Hydrogeological exploration and characterisation of the aquifers found in Middelburg, Eastern Cape, for town water supply

[摘要] English: The Karoo town of Middelburg in the Eastern Cape Province is solely dependent on groundwater for water supply, as no large surface water bodies are present in the town's vicinity. Previously (1987) water supply from existing municipal production boreholes was adequate to supply the town, but town growth has now resulted in a water supply shortage. The shallow aquifer with alluvial character currently used as sole source is under severe stress and water levels are declining steadily. Piping water from the Orange-Fish River scheme was considered, but this option was shown to be very expensive (±R180 million). Instead, regional and deep groundwater potential was investigated. Deep exploration borehole drilling to around 300 mbgl was used as discussed in Woodford et al. (2002).A detailed GRIP hydrocensus and groundwater sampling was conducted across the study area. A total of 1695 geosites were surveyed. Of 414 boreholes sampled, the water quality of 16% were good, 45% were marginal, 30% were poor and 9% were dangerous. Six sub-catchments were delineated based on hydrogeological principles and groundwater potential. For each sub-catchment a minimum groundwater balance was calculated, based on a 95% assurance level. The Glen, Dunblane, Karmel and Grootfontein sub-catchments were targeted for groundwater exploration based on their favourable groundwater balances.Geophysical surveys comprised magnetic, electromagnetic and electrical resistivity profiling to determine the orientation of dolerite intrusions. Drilling targets were mainly deep fracturing associated with dolerite ring structures and dyke intrusions. The 18 new boreholes drilled using rotary air-percussion and water-hammer drilling techniques delivered an accumulated airlift yield in excess of 187 �?s with the deepest strike at 236 mbgl. Aquifer tests were conducted and aquifer parameters were calculated using the constant discharge test data and applying the Cooper-Jacob-, FC- and derivative-equations. A total of 26 pumping tests were performed on selected new and existing boreholes. An accumulated sustainable yield was calculated to be approximately 80 �?s.Detail analytical water balances were calculated for the sub-catchments to estimate the amount of groundwater available in each one. Different recharge and abstraction methods were used and compared through analytical balance scenarios. A higher confidence conservative scenario using the saturated volume fluctuation recharge method and a water level-transmissivity based abstraction estimate per sub-catchment showed abstraction:recharge ratios of: The Glen, 15% (33 �?s); Karmel, 32% (20 �?s); Middelburg Municipal, 447% (-100 �?s); Dunblane, 93% (4 �?s); Lusernvlei, 104% (-1 �?s) and Grootfontein compartment, 94% (2 �?s). The chloride mass balance recharge provided even higher values of recharge in the order of 6.7% of MAP for mountainous sub-catchments like The Glen and Karmel, which is in line with Vandoolaeghe's (1979) mountainous catchment recharge.Numerical modelling was performed to determine whether the estimated recharge-based volumes available and abstraction thereof is feasible given hydraulic parameters. A good (R2 = 0.94) snapshot data calibration was obtained, but history matching (transient calibration) was required to get correlation in hydraulic head trends using time series municipal abstraction. The Present day numerical scenario even with inclusion of the new production boreholes in The Glen showed more favourable balances that the analytical balances: The Glen, 41% (40 �?s); Karmel, 17% (45 �?s); Middelburg Municipal, 184% (-47 �?s); Dunblane, 65% (29 �?s); Lusernvlei, 104% (-1 �?s) and Grootfontein compartment, 48% (36 �?s).The 2 �?5 years of water level information from the new monitoring network developed are also not as negative as expected, however longer groundwater level fluctuation data is required to make conclusive statements. There was also a drought from 1997 �?2002/2003.Current municipal abstraction is sustainable, but should be more spatially distributed. The shallow Middelburg aquifer has excellent hydrogeological properties, except for aquifer thickness. Production boreholes were historically developed in too close proximity to each other, resulting in the local aquifer being stressed beyond its capacity. The key during new groundwater resource development is a wider borehole distribution and monitoring network and the exploration and development of deeper boreholes associated with fracturing within and adjacent to dolerite ring structures. Results predict a connection between the shallow and deep aquifers. The optimised use of shallow aquifers, combined with monitoring and the use of newly developed deep regional boreholes will ensure a more sustainable groundwater supply to Middelburg. Opportunities for artificial recharge are also seen in the Middelburg study area.

[发布机构] University of the Free State