Caratterizzazione chimica preliminare delle acque sotterranee di Roma Capitale.

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.