The inorganic chemistry and geochemical evolution of pans in the Mpumalanga Lakes District, South Africa
- Authors: Russell, Jennifer Lee
- Date: 2009-11-06
- Subjects: Pans (Geomorphology) , Water chemistry , Chemistry, Inorganic , Mpumalanga (South Africa)
- Type: Thesis
- Identifier: uj:8636 , http://hdl.handle.net/10210/2994
- Description: Master of Science , 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 of the 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. Results from the above analyses show that each pan in the MLD has a unique chemistry, which cannot be inferred from neighbouring pans. The inorganic chemistry differs from pan to pan as a result of these separate, closed systems being at different stages of the evaporation process. Throughout the path from groundwater to the pan, waters are subject to mineral dissolution and precipitation, adsorption and biological mechanisms, which continually add or remove solutes from solution. Although seemingly simple, there are certainly other factors that play a role in the evolution of the water chemistry. Key to the current inorganic chemistry is the balance between import and export of solutes. The groundwater, predominantly the perched aquifer water, brings solutes into the pans and blowouts of precipitates on the pan floor at the end of the dry season, when the wind is strongest, results in the export of solutes. This process is significant in maintaining the overall freshness of the pans in the MLD, contrasting to their western counter parts that evolve to highly saline saltpans. Other factors such as the periodicity of pans drying completely, the surface area to catchment area ratio (CA/SA), the formation and dissolution of efflorescent crusts and the presence or absence of reeds all have varying effects on the water chemistry of the lakes and pans. Significantly, the amount of evaporative concentration that a pan evolves through has been shown to be dependant on the CA/SA ratio with pans having larger ratios being lower in salinity compared to those with low ratios being the most saline pans. The reservoir available to the pans with the large catchment areas sustains these pans through the dry months and slows the progression of evaporation. It is clear that the factors affecting the hydrochemistry of the pan waters can not be simplified to a single process affecting a single dilute inflow of water to produce our final solution of evaporated pan water. Instead, water in the pans reflects a long-term evolution of solute species, with some memory effect remaining after each season of evaporation. The result is an accumulation of solutes as they are added continuously via dilute inflow and then removed from the waters at various times, particularly during dry periods when evaporite minerals are formed and transported out of the system.
- Full Text:
- Authors: Russell, Jennifer Lee
- Date: 2009-11-06
- Subjects: Pans (Geomorphology) , Water chemistry , Chemistry, Inorganic , Mpumalanga (South Africa)
- Type: Thesis
- Identifier: uj:8636 , http://hdl.handle.net/10210/2994
- Description: Master of Science , 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 of the 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. Results from the above analyses show that each pan in the MLD has a unique chemistry, which cannot be inferred from neighbouring pans. The inorganic chemistry differs from pan to pan as a result of these separate, closed systems being at different stages of the evaporation process. Throughout the path from groundwater to the pan, waters are subject to mineral dissolution and precipitation, adsorption and biological mechanisms, which continually add or remove solutes from solution. Although seemingly simple, there are certainly other factors that play a role in the evolution of the water chemistry. Key to the current inorganic chemistry is the balance between import and export of solutes. The groundwater, predominantly the perched aquifer water, brings solutes into the pans and blowouts of precipitates on the pan floor at the end of the dry season, when the wind is strongest, results in the export of solutes. This process is significant in maintaining the overall freshness of the pans in the MLD, contrasting to their western counter parts that evolve to highly saline saltpans. Other factors such as the periodicity of pans drying completely, the surface area to catchment area ratio (CA/SA), the formation and dissolution of efflorescent crusts and the presence or absence of reeds all have varying effects on the water chemistry of the lakes and pans. Significantly, the amount of evaporative concentration that a pan evolves through has been shown to be dependant on the CA/SA ratio with pans having larger ratios being lower in salinity compared to those with low ratios being the most saline pans. The reservoir available to the pans with the large catchment areas sustains these pans through the dry months and slows the progression of evaporation. It is clear that the factors affecting the hydrochemistry of the pan waters can not be simplified to a single process affecting a single dilute inflow of water to produce our final solution of evaporated pan water. Instead, water in the pans reflects a long-term evolution of solute species, with some memory effect remaining after each season of evaporation. The result is an accumulation of solutes as they are added continuously via dilute inflow and then removed from the waters at various times, particularly during dry periods when evaporite minerals are formed and transported out of the system.
- Full Text:
The inorganic chemistry and geochemical evolution of pans in the Mpumalanga Lakes District, South Africa
- Authors: Russell, Jennifer Lee
- Date: 2014-07-28
- Subjects: Pans (Geomorphology) - Research - South Africa - Mpumalanga , Water chemistry - Research - South Africa - Mpumalanga , Chemistry, Inorganic - Research - South Africa - Mpumalanga
- Type: Thesis
- Identifier: uj:11868 , http://hdl.handle.net/10210/11599
- Description: 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...
- Full Text:
- Authors: Russell, Jennifer Lee
- Date: 2014-07-28
- Subjects: Pans (Geomorphology) - Research - South Africa - Mpumalanga , Water chemistry - Research - South Africa - Mpumalanga , Chemistry, Inorganic - Research - South Africa - Mpumalanga
- Type: Thesis
- Identifier: uj:11868 , http://hdl.handle.net/10210/11599
- Description: 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...
- Full Text:
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