2.7 Selection of Case Studies

MAR systems are generally discussed in terms of five major components (National Research Council, 2008):

1. source of water to be stored;
2. recharge method;
3. storage method and management approach;
4. recovery method; and,
5. end use of recovered water.

Opportunities and issues related to the selection, development, use, and regulation of MAR systems are typically tied to these components, and discussion regarding hydrogeology and hydraulics, water quality, legal, regulatory and economic issues, and management of systems are usually tied to one or more of these components. While issues related to water sources and end uses may be common to both underground and surface storage of water, many of these issues are unique to underground storage systems, such as potential interactions between stored water and native water in the surrounding aquifer.

Figure 6 illustrates the above-mentioned major MAR components, along with some associated criteria, that affect system selection and design (National Research Council, 2008). Note that many MAR systems contain some form of pretreatment before recharge and post-treatment during recovery. Monitoring of the stored water is often required. A source of water is required for all systems, but selection of the source is tied to the end use (particularly with respect to whether that end use is to be potable or not), as are treatment and management during recharge, storage, and recovery. Major factors that impact the selection of recharge methods include aquifer type, land availability, and proximity to the water source.

Figure 6 – Technical components of a MAR system (after National Research Council, 2008).

With this as background, six cases have been selected covering different physical and management environments and recharge methods in Southern Africa. Table 1 includes the Cape Flats and Sedgefield cases, as well as the widespread use of sand dams, but these are not discussed as case studies. Each of the different recharge methods in these selected Southern African cases is illustrated in general in Figure 7, using a conceptual diagram and some pointers to its applicability and relative cost (from Murray and Harris, 2010).

	Infiltration basin Basins constructed in sand or gravel aquifers. Surface water is diverted to basins and allowed to infiltrate through an unsaturated zone to the underlying unconfined aquifer. For sandy unconfined aquifers – not suited to clayey soils. Treatment: infiltration is more rapid with clean water than turbid water; treatment lengthens the “runs” before having to scrape the basins and remove fine material. The unsaturated zone provides natural treatment. Costs: moderate – can recharge up-gradient of existing boreholes.
	Dune Infiltration The infiltration of water through a sand dune and abstraction from boreholes/wells/ponds downstream For unconfined sedimentary aquifers. Treatment: minimal pre-treatment; source water should be reasonably clear to prevent clogging Costs: low – only shallow abstraction required
	Sand dam Built in ephemeral streams in arid areas on low permeability lithology. They trap coarse sediment when flow occurs and following successive floods, the sand dam is raised to create an “aquifer”. Often, they are built on fracture features in the landscape to speed up recharge of the natural aquifer underneath. Abstraction can also be from the sand aquifer. It takes years to create the artificial aquifer, but the approach allows for a phased implementation.
	Borehole Injection – Aquifer Storage and Recovery (ASR is a term commonly used for MAR using wells in the United States and Australia) The injection of water into a borehole for storage and recovery (mostly from different boreholes) Suitable in both confined and unconfined aquifers. Treatment: usually high treatment required – need to remove sediment/debris to prevent borehole clogging Cost: moderate to high

Figure 7 – General description of recharge methods discussed in Southern Africa case studies (Murray and Harris, 2010).

Table 1 – Selected case studies of active MAR schemes in Southern Africa.

¹ Million cubic meters per year
² Discussed in concluding section
³ Referred to under Plettenberg Bay
⁴ Under traditional technologies

For purposes of comparison and overall assessment, each case study will be discussed under the following headings:

• need for artificial recharge – setting the scene;
• source water;
• aquifer hydraulics;
• water quality;
• scheme elements;
• water resource management environment; and,
• evaluation and way forward.

The case study section will conclude with some thoughts on the roll-out of MAR to date as part of sustainable groundwater resource development in the region.