Neptunium and americium are relatively insoluble in ground waters containing high sulfate concentrations, particularly at 90??C. The insoluble neptunium species is Np(IV); hence reducing waters should enhance its formation. Americium can exist only in the trivalent state under these conditions, and its solubility also should be representative of that of curium.
Additional publication details
Neptunium and americium speciation in selected basalt, granite, shale, and tuff ground waters