Event Debrief: Talk on Stable isotopes in precipitation and the assessment of groundwater recharge mechanisms in the Johannesburg region (GWD Gau)

Event Debrief: Talk on Stable isotopes in precipitation and the assessment of groundwater recharge mechanisms in the Johannesburg region (GWD Gau)

17 Jun 2021
Elanda
Thank you to Dr Khahliso that took the technical difficulties in his stride and availed himself to present his Talk again. Special thank you also to Mr Kwazi Majola (Chair GWD Gauteng) for inviting and facilitating this very interesting talk.
Home News Event Debrief: Talk on Stable isotopes in precipitation and the assessment of groundwater recharge mechanisms in the Johannesburg region (GWD Gau)

Talk was re-broadcasted on 17 June due to technical issues during the 10 June broadcast.

Thank you to Dr Khahliso that took the technical difficulties in his stride and availed himself to present his Talk again. Special thank you also to Mr Kwazi Majola (Chair GWD Gauteng) for inviting and facilitating this very interesting talk.
 

Leketa

 

ABOUT THE TALK:

Characterising Stable moisture trajectories and groundwater recharge in Johannesburg South Africa 


This study is about assessment of the isotope signature of rainfall events between November 2016 and October 2018 in Johannesburg, South Africa.

Additionally, the study uses the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) to assess the moisture sources and trajectories for rainfall events of extreme isotopic signatures. Also, the Johannesburg Local Meteoric Water Line (JLMWL) is used to identify direct and indirect groundwater recharge mechanisms in the Johannesburg region. The results indicate temperature and amount effects, however, with low coefficient of determination (R2) values of 0.21 and 0.12 for the respective effects. The rainfall with amounts of <20 mm yielded a meteoric water line (MWL) with a slope of +6.9 and deuterium excess (d-excess) of +11.9‰. Rainfall with amounts of ≥20 mm had slope and d-excess values of +8.2 and +18.6‰, respectively. The lower slope in rainfall of <20 mm indicated the preferential occurrence of sub-cloud re-evaporation on light rainfall. Considering the lack of re-evaporation in heavy rainfall (≥20 mm), its slope and d-excess were approximated to those of incoming or in-cloud moisture prior to condensation. Therefore, a high d-excess of +18.6‰ in incoming moisture indicates evaporation that mainly occurs under warm sea surface temperature and low relative humidity. HYSPLIT shows that the moisture for the most depleted and the most enriched rainfall originates in the higher latitudes but differ in their trajectory. The moisture for the most depleted rainfall had long residence in the higher latitudes following a semi-direct trajectory to Johannesburg, over the Atlantic and Indian Oceans. The most enriched rainfall followed a curved anticlockwise trajectory with long residence in the lower latitudes over the Indian Ocean leading to a loss of light isotopes, thereby, enhancing enrichment of moisture that moves further inland. This indicated the importance of atmospheric conditions along the trajectory on the signature of Johannesburg rainfall.  The stable isotopes indicate direct recharge in the fractured quartzite aquifer in Johannesburg and indirect recharge from Hartbeespoort Dam through the Brits faults. The study further continues with an assessment of continental moisture circulation through the ongoing monitoring of stable isotopes in rainfall of Lesotho and Johannesburg.

ABOUT THE SPEAKER

Dr Khahliso Leketa obtained a Doctor of Philosophy at the University of the Witwatersrand (2019), a Master of Science and BSc Honours in Hydrogeology at the University of the Free State (2009 and 2011) and BSc at the National University of Lesotho (2008). After serving as a Senior Hydrogeologist at the Department of Water Affairs in Lesotho from 2011 to 2019, Dr Leketa is now a lecturer at the National University of Lesotho where he lectures and researches on water resources. He has authored eight (8) peer-reviewed publications on groundwater recharge assessment, groundwater characterisation, modelling the impacts of changes in climate on water resources, characterisation of environmental isotopes in precipitation, moisture source identification for precipitation and groundwater dating using radioactive isotopes.

 

Leketa2

 

TALK RESOURCES 

MORE READING ON THE PRESENTED FINDINGS

(1) Leketa Khahliso, Abiye Tamiru (2021). Using environmental tracers to characterise groundwater flow mechanisms in the fractured crystalline and karst aquifers in Upper Crocodile River Basin, Johannesburg, South Africa. Hydrology 8, no. 1: 50. https://doi.org/10.3390/hydrology8010050

(2) Leketa Khahliso, Abiye Tamiru, (July 2020). Investigating the traces of stable isotope effects in the rainfall from Johannesburg, South Africa. Water SA Journal. (3) 46, 429. https://doi.org.10.17159/wsa/2020.v46.i3.8653

(3) Leketa Khahliso, Abiye Tamiru, Butler Michael. (2018). Characterisation of groundwater recharge conditions and flow mechanisms in the crystalline aquifers of the Johannesburg area, South Africa. Environmental Earth Sciences. 77, 727. https://doi.org/10.1007/s12665-018-7911-7.

(4) Leketa Khahliso, Abiye Tamiru, Zondi Silindile. (2018). Assessing groundwater recharge in crystalline aquifers of the Upper Crocodile River Basin, Johannesburg, South Africa. Groundwater for sustainable Development. 31-40. https://doi.org/10.1016/j.gsd.2018.08.002

 

Stats leketa