale soil geochemistry data and, along with the soil archive, provides a robust soil data framework for the U.S. now and into the future.