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

Abstract A geochemical survey, in shallow aquifers and soils, has been carried out to evaluate the feasibility of natural gas (CH 4 ) storage in a deep saline aquifer at Rivara (MO), Northern Italy. This paper discusses the areal distribution of CO 2 and CH 4 fluxes and CO 2 , CH 4 , Rn, He, H 2 concentrations both in soils and shallow aquifers above the proposed storage reservoir. The distribution of pathfinder elements such as 222 Rn, He and H 2 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 CH 4 -rich reducing waters are probably related to organic matter (peat) bearing strata which generate shallow-derived CH 4 , as elsewhere in the Po Plain. On the basis of isotopic analysis, no hints of thermogenic CH 4 gas leakage from a deeper reservoir have been shown. The δ 13 C(CO 2 ) both in ground waters and free gases suggests a prevalent shallow origin of CO 2 (i.e. organic and/or soil-derived). The acquisition of pre-injection 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 CO 2 /CH 4 storage projects.