USING QUANTITATIVE MINERALOGY TO ENHANCE INTERPRETATIONS OF NATIONAL-SCALE PATTERNS OF TRACE ELEMENT DISTRIBUTIONS IN SOIL—RESULTS FROM THE USGS SOIL GEOCHEMICAL AND MINERALOGIC SURVEY OF THE CONTERMINOUS UNITED STATES

The USGS has completed a geochemical soil survey of the conterminous 48 states by sampling multiple soil horizons at nearly 5000 sites. In addition to chemical analyses for a large suite of trace and major elements we have determined quantitative soil mineralogy by x-ray diffraction and Rietveld refinement calculations for all A-horizon and C-horizon samples. The mineralogic data are significant in their own right for variables such as carbonate mineral content, which determines soil pH and buffering capacity, and clay content, which affects a wide variety of both chemical and physical properties of soil. More significantly, combining information on the mineral content of soils with trace element geochemistry allows a level of understanding of the causes and significance of trace element variations that is not possible with trace element geochemistry alone. Quartz is a very significant variable. It is the most abundant mineral in most soils and varies nationally from zero to nearly 100%. Because quartz contains essentially no trace elements, it acts as a dilutant to trace element-bearing phases so that the concentration of trace elements strongly reflects the inverse of the quartz content of soils. Many prominent patterns of trace element distribution on the national scale appear to be explainable largely by the amount of quartz dilution. However, knowing the quartz content of soil allows a calculation of the composition of the fraction of the soil that is not quartz. Maps of the trace element concentrations in this non-quartz fraction reveal patterns very different from those shown by the bulk samples and provide information on the source and behavior of these elements that is difficult to decipher from the bulk chemistry alone. Quantitative mineralogy also allows inferences on the mineralogical residence of trace elements, information applicable to predicting their geochemical behavior and environmental availability. For instance, lead is a common trace element in feldspars, so normalizing lead concentrations of soil to feldspar concentrations reveals regions where lead is mostly bound in feldspars and thus relatively immobile versus regions where it is mostly in other minerals, such as clays or ferromanganese minerals, and more likely to be environmentally accessible.