Calculating flux to predict future cave radon concentrations

Cave radon concentration measurements reflect the outcome of a perpetual competition which pitches flux against ventilation and radioactive decay. The mass balance equations used to model changes in radon concentration through time routinely treat flux as a constant. This mathematical simplification...

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Bibliographic Details
Published in:Journal of environmental radioactivity 2016-06, Vol.157, p.16-26
Main Authors: Rowberry, Matt D., Martí, Xavi, Frontera, Carlos, Van De Wiel, Marco J., Briestenský, Miloš
Format: Article
Language:English
Subjects:
Air Movements
Air Pollutants, Radioactive - analysis
Anomalies
Cave radon concentration
Cave radon flux
Cave ventilation
Caves
Confined Spaces
Fault slip
Faults
Flux
Mathematical analysis
Mathematical models
Models, Theoretical
Numerical modelling
Radiation Monitoring
Radioactive decay
Radon
Radon - analysis
Radon levels
Slovakia
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Summary:Cave radon concentration measurements reflect the outcome of a perpetual competition which pitches flux against ventilation and radioactive decay. The mass balance equations used to model changes in radon concentration through time routinely treat flux as a constant. This mathematical simplification is acceptable as a first order approximation despite the fact that it sidesteps an intrinsic geological problem: the majority of radon entering a cavity is exhaled as a result of advection along crustal discontinuities whose motions are inhomogeneous in both time and space. In this paper the dynamic nature of flux is investigated and the results are used to predict cave radon concentration for successive iterations. The first part of our numerical modelling procedure focuses on calculating cave air flow velocity while the second part isolates flux in a mass balance equation to simulate real time dependence among the variables. It is then possible to use this information to deliver an expression for computing cave radon concentration for successive iterations. The dynamic variables in the numerical model are represented by the outer temperature, the inner temperature, and the radon concentration while the static variables are represented by the radioactive decay constant and a range of parameters related to geometry of the cavity. Input data were recorded at Driny Cave in the Little Carpathians Mountains of western Slovakia. Here the cave passages have developed along splays of the NE–SW striking Smolenice Fault and a series of transverse faults striking NW–SE. Independent experimental observations of fault slip are provided by three permanently installed mechanical extensometers. Our numerical modelling has revealed four important flux anomalies between January 2010 and August 2011. Each of these flux anomalies was preceded by conspicuous fault slip anomalies. The mathematical procedure outlined in this paper will help to improve our understanding of radon migration along crustal discontinuities and its subsequent exhalation into the atmosphere. Furthermore, as it is possible to supply the model with continuous data, future research will focus on establishing a series of underground monitoring sites with the aim of generating the first real time global radon flux maps. •Cave radon concentrations pitch flux against ventilation and radioactive decay.•Exhalation is dominated by radon advection along discontinuities such as faults.•Our numerical model isolates flux
ISSN:0265-931X
1879-1700
DOI:10.1016/j.jenvrad.2016.02.023