Trace and Rare-Earth Elements in Natural Ground Waters: Weathering Effect of Water-Rock Interaction

The distributions of Ca, Cr, Fe, Co, Zn, Ga, Se, Rb, Ba, Tl, Bi, Th, U and rare-earth elements (REEs), measured by Inductively Coupled Plasma Sector Field Mass Spectrometry (ICP-SFMS) in ground natural waters characterized by near-acidic to alkaline pH (5.7 - 9.0), were evaluated to examine the major processes of water-rock interaction. Cluster analysis of ground water quality parameters resulted in three clusters: 1) transition elements (Cr, Co and Zn); 2) REEs; and 3) geogenic elements (Rb, Ga, Ba and Th) with 55%, 60% and 75% similarity levels, respectively. Also, natural ground waters with relatively high salinity (5.3 - 6.8 g/L) were found to be significantly enriched with Ca, Cr, Co, Zn, Ga, Rb and Ba compared to lower-salinity waters (0.5 - 3.7 g/L). Negative Ce (from 0.4 to 0.9) and positive Eu (from 1.2 to 17.5) anomalies were found in the investigated waters and most of them were associated with heavy rare-earth elements (HREEs) enrichments. It was established that the relative proportion of total REE concentration (range 75 - 470 ng/L) to Ca abundance (from 1 to 180 mg/L) could be a useful indicator of weathering processes in studied ground waters. This observation is attributed to the increase in the calcite dissolution during the weathering process, as well as to REEs leaching due to the complex formation of bicarbonate. The reliability of the ICP- SFMS method was checked by using certified reference materials (SLEW-3, SLRS-4 and TMRAIN-04). The relative standard deviation (RSD) was < 10%.