Assessing groundwater recharge in crystalline and karstic aquifers of the Upper Crocodile River Basin, Johannesburg, South Africa

Abstract

Groundwater recharge and flow conditions were assessed in the Upper Crocodile River Basin, Johannesburg South Africa. This study revealed high elevation recharge from rainfall that is depleted in stable isotopes of water molecule (δ18O and δ2H) occurring prior to extreme evaporation. The effective recharge occurs from rainfall of varying moisture source and humidity conditions. The δ13C (‰) value in groundwater indicates both open and closed system carbon evolution. Open system was observed in shale, andesite and granitic gneiss aquifers indicating that soil CO2 is a dominant source of carbon in bicarbonate. Enriched δ13C observed in Malmani dolomite aquifers deduced the presence of carbonate dissolution, which indicates a closed system carbon evolution. Mixing of water from open and closed systems, or evolution from closed to open system is also observed in groundwater samples that had intermediate δ13C (‰) values, particularly groundwater samples that are positioned at the periphery of the Basement Complex granitic gneiss near the Malmani dolomite aquifers. The integrated mean annual recharge estimates obtained from BFS for the UCRB is 6.7% while the distributed point recharge estimates in the Malmani dolomite obtained from application of WTF on 43 individual boreholes ranged between 2.5% and 39% with an average of 15% as a percentage of MAP. A comparison between estimated recharge and groundwater abstractions conducted for each sub-catchment revealed that there is an excess of groundwater recharge over groundwater use in most sub-catchments. The CMB point recharge estimates obtained from the Albert Farm spring and the Malmani dolomite springs are 2.5% and 27.8% of MAP, respectively. The low CMB recharge estimates in the shale and quartzite has been attributed to possible anthropogenic chloride sources such as leakage of chlorinated potable water in the pipelines of the city of Johannesburg, which elevate chloride concentrations in groundwater.