DISTRIBUTION OF RADON SOURCES AND EFFECTS ON RADON EMANATION IN GRANITIC SOIL AT BEN-LOMOND, CALIFORNIA

The abundance and distribution of radioelements on bulk and microscopi c scales were investigated in residual granitic-derived soil at a faci lity for investigating the movement of radon into structures. In bulk soil samples, Ra concentrations range from 0.6 to 1.3 pCi/g, and varia tions in Ra, Th, and K appear to be controlled mainly by heterogeneiti es inherited from the parent granitic rock, which contains abundant di kes and inclusions. U in soil and parent rock is concentrated in prima ry minerals (mainly zircon and sphene), and in secondary sites that ar e of greater importance for Rn emanation. The main U-bearing secondary sites are weathered sphene, grain boundary coatings, weathered biotit e and plagioclase, as well as dense Fe-rich coatings and a REE-phospha te mineral present in near-vertical fracture zones in saprolite underl ying shallow loam. Elevated U in these sites generally correlates with high Ti, Al, Fe, and/or P. Preferential distribution of U and Ra on g rain boundaries and porous weathered minerals is reflected in relative ly high Rn emanation rates in the soil. Highest emanation occurs betwe en 1.3 and 2.3 m depth, where fine pedogenic phases-gibbsite and amorp hous silica and Fe-OOH-are most abundant; it is related to fixation of Ra by these phases, which precipitate close to the surface and accumu late at these depths by illuviation. Separation of Ra from U may occur locally, given remobilization of U-series elements from secondary sit es, and large differences between Ra and U sorption capabilities of se veral phases present in the soil. Concentration of U along permeable f racture zones in saprolite suggests that contribution of soil-gas Rn f rom depth (> 2 m) could be significant to Rn availability near the sur face.