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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) PAAS. The observed relationship between Ce anomalies and dissolved oxygen (DO) supports the idea that Ce oxidative 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 (LnCO 3 +) whereas the percentage of dicarbonato complexes (Ln(CO 3) 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 LnSO 4 + species is not negligible. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) distribution in groundwater is an important tool for investigating water-rock interaction processes as REEs content in aqueous systems strongly depends on the degree of rock weathering. However, subsequent to REEs' dissolution from the aquifers rocks, REEs' removal from solution by sorption or coprecipitation can occur. In this work, the concentrations of dissolved REEs were determined in groundwater of Mount Vulture volcanic aquifer. Mt. Vulture is a Pleistocene composite volcano located at the easternmost border of the Apennine compressive front whose aquifer core is mainly composed of pyroclastic and subordinate lava flow of different permeability, which give rise to distinct aquifer layers. The collected water samples were cold and slightly acidic water with electrical conductivity (EC) that varies over a wide range of values; in addition, Eh and dissolved oxygen (DO) of groundwater were also measured. The water samples, due to the high variability of some physical-chemical parameters such as EC and DO, were the object of a cluster analysis, which showed the presence of two main subset having substantial differences in the measured parameters. Thus the dataset was divided into a high salinity subset (EC > 800 µs/cm) with low Eh and OD values and a low salinity subset (EC < 800 µs/cm) characterized by higher OD and Eh values. The concentration of dissolved REEs in groundwater fluctuates over several order of magnitude and shows a slightly positive correlation with water temperature and a slightly negative correlation with the dissolved oxygen (DO); relationships with EC, pH and Eh are not observed. The Post-Archean Australian Shales (PAAS) normalized pattern of the high salinity subset showed an important fractionation between LREE and HREE and moderate CeN* and EuN* anomaly. Instead, the low salinity subset showed a minor fractionation between LREE and HREE and an important EuN* anomaly compared to the previous group. The EuN* anomaly of the whole dataset is in a positive correlation with the Eh whereas the CeN* is in weak negative correlation. These preliminary results confirm the usefulness of REEs' distribution in groundwater for assess water-rock interaction processes. [ABSTRACT FROM AUTHOR]