Radon: A Tracer for Geological, Geophysical and Geochemical Studies

Radon is the heaviest of all noble gases and has a total of 36 isotopes ranging from ¹⁹³Rn to ²²⁸Rn all of which are radioactive. Over a large portion of the past 115 years since its first discovery in 1900, radon has been widely studied including for its impact on human health. The three naturally-occurring radon isotopes (²²²Rn, ²²⁰Rn and ²¹⁹Rn) with half-lives varying over 3 orders of magnitude are useful as a tracer in several branches of geosciences. A thorough understanding of its physical, chemical and nuclear properties are required for its effective use as an environmental tracer. For example, the radon partition coefficient between water and air and the octanol-water partition coefficient (K_ow) are reported to be 0.2395 and 32.4, respectively. This chapter lays the foundation for the utility of radon as a geochemical tool.

The specific activities of ²²²Rn in environmental samples (atmospheric and soil air, surface and groundwater, etc) can vary over 5 orders of magnitude and hence the methods and instruments to measure radon activities vary widely. The measurement techniques are classified based three characteristics—i) whether measurement involves ²²²Rn or its progeny; ii) time resolution; and iii) detection of the type of emission, whether alpha or beta particle or gamma radiation resulting from radioactive decay. The three broad classes of time resolution, based on sampling and analysis, include i) grab-sample technique; ii) continuous technique and iii) integrating technique. The preferred technique is based on the purpose of the study and the research question addressed. Precise determination of radon activity in surface waters of the ocean and lakes for tracer studies requires careful sampling. A review of most commonly-used techniques for measuring radon in air and water samples along with quality control, precision and accuracy in radon measurements are presented in this chapter.

The temporal and spatial variations in the release rates of radon from earth materials in to air and water have been widely recognized for several decades. One of the most important factors that affect the radon concentration in indoor (note that indoor air radon concentrations have direct health implications, chapter-11) and outdoor air is the rate of release of radon and its subsequent transport. The radon emanation coefficient in naturally-occurring rocks, minerals and soils varies over five orders of magnitude. Towards the effective use of radon as a tracer for geological, geophysical and geochemical studies, investigations on the factors and processes that cause concentration gradients of ²²²Rn in both air and water are essential. In this chapter, the mechanisms of and factors affecting radon release rates are presented. In-depth discussion on how key parameters such as pressure, temperature, moisture, mineralogy and grain size affect the release rates of radon and its subsequent transport is presented. Large-scale well-coordinated efforts around the globe, both on continental and national scale, on radon flux measurements have been conducted and results from those efforts are summarized. Radon flux studies from continents and oceans are reviewed and a compilation of long-term global ²²²Rn flux is presented. Radon emanation rate averaged over 32 years (based on the inventory of ²¹⁰Pb) along with the global radon emanation rate curve is given.

Radon is an ideal atmospheric tracer due its mean-life, which is long compared to turbulent timescales, but short enough to constrain ²²²Rn activities in the free troposphere; the mean-life is comparable to the transit time of air masses across major continents, but much shorter compared to the global mixing time scale of the atmosphere. A well-defined, yet, simple source function (~99 % from continents and ~1 % from oceans) and sink (100 % removal by radioactive decay), as well as large observed gradient in radon concentrations between oceanic and terrestrial air masses aid in identifying and quantifying the sources of air masses, thereby serving as an unambiguous indicator of recent terrestrial influence on the oceanic air mass. Vertical profiles of atmospheric radon in different seasons combined with modeling efforts have provided insights on the inter-seasonal variations of the fractional escape of other trace gases from the planetary boundary layer to the upper atmosphere. Future promising areas of research include investigations on the monsoon dynamics in the Indian subcontinent by combining air trajectory analysis and temporal variations of ²²²Rn in the upper air and identifying the possible link (if any) between radonic storms observed in the Polar Regions and transport of water vapor by atmospheric rivers from low to high latitudes.

The distribution of ²²²Rn and its progeny (mainly ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po) have provided a wealth of information over the past five decades as tracers to quantify several atmospheric processes that include: i) source tracking and transport time scales of air masses; ii) the stability and vertical movement of air masses iii) removal rate constants and residence times of aerosols; iv) chemical behavior of analog species; and v) washout ratios and deposition velocities of aerosols. Most of these applications require that the sources and sink terms of these nuclides are well characterized.

