Hydrogeological characterization of the Kosi Bay Lakes system, north-eastern South Africa

This paper reports a hydrogeological characterization of a groundwater-dependent lakes system, the Lakes Kosi Bay catchment located in the north-eastern coast of South Africa. Conventional hydrogeological techniques including water balance analysis, hydrogeochemical and environmental isotope methodologies were applied in the study. Three distinct aquifer systems are recognized in the area, namely the unconfined Holocene cover sands, the Kosi Bay and Port Durnford Formations, and the Umkhwelane and Uloa Formations. Groundwater recharge to the topmost aquifer, which is connected to the lakes, is estimated using the chloride mass balance method to be 12 % of the mean annual precipitation (MAP = 939 mm/a). Evaporation rate from the lakes and evapotranspiration from the catchment are 1310 and 1135 mm/a, respectively. The mean annual water balance components indicate a positive balance between inputs and outputs. Groundwater flows from the west through the lakes (flow-through lakes) to the Indian Ocean. Groundwater in the shallow Holocene aquifer and streams has similar hydrochemical and isotopic signature indicating strong interconnection. These groundwater and streams have a hydrochemistry dominated mainly by a Na–Cl–HCO3 water type, a very low salinity (ranging from 88 to 400 µS/cm) and a negative isotopic signature (negative δD and δ18O values). The lakes are characterized by Na–Cl hydrochemical water type, have salinity that ranges from 1014 to 25,300 µS/cm, and positive stable isotopic signature (positive δD and δ18O signature) indicating strong evaporation effects. The study further reveals a salinity and isotopic series among the lakes from south to north, towards the estuary.