Hydrogeological setting and hydrogeochemical characteristics of the Durban Metropolitan District, eastern South Africa

Abstract

Abstract Population and economic growth within the Durban Metropolitan region in eastern South Africa have increased the demand for water supply. Though the region’s water supply comes mainly from surface water sources, the ever-increasing demand means that all available water supply sources including groundwater will be looked at, particularly in the peri-urban areas. However, the state of the groundwater resource in the region is poorly understood. This study aims to contribute towards improved understanding of the state of groundwater resources in the Metropolitan District through an integrated hydrogeological, hydrochemical and environmental isotope investigations. Results of the hydrogeological and hydrogeochemical characterization identified at least five hydrostratigraphic units of varying hydraulic and hydrochemical characteristics: the weathered and fractured basement aquifers of the Mapumulo Group, Oribi Gorge, Mzimlilo and Mkhomazi Suites characterized by average borehole yield and transmissivity (T) of 1.2 l/s, and 3.9 m2/day, respectively, and hydrochemical facies of Ca-Mg-HCO3;the fractured Natal Group sandstone characterised by average borehole yield and hydraulic conductivity (K) of 5.6 l/s and 2.8 m/day, respectively and with Na-Mg-HCO3-Cl dominant water type;the fractured aquifers of the Dwyka Group diamictite and tillite characterized by average borehole yield of 0.4 l/s, transmissivity of 1.3 m2/day and Na-Cl-HCO3 dominant water type;the Vryheid Formation of the Ecca Group characterized by average borehole yield of 2.5 l/s, T of 4.9 m2/day, K values 0.17 m/day, and Na-Cl-HCO3 water type. The Pietermaritzburg Formation of the Ecca Group is characterized by a shale lithology with very low borehole yields and average transmissivity of 0.28 m2/d with Na-Ca-Cl dominant water type. It is considered as an aquiclude than an aquifer;the intergranular aquifer of the Maputaland Group which comprises the Bluff, Berea type sands and harbour beds (recent alluvium and estuarine deposits). These units collectively have average borehole yield of 14.8 l/s, transmissivity of up to 406 m2/day and a mainly Na-Cl-HCO3 hydrochemical signature. The region receives mean annual precipitation (MAP) of 935 mm/yr of which an estimated 6.6% recharges the various aquifers. Environmental isotope data (2H, 18O and 3H) indicated that groundwater is recharged from modern precipitation. High concentrations of tritium, as high as 92 T.U., measured around landfill sites, indicates groundwater contamination from leachate leakage posing a risk to human and environmental health.