SOIL GEOCHEMISTRY AND MINERALOGY FOR THE CONTERMINOUS UNITED STATES – THE USGS GEOCHEMICAL LANDSCAPES SOIL SURVEY

The U.S. Geological Survey has completed a geochemical and mineralogical survey of soils across the conterminous United States. Soils were collected from about 4850 randomly selected sites in a spatially balanced array. At each site, three samples were collected: 1) 0 to 5 cm in depth, regardless of soil horizon, 2) a composite of the uppermost mineral soil (A horizon), and 3) a composite of deeper subsoil (B or C horizon). The <2 mm fraction of each sample was analyzed for more than 40 major and trace elements using a near-total digestion method. Major mineralogical components were quantified in A- and C-horizon soils by X-ray diffractometry. A split of each sample was archived for future use. The data provide a unique and comprehensive three-dimensional framework of soil geochemistry and mineralogy at continental and regional scales. Patterns of element and mineral distributions are revealed that reflect the effects of a variety of soil-forming processes across the highly diverse surficial geology and strong continental climate gradients, as well as human influences. For example, soil geochemistry and mineralogy can be correlated to geographic regions with distinctive parent materials, such high P in the outcrop area of the Phosphoria Formation, enriched base and precious metals in mineralized rock in Nevada, and calcite in soils developed on limestone in the Edwards Plateau of Texas. Prolonged weathering in the humid subtropical climate of the southeastern United States results in soils depleted in many elements and soluble minerals, but enriched in resistant quartz and secondary kaolinite. Sediments deposited by individual glacial lobes in the Midwest can be traced to their sources by distinctive mineralogy and chemistry. Element and mineral distribution in transported alluvium along major rivers, such as the Mississippi, bisect broad regional patterns that formed in situ. Impacts from agriculture, industrialization, and urbanization are apparent when surface soil concentrations are compared to deeper soils. For example, a region of high P in surface soils across the Midwest corresponds to intense harvested crop agriculture and use of phosphate fertilizers. High Pb in surface soils is found near urban and highly industrial regions, such as eastern New Jersey and the Chicago/Gary region.