The origin of radon anomalies along normal faults in an active rift and geothermal area

Jonathan Robert Joseph Davidson,Andrew Nicol,Uwe Ring,Jerry Fairley,Darren Gravley

doi:10.1130/ges01321.1

Jonathan Robert Joseph Davidson, Andrew Nicol + Show 3 more

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https://doi.org/10.1130/ges01321.1

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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 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.

Highlights

IntroductionMany investigations have reported radon anomalies at concentrations significantly higher than background levels along active faults (Tanner, 1978; Whitehead, 1984; King et al, 1996; Ciotoli et al, 1999; Al-Tamimi and A bumurad, 2001; Atallah et al, 2001; Ioannides, 2003; Burton et al, 2004; Font et al, 2008; Katsanou et al, 2010)
Whatever the precise mechanism that results in elevated radon emanation, our results indicate that the greatest values of radon occur in darker soils that occur below the ca
Elevated radon gas concentrations close to scarps could be due to a number of processes, including flow of radon gas within fault zones and/or local variations in radon production arising from fault-related changes in near-surface stratigraphy

Summary

Introduction

Many investigations have reported radon anomalies at concentrations significantly higher than background levels along active faults (Tanner, 1978; Whitehead, 1984; King et al, 1996; Ciotoli et al, 1999; Al-Tamimi and A bumurad, 2001; Atallah et al, 2001; Ioannides, 2003; Burton et al, 2004; Font et al, 2008; Katsanou et al, 2010). The consensus in recent publications is that the observed radon anomalies result from advective transport by carrier gases through high-permeability fault zones (King et al, 1996; Ioannides, 2003; Zarroca et al, 2012). In support of this hypothesis, it has been argued that faults are commonly associated with anomalous fluxes of a wide range of gases (CO2, He, H2) in addition to radon (King et al, 1996; Zarroca et al, 2012). Investigators have pointed out a lack of correlation between the concentration of radon parent isotopes radium (Ioannides, 2003) or uranium (King et al, 1996) and radon gas concentration in specific locations along the studied faults that they interpret as an indication of advective transport of radon-enriched gas, rather than as a reflection of changes in parent isotope concentration in the soils

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