Innovative methodology for quantitative and qualitative assessment of water resources in karst aquifers: a case study in southern Lazio region, central Italy

This paper outlines the results of a hydrogeological study carried on from July 2014 to November 2106 to assess quantitatively and qualitatively water resources (groundwater and surface water) emerging in the Upper Valley of the Aniene River (Latium Region, Central Italy). This work deals with the Environmental Monitoring Plan, related to the catchment project of the Pertuso Spring, which is going to be exploited to supply an important water network in the South part of Roma district. The study area is located in the Upper Valley of the Aniene River, in the outcrop of Triassic-Cenozoic carbonate rocks, and belong to an important karst aquifer. Pertuso Spring is the main outlet of this karst aquifer and is the one of the most important water resource in the southeast part of Latium Region, used for drinking, agriculture and hydroelectric supplies. This hydrogeological system is characterized by a strong local hydraulic connectivity between the Aniene River surface water and groundwater coming from the Pertuso Spring. First at all, in this study, in order to estimate the vulnerability degree of the karst aquifer feeding the Pertuso Spring, the COP method has been applied and vulnerability maps are proposed. Thus, with the aim of highlighting the karst features key-role in the unsaturated zone, a new vulnerability approach has been set up, starting from two discretization approaches. The aim of this work is to compare both results of the intrinsic vulnerability mapping, in order to evaluate which one is the most suitable for the study area. On the basis of the hydrogeochemical data and their interpretations for groundwater and surface water, monitored from July 2014 to November 2016, a hydrogeological study has been carried on to identify flowpaths and hydrogeochemical processes governing groundwater-surface water interactions in this region. To this end, discharge surveys were carried out on four monitoring sections along the Aniene River. The proposed conceptual model shows that the karst aquifer feeding the Pertuso Spring supplies the Aniene River, highlighting seasonal variability depending on the rainfall regime. The analysis of solute contents in the monitoring points has suggested the identification of the Magnesium ion as a conservative tracer in this specific system and, consequently, to the development an indirect method for the evaluation of karst spring discharge based on discharge measurements and water geochemical data. This method is based on the elaboration of surface water discharge measurements in relationship with Mg2+ concentration values, determined as for groundwater, coming from the Pertuso Spring, as for surface water samples, collected upstream and downstream the spring, along the Aniene River streamflow. This method has been validated by the comparison with discharge values obtained using the current meter method and by geochemical data. This study suggests that the application of the Magnesium ion as an environmental tracer may provide a means to evaluate discharge of the Pertuso Spring, as it came up to be a marker of the mixing of surface water and groundwater. On the other hand, the Magnesium ion concentration provides information for the identification of groundwater flow systems and the main hydrogeochemical processes affecting the composition of water within the karst aquifer feeding the Pertuso Spring