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
Thesis (MSc)--Stellenbosch University, 2020.
ENGLISH ABSTRACT: 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. The two exceptions to this were: (1) the TMG aquifer where the lack of U and Ra in the aquifer host rocks, meant that the activity concentration of radon was lower in this aquifer than the other aquifers; and (2) the Cape Granite Suite aquifer system, where the high concentrations of U and Ra in the host rocks resulted in higher radon activity concentrations in groundwater hosted by these rocks. The radon activity concentration in groundwater in different locations changed as a consequence of groundwater recharge. As rainwater contains negligible radon activities, a dilution effect was noted in response to groundwater recharge in some of the aquifer systems. Three radon activity concentration trends were noted: (1) an immediate dilution in the radon activity concentration was recorded due to direct recharge; (2) a delayed dilution in the radon activity concentration was recorded due to a lag time in the recharge; and (3) radon activity concentrations were stable indicating little or no recharge response within a period of ~ 20-25 days after the recharge event (precipitation event). The radon data was compared with radiocarbon data (collected as part of a separate parallel study) for the same sample locations. The 14C data was consistent with the three radon activity concentration trends above being associated with groundwaters of different ages. The groundwater samples with the stable radon activity concentrations were associated with lower 14C activities, implying older residence times and hence a disconnection from modern recharge. In utilising the groundwater radon activity concentrations, sites of rapid groundwater recharge were delineated and mixing behaviour between surface water and groundwater was evaluated. This contributed to a better understanding of the groundwater recharge dynamics and allowed assessment of which aquifer systems were more sustainable. Groundwater from the TMG aquifer system has low radon activity concentrations. After precipitation events, these values dropped rapidly implying a direct recharge response. 14C data for the same groundwater samples, indicates the groundwater is typically young (± ≥ 100 pMC) and thus its sustainability is directly linked to current precipitation patterns. Hence, during periods of little or no rain, the aquifer is vulnerable to overexploitation and should be closely monitored and used sparingly. The results presented here introduce new perspectives in the application of groundwater isotopic tracers to understanding the TMG aquifer system and how it is recharged.
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