The inorganic chemistry and geochemical evolution of pans in the Mpumalanga Lakes District, South Africa

M.Sc. (Geology)Despite Chrissie Lake being South Africa's largest freshwater lake, the chemistry of this lake and the surrounding lakes and pans in the Mpumalanga Lake District has never been studied in detail. These closed systems show varying chemistry while being in very close proximity to one another, adding to the uniqueness of this area where pans, usually typical of arid regions, are found in a humid area. The factors affecting the water chemistry of these lakes needed to be identified and explained. In order to evaluate the water chemistry in this unique environment, water samples were taken at the end ofthe wet and dry seasons, in April and September 2007 respectively. The major pans were sampled, as were adjacent fountains or springs, indicative of the perched groundwater aquifers found in this area, as well as borehole water from the surrounding farms. Alkalinity was determined by manual titration upon returning from the field while pH and conductivity measurements were performed on site. Major cations and anions were analysed for using ICP-OES and Ion Chromatography respectively. Sediment samples were collected from the floor of each pan in the summer sampling and the mineralogy determined by X-ray diffraction (XRD). During September 2007 sampling, precipitates found on the floors and banks of the pans were also collected and analysed using XRD, to identify mineral species precipitating from solution. Initial results show pH values ranging from 7.0-10.5 for the lakes and pans and from 6.0-8.0 for the borehole water and springs. Values as low as 100 mglL Total Dissolved Solids (TDS) were measured for the pans, with maximum values set at 10 giL for the most saline of these bodies of water in the wet season and as much as 90glL for a pan almost completely dried out in the dry season. The water in the closed pan systems are dominated by Na-CI- HCO~ and have varying concentrations of major cations. The dilute spring waters have TDS values ranging from 20-200 mg/L, indicating the excellent quality of the groundwater, while some boreholes reach values of I 000 mg/L TDS showing possible linkage to pans or leaking of the pan water into the surrounding strata. To understand the main processes affecting the inorganic chemistry of the surface and shallow groundwater of this area, major ions were plotted against chloride. The latter behaves conservatively and can thus be used to monitor the behaviour of solutes in the pan waters. These plots illustrate that the dominant process in the evolution of the waters in the MLD is evaporative concentration. Removal of species through mineral precipitation is clearly seen; carbonate species..