Abstract: Four different techniques for measuring radon activity in soil gases were analyzed in the natural analogue of Campo de Calatrava, Spain, to study its application in superficial monitoring of CO2 storage sites. The following methods were applied: (i) alpha spectroscopy monitors (SARAD, model RTM-2100), (ii) pulse ionization chamber detector (RADON v.o.s. model RM-2), (iii) solid-state nuclear track (CR-39) detectors and (iv) Lucas cell detectors (EDA model RD-200). Measurement protocols and selection criteria were defined according to the different phases of a CCS project, i.e., pre-injection, injection and post-injection. Radon measurements and CO2 fluxes were determined at 36 points around a natural CO2 leak. The concentrations of 222Rn in the soil gases by ionization chamber detectors (RM-2) and alpha spectrometry monitors (SARAD RTM-2100) showed consistent data, whereas passive detectors (CR-39) did not produce realistic 222Rn activity values although anomalous concentrations related to the CO2 leak were recognized. This implies that the latter can be used to create a control grid to estimate whether an increase of radon concentration due to a possible CO2 leak occurs. The Lucas cell (EDA RD-200) produced the greatest analytical uncertainty and, in the present configuration, this method appears the least appropriate for monitoring CO2 storage sites.

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