Conceptual Modelling Of Lake–Groundwater Interactions For The Lake Sibayi Catchment, North-Eastern South Africa

Lake  Sibayi  (a  topographically  closed  freshwater  lake)  and  coastal  aquifers  around  the  Lake  in eastern South Africa are important water resources and are used extensively for domestic water supplies. Both the Lake and groundwater support an important and ecologically sensitive wetland system   in   the   area.   Surface   and   subsurface   geological   information,   groundwater   head, hydrochemical and environmental isotope data were analysed to develop a conceptual model of aquifer–lake interaction for further three-dimensional numerical modelling. These local geologic, groundwater head distribution, lake level, hydrochemistry and environmental isotope data confirm a direct hydraulic link between groundwater and the Lake. In the western section of the catchment, groundwater flows to the lake where groundwater head is above Lake stage, whereas along the eastern section, the presence of mixing between Lake and groundwater isotopic compositions indicated that the Lake recharges the aquifer. Stable isotope signals further revealed the movement of lake water through and below the coastal dune cordon and eventually discharges into the Indian Ocean. Quantification of the 14-year monthly water balance for the Lake shows strong seasonal variations of the water balance components. Recent increase in rate of water abstraction from the lake combined with decreasing rainfall and rapidly increasing pine plantations may result in a decrease in lake level which would have dramatic negative effects on the neighboring ecosystem and a potential seawater invasion of the coastal aquifer.