Hydrogeochemistry of the groundwater system in the Mt. Catria-Mt. Nerone carbonate ridge (northern Marche, central Italy)

The Mt. Catria-Mt. Nerone (northern Marche, central Italy) carbonate ridge contains a defined and isolated hydrogeological system composed by three main sedimentary aquifers hosted in the Scaglia, Maiolica and Massiccio Formations. These calcareous and marly-calcareous aquifers belong to the Umbria-Marche series and are divided by marly and marly-clayey aquicludes. The groundwaters from the Mt. Nerone-Mt. Catria calcareous formations represent the main water supply for 360k inhabitants of the Province of Pesaro-Urbino. Despite the social and economic importance of these water reservoirs, the scientific knowledge is rather scarce (e.g., Bison et al., 1995; Capaccioni et al., 2001). Therefore, in this study the number of sampling points (31) as well as that of analysed geochemical parameters (i.e., major and trace solutes, water stable isotopes and dissolved gases) was implemented with respect to previous investigations, in order to better characterize the geochemical composition of the waters discharging in the area. The aims were to (i) define the main geochemical processes, (ii) understand the hydrogeological pathways and fluid circulation patterns and (iii) assess the quality of these waters. The waters can be classified into four different groups: (a) Ca-HCO3 waters with TDS < 500 mg/L; (b) Ca-HCO3(SO4) waters with slightly high TDS; (c) Ca-SO4 waters associated with mineral springs (TDS > 1100 mg/L); (d) Na-HCO3 waters with pH > 9 and negative Eh values. The isotopic values of hydrogen and oxygen clearly indicate a meteoric origin for these waters. The composition of Ca-HCO3 waters is almost exclusively related to the dissolution of carbonate-bearing formations, whereas those showing an SO4-enrichment (up to 200 mg/L) suggested a deeper circulation and the interaction with the gypsum-anhydrite formation (i.e., Burano Formation). The Ca-SO4 waters are related to a deep circulation within the Messinian formations. Finally, the Na-HCO3 waters are due to long-lasting interactions between meteoric waters and Na-rich silicate rocks (e.g., Marnoso-Arenacea Formation) in saturation/oversaturation conditions for carbonate-bearing minerals. The dissolved gases of Na-HCO3 waters are enriched in CO2 and CH4, whose source is likely pertaining to layers characterized by a higher content of organic matter where anaerobic degradation occurs.