The occurrence and distribution of rare earth elements in mineral and thermal waters in the Polish Lowlands

Abstract

The occurrence and distribution of rare earth elements (REEs) in mineral and thermal waters encountered in Quaternary, Mesozoic and Permian aquifers of the Polish Lowlands are described, with REE concentrations analyzed in 35 groundwater samples and 25 aquifer host rock samples. The waters analyzed are characterized by high variability of total dissolved solids (TDS: 0.44 to 340 g/L) and of outflow temperatures (9 to 89 °C). Values of ultralow concentrations (pg/L-μg/L) of REEs in groundwaters of variable salinity were obtained using a novel method combining a seaFAST-pico preconcentration system with inductively coupled plasma mass spectrometry (ICP-MS). Very low concentrations of dissolved (<0.45 μm) REEs and Y are present in these waters, ranging from 0.6 ng/L to 19.1 μg/L and from 1.7 to 54.5 ng/L, respectively.The REE-patterns, normalized to European Shale (EUS), were used to assess the range of REE fractionation between water and host rocks and to show differences in REE distribution in various hydrogeological systems.Three groups of REE-patterns were distinguished: (i) upwards-sloping with HREE-enrichment (74% of all samples), (ii) convex with MREE-enrichment (20%), and (iii) near-flat with very slight depletion in HREE (6%). REE fractionation in shallow waters was attributed to the adsorption of LREE on Fe/Mn oxide/oxyhydroxide particles and the simultaneous complexation of REEs with carbonate and bicarbonate ions. In waters occurring below the zone of active water exchange, in reducing conditions, HREE-enriched patterns developed, mainly as a result of reductive dissolution of previously adsorbed and/or precipitated REEs. The convex, MREE-enriched patterns resulted from a combination of several geochemical processes controlling REE fractionation: (i) high temperature (T > 60 °C) favouring a release of the more easily soluble MREEs from rocks into the water, (ii) the presence of organic material showing a strong ability to form complexes with REEs, (iii) reductive release (dissolution) of previously adsorbed REEs on mineral particles, and (iv) dissolution of rocks by acidic waters.Our findings confirmed that REEs can be a useful markers for identifying basic hydrogeochemical processes shaping the chemistry of waters.