Conceptual modelling of groundwater-surface water interactions in the Lake Sibayi Catchment, Eastern South Africa

Lake Sibayi, a topographically closed fresh water lake in northern KwaZulu-Natal, South Africa, and coastal groundwater aquifers surrounding the lake, are important water resources and are used extensively for domestic water supplies. Both the lake and groundwater support 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, upon which a three dimensional numerical model will be based. Local geologic, groundwater head distribution, lake level, hydrochemical and environmental isotope data confirm 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 indicates that the lake recharges the aquifer. Stable isotope signals further revealed the movement of lake water through and below the coastal dune cordon before eventually discharging into the Indian Ocean. Quantification of the 15 year monthly water balance for the lake shows strong seasonal variations of the water balance components and average residence time of water in the lake is about 7.5 years. Recent increase in rate of water abstraction from the lake combined with decreasing precipitation and rapidly increasing pine plantations appears to be responsible for lake level reduction which would have dramatic negative effects on the neighbouring ecosystem and a potential seawater invasion of the coastal aquifer.