DISTRIBUTION OF PB, CD, CR, CU, ZN, NI, AND BA IN SOILS OF THE CONTERMINOUS UNITED STATES

The U.S. Geological Survey recently completed a soil geochemical survey of the conterminous United States based on 4,857 sample sites (1 site per approximately 1,600 km²). At each site, a sample from a depth of 0 to 5 cm was collected along with samples from the soil A and C horizons. Each sample was sieved to <2 mm and then ground to <150 μm prior to chemical analysis for 45 major and trace elements. Results for the seven “heavy metals” Pb, Cd, Cr, Cu, Zn, Ni, and Ba are discussed in terms of the broad-scale processes that cause the observed element distribution in the conterminous United States. All seven of these elements were analyzed by inductively coupled plasma-mass spectrometry or inductively coupled plasma-atomic emission spectrometry following a four-acid digestion. The concentrations of these elements in soils collected from a depth of 0 to 5 cm varied by a factor of 10³ (Cd, Cr, Cu, Ni, and Ba) to 10⁴ (Pb and Zn). This large variation primarily reflects soil formation from parent materials of widely differing composition. Soils formed from quartz-rich sediments show the lowest metal concentrations and soils formed from marine shales, ultramafic rocks, and mineralized bedrock tend to have the highest concentrations. In addition, the distribution of Ba clearly shows the effects of climate as expressed by average annual precipitation. Barium, along with mobile major elements such as Ca and Na, tends to be leached from the soil in areas of high precipitation such as the southeastern United States. Human influence is superimposed on the highly variable natural background distribution, but may be assessed by comparing the concentration in the soil C horizon with that in the near-surface soils. Lead gives some indication of this influence in highly populated areas.