Deciphering and Predicting Microscale Controls on Radon Production in Soils, Sediments and Rock

The dynamics of Rn-222 cyclic flow within the shallow geological subsurface media as a daily temporal variated source for exhalation into the air

Extensive research on the dynamics of radon gas (Rn-222) originating from the radioactive decay of radium (Ra-226) in geological subsurface media, sheds light on its periodic release into the atmosphere. Radon is a product of the uranium-238 decay chain found within rock and soil grains. While only a fraction of the generated radon escapes (emanates) into porous spaces due to nuclear recoil, it serves as the source for subsurface gas flows and for cyclic exhalation into the soil-atmosphere interface. Ongoing study of radon movement in shallow and deep subsoil, and its emergence at the surface, reveals complete semi-diurnal, diurnal, and seasonal gas flow cycles in the subsoil. Complementary emissions occur nocturnally as radon is released into the atmosphere. Moreover, two natural driving forces govern complex semi-diurnal and diurnal flows below and above the surface. Subsurface gas movement in porous media exhibits nonlinear behavior influenced by surface temperature gradients, resulting in downward flow to depths of up to 100 m. This flow exhibits daily periodicity with depth-dependent time delays, correlating with the diurnal surface temperature cycle. Additionally, pore gas transport into and out of open boreholes responds linearly to semi-diurnal barometric pressure changes, known as barometric pumping. Beyond subsurface phenomena, Europe and Australia increasingly employ nocturnal radon measurements to study atmospheric stability and air quality, assuming that variations in local stationary near-surface radon concentrations reflect atmospheric mixing processes. Recognizing mechanisms governing radon's temporal changes within geological subsurface media highlights the need for continuous underground radon monitoring to validate variations in daily radon exhalation to the surface. On the other hand, monitoring radon at considerable depths minimizes climatic contributions and enhances the ability to discern non-periodic pre-seismic radon signals, independent of atmospheric compulsion. This research offers potential insights into seismic precursors and the complex interplay between subsurface geodynamics and atmospheric conditions.

Application of radon (222Rn) as an environmental tracer in hydrogeological and geological investigations: An overview

Radon (²²²Rn) is a colourless, odourless, inert, and radioactive noble gas (t_1/2 = 3.8 days) that emanates from rocks and soils as a result of the alpha decay of its parent, radium (²²⁶Ra) in the decay series of uranium-238, is the focus of this study. Radon is produced in the crystal lattice of the minerals and emanates out through alpha recoil. It dissolves in water, and is also found in soil and air. Its distribution in water is more pertinent for scientific investigations. It can be measured by various methods. Certain properties of radon enable it to serve as an ideal tracer, viz., short-half life, inertness, high abundance in groundwater than surface water, preferential partitioning, sensitivity to sudden changes in subsurface conditions, non-invasiveness etc. This paper reviews the state-of-the-art techniques on the measurement of dissolved radon in water and its potential applications as a tracer and precursor in several hydrogeological and geological applications like understanding the surface water - groundwater interactions, hydrograph separation of streams, estimation of Submarine Groundwater Discharge (SGD), study of hydrodynamics and water balance of lakes, earthquake predictions, locating geological structures (faults/lineaments), geochemical explorations, NAPL contamination studies etc. Among the various applications presented, radon based approach is found to be more reliable in water resources domain than seismic precursory studies. The interpretations based on radon study in the above applications will pave the way for the improved understanding of the hydrological processes, and thus, help the planners and water managers for the sustainable development and management of water resources.

Geochemical conditions conducive for retention of trace elements and radionuclides during shale-fluid interactions

Produced water generated during unconventional oil and gas extractions contains a complex milieu of natural and anthropogenic potentially toxic chemical constituents including arsenic (As), chromium (Cr), and cadmium (Cd), naturally occurring radioactive materials (NORMs) including U and Ra, and a myriad of organic compounds. The human-ecological health risks and challenges associated with the disposal of produced water may be alleviated by understanding geochemical controls on processes responsible for the solubilization of potentially hazardous natural shale constituents to produced water. Here, we investigated, through a series of batch treatments, the leaching behavior of As, Se, Cu, Fe, Ba, Cr, Cd, and radioactive nuclides U, Ra from shale to produced water. Specifically, the effect of four major controls on element mobility was studied: (1) solution pH, (2) ionic strength of the solution, (3) oxic-anoxic conditions, and (4) an additive used in fracking fluid. The mobilization of metals and metalloids from shale was greatest in treatments containing sodium persulfate, an oxidant and a commonly used additive in fracture fluid. In the high ionic strength treatments, dissolved Ba concentrations increased 5-fold compared to low ionic strength treatments. Overall, anoxic conditions superimposed with low pH resulted in the largest increase of dissolved metals and radionuclides such as Ra. Overall, our results suggest that (1) limiting pore water acidification by injection of alkaline fluid in carbonate-low shale and (2) minimizing strong oxidizing conditions in shale formations may result in cost-effective in situ retention of produced water contaminants.