Investigation of sustainable groundwater resource supply in the crystalline basement aquifers at Relela Village, Greater-Tzaneen Municipality, Limpopo province, South Africa.

Abstract

South Africa is one of the countries in the world having challenges on the untreated surface and groundwater. In arid and semi-arid areas groundwater is the main source of supply, these areas consist of crystalline basement aquifers which store water either in rock fractures, geological structures (i.e dykes), and contact zones. The water supply crisis is caused by the combination of low rainfall, high evaporation rates, and population growth whose geographical demand for water supply cannot conform to the distribution of exploitable water. The current study aims to evaluate groundwater resource supply in the crystalline basement aquifers at Relela villages, in South Africa, Limpopo province. Based on the literature review, two geophysical techniques were used for groundwater exploration. The proton precession G856 magnetometer was used in magnetic data collection which covered six profiles in four villages. PQWTC-300 (resistivity) was used to study the resistivity of the subsurface lithologies. The traverse lines were set perpendicular to the geological structures trending in the NW-SE direction. The magnetic results presented in x-y graphs showed the variation in magnetic intensities, with the maximum magnetic intensity of 1431 nT and the minimum magnetic intensity of -3561 nT. High magnetic intensity indicated the presence of subsurface geological structures and the low magnetic intensity indicated the presence of weathered subsurface lithologies. The resistivity results indicated low, medium to high (1.2-8.6 ohms) resistivity values throughout the study area. Low resistivity values indicated the presence of weathered subsurface lithologies whereas high resistivity values indicated solid/hard subsurface lithologies. The drilling targets were selected based on the magnetic and resistivity results gathered during groundwater exploration. A drilling machine was used to exploit groundwater resources, several lithologies which were encountered during drilling are; clay soil, dolerite dykes, gneiss, and granites. The blow yield values ranged from 0.1-2.0 l/s. After the completion of the drilling pump testing was conducted to determine the aquifer parameters which had safe yields ranging from 0.01 l/s – 0.5 l/s; transmissivity ranging from 1 m2/day-2.9 m2/day and the specific capacity ranging from 0.01 l/s per m- 0.15 l/s per m. Low transmissivity values indicated low-yielding borehole whereas high transmissivity values indicated high-yielding borehole. Water samples were collected and sent to the laboratory for chemical analysis. Hydrochemical processes were also evaluated using the Durov diagram. The laboratory results suggested class 0 (good water quality) in borehole H07-2016 and H07-1685, class 1 (good water quality) in borehole H07-2023, class 2 (marginal water quality) in borehole H07-2015 and H07-2022, and class 3 (poor water quality) in borehole H07-2014. The pH values were in a range of 6.8 to 7.5, the electric conductivity of 30.2 – 332 μS/cm, Total alkalinity of 88.8 to 690.4 mg/l, and Total hardness of 94.48-776.79 mg/l. All metals (in boreholes H07-2015, 2022, 2016, 1685 and 2023) including As, Ca, Mg, Mn, K, Na, and Zn, had concentrations in a range of 0 mg/l -0.03 mg/l, 24.36 mg/l -155.90 mg/l, 8.19 mg/l – 94.40 mg/l, 0.01 mg/l – 0.76 mg/l, 0.71 mg/l – 4.80 mg/l, 21.75 mg/l – 418.45 mg/l, and 0.29 mg/l – 1.06 mg/l, respectively. All constituents were within the South African National Standards 241 (SANS): 2011 except borehole H07-2014 with a sodium concentration of 418.45 mg/l. The Durov specified the dominance of dynamic water within the study area, this can be attributed to the fresh recent recharge water exhibiting Mg and HCO3 dominance. In conclusion, all recently drilled boreholes, as well as the existing boreholes, can supply groundwater that meets the water needs of the current population (of about 6 500 people). It was estimated that all boreholes can supply about 325 000 l/day.