## 6 Concluding Remarks  
The in-depth analysis carried out at the study site made it possible to refine knowledge about the hydrogeological behavior of evaporite-bearing low-permeability successions at a representative site. In more details:  
* Hydrogeological investigations and isotopic analyses demonstrated the coexistence of relatively brief to very prolonged groundwater pathways, but everywhere recharged by local precipitation;
* The deuterium content of some samples showed unusual values for groundwater; however, the coupled isotopic-biomolecular approach allowed to demonstrate that these unusual values are due to 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;
* At this stage of the research, these unusual isotopic content was detected in only one sampling sub-area (C5-cluster); nevertheless, this discrepancy between C5-groundwaters and those sampled in the other available wells is not surprising when taking into consideration the (hydro)geological heterogeneity of the investigated system, as well as the results obtained in a similar setting from the biogeochemical points of view [14; 15; 16]; in this previous case, a \"patchwork\" of geochemical and microbiological sub-environments was detected within the whole heterogeneous system, therefore demonstrating the coexistence of different microbial communities (with different microbial activity) also at a metric or decametric scale.  
Taking into consideration the interesting results obtained at this stage concerning the heterogeneous distribution of biogeochemical processes, and the importance of moving from speculations to statements concerning the role of H\({}_{2}\)S/CH\({}_{4}\) in influencing the local anomaly in deuterium content, a second step of the research is in progress. During this new phase, the study area will be significantly expanded, and the actual data set will be integrated also with measures of gas concentration and their isotopes, organic carbon percentage in both sediments and groundwater.  
In a wider, and methodological context, the present study further demonstrates the importance of interdisciplinary approaches when studying such complex hydrogeological systems. An interdisciplinary approach combining well-established investigation methods (in this case, hydrogeological, isotopic and biomolecular methods), allows refinement of knowledge about the hydrogeological features and behavior of heterogeneous media characterised by evaporite-bearingsuccessions, and can help ensure correct interpretation of the meaning of each data type, avoiding misunderstandings and misinterpretations. Thus, this work can be used to refine the guidelines used to study heterogeneous media through purpose-designed interdisciplinary approaches and plan optimal monitoring networks and protocols for several anthropogenic purposes (e.g., environmental monitoring of landfills or contaminated sites).  
Conceptualisation, P.R., C.C., P.I., E.R., A.M.S., F.C.; methodology, P.R., C.C., P.I., E.R., A.M.S., F.C.; investigation, P.R., C.C., G.Z.; data curation, P.R., C.C., G.Z.; writing--original draft preparation, P.R., C.C.; writing--review and editing, P.R., C.C., P.I., E.R., A.M.S., F.C.; visualisation, P.R., C.C., G.Z.; supervision, P.I., E.R., A.M.S., F.C.; project administration, F.C. and E.R.; funding acquisition, F.C. and E.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Catanzaro Cestruzioni S.r.l. (Heads of Funds: [PERSON], [PERSON]).  
We warmly acknowledge [PERSON], [PERSON] and [PERSON] and [PERSON] for useful discussions. The authors would like to thank anonymous reviewers for their fruitful comments. This work has benefited from the equipment and framework of the COMP-HUB Initiative, funded by the 'Departments of Excellence' program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022).  
The authors declare no conflict of interest.