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

¹ Department of Earth Sciences, University of Parma, Parco Area delle Scienze 157/A, 43100 Parma, Italy (e-mail: tosco@unipr.it; tiziano.boschetti@unipr.it)
² Department of Inorganic Chemistry, Analytical Chemistry and Physical-Chemistry, University of Parma,Parco Area delle Scienze, 43100 Parma, Italy
³ Department of Earth Sciences, University of Rome ''La Sapienza'', P.le A. Moro, 00185 Rome, Italy

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 SO₄^2–. 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 SO₄^2–. 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 ²H and ¹⁸O 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 ³⁴S (SO₄^2–). The studied saline sample (27.8 g/l TDS) show a ³⁴S (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 statistical, and multi-isotope approach.

KEYWORDS: groundwater, dissolved gases, H₂S, methane, redox processes, mixing, Berner zones, isotopes, Po Plain, Italy