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Radon-220 (Thoron in ground water)

English title Radon-220 (Thoron in ground water)
Applicant Hoehn Eduard
Number 119802
Funding scheme Project funding
Research institution Wasserressourcen und Trinkwasser EAWAG
Institution of higher education Swiss Federal Institute of Aquatic Science and Technology - EAWAG
Main discipline Hydrology, Limnology, Glaciology
Start/End 01.04.2008 - 31.03.2011
Approved amount 192'725.00
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Keywords (4)

isotope tracer; groundwater age; mixing; hydrology

Lay Summary (English)

Lead
Lay summary
Isotopes of tThe radioactive noble gas radon has an isotopeare produced in both natural radioactive decay series of 238U and 232Th, 222Rn (“radon”, half-life t1/2 = 3.8 d), and 220Rn (“thoron”, t1/2 = 56 sec), respectively. Because of dissolution of uranium and re-precipitation of thorium, the relatively long-lived parent radionuclides of radon, 226Ra and 230Th, are enriched on surfaces of aquifer materials. Thus radon has the potential to emanate from rocks into ground water and soil gas. The emanation process reaches a steady state, when the radioactive decay and the emanation rate rend up witheach a constant concentrations in the fluids. Today radon is used as a tracer for dating very young ground water (up to 15 days). A major drawback of the radon dating with radon is that we do not know, whether the steady-state concentration is really reached, or whether an observed steady radon concentration resulted from mixing of if the sampled ground water is a mixture of fractions of different ages. We intend to use the thoron/radon ratio as a proxy of the radon steady-state concentrationwishto combine radon concentrations and thoron/radon isotope ratio at steady state (220Rn/222Rn = ?222Rn), and hence to make an additional step to increase the importance of radon as a groundwater tracer. If the ?222Rn isotope ratio remains constant along the flow path, the short half live of thoron allows us to inferwe could infer the local steady-state radon emanation, because the ?222Rn is not affected by groundwaterwithout mixingg of ground waters. Due to the different geochemical behavior of the parent radionuclides of radon and thoron, wWe hypothesize that the emanation dynamics of thoron is different from that of radon. However, we understand only poorly the geochemical behavior in ground water ofFor the parent radionuclides of thoron, 228Ra, 228Th and 224Ra, there is only limited knowledge about their geochemical behavior in ground water. In contrast to radon, thoron has almost never been analyzed in shallow phreatic ground water, as its detection is experimentally challenging. As first resultsWe did not detect thoron in oxic phreatic ground water and spring water, even when sampled directly at its emergence from the formation., w We, however, detected thoron in anoxic but not in oxic ground water, at the outlet of a mineral spring. We hypothesize that Ra dissolved in the anoxic ground water is scavenged as a co-precipitate of Fe and Mn oxides/hydroxides, upon contact with the atmosphere. HThese Ra isotopesence 228Ra and 224Ra seem tomight be concentrated in Fe and Mn oxide/hydroxides that precipitates onf aquifer materials, at the interface between oxic and anoxic ground water. TIn the futurehe occurrence of thoron under re-oxidized conditions will probably allow to detect this isotope in some hyporheic ground water., we will try to detect thoron in hyporheic ground water. In summer, many hyporheic zones with a high content in organic carbon tend to have suboxic and anoxic ground water from the infiltration of river water. Here thoron could be used as a proxy for the steady-state radon concentrations and become part of the radon groundwater dating methodwill broaden the application in environmental science to characterize short groundwater residence times.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
135513 Rn-220 (Thoron) in ground water and soil gas (Continuation) 01.04.2011 Project funding
135513 Rn-220 (Thoron) in ground water and soil gas (Continuation) 01.04.2011 Project funding

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