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

Abstract

This paper describes the chemical and isotopic characterization of H 2 S-bearing groundwaters of the Fontevivo area, northern Italy. Groundwaters from Fontevivo (Parma Province) contain dissolved H 2 S 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 2 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 NH 4 + and SO 4 2− . Groundwaters are dominantly Ca-HCO 3 type with lesser Ca-SO 4 and Na-Cl types and display two compositional trends: group A, waters from Ca-HCO 3 to Ca-SO 4 ; group B, waters from Ca-HCO 3 to Na-Cl. Group A water compositions are influenced by redox processes including the oxidation of H 2 S to SO 4 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 δ 2 H and δ 18 O 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 H 2 S is depleted in 34 S consistent with biogenic sulphate reduction. Secondary origin from oxidation of H 2 S in shallow groundwater is invoked for depleted δ 34 S (SO 4 2− ). The studied saline sample (27.8 g/l TDS) show a δ 34 S (H 2 S) value of +24.9‰, suggesting a nearly complete reduction of Messinian sulphate within a system closed to H 2 S. A hydrogeological model is presented based on a chemical-thermodynamic, trace element statistical, and multi-isotope approach.