The hydrogeology of the Uitenhage Artesian Basin with reference to the Table Mountain Group Aquifer

Abstract

The Uitenhage Artesian Basin (UAB) lies in the Eastern Cape and is South Africa’s most important artesian groundwater basin, supplying approximately 1 400 Ml/yr (44 l/s) of water from springs for domestic use to Uitenhage, as well as supporting large citrus irrigation schemes. Groundwater from this basin has been extensively utilised from the early part of the 20 th century, including periods of over-exploitation resulting in the declaration of a groundwater control area to limit abstraction to sustainable rat es. The aquifer comprises fractured Table Mountain Group (TMG) sandstones confined in the eastern part of the basin by overlying Cretaceous siltstones and mudstones, resulting in artesian conditions. The Coega Fault is a major structural feature dividing the basin into separate systems, viz. the southern Swartkops Aquifer and the northern Coega Ridge Aquifer, that are hydrogeological ly independent of each other. The Elands River syncline divides the Swartkops Aquifer further into the Kruisrivier and Bethelsdor p Units. Borehole yields commonly range from 5 to 10 l/s and the groundwater quality is excellent with low salinities. Water hardening, however, is required due to the acidic and corrosive nature of the groundwater, typical of other Table Mountain Group aquifers in South Africa. Using 14 C data, the age of the groundwater in the basin ranges from 1 500 to 28 000 years with a calculated flow rate of 0.8 m/a. From the chloride mass balance method, recharge rates are determined to be 25 to 55% of annual rainfall. Groundwa ter temperatures generally show that depths of groundwater strikes do not necessarily correspond with depth of origin, indicating a complex groundwater circulation pattern within the basin. Whilst the UAB has been well studied locally, a basin-scale hydrogeological characterisation is considered to be necessary, followed by recommendations and formulation of a management plan to ensure the continued sustainability of groundwater supply from this national asset.