Hydrogeological investigation of the Rietvlei sandstone, Robertson, South Africa
The study site is located 15 km south west of Robertson and 10 km north west of McGregor. The Klipberg Mountain forms the southern boundary of the site and the Breede River, from which the bulk of irrigation water is currently sourced, makes up the northern border of the study site. Recent drought has necessitated a hydrogeological investigation to determine groundwater potential to augment the current supply, however little is known about the Rietvlei Formation in the area. The investigation comprised of a detailed desktop survey, making use of satellite imagery, geological maps, existing literature and hydrogeological maps. Areas of interest were selected for geophysical survey. This proved challenging due to rugged terrain typical of the Table Mountain Group (TMG), the presence of a high voltage power line over target areas, and the associated low conductivity of the quartzitic sandstone. A successful borehole sited on an electromagnetic survey did however provide the ideal geological setting for further sitings using lineament mapping and geological survey. Drilling followed by Pumping Tests of borehole with blow yields in excess of 15 000 L/hr followed allowing aquifer parameters to be determined. Radial acting flow proved to be the dominant flow regime of the Rietvlei Formation. An average transmissivity of 23.32 m2/day was estimated which matches existing literature, while the average storativity of 4.8 x 10-4 was slightly lower. The groundwater quality varies across the site with exceptional quality found within the only existing borehole, drilled into the Sewefontein Fault with an electrical conductivity (EC) of 13.8 mS/m. This borehole did not show connectivity to other boreholes during the Pumping Tests and comprised Na – HCO3– type water. The boreholes drilled into the Klipberg Mountain have electrical conductivities ranging from 22.9 mS/m to 207 ms/m and are Na – Cl type waters. A number of irrigation classifications deem the groundwater suitable for irrigation, while some boreholes are not suitable according to other classification methods. Regular sampling of both water and soil should be conducted to determine long term affect (if any). Fracture size increased with depth in the direction of the syncline axis. There is also an associated decrease in groundwater quality towards the axis of the syncline and away from the mountains where recharge occurs. Borehole siting in similar conditions where extensive folding and faulting have occurred should take this into consideration to improve probability of intersecting good quality groundwater.