Investigating the possibility of using groundwater associated with dolerite structures to augment the municipal water supply to the city of Bloemfontein: investigations in the central business district

[摘要] This study investigates the potential of using groundwater resources associated with an intrusivedolerite ring-dyke underlying the central business district (CBD) of Bloemfontein to augment thecurrent water supply to the Mangaung Metro Municipality (MMM). The current water supply to themunicipality is wholly dependent on surface water sources, which have proved to be unreliable andinsufficient to meet the increasing water demand in the municipality.During the investigations, ground geophysical methods were used to detect and delineate the ringdykein areas within the CBD where the surface infrastructure allowed the geophysical surveys. Themagnetic and electrical resistivity (ERT) methods were used during the surveys. The magneticmethod is sensitive to both the presence of metallic infrastructure at surface or in the shallowsubsurface and to the presence of electromagnetic noise generated where electrical currents flow.Since urban and industrial environments are characterised by such sources of noise, the magneticsurvey was severely restricted in terms of the locations available for the recording of reliable data.The ERT survey, in turn, was severely restricted in terms of the space available within the CBD, aswell as the presence of hard surface covering (roads, pavements, concrete slabs) prohibiting theinstallation of electrodes into the ground. Despite these limitations, the geophysical surveys weresuccessful in detecting the presence of the ring-dyke at certain positions within the CBD.Based on the results of the geophysical investigations, positions for the drilling of investigative andproduction boreholes were proposed. The drilling of these boreholes was to form part of the currentinvestigations, but due to factors beyond the control of the researcher, these boreholes are yet to bedrilled. These boreholes would have allowed the researcher access to the aquifers system associatedwith the ring-dyke in order to perform hydraulic tests and assess the groundwater quality. The factthat these boreholes were not drilled in time should be seen as a significant limitation of the currentstudy.A limited hydrocensus was conducted in the vicinity of the ring-dyke. The purpose of the hydrocensuswas to locate points of groundwater abstraction near the ring-dyke in order to obtain information onthe use and quality of the groundwater, as well as to investigate the aquifer system(s) hosting thegroundwater. During the hydrocensus, several boreholes were located within 300 m from the ringdyke.Of these boreholes, access to only three could be obtained as the owner of the properties onwhich the remaining boreholes were located would not allow access to these boreholes. Thehydrocensus revealed that the boreholes near the ring-dyke are currently mostly used for irrigation.No hydraulic tests could be performed on the boreholes identified during the hydrocensus due to thepresence of infrastructure. Hydraulic tests were, however, performed on a single borehole located on the premises of the Central University of Technology (CUT). Analyses of the results of the hydraulictests indicated transmissivity values in the order of hundreds of metres squared per day, indicatingthat the aquifer system associated with the ring-dyke can be expected to be high-yielding.Hydrochemical analyses of the groundwater samples collected from the boreholes identified duringthe hydrocensus showed that the groundwater quality ranges from good to ideal. No clear evidencefor contamination was visible in the results of the inorganic analyses. The good quality of thegroundwater suggests that it can be incorporated into the municipal water supply without requiringtoo much treatment. However, the investigations into the groundwater quality did not considerhydrocarbon or bacteriological contamination of the groundwater. No conclusions can therefore bedrawn on the groundwater quality in terms of possible organic and bacteriological contaminants.The results of the investigations indicate that that groundwater associated with the ring-dyke couldsuccessfully be used to augment the municipal water supply. Depending on the quality of thegroundwater, it may find different applications in the municipality, including: domestic water,irrigation, and industrial use. However, each of these applications has its own challenges in terms ofmonitoring, aquifer management, water treatment and infrastructural requirements. To ensure the safeand sustainable use of the groundwater resource, a groundwater monitoring and managementprogramme should be implemented. Such a programme will aim to ensure that the volumes ofgroundwater abstracted do not exceed the long-term capacity of the aquifer system to deliver water,while ensuring that the quality of the water delivered to the municipal supply system is of a suitablequality for the its intended purpose. Routine monitoring of the water quality should be done to detectpossible contamination with organic compounds and bacteria, as well as industry-specificcontaminants.

[发布机构] University of the Free State