Geohydrological studies of the Western Shores of lake St Lucia

A dissertation submitted to the faculty of Science at the University of Zululand, in partial fulfilment of the requirements for the degree Master of Science in Hydrology, South Africa, 1998.

Lake St Lucia is a sensitive estuarine system of international importance which is threatened with over-development within its catchment area by afforestation, overgrazing, proposed dredge mining on the eastern shores and other exploitations. These disturbances and proposed developments have invoked considerable public concern for the sustainability of the Lake St Lucia system. This study looks at the contribution of the groundwater from the western shores area to the lake system in order to evaluate its importance to sustaining the ecology of the system. Under severe drought conditions, the lake experiences considerable lowering of water levels which are accompanied by high rises in salinity levels because of the low freshwater recharge. While the western shores catchment of Lake St Lucia has a number of rivers that contribute freshwater to regulate the salinity of St Lucia, most of these rivers are non-perennial and have been exploited by developments within their catchments. In this study the aquifer system of the southern portion of the western shores area of Lake St Lucia is delineated with the specific intention of defining the coastal geohydrology of the western shores in order to determine its contribution to the water balance of Lake St Lucia. The geological succession of the Zululand coastal plain is described using geophysical information and borehole data which have been compared with field observations and findings in adjacent areas. The general stratigraphic succession, constructed from a number of electrical resistivity soundings and a few boreholes drilled around the western shores area of Lake St Lucia, is described as the first step in defining a conceptual model of the present system for numerical simulation studies. The resistivity soundings have shown that the permeable succession is restricted to post-Cretaceous sediments. The low permeable Cretaceous siltstones, the Lebombo volcanics and the Karoo sediments were all selected to coincide with the base of the numerical model. The overlying, semi-permeable to permeable, upward-fining calcareous and clayey sands of Pleistocene age, known as the Port Durnford Formation, were generally designated as the second layer (LAYER 2) of the model. The electrical resistivity soundings reveal that, on the western shores, the Port Durnford Formation has a thickness of 0 to 30 metres. Overlying the Port Durnford Formation is a relatively thin cover of unconsolidated aeolian, alluvial and estuarine sands of Holocene age which were generally selected as the top layer (LAYER 1) of the model. The upper surface of these coversands coincide with the surface topography, and they range in thickness from 0 to 8 metres and play a major role in the hydraulics of the shallow coastal aquifer. A three-dimensional finite difference model (INTERSAT), was developed from the conceptual geological model. This model was used to determine piezometric surfaces and subsurface flow into Lake St Lucia. Geohydrological and other hydrological parameters assigned to the model were estimated from existing literature and published reports because there were no observation data available. The model parameters for effective rainfall and evapotranspiration were derived from consideration of landuse features to account for surface-groundwater interactions. After the steady state condition had been achieved, the model was validated by comparing the simulated piezometric surfaces (heads) and simulated stream recharge against the observed water table elevations and estimated outflow from rivers, respectively. The simulation results have shown that the groundwater contribution from the western shores to the lake is approximately 40x106m3/year. This component of the water budget may be sufficient to have a buffering effect on the salinity of the lake along the edges of the shores if it is released in times of surface flow droughts. To quantitatively evaluate the groundwater contribution from this area and other sections of the lake system, the presently existing network of electrical resistivity surveys needs to be supplemented with the boreholes or observation wells that should be scattered over the area. From these records a good groundwater database could be built. It is recommended that simulation studies should be extended to the northern shores in order to evaluate its contribution to the water budget of Lake St Lucia.

Water Research Commission (WRC)