FORM AND BEHAVIOR OF RADIUM, URANIUM, AND THORIUM IN CENTRAL PENNSYLVANIA SOILS DERIVED FROM DOLOMITE

In a pair of well‐ and poorly‐drained soils developed on Ordovician carbonate rocks in Central Pennsylvania, the distribution with depth and chemical form of 238U, 232Th, and 226Ra have been compared to soil properties. Concentrations range from 3.3 to 8 ppm U and 6.3 to 11 ppm Th in the soils. Although radium is in equilibrium with U at depth in both soils, in surficial A‐horizons Ra is well in excess of U (165–180%). Ra and U are enriched 10X‐12X over the bedrock. Th is enriched, along with Al and Fe, by 20X. Progressive selective extractions for U, Th, and Ra in soil show half the radium occurring in pedogenic phases, chiefly in the organic fractions (24% of total Ra). By contrast, <1% of the U and Th occurs in any of the organic fractions; up to 80% occurs in the silt and clay fractions with a significant occurrence in the Fe‐oxides, up to 50% for Th. Ra and chemically similar Ba are cycled by vegetation but Ra is retained in soil, bound in humified soil organic matter while Ba is not. Radon emanation of these soils (8–27%) can be almost entirely attributed to highly emanative Ra bound in organic matter. In these soils the action of vegetation leads to high soil gas radon by maintaining high Ra concentration and enhancing its emanating power. Copyright 1990 by the American Geophysical Union.