Hydrogeological characterisation of Fountains East and Fountains West Karst compartments

Abstract

The Fountains East and Fountains West groundwater compartments (by means of the Upper and Lower Fountain springs) have been supplying the City of Tshwane with water since the founding of Pretoria in 1855. These adjacent compartments, which are underlain by the Malmani dolomites of the Chuniespoort Group, are separated by the Pretoria syenite dyke and are bounded to the north by the rocks of the Pretoria Group (Timeball Hill Formation). The perennial, artesian springs that drain the compartments are situated within the Groenkloof Nature Reserve and currently supply the citizens of Pretoria with 46 ML/day of water. Inorganic chemistry data (2007-2012) as well as spring discharge volumes (2011-2012) for the Upper and Lower Fountain springs along with water levels obtained from DWA boreholes in the Fountains East and Fountains West compartments (1984-2013) and isotope data for both springs and numerous rainfall stations in the City of Tshwane (1979-2007) were used in order to aid in the characterization of the springs and the compartments to which they belong. This was done by means of statistical analysis, Piper diagrams, bar graphs and temporal plots. Interpretation of the water levels indicates that that the Fountains East compartment generally has more shallow water levels while the Fountains West compartment has average water levels that are approximately 8.5m deeper and irrespective of the compartment, groundwater flow is generally from the south to the north in the karst aquifer . From the chemistry data the hydrochemical characteristics of both springs are found to be similar with the groundwater signature for both springs being Ca(Mg)-HCO3 which is indicative of fresh, recently recharged groundwater. Isotopically both springs are found to be depleted (as a result of the rainout effect) and may indicated that recharge of the compartments did not occur in the Pretoria area.