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Stellenbosch University (2020)

THE APPLICATION OF RADON-222 IN CONSTRAINING ZONES OF RECENT GROUNDWATER RECHARGE IN THE TABLE MOUNTAIN GROUP AQUIFER IN THE CITY OF CAPE TOWN AND ITS SURROUNDING AREAS

Agyare-Dwomoh, Yaa

Titre :

Auteur : Agyare-Dwomoh, Yaa

Université de soutenance : Stellenbosch University

Grade : Master of Science (MS) Geology 2020

Résumé partiel
The world’s population is expected to increase by 2.14 billion people by the year 2050, and therefore finding sustainable water resources to satisfy humanity’s water demand has become one of the most urgent challenges of the 21st century. Due to population growth, poor water management practices and climate change, water scarcity has become a critical issue in arid regions like South Africa. Drought conditions affected the available water resources in the Western Cape during 2015-2018, and groundwater was considered as a sustainable water resource to mitigate recurrent water stresses. The Table Mountain Group (TMG) aquifer that surrounds the City of Cape Town is considered an excellent target for large scale abstraction to supplement the municipal supply. Although the high quality of the water (judged on the basis of very low TDS) makes this aquifer suitable for domestic abstraction purposes, for long term sustainability of abstraction still needs to be evaluated. 222Rn is an inert radioactive noble gas with a half-life of 3.82 days. This isotope is present in very low concentrations in precipitation but much higher concentrations in groundwater. Therefore, changes in the radon activity concentrations in groundwater might reflect dilution due to rapid recharge. Thus radon has the potential to be a means of evaluating groundwater sustainability where sustainable groundwater is defined as groundwater that is regularly recharged by modern precipitation. Analysis of 222Rn as undertaken in groundwater from different aquifer systems that surround the City of Cape Town in order to understand the groundwater recharge dynamics of each system. The groundwater systems examined were the Table Mountain Group aquifer, the Malmesbury Group aquifer, the Cape Granite Suite aquifer, the Bokkeveld Group aquifer, the Witteberg Group aquifer and the Quarternary sediments aquifer. As the groundwater was not further differentiated into specific formations or rock units within each of these stratigraphic units, they are referred to as aquifer systems. The hydrochemistry including stable isotopes of each groundwater sample was used to assign each groundwater sample to a host aquifer system. These host aquifer groupings were then used to characterise radon activity concentrations in each aquifer. Radon activity concentrations were variable in each aquifer system and these significant ranges meant that each aquifer system did not have a distinct radon activity concentration character.

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