The Windhoek aquifer is located to the south of the city, extending northwards from the Auas Mountains for 20 to 25 km to the city center. For MAR to succeed, an aquifer is needed that allows easy access to a relatively large storage capacity.

Favorable Conditions in Hard-Rock Aquifer

Whereas the rock hosting the Windhoek Aquifer consists largely of quartzite with no primary porosity, secondary porosity is present in the aquifer for several reasons. The geological formations within the area were folded in the process of orogenesis and subjected to a number of episodes of faulting including thrusting and rifting. Quartzite and schist horizons with transverse faults and fractures are prevalent throughout the aquifer. The quartzite, being brittle, is highly fractured because of folding and faulting and has developed secondary porosity and permeability. The schist on the other hand is ductile and does not have well developed secondary permeability. Therefore, fractured quartzite has a larger storage capacity per unit volume compared to an equal volume of fractured schist (Murray & Redox, 2002; Redox et al., 2009).

The transmissivity values obtained from the highest-yielding boreholes range between from 100 to 1000 m²/d for the early-time fracture flow component of the constant discharge pumping tests; late-time transmissivities, which reflect the permeability of the micro-fracture network, range between 50 and 350 m²/d (Murray, 2002). A tracer test between two boreholes located approximately 800 m apart along a highly permeable fault zone established a surprisingly rapid flow velocity of 216 m/hour, suggesting low effective porosity. The storage coefficients reflect the predominantly confined nature of the aquifer: with the pure quartzites on the order of 0.009 to 0.010, micaceous quartzites ranging from 0.005 to 0.008 and schists with a value of about 0.001 (Murray, 2002).

Hydrogeologically, the aquifer can be divided into three main units of decreasing permeability: quartzite, micaceous quartzite, and schist. The dominant groundwater flow direction is northwards from the quartzite mountains south of the city towards the city which is underlain by schists (Figure 27). The flow follows preferential pathways along the numerous faults and fracture zones that transect the area. The aquifer is bounded by impermeable formations on all sides. (Tredoux et al., 2009c; Murray, 2017).

Figure 27 – Simplified geology of the Windhoek Aquifer (Murray, 2002).

Over-Abstraction from Aquifer Storage

Since the onset of large-scale abstraction from the Windhoek Aquifer in the 1950s, in particular during extended drought periods, water levels have dropped tens of meters. Even after five-year rest periods (e.g., from 1970 to 1975), water levels did not recover to their original levels (Figure 28). The aquifer had effectively been over-pumped or “mined”. For some periods of high abstraction, water levels would nearly recover to their pre-abstraction levels more than a decade after the event, but then a new period of increased demand would occur. In 2002, the volume of water that had been abstracted from storage since 1950 was estimated to be 28 Mm³. This available storage presented a major opportunity for artificial recharge (Murray et al., 2018).

Figure 28 – Production rates and water levels in the Windhoek Aquifer (Kirchner and van Wyk, 2001).