###### Abstract  
Knowledge about the hydrogeological behaviour of heterogeneous low-permeability media is an important tool when designing anthropogenic works (e.g., landfills) that could potentially have negative impacts on the environment and on people's health. The knowledge about the biogeochemical processes in these media could prevent \"false positives\" when studying groundwater quality and possible contamination caused by anthropogenic activities. In this research, we firstly refined knowledge about the groundwater flow field at a representative site where the groundwater flows within an evaporite-bearing low-permeability succession. Hydraulic measurements and tritium analyses demonstrated the coexistence of relatively brief to very prolonged groundwater pathways. The groundwater is recharged by local precipitation, as demonstrated by stable isotopes investigations. However, relatively deep groundwater is clearly linked to very high tritium content rainwater precipitated during the 1950s and 1960s. The deuterium content of some groundwater samples showed unusual values, explained by the interactions between the groundwater and certain gases (H\({}_{2}\)S and CH\({}_{4}\)), the presences of which are linked to sulfate-reducing bacteria and methanogenic archaea detected within the saturated medium through biomolecular investigations in the shallow organic reach clayey deposits. In a wider, methodological context, the present study demonstrates that interdisciplinary approaches provide better knowledge about the behaviour of heterogeneous low-permeability media and the meaning of each data type. **Keywords:** conceptual model; evaporites; bacterial community; stable isotopes; tritium; Southern Italy +
Footnote †: journal: Applied  
of tectonic and glacio-eustatic processes [1; 2; 3; 4] that resulted in the deposition of large volumes of evaporitic sediments [5; 6]. The Messinian Sicilian Basins are very important geologic systems for analysing these evaporite successions, in view of their lateral variations and subsequent deformation [7]. Their syn-tectonic evolution is in fact fundamental to reconstructing the timing and geometry of the propagating thrust belt [8; 9]. In particular, progressive filling of sub-basins led to the formation of aquifer systems characterised by the coexistence of very low permeability clay successions and evaporitic lenses/horizons [10]. From a hydrochemical point of view, both low- and high-salinity groundwaters may be found associated with the mineralogical features described above. Several hydrogeological and geochemical studies have been carried out in such settings. Some of these studies provided preliminary characterisations through the analysis of single sampling campaigns [11; 12; 13], whereas others investigated in greater detail the hydrogeological behaviour and the hydrochemical evolution over time based on more prolonged and multidisciplinary approaches [14; 15; 16]. Worldwide, a variety of conceptual models have been proposed for groundwater circulation in evaporite deposits. In some systems, the hydrogeology and hydrochemistry of the evaporitic aquifers are significantly influenced by deep ascending regional fluids [17; 18; 19; 20]; conversely, at other sites, the hydrogeology and hydrochemistry of the studied system are influenced by the mixing of saline groundwater (flowing through evaporitic rocks) with more diluted waters flowing through nearby porous aquifers [21], with the possible influence of surface waters [22; 23] or peat layers [24]. The main objective of this study was to refine knowledge about the hydrogeological behaviour of such systems. Because of the expected complexity, the study was carried out by merging hydraulic head measurements, isotopic analyses, and microbial community investigations, as to acquire a broad spectrum of complementary hydraulic and biogeochemical information.