Ra-226/U-238 disequilibrium in an upland organic soil exhibiting elevated natural radioactivity

This paper presents the results of a study into the anomalous Ra-226/U-238 disequilibrium (Ra-226/U-238 of 0.5-9) exhibited by an upland organic soil in Co. Donegal, Ireland. Radiochemical speciation of Ra-226, U-238 and Ra-228 indicates that in this organic soil the high Ra-226/U-238 ratio is due to loss of U-238 relative to Ra-226 via oxidation and mobilisation of U-238 in the upper layers of the soil and subsequent loss in solution. At the lower, more reducing depths of the soil profile, U-238 and Ra-226 are essentially in equilibrium. Loss of (238) U appears to occur primarily from the easily oxidised organic and iron oxide fractions of the soil, samples exhibiting high Ra-226/U-238 ratios displaying significantly lower U-238 levels in these fractions than samples whose ratio is below the average value for the soil of the valley. Selective enrichment of Ra-226 by plants or preferential leaching of Ra-226 from the underlying rock is not supported by the results of this study. (C) 2002 Elsevier Science Ltd. All rights reserved.