Abstract

Radon anomalies are widely reported in the vicinity of active faults, where they are often inferred to result from upward migration of fluids along fault zones. We examine the up-fault flow hypothesis by measuring radon (220Rn and 222Rn) in soil gas above two active normal fault zones within the central Taupo rift, New Zealand. In agreement with previous investigations, we find that the average concentrations of both radon isotopes are generally higher near mapped faults, although in some cases we find that the difference with background populations is not significant. Soil samples recovered from 1 m depth indicate that some of the radon anomalies along faults may reflect local changes in soil types. The 220Rn isotope emanation measured from extracted soil samples shows a linear correlation with the field concentration measurements (R2 = 0.90, p value = 3 × 10–6), whereas 222Rn emanation shows no linear correlation (R2 = 0.17, p value = 0.17). The soil gas isotopes measured show a significant linear correlation of 220Rn and 222Rn concentrations (R2 = 0.44–0.55, p value <10–5) near faults. This correlation suggests a constant radon isotopic ratio is emitted from the soils tested, and this finding is supported by emission data measured on extracted soil samples. The distribution of 222Rn concentration compared to 220Rn can be explained by small-scale diffusion for >90% of the soil gas measurements, showing that a majority of radon anomalies along faults are not necessarily caused by advection of gases along fault planes and can be explained by an increase in radon soil emanation. However, diffusion cannot account for all of the observed patterns in the data, and in some specific locations along faults, 222Rn concentrations are most likely produced by advective flow of subsurface gases, suggesting channelized gas flow in portions of some faults.

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