3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
4 results
Search Results
Item Holistic approach to groundwater recharge assessment in the Upper Crocodile River Basin, Johannesburg, South Africa(2019-07) Leketa, Khahliso CliffordIn the highly urbanised and water stressed Upper Crocodile River Basin (UCRB), Johannesburg, South Africa, which has wastewater induced surface water resources and a complex land use and geological setting, a holistic assessment of groundwater recharge was undertaken to understand the groundwater provenance and to address the prevailing water security issues. This work emanated from a project entitled, “Understanding Groundwater Recharge in the Limpopo River Basin (GRECHLIM)”. Samples from Johannesburg rainfall, surface water and groundwater were collected and analysed for δ18O, δ2H and 3H, while selected groundwater sources were additionally sampled and analysed for δ13C and 14C. Albert Farm spring was monitored monthly for stable isotopes and yield over a period of 14 months. The Thornthwaite Monthly Water Balance Model (TMWBM), Stream Segment Water Budget, Reservoir Water Budget, Baseflow Separation and Water Table Fluctuation methods were applied to assess groundwater recharge, while Precipitation Runoff Modelling System (PRMS) was applied to assess the impacts of changes in climate variables on the hydrology of the UCRB. The temperature and amount effects were observed in rainfall and the Johannesburg Local Meteoric Water Line (JLMWL) was constructed as ��2��=6.7��18��+10‰, while the temperature-δ18O regression line was ���� 18=0.552��−14.1‰. A high stable isotope variability was observed in the Johannesburg rainfall with the most depleted sample having δ2H and δ18O values of -131.6‰ and -17.82‰, respectively, and the most enriched rainfall having 51.4‰ and 7.03‰, respectively. A high variability was also observed in the monthly stable isotope samples of the Albert Farm spring with the signature ranging from -6.01‰ to -2.28‰ for δ18O and from -21.6‰ to -10.48‰ for δ2H with d-excess ranging from 3.23‰ to 18.7‰ for the review period. The majority of the groundwater samples across the UCRB plot close to the JLMWL and the most depleted samples were those from the dolomitic aquifer, while the most enriched were the boreholes positioned downstream of the Hartbeespoort Dam. The samples from the dolomite aquifer were enriched in stable carbon isotope (δ13C), while those from the granites, shales and quartzites were depleted, with intermediate values observed at the margins of granites and dolomites. The 14C results indicated that groundwater in the UCRB has an MRT between 0 (present) and 3856 years, deducing high aquifer heterogeneity. The estimated daily air temperature at the time of recharge is similar to the current daily air temperature during rainfall, while the estimated annual air temperatures are lower than the present annual temperatures. An average potential recharge estimate of 4.3% was obtained in the UCRB using the TMWBM with low amounts estimated in the warmer low elevation northern part of the UCRB and higher amount in the cooler elevated southern portion. This indicated the influence of temperature on recharge. The Stream Segment and Reservoir Water Budget methods indicated spatial and temporal net gain and net loss across the UCRB and the Reservoir Water Budget indicated that annually, the Hartbeespoort Dam experiences a net loss of 7% of its inflow, which is referred to as focused recharge amounting to 2 084 131 m3. The focused recharge is deduced to occur through the Brits fault lines and the bedding planes in the Magaliesberg quartzite at the rate of 202 m/year deduced from tritium analysis. The application of the Baseflow Separation method in different quaternary catchments of the UCRB gave an area weighted average recharge estimate of 6.7% for the UCRB, while the Water Table Fluctuation method gave an average recharge estimate of 11.4% in the dolomitic aquifers. The hydrograph and stable isotope analysis of the piezometers indicated the influence of interflow contribution, deducing that the baseflow estimate in the basement granites is inclusive of interflow. It is concluded that change in seasons and moisture sources are the causes of the high variability in rainfall isotopes. The high temporal variability in stable isotopes and d-excess values from the same groundwater source further confirms that the UCRB receives rainfall from different moisture sources. This indicates that in regions that receive rainfall of different isotopic signature, the use of a single sample to characterise recharge may not be a good practice as this may erroneously yield biased interpretations. Effective recharge in the UCRB predominantly occurs through diffuse mechanisms, while