Utility of ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po as atmospheric tracers requires that data on the ²²²Rn emanation rates is well constrained. Due to ~4 orders of magnitude higher specific activities of ²²⁶Ra (mBq g-1) in rocks/minerals compared to water, the ²²²Rn emanation rates from the continent is about two orders of magnitude higher than that of the ocean. This has led to distinctly higher ²¹⁰Pb activities in continental air masses compared to oceanic air masses. The highly varying activities of ²¹⁰Pb in air as well the depositional fluxes have yielded insights on the sources and transit times of aerosols. In an ideal enclosed air mass (closed system with respect to these nuclides), the residence times of aerosols obtained from the activity ratios of ²¹⁰Pb/²²²Rn, ²¹⁰Bi/²¹⁰Pb, and ²¹⁰Po/²¹⁰Pb are expected to agree with each other, but a large number of studies have indicated discordance between the residence times obtained from these three pairs. Relatively recent results from the distribution of these nuclides in size-fractionated aerosols appear to yield consistent residence time in smaller-size aerosols, possibly suggesting that larger size aerosols are derived from resuspended dust. The residence times calculated from the ²¹⁰Pb/²²²Rn, ²¹⁰Bi/²¹⁰Pb, and ²¹⁰Po/²¹⁰Pb activity ratios published since 1970’s are compared to those data obtained from size-fractionated aerosols and possible reasons for the discordance is discussed with some key recommendations for future studies.

The existing global atmospheric inventory data of ²¹⁰Pb and ²¹⁰Po is re-evaluated and a ‘global curve’ for the depositional fluxes of ²¹⁰Pb is established. A current global budget for atmospheric ²¹⁰Po and ²¹⁰Pb is also presented. The relative importance of dry fallout of ²¹⁰Po and ²¹⁰Pb at different latitudes is evaluated. A synthesis of the global data of the deposition velocities of aerosols using ²¹⁰Po and ²¹⁰Pb is also given.

The observed disequilibrium between ²²²Rn and ²²⁶Ra in aqueous system is one of the most widely used and successful applications of U-Th-series radionuclides. The observed radon concentration gradients at key interfaces such as sediment-water and air-water have been widely utilized for the past five decades to investigate several geochemical and geophysical processes in marine and lacustrine environments. In this chapter, the most important applications of radon as a tracer in aqueous system are reviewed and presented. Those include: i) determination of gas exchange rate coefficient at air-sea interface; ii) estimation of isopycnal and diapcynal mixing coefficients in the upper ocean as well as the bottom ~200 m of the water column; and iii) quantification of amount of groundwater discharge in rivers, coastal ocean, and lake water using a mass balance approach of ²²²Rn. From the measurements of nutrients and other key trace metals in the advecting fluids, the fluxes of these species to aqueous systems can also be quantified. Radon-derived exchange rate coefficient can also serve as a proxy to determine exchange coefficient for other gases including O₂, CO₂, N₂, etc. Vertical transport rates of ²²²Rn can be utilized to deduce the transport rates of other tracers as well as estimation of buoyance fluxes or heat flux rates. The excess inventory of ²²²Rn in the water column provides direct information on the rates of sedimentation, including possible information on the boundary scavenging.

One of the most widely used progeny of radon is ²¹⁰Pb, which is utilized both as a chronometer and tracer. Diffusion of radon out of the Earth’s surface results in horizontal redistribution of ²¹⁰Pb produced from the decay of atmospheric ²²²Rn, which eventually leads to higher activities of ²¹⁰Pb in the surface layers of land, oceanic water column and snow/ice fields than that expected from equilibrium with ²²⁶Ra. The vertical variations of excess activity is used to obtain ages of the sedimentary layers which are useful for a large spectrum of researchers including biologists, paleoecologists, limnologists, geochemists, speleologists, atmospheric chemists, etc. Pb-210 is also used as a tracer to assess rates and patterns of soil redistribution and for tracing sediment movement in terrestrial and aquatic environments. In this chapter, a brief review of the geochemical behavior of Pb-210 in the environment is presented. Applications of ²¹⁰Pb as a tracer for soil erosion studies require understanding of the variations of the inter-annual atmospheric depositional fluxes of ²¹⁰Pb which is also briefly presented. Ice-rafted sediments in the Arctic sea-ice tend to accumulate large amounts of atmospherically-delivered ²¹⁰Pb and its application as a tracer in the Arctic is summarized. In-depth discussion on most commonly employed ²¹⁰Pb-based sedimentation-rate models that include constant flux:constant sedimentation (CF:CS) and variable sedimentation models (constant rate of supply (CRS) and constant initial concentration (CIC)) are included. It is now agreed by the scientific community that ²¹⁰Pb-based chronology must be validated at least by one another independent method and a comparison of ²¹⁰Pb method with another method (either 137Cs and/or historical time marker, e.g. Hg discharge) is discussed. An example for both cases, one in which there is a good agreement between ¹³⁷Cs- and excess ²¹⁰Pb-based chronologies and a case in which there is no agreement between the same nuclides-based chronologiesare presented. A brief discussion on the calculation of residence time of ²¹⁰Pb in the oceanic water column is also included.

