HYDROGEOLOGICAL AND HYDROGEOCHEMICAL CHARACTERISTICS OF THE NATAL GROUP SANDSTONE, SOUTH AFRICA

Abstract

The Palaeozoic age Natal Group Sandstone (NGS) that outcrops from Hlabisa (in the north) to Port Shepstone (in the south) and Greytown (to the west) to Stanger (to the east) in KwaZulu-Natal Province, South Africa, is investigated in terms of its hydrogeological characteristics. The NGS, which consists of a Lower Durban Formation and an Upper Mariannhill Formation, forms a secondary/fractured aquifer system that has variable to good productivity across its members. This aquifer is characterised by borehole yields ranging from 0.2 L/s to 25 L/s, with more than 50% of the boreholes having blow yields greater than 3 L/s. Preliminary analysis indicates that higher-yielding boreholes are associated with a network of intersecting fractures and faults, that are recommended targets for future borehole drilling in the area. Groundwater recharge increases from west to east and from north to south. The south-eastern sector receives up to 120 mm/a of recharge compared to the north-eastern sector where recharge declines to less than 30 mm/a. Depth to groundwater decreases southwards commensurate with an increase in the rate of recharge. The general direction of groundwater flow is eastwards, to the Indian Ocean. Groundwater in the NGS is of good quality in terms of major and trace element composition. The electrical conductivity (EC) values measured are mainly less than 100 mS/m except areas north of the Mhlathuze River where EC values as high as 449 mS/m have been determined. EC, total dissolved solids (TDS) and major ion contents in the sandstone aquifer groundwaters decrease from north to south, controlled by geochemical composition of the aquifer material, the rate of recharge and impacts of sugarcane cultivation. Groundwater hydrochemical facies are either Na-HCO 3 or Na-HCO 3 -Cl. The electrical conductivity increases from inland to the coastal zone, indicating marine influences. Environmental isotope data (δ 2 H, δ 18 O, tritium) indicate that the groundwater recharge is from modern precipitation.