DISTRIBUTION OF TRACE AND POTENTIALLY TOXIC ELEMENTS IN SOILS AT THE NATIONAL SCALE – THE USGS GEOCHEMICAL LANDSCAPES PROJECT

The U.S. Geological Survey is nearing completion of a low-density geochemical and mineralogical survey of soils within the conterminous United States. The goals are: 1) develop a national-scale framework for generating and managing geochemical and mineralogical data now and in the future; 2) produce a national array of soil data using consistent sampling and analytical protocols; and 3) establish an archive of soil samples for future researchers. Sampling began in 2007 and was completed in 2010. The nominal sample spacing is 1 site per 1,600 km² within a spatially balanced random array, resulting in about 4,800 sites. At each site, three soil samples were collected: 1) the uppermost 5 cm of soil, regardless of horizon; 2) a composite of the A horizon; and 3) a deeper subsoil, typically B or C horizon, from about 1 m depth. This dataset will help to define regional ambient background levels for a number of elements of environmental concern. Analytical chemistry and mineralogy determinations for approximately 14,400 soil samples are currently in progress, with data for more than half the country now available. The <2 mm fraction of each sample is analyzed for a broad suite of major and trace elements by a combined inductively coupled plasma-mass spectrometry/atomic emission spectrometry method following near-total digestion in a 4-acid mixture; separate methods are used for As, Hg, Se, and total C. Major mineralogical components are determined by quantitative X-ray diffraction. Understanding the spatial variations of these potentially toxic elements in surficial soils is critical for estimating environmental exposure via soil pathways. Current data reveal coherent, continental- to regional-scale geochemical and mineralogical patterns dependent on underlying soil parent materials, soil age, land type/land use, and climate. Human-induced modifications to soil geochemistry related to current and historic industrial and agricultural activities are recognized by comparing element concentrations in surface soils to deeper soils at individual sites. However, regional enrichments of multiple metal suites in surface soils resulting from mineralization by geologic process can be as high (or even higher) than enrichments from anthropogenic contamination.