Abstract: The Mazarrón Tertiary basin is characterised by the presence of a deep saline aquifer (>500m) oversaturated in CO2. This system can be considered as a natural analogue of a deep geological storage for anthropogenic CO2. The CO2 of this natural system remained sealed since its natural storage until the sixties of the past century, when an over-exploitation of the shallow aquifers occurred due to a lot of hydrogeological wells for agricultural purposes. This overexploitation originated a very important drop in the hydrostatic pressure of the system, causing the rise of the deep-seated CO2 that affected the shallowest aquifers. Later on, this natural CO2 was evidenced when two geothermal wells (namely El Saladillo and El Reventón) were drilled. As CO2 acts as a carrier of 222Rn, this radioisotope has also been detected during this research work in both the hydrogeological and geothermal wells. The relationships among the surficial flux of both gases, their respective sources, as well as the effectiveness of the Tertiary marly formation filling the basin, as sealing materials, are presented and discussed in this work, in the case that the system remains undisturbed.

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