Recharge rates and processes in the upper Crocodile catchment

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2017.A study on groundwater recharge and processes controlling recharge was conducted in the Upper Crocodile catchment, located in the Johannesburg region. The catchment extends from the water divide south of Johannesburg, to the Hartbeespoort Dam in the North-West Province. The study area is predominantly underlain by the crystalline basement and meta-sedimentary rocks. The Upper Crocodile catchment is classified as a semi-arid region, receiving a mean annual rainfall of 699.3 mm/yr. Groundwater recharge was quantitatively and qualitatively assessed using the water balance, baseflow separation, water table fluctuation and environmental isotope methods. The water balance and the baseflow separation methods resulted in recharge amounts of 4 and 5.8% of mean annual rainfall, respectively. The water table fluctuation method was only applied to the dolomitic aquifer and yielded a mean annual recharge estimate of 14% of the mean annual rainfall. Application of the isotopic shift method, which makes use of isotopically enriched water samples, resulted in a recharge amount of 10.19 to 23.90 mm/month obtained for the quartzites of the Witwatersrand Supergroup, south of the study area. Tritium was used to determine the residence time of stream water samples, collected during winter to represent baseflow. Additionally, it was used to understand the range of groundwater contribution to streams. The tritium values revealed that there are three types of water; i) relatively old water with lower tritium values, ii) intermediate tritium values indicating the possibility of mixing of older groundwater with more recent recharge and iii) high tritium values suggesting contamination from a local source/recent rainwater. The results of groundwater recharge from the quantitative methods showed a temporal and spatial variability of recharge; this was attributed to the different processes that govern groundwater recharge. Climate appeared to have the most influence on potential groundwater recharge, with rainfall controlling the temporal variability of recharge while land cover, soil characteristics and geology influenced the spatial distribution of groundwater recharge. Approximately 153 x 106 m3/yr of wastewater was discharged into streamflow from wastewater treatment works as of 2008. The wastewater flow into streams overshadowed the baseflow contribution. The consequence of the presence of wastewater was reflected in the overestimation of groundwater recharge.LG201