Hydrogeological conceptual modeling of the Kosi Bay Lakes system, north eastern South Africa.
This M.Sc. Thesis focuses on the hydrogeological study of the Kosi Bay Lakes system, located in the north-eastern KwaZulu-Natal (KZN) Province of South Africa. The research catchment covers an area of about 659 km². It is characterised by four interconnected lakes, two isolated lakes and an estuary with a combined area of about 48 km². Two fresh water streams; namely, Sihadhla and Gezisa drain into the lakes. The study was initiated due to information gaps and the importance of the area with respect to conservation, ecology and water resources. The main objectives of the research was to characterize the groundwater and surface systems, in terms of their interconnection, flow and hydrochemistry; conduct a water balance study and develop a conceptual hydrogeological model on the occurrence and interaction of groundwater and surface water within the study area. The study has been undertaken by collecting primary data through a series of field campaigns in April 2013, May 2013 (onsite measurements and water, and water sampling) and October to December 2014 (geophysical data collection and supervision of borehole drilling). Original data generated in this study was complimented with data from KZN Groundwater Resource Information Project (GRIP), the National Groundwater Archives (NGA) and geophysical data, borehole logs, chemistry, and borehole yield data from consultant reports. Geophysical sounding data were calibrated using borehole logs and aquifer pumping tests, which indicate the presence of three hydrostratigraphic units in the study area, namely; the unconfined Holocene cover sands, the Kosi Bay and Port Durnford Formations, and the leaky-confined aquifer made up of the Umkhwelane and Uloa Formations, from top to bottom, respectively. The mean annual precipitation (MAP) for the study area based on data collected at Ingwavuma Kosi Bay and Ingwavuma Manguzi meteorological stations is 939 mm/year. The mean annual groundwater recharge estimated using the chloride mass balance method is 13% of the MAP. Surface water runoff from the catchment to the lakes derived using the Runoff Curve Number method is 14% of the MAP. Evaporation rate from the lakes and evapotranspiration from the catchment area estimated using the Penman and FAO Penman-Monteith approach are 1341 mm/a and 1135 mm/a, respectively. The water balance parameters indicate that inputs into the lakes are greater than the output as indicated by the positive change in storage (ΔS). The lake water balance result was supported by long-term lake level records that show an increasing trend over time. The measured electrical conductivity (EC) for the Kosi Bay Lakes range from 1024 μS/cm (Amanzamnyama) to 24600 (Makhawulani) μS/cm, for the groundwater from 86 to 400 μS/cm and for the streams, it ranges from 227 to 341 μS/cm. The high EC and TDS values of some of the Kosi Bay Lakes are attributed to the high evaporation and connection to the sea through the estuary. The shallow aquifers are characterized by Na-HCO₃-Cl, whereas the deep aquifers have a Na-Ca-Cl hydrochemical facies. All groundwater, stream and lake water samples have δ¹⁸O and δ²H values that plot on the local and global meteoric water lines indicating recharge from meteoric sources. Groundwater in the shallow Holocene aquifer and streams has similar hydrochemical and isotopic signature, indicating strong interconnection. On the other hand, the lakes are characterized by Na-Cl hydrochemical water type and an enriched stable isotopic signal (positive δD and δ¹⁸O signals) indicating evaporation and terminations of the local surface and groundwater flow system. The detectable tritium signal along with the low salinity of groundwater in the shallow aquifer reflect recent (< 50 years) recharge.