The Deep Artesian Groundwater Exploration for Oudtshoorn Supply (DAGEOS) Project is culminating in development of the Blossoms Well-field (C1 Target Zone), about 20 km south of the town. The target Peninsula Aquifer is located at depths >300 m below ground level, geopressured to ~800 kPa (8 bar) artesian head. Each production well has to be uniquely designed for site-specific hydrogeological, hydrochemical and aquifer hydraulic conditions. Hydrostratigraphy rather than stratigraphy must inform the final well design. It is a recipe for unnecessary expense and deleterious consequences for aquifer management, to design and commence the drilling of wide-diameter production wells without the data and information provided by necessary exploration and essential pilot boreholes, yielding broader hydrogeological insights.
During discovery exploration at the C1b Target Site Area (TSA), drilling of a 715 m-deep diamond-core exploration borehole (C1b2) was essential for the proper siting and safe design of a production well (C1b3). Following confirmation of the artesian nature of the Peninsula Aquifer, the C1b2 borehole was equipped for monitoring, prior to the drilling of the nearby (~25 m distant) C1b3 production well, which was piloted with a core borehole down to a low level (~290 m) within the Goudini Aquitard, where it became marginally artesian and was then plugged and sealed. This pilot borehole was reamed with wide-diameter percussion tools to a depth where casing could be firmly cemented within the Goudini, above a solid, relatively unfractured zone. The final stage of drilling into the Peninsula Aquifer, using the Wassara water-hammer method, was thereafter continuously monitored from the C1b2 site, and the subsequent recovery history of C1b3 is comprehensively documented. The DAGEOS drilling and deep-groundwater monitoring provides significant experience in solving technological problems likely to be encountered in the future development of shale-gas in the main Karoo basin. The confined, artesian aquifer behaves very differently to other, conventional groundwater schemes and requires a different management approach that focuses on managing the artesian pressure within the basin and its response to abstraction.
The potential adverse influences of high and/or extended abstraction on the Peninsula Aquifer may be divided into two general categories: 1) depletion or degradation of the groundwater resource, and 2) environmental or ecological consequences. Depletion in the case of a confined aquifer refers to depletion in storage capacity due to non-elastic behaviour. Environmental/ecological impacts of groundwater extraction arise only when the ‘radius-of-influence’, defined by the distance from the centroid of a well-field to the perimeter of the cone of depression in the ‘potentiometric surface’ (surface of pressure potential in the aquifer), reaches recharge and or discharge boundaries. The new Oudtshoorn Groundwater Scheme affords an opportunity to stage a transition from an increasingly risky reliance on surface water that is prone to severe reduction through climate change, to a deep groundwater resource that is capable of acting as a sustainable buffer against water-scarcity through drought intervals that may endure over decades rather than years, and can be operated without electricity supply by utilising the artesian pressure in the aquifer. This approach was demonstrated in a 3- month artesian flow test during 2009.