Declaration 1 Plagiarism
3.3. Examples of managed aquifer recharge schemes
3.3.2 Southern African experience with Managed Aquifer Recharge
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Figure 3.9. Southern Africa's artificial recharge sites (DWAF, 2007) Windhoek (Namibia)
The City of Windhoek decided on large-scale artificial recharge before introducing other supply options, such as transferring water from aquifers in the northern parts of the country to the Okavango River (DWAF, 2007). Windhoek water requirement is at 21 million m3/a. Most of this water comes from three dams, while others are sourced from a quartzite aquifer and reclaimed water (fully treated recycled water) (Tredoux et al., 2012). A major achievement for the city is a reduction in cost through the use of artificial recharge to increase assurance of water supply. Surface water transfer from the Okavango River was estimated at R1.79 billion in comparison to the R242.5 million required for the artificial recharge scheme (Tredoux et al., 2012). The artificial recharge takes the form of water banking, where surface water is “banked”
in the aquifer as security against drought. This allows the dams to be used at greater risk levels, where security lies in sub-surface storage and evaporation and aquifer loss are negligible (DWAF, 2007). The main aim of the Windhoek scheme is establishing an aquifer that can meet the entire city’s demand when full, with the ability to rapidly and fully recharge thereafter (Tredoux et al., 2012).
39 Atlantis (Western Cape)
Atlantis is located along the semi-arid to arid west coast of South Africa. According to DWA (2010b), Atlantis provides an example of wise water usage. The town has a population of 67 491 people (2011 census).The Atlantis Water Resource Management Scheme (AWRMS) was designed to optimise the use of water in the town of Atlantis, situated along the arid west coast of southern Africa (DWAF, 2007). It was initially fully dependent on groundwater; however, the reserves were insufficient, and artificial recharge was introduced to augment local groundwater supplies (Tredoux et al., 2002). Treated wastewater and storm water is diverted to large basins where it infiltrates into a sandy aquifer where it is abstracted and reused for municipal supplies (DWA, 2010b).
The primary coastal aquifer system in the Atlantis area is comprised of unconsolidated sediments that are Tertiary to recent in age. These sediments overly Malmesbury Group bedrock composed of greywacke and phyllitic shale (Van der Merwe, 1983). Natural recharge is estimated to be in the order of 15-30% of the annual rainfall (450 mm) (Tredoux et al., 2002).
The Atlantis scheme has successfully recharged and recycled water for nearly three decades.
Approximately 7500 m3/d of storm water and wastewater is currently recharged, thereby augmenting the water supply by more than 2.7 million m3/a (DWA, 2010b). Moreover, 30%
of Atlantis’s groundwater supply is augmented through artificial recharge.
Managing water quality, especially salinity, has been one of the greatest challenges for the Atlantis Water Scheme (Tredoux et al., 2002). The recent importation of limited quantities of surface water beyond the catchment is an important additional source of low salinity fresh water entering the system (Tredoux et al., 2002). A decline in the yield of the boreholes in the Atlantis aquifer led to the discovery of iron-related clogging problems (DWAF, 2007). The first well clogging occurred in the early 1990s when the boreholes were over-pumped and air filled the screened section of the borehole due to drought conditions (DWA, 2010b).
The Atlantis scheme has proven itself as an innovative and highly successful scheme that has worked extremely well and won various awards (DWA, 2010b). A major component of the scheme has been the separation of the source water into different fractions, which allowed recharge of the good quality water in the areas of importance (Tredoux et al., 2002). The Atlantis groundwater scheme provides a cost-effective water supply option when coupled with the careful management of the water sources and aquifer (Tredoux et al., 2002).
40 Polokwane (Limpopo)
Polokwane grew rapidly over the past decade, with an estimated population of 400,000 and water requirements of about 12 million m3/a (Tredoux et al., 2002). However, the city has an elaborate groundwater abstraction infrastructure that supplies domestic water to meet daily peak demand, and also serves as a back-up during periods of surface water shortage. For example, during the 1992–1994 drought, groundwater accounted for a large proportion of the city’s supply (3.7 Mm3/a) (DWA, 2010b). The reliability of this source is largely due to the infiltration of Polokwane municipal treated wastewater into the alluvial and gneissic aquifers (Tredoux et al., 2002). Treated wastewater is discharged into the ephemeral Sand River, which flows over a 20 m thick layer of alluvium that is 300 m wide (DWAF, 2007). Underlying the alluvium are granite-gneiss rocks that are weathered and fractured to depths of 60 m (Tredoux et al., 2002).
The reliability of groundwater for Polokwane is largely due to the artificial recharge of 3-4 million m3/a of treated municipal wastewater infiltrated into the alluvial and gneissic aquifers (DWAF, 2007). This water is used by both the municipality and farmers for large-scale irrigation. To recycle as much water as possible, Polokwane municipality need to continuously abstract groundwater from the gneissic aquifer (Tredoux et al., 2002).
Kharkams (Northern Cape)
Kharkams is a small village in the semi-arid Namaqualand region that depends solely on groundwater from a granitic aquifer. Surface runoff, whenever it rains, is directed into the aquifer through the lowest yielding borehole of the village’s three production boreholes (Tredoux et al., 2002). This has had the effect of tripling the borehole’s yield and bringing the salinity of the water from virtually undrinkable (EC of 300 mS/m) to an acceptable quality (EC
< 100 mS/m) (Tredoux et al., 2002). This method ensures that the surface runoff water used for this artificial recharge is not lost to evaporation and evapotranspiration.
This scheme demonstrated the value of opportunistic artificial recharge in semi-arid areas, even if it is practised on a small scale (DWAF, 2007). Unfortunately, inadequate maintenance of the sand filter resulted to poor operation of the scheme over the past few seasons (Tredoux et al., 2002).
41 Prince Albert
The demand for water in Prince Albert, located in the Karoo region of South Africa, increased threefold over the summer months from about 1000 to 3000 m3/day (DWAF, 2007). Water is sourced from untreated river water that runs off from the Swartberg Mountains and enters the furrow and from boreholes in a sandstone aquifer. These boreholes are used to bridge the summer months’ supply shortfall. The mountain water is supplied via a furrow and the aim is to divert this water into the aquifer during winter when the furrow is being cleaned (DWAF, 2007). This would add 4000 m3 to the aquifer for use during summer.
3.4 Challenges of MAR