Determination of baseline values in evaporitic aquifers: A case study from southern Sicily (Montallegro-Siculiana area)

In Italy, groundwater quality is regulated by predefined values, known as Threshold Concentration Limits ("Concentrazione Soglia di Contaminazione": CSC), are established by the Legislative Decree 152/2006 (Annex 5, Part IV, Table 2 – Groundwater Contamination Threshold Concentrations). However, these values cannot be considered universally applicable across the entire national territory, necessitating a highly sitespecific approach for an accurate assessment of real baseline levels, coherent to the hydrogeochemical characteristics of groundwater systems. Natural baseline values for an evaporitic aquifer in the Montallegro-Siculiana (Southern Sicily) area were investigated. In facts, in this area a wide folded sector of Miocene (Serravallian to Messinian) formation rocks results in a very specific hydrogeological setting, with the superimposition of highly permeable fissured gypsum sequences on a very low permeable clayey substratum (Feo et al., 2023; Rizzo et al., 2020). Along the valleys of the drainage network alluvial and colluvial permeable deposits are also found. In light of their limited interest for domestic or agriculture uses, few data are available for characterizing these groundwater systems. At the same time, any land use planning issues (e.g., crops, landfill, mines) always requires comparing monitoring data to a reference baseline, dependent on the local geological, hydrogeological, and environmental context. This study was conducted in accordance with the methodology outlined by ISPRA (Italian Institute for Environmental Protection and Research) guidelines (SNPA, 2018) and adopts an integrated approach that combines environmental monitoring data with advanced geochemical characterization of the local context. Several studies (Bulut et al., 2020; Zanotti et al., 2022) have emphasized the need to define baseline values, which are essential for distinguishing natural from potential anthropogenic contaminated conditions, thereby enabling a more precise evaluation of groundwater quality.