The behavior of the rare earth elements and yttrium in groundwaters of the Holy Cross Mountains, SE Poland

Abstract

In areas of contrasting geology, local but significant spatial changes in environmental conditions can occur suddenly and unexpectedly within aquifers, hampering accurate assessment of groundwater chemistry. Recently, the rare earth elements (REEs) have become extensively used in identifying geochemical processes in aqueous systems, due to their unique sensitivity to environmental changes. In this study, the REEs and Y (yttrium), combined with the chemical parameters of the main, minor and trace water components, and with the isotopic signatures of δ¹⁸O-H₂O, δ²H-H₂O, ³H-H₂O, δ³⁴S-SO₄, δ¹⁸O-SO₄, were investigated in order to gain a better understanding of the geochemistry of the groundwaters in the Holy Cross Mountains, which possesses a diverse hydrogeological system. The waters studied are ‘young’, meteoric-derived, of acidic to slightly alkaline pH (4.95–7.75) and TDS values of 38.97–2713.91 mg/L. They represent predominantly the HCO₃-Ca-Mg and less often the HCO₃-Ca-SO₄-(Mg) or SO₄-Ca-(Mg) types. The dissolved (<0.45 μm) concentrations of the REEs (5.55 to 13,857.35 ng/L) and Y (4.43 to 2450.22 ng/L) in the waters studied are the result of host rock dissolution, and tend to increase significantly, by up to several orders of magnitude, via interactions between the rocks and acidic waters. Speciation calculation reveals that dissolved REE + Y in neutral and alkaline waters are transported mainly as bicarbonate (CO₃)₂⁻ and carbonate CO₃⁺ complexes, while in a more acidic environment these elements occur preferably as free ions (REE³⁺+Y³⁺), and their abundance gradually increases along with decreasing pH value. The EUS (European Shale) normalized REE patterns of the waters studied show two distinct dominant types: (i) upwards-sloping with HREE-enrichment (LREEs<MREEs<HREEs), (ii) convex with MREE-enrichment (LREEs<MREEs>HREEs). The first pattern type reflects REE fractionation in alkaline and oxygenated waters, leading to preferential LREE adsorption onto mineral particles, accompanied by complexation of HREEs with carbonate and bicarbonate ions, while the second pattern type results from dissolution of the host rocks in more acidic conditions. The REE patterns in the waters studied are mostly different from the REE patterns characteristic of the host rocks, except for some water samples from carbonate aquifers. Ce and Eu anomalies were recorded in the waters studied. Some of the negative Ce and positive Eu anomalies were inherited from host rock dissolution, while other negative or positive Ce anomalies reflect oxygenic or more reducing conditions, respectively.