Hydrogeochemical assessment of the Mooiplaas dolomite quarry near Laudium in Gauteng, South Africa

Abstract

Karst aquifers within dolomite terrain in South Africa have been researched in the past for dewatering due to gold mining, sinkhole formation, and for its high value to supply sustainable, good quality groundwater. Karst aquifers are also known to be extremely vulnerable therefore, they should be protected and taken care of. Thus, understanding how anthropogenic activity influences the hydrogeochemistry of a karst aquifer is extremely important. Pretoria Portland Cement Mooiplaas (PPC Mooiplaas) quarry was chosen as the study area because the mining of dolomite, below the natural groundwater level, is a great example of anthropogenic activity within a karst aquifer. There are several factors that influence the movement of the groundwater within the study area such as dykes, faulting and the anthropogenic activity of mining activity itself; such as the plant and the Slimes Dams that produce the product. The main objectives of the study were to characterise the hydrogeochemistry of the PPC Mooiplaas and to compare the water quality of PPC Mooiplaas to that of it surrounding karst aquifer. To identify the movement of water within the study area by stable isotope analysis and to identify the source of water within the study area. Then lastly it is to understand how the anthropogenic activity impacts the karst aquifer This was carried out by sampling several sites of groundwater (West Pit Seep, Exploration Borehole North, Exploration Borehole South, West Pit Wall Seep, East Pit Wall Seep, Groundwater Flow 1 and Groundwater Flow 12); surface water (West Pit, East Pit and Fish Dam); mine water (Plant Inlet, Slurry Dam, Metallurgical Grade “Vergryser” Slurry Dam and Slimes Dam); and rainwater. The water samples that were collected at PPC Mooiplaas were tested for major inorganic chemistry and stable isotopes. Due to several factors that influence the water, it is possible to use water chemistry and stable isotopes to gain an understanding of the source of groundwater and the movement of water in the study area. It was found that the groundwater and the surface water at PPC Mooiplaas is mainly characterised by Ca2+-Mg2+-HCO3- type of water. The groundwater is mainly freshly recharged groundwater with lower average values of pH than the surface water. When the inorganic chemistry results of PPC Mooiplaas was compared to the surrounding karts aquifer chemistry it was seen that the mine has high amounts of NO3- in it. It was also seen that the Hennops River is contaminated by sewage due to the high concentrations of NH4+ and very small amounts of NO3-. This indicates that the NO3- contamination caused by PPC Mooiplaas has a very small impact on the river but it still has a substantial impact on the groundwater downgradient of the mine. The hydrogeochemical at PPC Mooiplaas is consistent with the understanding of what groundwater in a karst environment is. The study also showed that mining dolomite at PPC Mooiplaas does not have detrimental environmental effects besides elevated NO3-. In addition to assessing the hydrogeochemistry of the water samples collected at PPC Mooiplaas stable isotopes were used to identify the source and possible pathways of the water in the study area. The isotope data indicated that the WPS water is freshly recharged groundwater with the Expl BHs possibly being the source of water seeping at the WPS. The isotope data also showed that the wall leaks, WPWS and EPWS both have a component of SsD since both these sampling locations deviate from the LMWL. The major surface water bodies e.g. the WP, EP and the FD is groundwater with a slight evaporation signature. PPC Mooiplaas is a great example of the effects a dolomite mine has on a karst aquifer with the main impact being an increase in NO3- concentration. High amounts of NO3- can cause serious negative effects to human health if consumed therefore, it is recommended that the PPC Mooiplaas treats the water for NO3-. PPC Mooiplaas would be ideal to conduct further studies at like such as tracer test along the dykes and faults which will be valuable to understand movement of contamination in a karst aquifer. It is possible that the NO3- contamination is isolated within the compartment since these compartments are known to be barriers. It is recommended that NO3- is used as a tracer to investigate if water and its contaminants move across these boundaries. A hydrocensus was not included in this study. It is recommended the a thorough hydrocensus is conducted with the main objective to obtain groundwater level data in the surrounding area. This will enable a better understanding of the groundwater flow direction in the study area.