Rare earth elements distribution and geochemical behaviour in the volcanic groundwaters of Mount Vulture, southern Italy

The Mount Vulture basin, which mainly consists of pyroclastic and subordinate lava flow layers, is one of the most important aquifers for drinking water and irrigation supply in southern Italy. In this study, we investigated the geochemical behaviour of the rare earth elements (REEs) in the groundwater of this aquatic system, assessing fractionation patterns and performing speciation calculations to elucidate the geochemical processes affecting the REEs' distribution. The groundwater collected was separated into two subsets corresponding to recharge and discharge water. 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, a more positive average Eu anomaly, and a larger average (La/Yb). The observed relationship between Ce anomalies and dissolved oxygen (DO) supports the idea that Ce oxi- dative scavenging has a role in determining the size and shape of the Ce anomaly. Further, the less negative average size of the Ce anomaly in the discharge zone also suggests Ce desorption due to changes in redox conditions. As for the Eu anomalies, the largest Eu* are associated with the recharge zone, characterised by higher average Eh and DO values. The volcanics in the Mt. Vulture lithologies have post-Archean average shale (PAAS) normalised patterns showing positive Eu anomalies due to feldspars occurrence. This indicates that the Eu* of groundwater in the recharge zone is mostly produced by the dissolution of feldspars. In the discharge zone, most samples are in equilibrium with smectite and Eu2+ adsorption onto the clay phase surface or its inclusion in the mineral structure as exchangeable cations likely lead to the smaller average size of Eu*. The possible occurrence of secondary minerals preferentially scavenging light REEs may also explain the difference in the (La/Yb)PAAS index observed between the recharge and discharge zones. Speciation calculations indicate that dissolved REEs consist mainly of carbonato complexes (LnCO3+) whereas the percentage of dicarbonato complexes (Ln(CO3)2−) increases in samples with circa-neutral pH. At acidic pH, the Ln3+ species are numerous and, in samples with large amounts of dissolved sulphate, derived by feldspathoids dissolution, from the discharge zone, the percentage of LnSO4+ species is not negligible.