Groundwater resource exploration within the inner-city of Johannesburg

This study examined the hydrogeological characterisation of the Johannesburg inner-city. Residents reported a high water table that has resulted in groundwater seepage into building basements, posing a safety threat and limited functionality of these structures. Although this high groundwater table triggered this investigation, it took a deeper and broader approach by seeking to quantify the amount of groundwater resource that is available to be used to supplement the existing municipal supply. The study also aimed at establishing long-term sustainable abstraction rates and viable fit-for-purpose use of the available groundwater. The study then evaluated the abstraction rates against the water table response in order to achieve a safe water table level at which groundwater seepage into building basements does not occur. The methodology employed included assessing and verifying the number and distribution of buildings affected by groundwater seepage. Information was gathered by circulating a survey in the form of a questionnaire to a network database of property owners and managers. Field work, in the form of drilling and testing boreholes, and subsequent data processing were carried out to characterise the aquifers and establish the quantity of the resource. In order to determine possible uses of the groundwater, water chemistry testing was carried out. The characteristics and recharge dynamics of the aquifers that were established served as inputs into a numerical model which estimated the quantity of groundwater that can be sustainably abstracted for utilisation, while ensuring the water table is sufficiently suppressed. An estimated 2.47 million-m3 (or billion-litres) of groundwater is available as a total yield potential within the Johannesburg inner-city. This water can be sustainably withdrawn from the ground at a safe yield (abstraction rate) of over 56 million-litres per annum (56 902m3/a) from the nine drilled boreholes. The system response corresponding to this abstraction rate is predicted to cause a 2.3m decline in the water table level compared to the current scenario. This desired pumping scenario would suppress the water table level to be at an average of 8.6m below ground. The average basement depth in the Johannesburg inner-city is 8m below ground and hence, most building basements would consequently not be flooded under this scenario. Currently, 60% of building basements are subjected to the risk of flooding, and this would reduce to 30%, which is made up of those deeper than 9m. The current water demand in the Johannesburg inner-city is approximated to be 80Ml/day. Therefore, the estimated groundwater yield to supplement the municipal supply is insignificant (0.2%), and does not justify an investment into retrofitting the groundwater infrastructure into the bulk infrastructure system. Thus, developing a groundwater source infrastructure for bulk water supply purposes is not volumetrically viable in the case of Johannesburg inner-city, but can be beneficial for building strategic reserves and resilience into the system. It is recommended that in future, fine-tuning and continuous calibration of the numerical model should be carried out when the boreholes are in operation. Further research is also required to examine the aquifer geo-mechanics in order to assess the risk of land subsidence with decreasing water table levels, especially in a possible future scenario of over-exploitation of the aquifer
A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, 2020