Monitoring of soil gases in the characterization stage of CO2 storage in saline aquifers and possible effects of CO2 leakages in the groundwater system.

The main objective of this chapter is to describe which analytical methodologies and procedures can be applied at the surface to monitor and verify the feasibility of geologically stored carbon dioxide. The reported techniques are mainly focused on the measurements of diffuse soil gas. The soil-gas measurements include the determination of CO2 flux and the application to natural trace gases (e.g. radon) that may help to detect any CO2 leakage. In particular, the accumulation chamber method was used to measure the diffuse emission of CO2 at the soil-atmosphere interface. This technique was considered to be of utmost importance to adapt the optimum methodology for measuring the CO2 soil flux and estimate the total CO2 output. During the pre-injection phase CO2 fluxes are expected to be relatively low compared to the intra- and post-injection phases. If leakages are occurring, small variation in CO2 flux might be detected when the CO2 "noise" is overcoming that produced by the biological activity of the soil. Once the CO2 fluxmeasurements are completed and anomalies zones are detected, the total CO2 output is estimated to quantify the amount of CO2 released to the atmosphere. For the estimation of the CO2 output six statistical methods can satisfactorily be applied, namely, arithmetic mean, minimum variances unbiased estimator, bootstrap resample, partitioning of data into different normal populations with a graphical and a maximum likelihood procedures, and sequential Gaussian simulation. Leakages of CO2 toward the surface are also expected to modify the chemical composition of the groundwater system with which they may interact. Thus, a specific section of this paper will be dedicated to the expected variations by considering the equilibrium of the carbon species, which also includes the effects on the isotopic composition of dissolved CO2 and Total Dissolved Inorganic Carbon (TDIC), these parameters likely being the most sensitive and affected by any leakage.