Abstract
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...