A U.S. NATIONAL PERSPECTIVE ON THE LEAD CONTENT OF SOILS

New maps of the lead content of soil for the conterminous U.S. reveal numerous regions across the country in which typical lead content differs by as much as ten-fold between those regions. The national median lead concentration of soils increases significantly upward in the soil profile from 14.9 mg/kg at about 1 m depth, to 17.8 mg/kg for A-horizon soil, and 18.1 mg/kg for the top 5 cm of soil. Although geologic and biologic processes are known to produce upward migration of lead, there are substantial areas, particularly in the northeastern U.S., where the ratio of lead between A- and C-horizons exceed 2 and, more locally, exceeds 5. These are suspect areas for anthropogenic lead contamination of surface soils, although not all of these areas are urbanized. Quantitative mineralogical data gathered concurrently with geochemical analyses allow inferences of the mineralogical residence of much of the lead. Lead is a trace element in feldspars and comparing the lead and feldspar concentration of soils helps to identify the component of variability that appears to result from variable feldspar concentration. For the western two thirds of the country, variations of lead concentration appear to be caused largely by variations in feldspar concentration. Lead in feldspars is of geologic origin and, because it is tightly bound in the mineral structure, is not particularly mobile or biologically available. In contrast, soils in the southeastern and south central U.S., in areas of deep and prolonged weathering, have very little feldspar, and lead correlates most strongly with clay content, mostly kaolinite, as well as areas of limestone bedrock. Whether the lead in these soils is bound in the clay structure, adsorbed onto clay particles, or is contained in ferromanganese grain coatings cannot be determined uniquely from our data. But, in any case, lead in these forms may be more bioavailable than lead in feldspars. The use of mineralogical data thus provides a parameter, in addition to the lead concentration of soil, which allows an assessment of the potential mobility of lead, an important factor in judging its possible adverse effects .