Geochemistry of shallow aquifers and soil gas surveys in a feasibility study at the Rivara natural gas storage site (Po Plain, Northern Italy)

A geochemical survey, in shallow aquifers and soils, has been carried out to evaluate the feasibility of natural
gas (CH4) storage in a deep saline aquifer at Rivara (MO), Northern Italy. This paper discusses the
areal distribution of CO2 and CH4 fluxes and CO2, CH4, Rn, He, H2 concentrations both in soils and shallow
aquifers above the proposed storage reservoir. The distribution of pathfinder elements such as 222Rn, He
and H2 has been studied in order to identify potential faults and/or fractures related to preferential
migration pathways and the possible interactions between the reservoir and surface. A geochemical
and isotopic characterization of the ground waters circulating in the first 200 m has allowed to investigation
of (i) the origin of the circulating fluids, (ii) the gas–water–rock interaction processes, (iii) the
amount of dissolved gases and/or their saturation status. In the first 200 m, the presence of CH4-rich
reducing waters are probably related to organic matter (peat) bearing strata which generate shallowderived
CH4, as elsewhere in the Po Plain. On the basis of isotopic analysis, no hints of thermogenic
CH4 gas leakage from a deeper reservoir have been shown. The d13C(CO2) both in ground waters and free
gases suggests a prevalent shallow origin of CO2 (i.e. organic and/or soil-derived). The acquisition of preinjection
data is strategic for the natural gas storage development project and as a baseline for future
monitoring during the gas injection/withdrawing period. Such a geochemical approach is considered
as a methodological reference model for future CO2/CH4 storage projects.