Distribution of the rare earth elements in porewaters from a clay-rich aquitard sequence, Saskatchewan, Canada

Abstract

A 91.4-m thick porewater profile of the rare earth elements (REEs) is presented from a thick clay-rich Pleistocene till (0–80 m) and Cretaceous clay (80–156 m) aquitard sequence, Saskatchewan, Canada. The upper 3–4 m of till is weathered. Absolute aqueous concentrations of the REEs vary by more than one order of magnitude. Aqueous REE concentrations decrease with depth through the weathered till to a depth of about 15 m below ground, below which, the concentrations in the unweathered till remain relatively consistent. The only porewater sample from the clay (91.4 m) has the highest total REE concentration. Elevated REE concentrations in the oxidized zone attributed to geochemical weathering. There is no significant seasonal variation in aqueous REE concentrations within the water columns in the piezometers. Redox conditions and solution complexation reactions exert predominant controls on the aqueous REE concentrations. The majority of REE patterns are coherent, characterized by overall negatively fractionated REE, with convex-down profiles from La to Pr or Nd, and convex-up from Nd to Tb. In the porewater sample from the clay, the LREEs are an exception, with a flat normalized profile. HREE/LREE fractionations in the porewaters are depth-dependent and are mainly controlled by solution complexation with dissolved sulfate, phosphate, free carbonate ions and DOC. Porewaters in the unweathered till (5–76.2 m) are characterized by negative Ce anomalies, which may reflect in part, negative Ce anomalies in the host till geochemistry. The negative Ce anomalies are magnified relative to the host tills during water–rock interaction and REE transportation. Porewaters from the weathered till exhibit strong positive Ce anomalies relative to PAAS, the host rocks and their leachates. The Ce anomalies are predominantly controlled by the redox conditions and solution complexation. The large positive Ce anomalies in these porewaters are probably caused by the oxidation of Ce(III) to Ce(IV) that was stabilized in solution through complexation with high contents of inorganic and organic ligands. Positive Eu anomalies relative to PAAS, the host rocks and leachates of the host rocks, result from hydrolysis of plagioclase feldspar. The limited depth profile for total dissolved REEs across the weathered–unweathered till boundary is retarded with respect to that previously established for Na +, K +, Mg 2+, SO 4 2−, and TDS, suggesting that chemical reactions control the migration of dissolved REEs.