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The origin of radon anomalies along normal faults in an active rift and geothermal area
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Number of Authors: 5
2016 (English)In: Geosphere, ISSN 1553-040X, E-ISSN 1553-040X, Vol. 12, no 5, 1656-1669 p.Article in journal (Refereed) Published
Abstract [en]

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 (Rn-220 and Rn-222) 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 Rn-220 isotope emanation measured from extracted soil samples shows a linear correlation with the field concentration measurements (R-2 = 0.90, p value = 3 x 10(-6)), whereas Rn-222 emanation shows no linear correlation (R-2 = 0.17, p value = 0.17). The soil gas isotopes measured show a significant linear correlation of Rn-220 and Rn-222 concentrations (R-2 = 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 Rn-222 concentration compared to Rn-220 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, Rn-222 concentrations are most likely produced by advective flow of subsurface gases, suggesting channelized gas flow in portions of some faults.

Place, publisher, year, edition, pages
2016. Vol. 12, no 5, 1656-1669 p.
National Category
Earth and Related Environmental Sciences
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URN: urn:nbn:se:su:diva-139329DOI: 10.1130/GES01321.1ISI: 000390991300016OAI: oai:DiVA.org:su-139329DiVA: diva2:1073611
Available from: 2017-02-12 Created: 2017-02-12 Last updated: 2017-02-12Bibliographically approved

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Ring, Uwe
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