Abstract
Soils, sediments and rock are natural sources of radon (Rn), which poses an ongoing threat to human health. Numerous studies have measured Rn release from bulk earth materials, yet few have examined microscale controls on Rn flux from solids (emanation), which is required to develop a process-based framework for predicting the rate and extent of production. Here, we use a novel closed loop flow-through system to measure Rn emanation from two crushed rock types with disparate physical and geochemical characteristics, shale and granitic pegmatite. We relate the extent of Rn emanation from each sample to microscale characteristics examined using conventional and synchrotron-based techniques, such as Rn parent radionuclide distribution within mineral grains, porosity, and surface area. Our results illustrate that the extent of Rn release from solids is primarily dependent on the position of parent radionuclides within host mineral grains relative to the “recoil range”—the maximum distance a daughter product (such as Rn) may traverse within a solid and into an adjacent pore owing to alpha-recoil—and is less dependent on the bulk parent radionuclide (e.g., radium) activity. We also present a simple model for predicting the emanation coefficient for pure solids based on mineralogical and physical parameters, which is an initial step toward developing a framework for predicting Rn efflux (exhalation) from soils.
Highlights
Radon (Rn) is a chemically inert radioactive gas that poses a significant threat to human health [1,2], predominately through the accumulation of short–lived Rn decay products such as 210 Po, 214 Pb [2,3].These decay products are chemically reactive, and when inhaled are deposited within respiratory tract tissues, thereby increasing the risk of lung cancer [4]
We present a simple but novel model describing the Rn emanation coefficient for individual minerals, which is an initial step toward developing a framework for predicting Rn efflux from soils, sediments and rock
Autoradiograph analysis of pegmatite reveals widespread grain-scale heterogeneity in radionuclide distribution (Figure 2). This was observed in element maps acquired through electron microprobe analysis, where the spatial distribution of U was concentrated within veins spanning more than 50 μm in length and at least several μm in width (Figure 3)
Summary
Radon (Rn) is a chemically inert radioactive gas that poses a significant threat to human health [1,2], predominately through the accumulation of short–lived Rn decay products such as 210 Po, 214 Pb [2,3] These decay products are chemically reactive, and when inhaled are deposited within respiratory tract tissues, thereby increasing the risk of lung cancer [4]. Temporal and spatial variation in the release rates of Rn from soils and sediments to air and water have been used to locate subsurface uranium (U) and hydrocarbon deposits [6,7], as hydrologic flow path tracers of groundwater and streamflow [3,8,9,10], and as powerful tracers for quantifying atmospheric transport processes [11,12]. A thorough understanding of the factors and processes that control Rn production rates from soils, minerals and rocks is essential
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