While the final water quality from the Von Bach Dam Water Treatment Plant has low salinity (as indicated by electrical conductivity) and inorganic constituents, the dissolved organic carbon concentration is very high. Thus, further treatment is essential. Surface/dam water is blended with reclaimed water at a ratio of 3:1 and is further treated by granular activated carbon filtration and chlorination (Figure 32) to reduce the dissolved organic carbon concentration and minimize bacterial growth potential in the aquifer (Tredoux et al., 2009c; Murray et al., 2018).

Figure 32 – Final treatment of the source water (Murray, 2017).

Guiding Principles for Quality of Water Directly Injected into Aquifer

As the recharge water is injected directly into the aquifer without seeping through the unsaturated zone, it was concluded that the injection water quality must conform to drinking water requirements. The following guiding principles for injecting recharge were set (Tredoux et al., 2009):

• no significant negative environmental impact;
• sustainable use of water from the Windhoek Aquifer for drinking water purposes, preferably without treatment or at most with limited treatment such as stabilization and disinfection;
• the recharge water should meet modern drinking water standards;
• no additional health risk for the residents of Windhoek as compared to sources used in 2004;
• no significant technical problems should arise due to injection water quality such as clogging, corrosion and/or demand for extensive treatment before distribution; and,
• accept a deterioration of certain quality parameters of the water within the aquifer, provided that the water quality after abstraction complies with acceptable water quality guidelines.

Aquifer water quality aspects that had a bearing on the decision regarding pre-treatment of the injected water, are briefly touched on below (Tredoux et al., 2009).

• Salinity: Low salinity water occurs over most of the aquifer. Groundwater in the Auas Formation quartzites generally has very low chloride concentrations from 4 to 10 mg/L, increasing to a maximum of ~60 mg/L in the deep circulating hot water issuing forth in the city center, reflecting the long residence time at depth.
• Sulfate: Sulfate occurs in high concentrations throughout the aquifer due to iron sulfide in the form of pyrite in the host rocks of the Windhoek Aquifer. Dissolved oxygen entering the aquifer with the rainwater during natural recharge oxidizes the sulfide to sulfate. In this way, sulfate is generated in the aquifer until the dissolved oxygen is exhausted. Under natural conditions, the groundwater easily attains sulfate concentrations of 200 to 300 mg/L. In areas where the soil has been disturbed, as in residential areas, the sulfate concentration increases as more of the pyrite is exposed to oxygen entering the aquifer with the rainwater.
• Iron and manganese: Iron is ubiquitous in the groundwater in the Windhoek Aquifer. Similarly, manganese is present in the aquifer, which is also mobilized by changes in the redox potential in the aquifer. As a result, borehole clogging is a distinct possibility. Monitoring of the various iron species will be important in order to gain a better understanding of clogging potential.
• Arsenic: Fluid-rock interactions in some MAR systems have released arsenic and metals into recharged waters causing an unacceptable deterioration in water quality. The arsenic concentrations in the Windhoek Aquifer are generally very low and close to the detection limit of 0.005 mg/L. The highest concentration of 0.013 mg/L occurs in groundwater associated with mineralized faults in the schists in the northernmost part of the aquifer in a zone of significantly higher temperatures. The water bank is located away from these low-permeability, schistose areas with the slightly elevated arsenic concentrations. Hence it is considered highly unlikely that the arsenic guideline levels will be exceeded because of the MAR operations, and ongoing monitoring confirms this.
• Water temperatures: The concern around the potential for clogging due to water temperature differentials between the injected and the aquifer water is unlikely to be a significant problem. Injection boreholes are all located in the pure and micaceous quartzites where temperatures range between 25 and 30 °C, similar to the temperatures of the injectant (Murray et al., 2018).
• Dissolved organic carbon: The concentration of dissolved organic carbon (DOC) in the aquifer is very low, with values generally being < 1 mg/L, except in polluted areas such the Kupferberg waste disposal site and shallow boreholes in the schist (Murray, 2002).

Special Water Quality Concerns Associated with the Borehole Injection

From the available data, it would seem that changes in the oxidation-reduction potential in the subsurface might be the main factor affecting the hydrochemical environment of the Windhoek Aquifer. Major changes in salinity and sulfate were associated with pilot scale injection tests, but such changes are also due to variations in the abstraction regime due to water level fluctuations in the aquifer. The intensity of the effects will depend on the recharge technique, method, and abstraction regime. Injection through a deep borehole at depth will be the preferred technique as cascading of the water through the unsaturated zone will exacerbate any adverse reaction, such as the oxidation of pyrite (Tredoux et al., 2009).

Managing injection water quality according to the set criteria should maintain the high quality of the water stored in the aquifer. To date, the recovered water has had an average salinity of 91 mS/m or 610 mg/L TDS and an average dissolved organic carbon of 1.1 mg/L (Murray, 2017). Mass transport modeling is seen as essential for determining the longer-term impact of the injected water on aquifer water quality (Tredoux et al., 2009).

Special attention will need to be given to protection of the aquifer against all forms of pollution. The threat is illustrated through the Kupferberg waste disposal site in the south-western part of the aquifer. This area has elevated concentrations of chloride, sulfate, organic compounds and iron. Once artificial recharge is fully operational, the water level will rise and the need for protection will further increase (Tredoux et al., 2009, Mapani, 2005).