Rare Earth Elements distribution and geochemical behavior in the Mt. Vulture groundwater, southern Italy

The Mount Vulture basin is one of the most important aquifers for drinking water and irrigation supply in southern Italy. The volcanic aquifer mainly consists of pyroclastic and subordinate lava flow layers. Aim of this study is to investigate for the first time the REEs’ geochemical behaviour in the Mount Vulture groundwater assessing concentrations and fractionation patterns and performing speciation calculation, in order to verify whether REEs’ geochemistry records the geochemical processes occurring in this aquatic system. The study will also discuss the flow system of the groundwater’s basin and possible controls on the REEs’ distribution along the studied groundwater flow paths in the Mt Vulture. The water samples were collected from springs and drilled wells during a field trip in the dry season to record the highest concentration of dissolved elements. A cluster analysis highlights the existence of two main subsets: a high EC (HEC, average EC = 2,495 µs/cm) subset with lower pH (average = 6.12), Eh (average = 421 mV), and DO (average = 2.48 µmol/kg), and higher T (average = 18.7 °C) values; and a low EC (LEC, average EC = 382 µs/cm) characterised by higher pH (average = 6.50), Eh (average = 543 mV), and DO (average = 7.31 µmol/kg), and lower T values (average = 15.4 °C). In the HEC subset, the Na+ and HCO3- are significantly higher (Na+ average = 410.7 mg/l, HCO3- average = 1176.6 mg/l) than in the LEC subset (Na+ average = 31.7 mg/l, HCO3- average = 198.6 mg/l). That suggests that the HEC and LEC waters correspond roughly to the waters from the discharge zone and recharge zone, respectively. Overall, the groundwater exhibited REE concentrations well below 1000 ng/l, and the average content measured in the discharge water is approximately three times higher than that of the recharge water. In the recharge water, the shale normalised patterns are characterised by a more negative average Ce anomaly (logCe*PAAS = −0.56), a more positive average Eu anomaly (logEu*PAAS = 0.75), and a larger average (La/Yb)PAAS (log(La/Yb)PAAS =−0.12). The observed relationship between Ce anomalies (Ce*) and dissolved oxygen (DO) supports the idea that Ce oxidative scavenging, probably involving Fe/Mn oxyhydroxides, has a role in determining the size and shape of the Ce*. Further, the less negative average size of the Ce* in the discharge zone also suggests desorption of Ce as Ce3+, according to previous findings on the content of dissolved Fe and Mn. The desorption is related to changes in redox conditions, according to the lower Eh and DO average values observed in the discharge zone compared to the recharge zone. The lack of a significant correlation between Eh and Ce anomalies indicates that, in addition to the aqueous chemistry, the source composition affects Ce*. Although a significant correlation is observed between Eu anomalies (Eu*) and Eh and DO, the strongly reducing condition promoting Eu3+ and Eu2+ is never attained in the Mt. Vulture aquifer. The largest Eu* are associated with the recharge zone where the average Eh and DO values are higher. The volcanics occurring in the Mt. Vulture lithologies are characterised by PAAS-normalised patterns with positive Eu anomalies due to the presence of feldspars. This constraint indicates that, in the recharge zone, the size and shape of the Eu* of groundwater are largely due to the dissolution of feldspars. In the discharge zone, most samples are in equilibrium in terms of smectite content, and Eu2+ adsorption onto the clay phase surface or its inclusion in the mineral structure as an exchangeable cation, due to vicariance with Ca2+ and Sr2+, could explain the smaller average size of the Eu anomaly. Further, the occurrence of secondary minerals preferentially scavenging LREE also explains the difference in the LREE/HREE fractionation, indicated by the (La/Yb)PAAS index and observed between the recharge and discharge zones. Finally, speciation calculations reveal that dissolved REEs consist mainly of carbonato complexes (LnCO3+), whereas the percentage of the dicarbonato complexes (Ln(CO3)2-) increases in samples with circa-neutral pH. At acidic pH, the Ln3+ species are important, and, in the samples from discharge zone affected by a large amount of dissolved sulphate derived by feldspathoids dissolution, the percentage of LnSO4+ species is not negligible. Overall the REEs’ distribution in the Mt. Vulture aquifer is originated by several mechanisms, including weathering reactions involving volcanic rocks, the occurrence of secondary minerals, and pH and redox control. Therefore, the REEs’ geochemistry is a powerful tool in assessing the geochemical processes affecting the groundwater environment.