focused recharge is mainly observed near the surface water bodies. The depleted nature (in δ18O and δ2H) of groundwater samples in the dolomitic aquifers is deduced to be a result of altitude effect or possibilities of an exchange of oxygen between the dissolved CO2 and groundwater. However, the possibilities of oxygen exchange have been recommended for further investigation. The stable carbon isotopes (δ13C) indicated closed system carbon evolution in the dolomitic aquifer and open system in the granites, shales and quartzites. From the signature of stable isotope of carbon, oxygen and hydrogen, it is further concluded that there is mixing of the two water types at the periphery of the dolomitic and granitic aquifers. Combined interpretation of 14C age and the estimated annual air temperatures indicates that recharge of older groundwater occurred at lower temperatures, while recharge of more recent groundwater occurred at warmer temperatures showing evidence of increase in annual air temperatures with time. The observed similarity between the calculated and the current daily temperatures on one side, and the observed variability between the calculated and current annual air temperatures on another side, could be related to the fact that regardless of the changes in global temperatures, the physical in-cloud condensation conditions, which dictate the isotopic signature in daily rainfall are not changing. In general, the study indicates that the stable isotope effects in rainfall can successfully be used to assess the air temperature on the day of the rainfall that generated recharge and to determine the dependency of recharge amount on rainfall amount if the aquifer is recharged by rainfall without undergoing extreme evaporation prior to recharge. The climate simulations indicate that as baseflow decreases due to changes in climate variables and wastewater increases due to growth in urbanisation, the amount of wastewater shall dominate baseflow in the UCRB leading to more polluted surface water. Therefore, better treatment and constraints on pollution are recommended for the future.Item Hydrochemical and environmental isotope based investigation of the Masama Ntane Sandstone Aquifer, Botswana(2017) Mofokeng, ThelmaThe Masama Sandstone Aquifer is located in a semi-arid region of south-eastern Botswana where there are no perennial rivers. Groundwater is the main source of water supply for the communities. Historically many water drilling programs have been carried out in this area and the hydrogeological system has been conceptualized. An integrated approach coupling environmental isotopes, radioisotopes and multivariate statistical analysis of the hydrochemical variables was employed to study the origin, age, recharge conditions, rock-water interaction and the hydrological link between the aquifer and geological structures. The major ions in this area are Na+, Ca2+, Mg2+ and HCO-3. Groundwater in the Masama area fall in the transition from a Na-HCO-3 –type through Ca-Na-HCO-3 to Ca-Mg-HCO-3 -type waters from the western to the eastern part of the area. The water types are as a result of cation exchange, carbonate dissolution and rock-weathering processes. The δ18O and δ2H values vary spatially depending on the source of moisture, rainfall season, geology, topography and groundwater circulation depth. Deep circulating groundwaters are isotopically depleted whilst shallow circulating groundwaters are isotopically enriched with respect to winter rain. Low tritium values < 0.8TU and 14C values < 80pmc testifies for recharge. Recent rainfall amount in the area is not sufficient enough to make a profound replenishment in the aquifer. Tritium, 14C and Chloride Mass Balance helped in identifying recharge location and hydrologic connections between structures and the sandstone aquifer elucidating that recharge zones are in the NE and NW of the study area. High recharge rates occur in the north-eastern part and the Makhujwane fault act as a conduit for groundwater recharge. This study provides a better understanding of the aquifer and the information contained herein can be incorporated into future works for sustainable use of the groundwater resource.Item Application of fate and transport models to evaluate the efficiency of a Cr(VI) remediation pump and treat system(2016) Nkosi, Sifiso CollenGroundwater treatment by chemical precipitation is a popular form of remediation at mines that are in operation. The water quality status at the implementation of the PAT in this study was compared to the water quality status after a six-month period of active remediation. Chromium is very important as an industrial metal owing to its numerous uses in a variety of industries. The objective of the remedial action is to intercept