The groundwaters of Fontevivo (Parma Province, Italy): redox processes and mixing with brine waters

This paper describes the chemical and isotopic characterization of H2S-bearing groundwaters of the Fontevivo area, northern Italy. Groundwaters from Fontevivo (Parma Province) contain dissolved H2S and minor hydrocarbons, which are released from the truncated front of a buried geological structure (Calabrian–Miocene terrains) and through abandoned unsealed oil wells. H2S concentration is up to 5.54 mg/l in groundwaters from the topographical high of the village and its distribution in the investigated area is inversely related with those of NH4+ and SO42-. Groundwaters are dominantly Ca-HCO3 type with lesser Ca-SO4 and Na-Cl types and display two compositional trends: group A, waters from Ca-HCO3 to Ca-SO4; group B, waters from Ca-HCO3 to Na-Cl. Group A water compositions are influenced by redox processes including the oxidation of H2S to SO42-. Primary sulphate from dissolution of Messinian evaporite is rare. Group B waters represent mixing of meteoric water with small amounts of brine. The most Cl (Br, I)-rich groundwaters are located in the NE of the investigated area, where a Cl–Br–I-rich brine was encountered by an exploration well in the Calabrian stratum. Berner has provided a classification of redox environments starting from the observation of the sequence of reduction/oxidation processes as shown by groundwater composition. At Fontevivo the Berner redox zones show an areal distribution where the anoxic zones occur in the topographical high. The d2H and d18O values of groundwaters plot close to the Global Meteoric Water Line. They indicate that local precipitation recharges the shallow aquifer (<30 m deep), whereas rainwater from the higher elevated Apennine ridge recharges the deeper aquifer (>=30 m deep) via the Taro river and its alluvial fan. Dissolved H2S is depleted in 34S consistent with biogenic sulphate reduction. Secondary origin from oxidation of H2S in shallow groundwater is invoked for depleted d34S (SO42-). The studied saline sample (27.8 g/l TDS) show a d34S (H2S) value of +24.9‰, suggesting a nearly complete reduction of Messinian sulphate within a system closed to H2S. A hydrogeological model is presented based on a chemical-thermodynamic, trace element tatistical, and multi-isotope approach.