Rare Earth Elements in thermal and mineral waters from Mesozoic aquifers of the Polish Lowlands

Chemical composition including rare earth elements (REEs) was determined in thermal and mineral water samples collected from 33 spa and geothermal wells located in the Polish Lowlands (northern and central Poland). REE examinations have not been performed for this kind of thermal and medicinal mineral waters in Poland so far. Observation covered the Mesozoic aquifers, built mainly of Jurassic and less often Triassic or Cretaceous sandstone series, which occur at the depth ranging from 20 to 2,500 m below land surface. Selected elements in filtered (0,45 μm) and acidified (HNO3) water samples were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) methods with the use of Sea-Fast preconcentration tool. The examined waters, characterized by TDS value ranging from several to 114 g/L, represent mainly Na-Cl type and much less common HCO3 or SO4 types. The Cl ion concentration changes from 16,5 mg/l to 71,100 mg/l. The highest water temperature value 89 oC was measured at the well head in the Stargard geothermal plant, NW Poland. Total rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) concentration in groundwater samples changes from 0,06 to 59,9 ng/L, with the average being value 13,3 ng/L. Yttrium content ranges from 0,84 to 54,5 ng/L with the average being value 14,9 ng/L. In most of the water samples the ΣREE value is lower than 10 ng/L. Within the presented data population, ΣREE concentration as well as concentrations of individual REEs values display no clear correlation with basic physical and chemical parameters of the examined waters, such as pH, TDS, temperature and depth of occurrence. The obtained REE results normalized to European Shale allowed for describing 6 general types of REE fractionation patterns, expressing relation between 3 groups of the REEs: light (LREE: La-Nd), middle (MREE: Sm-Dy) and heavy (HREE: Ho-Lu). The following REE patterns were distinguished: 1)LREE < MREE < HREE -described for waters from most of the examined wells; 2)LREE < MREE > HREE -frequent; 3)LREE = MREE < HREE, 4)LREE > MREE > HREE; 5)LREE < MREE = HREE; 6)LREE = MREE > HREE. REE patterns of 3-6 types are less frequent. When we apply the division into two groups: (HREE: La-Eu) and LREE (Gd-Lu), we can notice a distinct quantitative advantage of HREE over LREE in the examined water samples. This is usually explained by the fact that LREE display a stronger tendency than HREE to be absorbed by sediment particles. The development of this process is favored by a long residence time typical of waters in deep seated aquifers.