Borehole injection is the preferred technique for applying managed aquifer recharge in Windhoek. In order to establish the feasibility of recharging this complex aquifer system, it was studied in detail to determine flow characteristics and boundaries. Four borehole injection tests were carried out in both the pure- and micaceous quartzites between 1997 and 1999. Despite the very different hydraulic characteristics between the two quartzite formations that are influenced by both preferential flow paths and barriers to flow, in total 0.5 Mm³ was successfully recharged (Murray et al., 2018).

The success of the injection tests showed that MAR had the potential to optimize the available resources in the area by injecting the excess water from the three surface reservoirs into the aquifer for use in times of short supply. The injection tests and the historic water level data showed that artificial recharge should be focused in the existing wellfield areas (the micaceous quartzites) and the main natural recharge and storage area (the pure quartzites). This would utilize as much of the aquifer’s available storage space as possible as shown on Figure 33 (Murray, 2017).

Figure 33 – Schematic of the main lithologies and injection areas (Murray, 2017).

MAR was developed in phases. Borehole injection started in 2006 and continued until 2012 when the targeted recharge area could not receive any more water. The first stage included six injection boreholes with a combined recharge capacity of 10,000 m³/day. The scheme’s success led to two expansion phases with a third planned for 2017.

The first expansion phase resulted in drilling an additional 10 recharge boreholes with a combined injection capacity of 16,000 m³/day, which is equivalent to 40 percent of the city’s annual water requirements, compared to the 10 percent achieved through the previous borehole abstraction scheme. To allow for abstraction of this water when required, 10 “deep” abstraction boreholes with a combined recovery capacity of 18,000 m³/day were also drilled. The second drilling phase will be completed in June 2017. Eleven of the planned 12 “deep” abstraction boreholes have been drilled and their abstraction capacities range up to 3,600 m³/day (Murray et al., 2018).

Benefits of Windhoek MAR Scheme

Windhoek’s city engineer outlined the expected benefits of the scheme in an interview in 2004 (van Rensburg, 2006):

• it will reduce the evaporation losses by banking water underground in years when there is surplus run off into the surface storage dams which would otherwise have evaporated, effectively increasing the yield of the reservoirs;
• it is estimated that with the use of deep wells (400 to 500 m depth) in the Windhoek aquifer there will be a “bank” of at least 100 Mm³ available for abstraction in years of water shortage that can be replenished again in years of abundance; and,
• having a large bank of water available at the point of consumption makes it possible to meet peak demand from the aquifer so the large-capacity, bulk-supply schemes of NamWater, which traverse hundreds of kilometers, will not have to be designed to supply the peak demand, but only the average demand, effecting a substantial cost saving.

Planning and implementation are continuing. The best estimate of the potential water bank is 90 Mm³ or about three times the current annual water use. At this stage the existing boreholes cannot access all this water and new deep boreholes are being drilled. The following goals were set:

• increase recharge to 12 Mm³/year by 2019;
• equip boreholes for drought abstraction of 19 Mm³/year from 2019; and,
• increase the storage capacity of the water bank from 41 Mm³ to 71 Mm³/year after completion of boreholes and infrastructure from 2018 to 2019 (Murray, 2017).