Actinide and technetium solubility limitations in ground waters of crystalline rocks

The solubility and speciation of U, Np, Pu, Am and Tc in deep groundwaters from crystalline rocks have been calculated. The solution chemistry of these elements is largely determined by the redox conditions (Eh = 0.24 - 0.06pH + 0.06 V, based on measurements of Fe(II)2), pH (usually 7 - 9) and the presence of carbonate (log[CO32-] from (pH - 14) to (0.76pH - 10.83), based on measurements of HCO3-). The oxidation states U(IV)(+VI), Np(IV), Pu(III)(+IV), Am(III) and Tc(IV) dominate, with the tetravalent oxides (U, Np, Pu, Tc) and Am2 (CO3)3 (s) as solubility limiting phases. Maximum concentrations in the range 10-9 - 10-10 M or below would be expected for U, Np and Tc under reducing conditions, somewhat higher for Pu and still higher (10-6 - 10-7 M) for Am. 11 references, 7 figures, 3 tables