Measurements and applications of radon in South African aquifer and river waters

Abstract

In the natural decay series of 238U an inert radioactive gas, 222Rn (radon) is formed in the decay of 226Ra. Because radon is relatively soluble in water, it migrates from places of its generation in rocks and soils to other places either by soil air, or travels with underground water. Therefore, there is a growing interest among hydrogeologists in using radon as a natural tracer for investigating and managing fresh water reservoirs. This work is aimed at investigating and developing radon-in-water measuring techniques applicable to aquifers and rivers. A gamma-ray spectrometry method using a hyper-pure germanium (HPGe) detector, based at iThemba LABS, Cape Town and Marinelli beakers, has been optimized to measure radon in borehole water via the g-rays associated with the decay of radon daughters 214Pb and 214Bi (in secular equilibrium with their parent). An accuracy better than 5% was achieved. Moreover, long-term measurements of radon in water from an iThemba LABS borehole have been carried out to investigate the role of radon for characterizing aquifers. These investigations led to the development of a simplified physical model that reproduces the time-evolution of radon concentration with borehole pumping and may be used to estimate the time for representative sampling of the aquifer. A novel method is also proposed in this thesis to measure radon-in-water in the field after grab sampling - a so-called quasi in-situ method. The quasi in-situ method involves inserting a y-ray detector in a container of large volume filled with water of interest. The g-ray spectra are analyzed using an approach involving energy intervals on the high-energy part of the spectrum (1.3 – 3.0 MeV). Each energy interval corresponds to contributions from one of the major g-ray sources: 40K and the decay series of 238U and 232Th, and cosmic rays. It is assumed that the U interval will be dominated by g-rays emitted from the radon daughters (214Pb and 214Bi). Minor contributions to an interval with major radionuclide are corrected using an MCNPX simulated standard spectra. The two methods in this thesis make a significant contribution to measuring and modelling of radon in aquifers and surface waters. It forms a basis for further development in an interactive mode with hydrological applications.