Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Rare earth elements (REE) were determined in groundwater samples collected from a thick till and clay-rich aquitard sequence located in southern Saska tchewan, Canada. The groundwaters are Na-Mg-SO4- type waters that range fro m highly concentrated brines (e.g., I up to 2 moles/kg) near the ground sur face to relatively dilute waters (I approximate to 0.04 moles/kg) at depth. The majority of these groundwaters have pH values between 7 and 8, althoug h the deepest samples are more alkaline (9 less than or equal to pH less th an or equal to 9.6). Groundwater REE concentrations are relatively constant in the overlying till but increase by up to a factor of 50 in the underlyi ng clay bedrock. Shallow groundwaters have heavy REE (HREE)-enriched shale- normalized patterns, whereas the REE patterns of the deep groundwaters are relatively flat. Solution complexation modelling indicates that variations in REE patterns reflect differences in solution complexation across the REE suite. In the shallow groundwaters, strongly adsorbed, positively charged carbonate complexes (LnCO(3)(+)), sulfate complexes (LnSO(4)(+)), and free metal ion species (Ln(3+)) dominate the speciation of light REEs (LREE), wh ereas HREEs occur chiefly as more stable, negatively charged dicarbonato co mplexes [i.e., Ln(CO3)(2)(-))]. For the deepest groundwaters, however, all of the REEs are predicted to occur in solution as dicarbonato complexes. Th e large HREE enrichments of the shallow groundwaters reflect the greater af finity of the positively charged LREE solution species to adsorb to clay mi nerals or coatings on clay minerals in the aquitard sequence compared to th e more stable, negatively charged HREE dicarbonato complexes. On the other hand, the flat REE patterns of the deep groundwaters reflect the dominance of the negatively charged dicarbonato complex for all REEs. The solution co mplexation model along with the strong positive correlation between REEs an d [CO32-](F)(0.79 less than or equal to r less than or equal to 0.95), and to a lesser extent pH (0.57 less than or equal to r less than or equal to 0 .72), indicates that carbonate ion concentrations, and thus pH, exert impor tant controls on aqueous REE concentrations in these groundwaters. Copyrigh t (C) 2000 Elsevier Science Ltd.