the Cr(VI) plume, abstract contaminated groundwater and chemically treat it on the surface. The long-term (15-year) objective is to eventually reduce Cr(VI) concentrations in the aquifer(s) to below 0.05 mg/ℓ. The PAT system was implemented as a mediumterm (5-year) strategy to intercept the Cr(VI) contamination plume during migration to prevent it from negatively impacting on groundwater users downstream of the mine. In the vicinity of the three PAT systems’ abstracting wells, water levels declined by an average of 2 m compared to the same period in 2014. Periodical fluctuations in the fractured aquifer are reflective of the influence of fractures on groundwater flow. In the aquifer, hydrochemical signatures show evidence of mixing between the primary and secondary aquifers. The treatment system has been successful in reducing Cr(VI) to Cr(III) and precipitating Cr(OH)3. The treatment system was designed to treat Cr(VI), other elevated constituents and generally high dissolved ions are not treated in this remedial process. Sulphate concentrations increase after treatment as a result of the addition of Fe(II)SO4 for chromate contamination treatment purposes. The simulated reaction path shows that the transformation of CrO4 2- to Cr2O3 in the treatment system is not immediate. The Cr(VI) to Cr(III) transformation is irreversible, this is beneficial as the water is abstracted from more reducing conditions, and the treatment ponds are open to the atmosphere thus the conditions following dosing with Fe(II)SO4 are oxic and chromate complexes are stable over a wider range of Eh-pH conditions than Cr(III) compounds. This ensures that the efficiency of the dosing system is not reversed in Settling Pond B. The modelled flow paths are similar to the inferred flow vectors in the plume capture zone. Fracture flow is the dominant type of flow, the fault zones and dykes create high permeability conduits to flow. Flow paths are parallel to fault lines or the lateral dimension of dykes; flow occurs along fractures and deformation zones. The reduction of Cr(VI) concentrations in some of the peripheral sampling points indicates that the PAT system has been successful in capturing the chromate contaminated water through pumping. Keywords: Hexavalent chromium, groundwater pollution, remediation, pump-and-treat, fractured aquifersItem Investigation of the impact of recharge water with respect to quality into the Khutala Colliery Rehabilitated block I opencast operation(2012-01-19) Repinga, Mandla RalphABSTRACT This study was an investigation of the impact of recharge water with respect to quality and quantity into the Rehabilitated Block I opencast operation, a former opencast coal mine in Mpumalanga, Witbank Coalfields western complex. The rehabilitated areas consisted of three mined mini-pit areas known as Block I, Block I Extension A and B. The area has been rehabilitated by backfilling and leveling of spoil material, subsoil material, placement of approximately 400-mm topsoil layer and grassing. As part of the vegetation maintenance lime is added per annum, in an effort to neutralize the soil cover and further assist in neutralizing the potential acid mine drainage. Additional monitoring boreholes were drilled to increase the monitoring of the water quantity and qualities. Ground and surface water samples were taken, analysed for pH, conductivity, redox potential, sulphates, carbonates and trace metals. The pH of the ground and surface water ranged from moderately acidic to alkaline. One of the monitoring boreholes located on the lowest elevation of the Block I area was observed to be filled up to the collar level of the borehole with water samples showing elevated Fe and Mn concentrations of 216 and 46.2 mg l-1 respectively. The water classification revealed the following facies: Ca-Mg sulphate type for the borehole water and Ca-Mg sulphate-bicarbonate type for the surface waters. Acid base accounting studies on the soil samples showed a negative net neutralising potential of up to -9.8 kg t-1 CaCO3 which indicated the potential of acid mine drainage in the area. The total metal analyses showed that the area was contaminated with heavy metals such as Fe, Cr, Mn, Ni and Zn and the metalloid As was also detected. The highest recorded concentrations of total metals were 78 252; 2 402; 1 959; 1 360 and 15 109 mg kg-1 respectively. The highest concentration of Arsenic was detected at 824 mg kg-1 respectively. The transmissivity of the boreholes in the spoil material was highly variable and ranges from 100 to 5 000 m2 day-1. Pump testing suggests that borehole yields of between 23 and 4 l s-1 can be expected in the spoil areas. The specific yield or the drainage porosity of the spoil material was in the range of 25 to 30 %.