Abstract: The Ga~nuelas-Mazarron Tertiary basin (SE Spain) hosts a deep saline CO2-rich aquifer from which CO2
escapes occurs. Consequently, this basin is considered as a natural analogue for CO2 storage and leakage.
Recent studies performed in wellheads showed a close association between CO2 and 222Rn, therefore
suggesting a further investigation of this association, particularly in diffuse escapes from soils. For that,
concentrations of CO2 and 222Rn in soils, at ~1 m depth, were measured by using the TRT-SEG04 and RM-
2 Radon devices, respectively. These measurements were performed on areas with high density of
lineament intersections, since they could be structural weakness zones and consequently favourable
pathways for these gases. Moreover, the measurements were correlated with their corresponding
geological cross-sections. The results allowed to conclude that: i) both gases only show a significant
correlation in faulted zones and/or wellheads; ii) the CO2 leakages could be doubly dangerous (toxicity
and radioactivity of the associated 222Rn); and iii) the Neogene marly cap-rock of the basin acts as a
sealing formation against CO2 and 222Rn leakages. Finally, it can be also stated that 222Rn can be used as
an alert of CO2 escapes from both CO2 natural reservoir and CO2 deep geological storage.
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