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In summer 2015 a geochemical survey on groundwater was carried out at 31 sampling points (wells and piezometers) belonging to the new “Official monitoring groundwater network of Rome Municipality” (GMNR). The following parameters were measured: temperature, pH, electrical conductivity (i.e. salinity) and alkalinity; these data were used to compute partial pressure of CO2 (pCO2). Furthermore, samples were collected to characterise waters from a chemical point of view (major elements). To implement our data - base, chemical analyses of 6 CO2 - rich mineral waters of Rome were considered. Hydrochemical survey was mainly devoted to: i) classify waters in chemical facies; ii) investigate the main water-rock interaction processes governing the water’s chemical evolution, also affected by variable amounts of dissolved CO2 and iii) define the pCO2 level in groundwater in the frame of the knowledge so far acquired in the Tyrrhenian sector of central Italy.. Groundwater shows a dominant Ca-HCO3 chemistry; some samples belong to Na-HCO3, Na-Cl and CaCl2 hydrochemical facies. In the dominant facies waters show a large variability in the abundance of chemical elements, in their salinity (ranging between 0.46 e 3.83 g/l) and pH (in the interval 5.87-7.22); these features are mainly due to different water-rock interaction processes together with the presence of variable CO2 contents. Na-HCO3 waters show the lowest salinity values (TDS up to 0.32 g/l) and strongly alkaline pH; cation exchange processes with clays, causing Na enrichment and Ca and Mg removal from solution, can be invoked to justify the observed chemistry. Waters of the Castel Fusano Natural Reserve (CFNR) belong to the Na-Cl and Ca-Cl2 facies; the different chemistry reflects the geochemical processes going on in the considered coastal aquifers such as: i) mixing between freshwater and saline waters of marine origin (fossil waters, seawater intrusion) and ii) cationic exchanges with clays that make up the less permeable sediments of the area. Two samples of the CFNR group have Ca-HCO3 chemistry and represent aquifers not affected by salinization processes. Calculated pCO2 distribution is highly variable, from low (0.03 bar) to high values (0.72 bar), implying different CO2 input (and origin) in the studied aquifers. Highest levels of carbon dioxide are linked to the degassing processes going on in the Tyrrhenian sector of Central Italy.
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