The use of Radon (Rn222) isotopes to detect groundwater discharge in streams draining Table Mountain Group (TMG) aquifers

Abstract

Environmental isotopes have been used for decades as natural tracers in studies aimed at understanding complex hydrogeological processes such as groundwater and surface water interactions. Radon (Rn²²²) is a naturally occurring, radioactive isotope which is produced from radium (Ra²²⁶) during the radioactive decay series of uranium (U²³⁸). Since U²³⁸ is present in most geological substrates, Rn²²² is produced in various lithological structures and subsequently transported with groundwater through fractures and
pore spaces in an aquifer towards surface water discharge points in rivers and springs. This study aimed to determine (i) the concentration of Rn²²² within both surface water and groundwater in Table Mountain Group (TMG) aquifer systems, and (ii) the feasibility of using Rn²²² isotopes as a natural tracer in groundwatersurface water interaction studies. This study was conducted in a highly fractured TMG aquifer system near Rawsonville, South Africa. Surface water from two perennial rivers (i.e. Gevonden and Molenaars), together with groundwater from a nearby borehole, were sampled and their corresponding Rn²²² concentrations measured. Our study found median Rn²²² concentrations in the Gevonden River of 76.4 Bq∙L^-1 and 67.2 Bq∙L^-1 in the dry and wet seasons, respectively. Nearly 12% of surface water samples exceeded 100 Bq∙L^-1. These abnormally high Rn²²² concentrations can only be attributed to the influx of groundwater with extremely high Rn²²² concentrations. Under ambient (no pumping) conditions, Rn²²² concentrations in groundwater range between 130–270 Bq∙L^-1. However, when the borehole was pumped, and inflowing water from the surrounding aquifer was sampled, even higher Rn222 concentrations (391–593 Bq∙L^-1) were measured. These extremely high
Rn²²² concentrations in groundwater are believed to be attributed to the underlying granitic geology and the prevalence of faults. The use of Rn222 isotopes as an environmental tracer in groundwater–surface water interaction studies is therefore regarded as a feasible option in similar highly fractured aquifer systems.