3.5 Scheme Elements

The various components of the Atlantis Water Supply Scheme are presented in Figure 12.

Figure 12 – Atlantis Water Supply Scheme with critical control points selected for monitoring shown as A1 through A13 (DWAF, 2010).

Domestic and industrial wastewaters are treated separately and only the domestic wastewater is recycled. The effluent from secondary settling tanks is polished in a series of maturation ponds. This is released to a reed bed basin. Stormwater runoff is collected via a stormwater collection system of 12 detention and retention basins. The peak flow and base flow in the stormwater system are channeled to different recharge basins to maintain good quality water in selected areas of the aquifer (Tredoux et al., 2009a). Treated domestic effluent from the maturation ponds is blended with low salinity urban stormwater runoff before being discharged into the main recharge basins (Tredoux and Cavé, 1997). The two recharge basins are shown in Figure 13.

The more saline effluent from the industrial wastewater treatment plant is discharged into coastal recharge basins and seeps into the ocean through the subsurface (Wright, 1991; Wright and Parsons, 1994). This also serves as a mitigation measure against potential seawater intrusion (Figure 14).

Figure 13 – Infiltration basins which form an integral part of the artificial recharge and recycling system in Atlantis. Cape Town’s Table Mountain is in the background (DWAF, 2010).

Figure 14 – Coastal recharge basin for treated industrial wastewater disposal (DWAF, 2010).

Groundwater is abstracted at the Witzand and Silwerstroom wellfields. Figure 15 shows the trend in abstraction at the respective wellfields. Abstraction rates have declined since the addition of surface water to the system in 1999. However, the artificial recharge of treated domestic wastewater and urban stormwater has continued, resulting in elevated groundwater levels, particularly in the vicinity of and downstream of the artificial recharge basins. The elevated groundwater levels have possible negative consequences in terms of attenuation of potential contaminants, due to reduced thickness of the unsaturated zone and shorter groundwater residence times. Abstraction is also reduced because of iron-related bacterial clogging of production wells and lack of effective routine maintenance to address the clogging problem (Bugan et al., 2016).

Figure 15 – Trend in production volumes at Witzand and Silwerstroom wellfields (Bugan et al., 2016).