The Neutron Difference – Isotopes
Warrick Daws, one of the newest additions to the Umvoto team, completed his geology Master’s degree at the University of Cape Town (UCT) in June 2022. His thesis focused on groundwater isotopes. Isotopes are atoms of the same element that have a difference in the number of neutrons. The isotope composition of groundwater can provide insights about aquifer recharge and the mixing of water bodies.
Differentiation
Different isotopes of the same element have varying atomic masses. Differentiation of hydrogen and oxygen isotopes occurs during physiochemical processes such as evaporation and precipitation. The strontium isotope composition (87Sr/86Sr) of modern seawater is constant, whereas the strontium isotope composition of rocks is related to time-dependent radioactive decay of rubidium. Isotopes are often presented in delta-notation (δ) relative to a standard.
Application
An effective technique for monitoring groundwater mixing and recharge is by analysing δD, δ18O, and 87Sr/86Sr composition of water samples. The δD and δ18O of 148 water samples (groundwater, rainwater, and stream water) from Hout Bay, a coastal valley 20 km southwest of Cape Town CBD, were considered in Warrick’s dissertation.
Three possible explanations were proposed for the δD and δ18O difference between Hout Bay monthly rainwater from March 2020 to February 2021, and Hout Bay February/March 2020 and November 2020 groundwater. The first is that intense rainfall events preferentially recharge groundwater with low δD and δ18O values. The second explanation was that Hout Bay groundwater might have retained more negative δD and δ18O values from rainwater recharge prior to March 2020. Lastly, recharge from mountain peaks at higher elevation than the Hout Bay groundwater sample locations adds a component of more negative δD and δ18O to Hout Bay groundwater than proximal rainwater.
Warrick analysed nine Hout Bay groundwater samples for strontium isotope composition (87Sr/86Sr) to determine the applicability of strontium to trace the source of groundwater. Minor interactions of Hout Bay groundwater with Cape Granite Suite produced relatively high 87Sr/86Sr ratios while lower 87Sr/86Sr ratios similar to that of modern seawater are attributed to dissolution of the marine sediments of the Quaternary Witzand Formation and marine aerosols in sea spray. Higher rainwater amounts were proposed to increase 87Sr/86Sr ratios of higher elevation groundwater sample locations by diluting the dissolution of marine aerosols in rainwater.
Warrick’s dissertation provides valuable insight into groundwater processes in Hout Bay and the findings will be utilized in future studies to further understand Cape Town’s groundwater behaviour. Professor Chris Harris from UCT, Warrick’s thesis supervisor, will present Warrick’s thesis results as part of a greater presentation on groundwater isotopes in Cape Town (of which Warrick is a co-author) at GeoCongress 2023 during mid-January 2023 at Stellenbosch University.
