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Release of Radium and Other Decay-Series Isotopes from Florida Phosphate Rock



The purpose of this research was to determine the conditions under which uranium decay-series isotopes are released from phosphate rock into the environment. Particular attention was given to the behavior of radium, radon, and polonium. This emphasis was justified because of several documented cases citing elevated concentrations of these radio-elements occurring in Florida groundwaters. When it became clear that polonium was occasionally present at exceedingly high concentrations in shallow groundwaters, the scope of the project was expanded to include a study of the distribution of Po-210 in the surficial aquifer of west central Florida.

Studies of a series of phosphate rock samples representing various degrees of chemical weathering show that almost all uranium-series radionuclides display higher activities in weathered samples compared to fresh material. Most samples display a Pb-210/Ra-226 activity ratio less than secular equilibrium because of Rn-222 leakage. An unexpected result was the deficiency of Po-210, relative to Pb-210 in several samples. This implies that polonium, under certain conditions, may be more mobile than lead.

Many wells in central Florida contain high concentrations of Po-210. Approximately one quarter of the 80 sites sampled had Po-210 concentrations higher than the federally- mandated limit for “gross” alphas (15 pCi/ L). Although the source of the polonium is radon, it is not clear what mechanism is responsible for maintaining polonium in solution. Characteristics which high-polonium groundwaters have in common include low pH, presence of sulfide and at least moderately high radon.