Mobility of radium and heavy metals from uranium mine tailings in acid sulfate soils

Abstract

This study was aimed at determining whether heavy metals in tailings from Ranger Uranium Mine (N.T.) change in chemical form in such a way that they will become more mobile, or bioavailable, after they are mixed with extremely acidic soils from downstream of the mine. Four soils were studied: two samples were acid sulfate (jarositic or pyritic) materials and two were acidic materials overlying acid sulfate horizons. Copper, iron, manganese, lead, uranium and zinc fractions were determined in soils to which uranium mill tailings had been added. Total and exchangeable 226Ra were also determined in selected samples. The tailings-soil mixtures were incubated for up to 4 months and included a comparison of reactions under continuously moist conditions and when subjected to a saturation and drying cycle. The tailings had considerably greater concentrations of total Mn, Pb, U and 226Ra than the soils. The heavy metals in the tailings occurred as relatively immobile forms. In the non-pyritic soils, the distribution of the metals between the fractions did not change much during 4 months of reaction. In the pyritic soil, which underwent oxidation and acidification during incubation, there were 2- to 3-fold increases in the exchangeable fractions of Fe, Mn, Cu and U. The metals in the tailings and soil behaved similarly. There appeared to be more likelihood of increased mobility of metals from oxidation of pyritic materials than from addition of tailings. The fraction of total 226Ra that was exchangeable decreased from 11% in the original tailings to 2-7% after reaction with three of the soils but increased to 44% in one soil. At estimated long-term erosion rates, the tailings are not likely to be a source of heavy metal pollution, but addition of 226Rato soils presents a possible radiological hazard.