SPECIATION OF THE RARE-EARTH ELEMENT NEODYMIUM IN GROUNDWATERS OF THENEVADA-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 Te st Site and one well located 3 miles east of the proposed high-level n uclear waste repository at Yucca Mountain, adjacent to the Nevada Test Site. The speciation of dissolved Nd was modelled using a combined sp ecific ion interaction (i.e., Fitter Model) and ion pairing model. The model provided a means of determining the activity coefficients of th e Nd3+ ion (gamma(M)), complexing anions (gamma(X)), and Nd3+-anion io n pairs (gamma M(MX)). The modelling indicated that between 92% and >9 9% of dissolved Nd is complexed with CO32- as either carbonate (NdCO3) or dicarbonato (Nd(CO3)(2)(-)) complexes. An elevated Nd concentrati on in one of the groundwater samples was associated with higher calcul ated free CO32- concentrations (i.e., [CO32-](F), where the subscript F denotes free concentrations). Based on the relatively large stabilit y constants for Nd CO3 complexes as compared with stability constants for other inorganic ligands, it is proposed that elevated [CO32-](F) c oncentrations in groundwaters can result in enhanced dissolved Nd conc entrations. The analogous chemical behavior of the trivalent REEs, esp ecially Nd3+, and the trivalent actinide series elements (Am3+, Cm3+, Cf3+), suggests that these actinides will be primarily inorganically c omplexed with CO32- in the local groundwaters and that elevated [CO32- ](F) concentrations may increase the ability of these groundwaters to solubilize and transport these actinides.