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. H,S 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 (HS)-S-2 to SO4. Primary sulphate from dissolution of Messinian evaporite is rate. 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 delta H-2 and delta O-18 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 S-34 consistent with biogenic sulphate reduction. Secondary origin from oxidation of H2S in shallow groundwater is invoked for depleted delta S-34 (SO2-). The studied saline sample (27.8 g/l TDS) show a delta S-34 (H,S) value of +24.9 parts per thousand, 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 statistical, and multi-isotope approach.