Underlying the township of Kwanokathula is the Peninsula Formation, a quartzite unit of the Table Mountain Group (Figure 40) that occurs as a roughly east to west, one-to-two-kilometer-wide band.

Figure 40 – Jointed and fractured Peninsula Formation Quartzite at Plettenberg Bay (Murray 2007).

MAR Considerations in a Fractured Aquifer

Aquifers of the Table Mountain Group have been exploited for their high-quality groundwater for decades by farmers, smallholders and other private users. The Table Mountain Group consists mainly of quartz-arenites, with subordinate and often thin shales and siltstones. It was intensely deformed and thickened by the Permo-Triassic Cape Orogeny, leading to often overturned folds, and strong fracture cleavage. Primary porosity and permeability in the quartz-arenites is negligible, and both storage and transmission of groundwater is via fractures, fault planes and other secondary features (Pietersen and Parsons, 2001). Borehole yields in these rocks can exceed 30 L/s. Key issues relating to possible MAR in this aquifer include (Murray and Ravenscroft, 2010):

• Is the aquifer sufficiently permeable to accept recharged water?
• What is the storage capacity of the aquifer?
• Will the recharged water be recoverable? Or put another way, will the recharged water remain in storage until it is needed?

The pre-feasibility work provided some important insights into these questions. The injection test (Figure 41) indicated that the aquifer is highly permeable. It has a very high storage capacity and can easily accept injection water at a rate of > 10 L/s, with water flowing rapidly away from the point of injection. Initial groundwater level interpretation indicated that aquifer storage should not be operated above an elevation of 60 m in order to minimize the chance of water losses. This must be confirmed in a more detailed study of the flow system. Considering the aquifer dimensions and a conservative storage coefficient of 0.3 percent, it was estimated that, to achieve an artificial recharge target of 400 million liters, it would require about 30 m of vertical aquifer thickness as shown on Table 10 (Murray and Ravenscroft, 2010).

Figure 41 – Production borehole converted for conducting injection test (Murray and Ravenscroft, 2010).

The boundaries of the Kwanokathula Aquifer are the Cedarberg shales to the north and Cretaceous rocks including the Enon conglomerates to the south (including a possible fault). It is considered unbounded for practical purposes to the east (where it underlies the ocean) and to the west, where it continues for many kilometers. A north-south cross-section of the aquifer is shown in Figure 42.

Figure 42 – North-south hydrogeological cross-section of the Kwanokathula Aquifer (Murray, 2007).

Table 10 – Summary of Kwanokathula Aquifer properties (Murray, 2007).