Groundwater resource exploration & development – Focus on groundwater to support surface water supply in the Lower Olifants River, South Africa

Elsevier, Journal of African Earth Sciences, Volume 180, August 2021
van Rooyen J.M., Veltman S., Botha F., Matthee J.

Many South-African towns in the Western Cape Province are dependent on surface water resources to satisfy the ever-increasing water demand. In recent years, the effects of climate change on the environment has challenged these areas continually and recently led to calls for major changes in water management. The Lower Olifants River Water Management Area experienced an extreme drought that persisted from 2016 to 2018, which led to the need for development of groundwater resources. The local economy of the Lower Olifants River is dependent on water releases from the Clanwilliam and Bulshoek Dams, located within the Olifants River Syncline (ORS), where ‘megafault’ zones form preferred flowpaths for groundwater in the Table Mountain Group (TMG) aquifers. The Lower Olifants River Water User Association (LORWUA) was mandated to proceed with the exploration and development of these aquifers to supplement the existing water use of the LORWUA. This will be available during periods of less rainfall and therefore increase the security of the water supplied. The groundwater project aimed at exploring and developing the fractured rock aquifers of the Peninsula (Op) and Skurweberg (Ss) Formations contained within the TMG. Through a high-level desktop assessment, remote sensing investigations and geophysical ground surveys, several target areas and drilling priorities were identified to obtain high borehole yields that access the resource in proximity to existing surface infrastructure. The projected water need from the groundwater resource was 8.6 million cubic metres per annum (Mm3/a). Faults/structure zones were the main hydrogeological targets, with drill target depths of structures planned for intersection deeper than 100 m below ground level (mbgl). Down-hole camera work and initial aquifer testing concluded that deep geological contact zones are prone to collapse when flow is induced in water-bearing fractures. This indicated the need for production borehole construction, even prior to aquifer testing for yield estimates. A major concern still to be resolved is the availability of water in storage and the realistic expectations of the well field over time. Although individual borehole testing provided information on the properties of specific water-bearing structures, long-term monitoring will provide a better understanding of the aquifer in response to abstraction and recovery in times of rest. The operating rules for the conjunctive use of the well field developed, with the existing surface water scheme, is still to be defined and modelled for optimisation, pending long-term monitoring.