Speciation of the rare earth element neodymium in groundwaters of the Nevada Test Site and Yucca Mountain and implications for actinide solubility

Concentrations of the rare earth element (REE) Nd and major ions were measured in groundwater samples obtained from 4 wells on the Nevada Test Site and one well located 3 miles east of the proposed high-level nuclear waste repository at Yucca Mountain, adjacent to the Nevada Test Site. The speciation of dissolved Nd was modelled using a combined specific ion interaction (i.e., Pitzer Model) and ion pairing model. The model provided a means of determining the activity coefficients of the Nd³⁺ ion (ym), complexing anions (γx), and Nd³⁺ -anion ion pairs (γM_MX). The modelling indicated that between 92% and > 99% of dissolved Nd is complexed with CO₃^2- as either carbonato (NdCO₃⁺) or dicarbonato (Nd(CO₃)₂^-) complexes. An elevated Nd concentration in one of the groundwater samples was associated with higher calculated free CO₃^2- concentrations (i.e., [CO₃^2-]_F, where the subscript F denotes free concentrations). Based on the relatively large stability constants for Nd CO₃ complexes as compared with stability constants for other inorganic ligands, it is proposed that elevated [CO₃^2-]_F concentrations in groundwaters can result in enhanced dissolved Nd concentrations. The analogous chemical behavior of the trivalent REEs, especially Nd³⁺, and the trivalent actinide series elements (Am³⁺, Cm³⁺, Cf³⁺), suggests that these actinides will be primarily inorganically complexed with CO₃^2- in the local groundwaters and that elevated [CO₃^2-]_F concentrations may increase the ability of these groundwaters to solubilize and transport these actinides.