The daughter products of 238U, 235U and 232Th have been utilized as tracers in groundwater systems primarily due to strong relative fractionation between a daughter and its immediate parent (generally). Of all the parent and daughter products in the U-Th series, ²²²Rn activity concentration is found to be the highest (excess radon of 102 to 105 times higher than that of ²²⁶Ra), as there is no removal of radon either by precipitation or sorption. Large scale spatial and relatively small scale temporal variations of ²²²Rn activities in worldwide groundwater systems have been reported. Attempts have been made to date groundwater using the measured 4He/²²²Rn ratios, although this method involves several key assumptions that need validation from more systematic studies, Radon has been used to investigate stream water-groundwater and surface water (lake, coastal ocean)—groundwater interactions and for the quantification of infiltration of meteoric water. Since almost all of the radon in groundwater is derived by recoil, from a comparison of the measured activities of ²²²Rn and radium isotopes (^{223,224,226,228}Ra), the rate constants of adsorption/desorption, and retardation factors have been determined for a few groundwater aquifers. One of the most powerful applications of radon as a tracer is in locating and quantifying the amount of non-aqueous phase liquids present in subsurface contaminated or industrial sites. With a sub-decameter spatial resolution, radon serves as a tool for in-situ monitoring of the location of free—phase plumes of LNAPLS.

Radon can be detected at extremely low levels of activity (e.g. one pico Curie) in air and water which provides the basis for a very sensitive geochemical exploration tool. Translation of the common observations on the spatial and temporal variability of radon activities in soil air and groundwater to the identification of uranium ore or hydrocarbon deposits or geothermal resources or impending earthquakes and/or volcanic eruption imply that radon transport processes below earth’s surface is well-understood. In this chapter, the two commonly used transport mechanisms, earth-mechanical and fluid convection mechanisms of transport are discussed. The meteorological parameters that affect the transport time scale of radon from subsurface to surface are presented. Earlier studies on the uranium exploration using aerial gamma-surveys, airborne and surface gamma-ray surveys to petroleum exploration are summarized. One of the most commonly used techniques in oil industry is the gamma radiation intensity measurements (which is a measure of the concentrations of potassium, uranium and thorium and their progeny) to obtain information on lithology, variations in organic carbon, permeability, and potential recognition of fracture systems in conventional downhole logging method is also summarized.

Radon is one of the most widely used tracers as a precursor for earthquake prediction studies due to its unique chemical properties (no adsorption, dissolution or precipitation). Its transport in subsurface environment is primarily controlled by radioactive decay, diffusion and advection. Seismic or volcanic activity in the subsurface environment results in the changes of stress-strain relationships of the subsurface material affecting the precursor signal. In this chapter, based on a large number of data reported in published literature, a summary of the mechanism of vertical transport of radon and the measured variations of radon concentration in soil-gas and groundwater in seismically-active regions (both earthquakes and volcanic eruption) is presented. A number of case studies linking ²²²Rn concentrations in soil-air and groundwater with earthquake activity, from major global earthquake regions, including Kobe Japan, North America (Southern California, Alaska and Hawaii), North-Western Himalaya, and Turkey are presented. Potential application of He/²²²Rn ratio as a precursor for predicting earthquakes is also presented. A summary of our current understanding on how effective the soil-air and groundwater radon concentrations are in predicting earthquakes and volcanic eruption is given and the potential future areas of research are highlighted.

Radon is the single major contributor to the ionizing radiation dose, about half of the total radiation exposure, received by the human population and is the second most frequent cause of lung cancer (3–14 % of all lung cancer is attributable to radon) after smoking. Radon was classified as a human carcinogen in 1988 by the International Commission on Radiation Protection, a cancer research agency under the World Health Organization. In this chapter, a historical development of studies related to indoor radon as health hazard is presented. A summary of burden of lung cancer from indoor radon and its progeny is given. Reference level (maximum accepted annual radon concentration in residential dwelling) and Action level (a level above which remedial action is required) for a number of countries are discussed. Factors that affect the indoor radon concentrations along with a mass balance approach for indoor radon in conjunction with the importance of ventilation rate on indoor air radon level is summarized. Key aspects on radon prevention methods and mitigation techniques, including design criteria for radon systems to minimize indoor air radon levels are discussed.