Fe and As geochemical self-removal dynamics in mineral waters: evidence from the Ferrarelle groundwater system (Riardo Plain, Southern Italy)

A theoretical pattern for Fe and As co-precipitation was tested directly in a groundwater natural system. Several monitoring wells were sampled to identify the different endmembers that govern the hydrodynamics of the Ferrarelle Groundwater System in the Riardo Plain (Southern Italy). In agreement with recent investigations, we found a mix of a deep and a shallow component in different proportions, resulting in a specific chemical composition of groundwater in each well depending on the percentages of each component. The shallow component was characterized by EC ~ 430 µS/cm, Eh ~ 300 mV, Fe ~ 0.06 µmol/L and As ~ 0.01-0.12 µmol/L, while the deep component was characterized by EC ~ 3400 µS/cm, Eh ~ 170 mV, Fe ~ 140 µmol/L and As ~ 0.59 µmol/L. A general attenuation of As and Fe concentration that was not due to a simple dilution effect was observed in the mixing process. The oxidation of Fe(II) to Fe(III) produces solid precipitates which adsorb As from solution and then co-precipitate. The reactions pattern of Fe(II) oxidation and As adsorption gave a linear function between [As] and [Fe], where the angular coefficient depends on the [O₂]/[H⁺] ratio. Chemical data obtained from our samples showed a very good agreement with this theoretical relationship. The investigated geochemical dynamics represented a natural process of attenuation of Fe and As, two undesirable elements that usually affect groundwater quality in volcanic aquifers in central-southern Italy, which are exploited to supply drinking water.

Tracing groundwater circulation in a valuable mineral water basin with geochemical and isotopic tools: the case of FERRARELLE, Riardo basin, Southern Italy

The Riardo basin hosts groundwater exploited for the production of high quality, naturally sparkling, bottled water (e.g., Ferrarelle water), and circulating in a system constituted by highly fractured Mesozoic carbonates, overlain by more impervious volcanic rocks of the Roccamonfina complex. The two formations are locally in hydraulic connection and dislocated by deep-rooted faults. The study aimed at elucidating groundwater origin and circulation, using isotopic tracers (δ¹⁸O, δ²H, δ¹¹B and ⁸⁷Sr/⁸⁶Sr) coupled to groundwater dating (Tritium, CFCs and SF₆). Besides recharge by local precipitation over the Riardo hydrogeological basin, stable isotope ratios in water indicated an extra-basin recharge, likely from the elevated surrounding carbonate reliefs (e.g., Maggiore and Matese Mts.). The mineralization process, promoted by the deep CO₂ flux, controls the B and Sr contents. However, their isotopic ratios did not allow discriminating between circulation in the volcanic and in the carbonate aquifers, as in the latter the isotopic composition differed from the original marine signature. Groundwater model ages ranged from ~ 30 years for the volcanic endmember to > 70 years for the deep, mineralized end-member, with longer circuits recharged at higher elevations. Overall, the results of this study were particularly relevant for mineral water exploitation. A recharge from outside the hydrogeological basin could be evidenced, especially for the more mineralized and valuable groundwater, and an active recent recharge was detected for the whole Riardo system. Both findings will contribute to the refinement of the hydrogeological model and water budget, and to a sustainable development of the resource.

Multiparameter daily time-series analysis to groundwater recharge assessment in a caldera aquifer: Roccamonfina Volcano, Italy

The definition of hydrogeological conceptual models sometimes suffers on uncertainties depending on the available dataset. The availability and the elaboration of long-term hydrogeological dataset allow ensuring a better definition of the aquifer dynamics and features. One of the main features, which can benefice on long-term datasets, is the recharge quantification. According to the literature, several methods exist to calculate the recharge and to get a deeper insight in recharge mode assessment. Most of them allow the quantification of infiltrating water by means of meteo-climatic data and land use information. In this research, a combined approach on hydrogeological time-series is proposed. The Cross-Correlation among the rainfall and the piezometric levels was applied to point out the response time of each aquifer layer. In addition, the Water Table Fluctuation method was used to indirectly quantify the recharge. The study area is the Roccamonfina Volcano caldera (Italy), where lateral inflows are absent, and the almost total natural groundwater discharge occurs in the Savone delle Ferriere stream. Here, a monitoring scheme of the major hydrogeological features was implemented. The local rainfall, the groundwater levels in 2 wells tapping the different aquifer layers, and the river stage of the Savone delle Ferriere stream were measured daily during the 2016-2017 period. The analyses defined different recharge modes for each aquifer layer. The aquifer recharge rates, calculated with different approaches, are comparable to each other and in line with the mean groundwater discharge through the Savone delle Ferriere streambed springs.

Dynamics of natural contamination by aluminium and iron rich colloids in the volcanic aquifers of Central Italy

The dynamics of natural contamination by Al and Fe colloids in volcanic aquifers of central-southern Italy were investigated. Localized perched aquifers, and their relative discharges, are strongly affected by the presence of massive suspended solids, which confer a white-lacteous coloration to the water. This phenomenon occasionally caused the interruption of water distribution due to the exceeding of Al and Fe concentrations in aquifers exploited for human supply. The cause was ascribed to water seepage from perched aquifers. Water discharges affected by such contamination was investigated for the Rocca Ripesena area (north-eastern sector of Vulsini Volcanic District) and for the Rianale Stream Valley (Roccamonfina Volcanic Complex). Hydrogeological survey of both areas confirmed the presence of perched aquifers not previously considered due to their low productivity. Pluviometric data and chemical parameters were periodically monitored. Water mineralization decreased with increasing rainfall, conversely Al and Fe concentrations increased. Statistical analysis confirmed the dependence of all the chemical variables on rock leaching, with the sole exception of Al and Fe which were imputed to colloids mobilization from local, strongly pedogenized pyroclastic material. The similarities in hydrogeological settings and mobilization dynamics in both areas suggest that the Al and Fe colloidal contamination should be more abundant than currently known in quaternary volcanic areas.