REE and Y in groundwater in the upper 1.2 km of Proterozoic granitoids (Eastern Sweden) – Assessing the role of composition and origin of groundwaters, geochemistry of fractures, and organic/inorganic aqueous complexation

Abstract

Yttrium and rare earth elements (YREEs) are studied in groundwater in the shallow regolith aquifer and the fracture networks of the upper 1.2km of Paleoproterozoic granitoids in boreal Europe (Laxemar and Forsmark areas, Sweden). The study includes groundwater sampled via a total of 34 shallow boreholes reaching the bottom of the regolith aquifer, and 72 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths in the fractured bedrock. The groundwater composition differs substantially between regolith and fracture groundwater and between areas, which affects the dissolved YREE features, including concentrations and NASC normalized patterns.In the fresh groundwater in the regolith aquifers, highest YREE concentrations occur (10th and 90th percentile; Laxemar: 4.4–82μgL−1; Forsmark: 1.9–19μgL−1), especially in the slightly acidic groundwater (pH: 6.3–7.2 – Laxemar), where the normalized YREE patterns are slightly enriched in light REEs (LaNASC/YNASC: 1.1–2.4). In the recharge areas, where redox potentials of the regolith groundwater is more moderate, negative Ce anomaly (Laxemar: 0.37–0.45; Forsmark: 0.15–0.92) and positive Y anomaly (mainly in Forsmark: 1.0–1.7) are systematically more pronounced than in discharge areas. The significant correlations between the YREE features and dissolved organic carbon, minor elements, and somewhat pH suggest a strong control of humic substances (HSs) together with Al rich colloids and redox sensitive Fe–Mn hydrous precipitates on the dissolved YREE pools.In the bedrock fractures, the groundwater is circumneutral to slightly basic and displays YREE concentrations that are at least one order of magnitude lower than the regolith groundwater, and commonly below detection limit in the deep brackish and saline groundwater, with some exceptions such as La and Y. At intermediate depth (>50m), where groundwater of meteoric origin percolates, the LaNASC/YNASC values moderately to substantially decrease (Laxemar: 0.24–2.65; Forsmark: 0.02–0.06) and Y and Ce anomalies are negligible as compared to the regolith groundwater. Aqueous speciation modeling predicts substantial binding of dissolved Y and La, respectively, to HSs. This, in turn, suggests that the features of the YREE pool in the meteoric fracture groundwater are dominantly controlled by the capacity of fracture minerals to sorb HS ligands inherited from the overlying terrestrial regolith. In the deep bedrock fractures (>100/200m), the YREE features vary substantially with the groundwater paleo-origin. In Laxemar, where groundwater with pronounced glacial origin percolates, the YREE concentrations decrease with increasing mixing fraction of glacial melt water. There, the dissolved YREEs are mostly bound to HSs, and inherited their fractionation features (LaNASC/YNASC: 0.15–2.1) from water–rock interaction in the intermediate bedrock fractures. In Forsmark, the YREE and heavy REE enrichment (LaNASC/YNASC: 0.007–0.23) are more systematic in the groundwater with pronounced marine origin, due to water–mineral interactions in the sea sediment and in the fractures while infiltrating and percolating. YREE features significantly change in the deep saline groundwater with a long residence time, which displays LaNASC/YNASC similar to those of the local bedrock. The findings of this study are relevant in terms of safety assessment for nuclear waste disposal in crystalline rock carrying groundwater influenced by various paleo-climatic recharges.