Spreading methods

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General information

1. Name technique: Infiltration pond South Africa

2. Pictures

Figure 1: Infiltration pond in dunes in Atlantis, South Africa. (Source: Tuinhof A.)

Figure 2: Map of Atlantis artificial recharge scheme. (Source: Tredoux G. 2002)

3. Brief description of the situation:

The town of Atlantis, located 50 km north of Cape Town, was fully dependent on groundwater at its inception in 1976. However, groundwater supplies are limited and artificial recharge through infiltration basins was introduced shortly afterwards for augmenting local groundwater supplies (Figure 1). Being a new development, the town was planned with fully separated residential and industrial areas. This fact contributed to the success of the artificial recharge operation, as diversion of stormwater and wastewater flows of inferior quality from the industrial area was possible (Figure 2). Since 1999, the scheme was augmented with a limited supply from surface water sources to meet peak demands, but the town is still mainly dependent on the local groundwater. The city of Atlantis has over 100 000 inhabitants, and water consumption without restrictions is around 7 Mm3/year (in 1999 and 2000). Atlantis is located along the semiarid west coast of South Africa. Most of the 450mm mean annual rainfall is received from April to September. As a result of the sandy surface over most of the area, recharge percentages of 15 to 30% of the annual rainfall are generally experienced, the higher recharge occurring in the unvegetated dune area.

Technical information

4. Brief description of the technique:
Infiltration ponds are used to infiltrate water into formations of good permeability, which are not overlain by an impervious layer. Ponds are either excavated, or are enclosed by dikes or levees which retain the recharge water until it has infiltrated through the floor of the basin.

In the Atlantis area, minimal runoff is generated under natural conditions due to the high infiltration capacity of the soil. It was realized that large volumes of stormwater runoff would be generated after urbanization and the associated hardening of surfaces. This was seen as a valuable water source for augmenting freshwater supplies in this region, and prompted the construction of a stormwater collection system. As an added water source, treated domestic waste water is recharged to the aquifer along with the stormwater.

5. Attributes overview:

6. Construction:
The system consists of twelve detention and retention basins and the necessary interconnecting pipelines with peak flow reduction features.

• Low salinity flows are channeled into two large spreading basins for subsequent infiltration in the Witzand aquifer, upgradient of the Witzand wellfield.
• Higher salinity baseflow is diverted to the coastal basins or to the Donkergat river.

A weir in the stormwater system separates the slightly more saline baseflow from the low salinity peak flow. Domestic and industrial wastewater is treated separately in twin wastewater treatment works. Only the final effluent from the domestic works is used for recharge upgradient of the Witzand wellfield Treated industrial wastewater, softening plant regenerant brine, and industrial area stormwater is discharged into the coastal recharge basins. In doing so, a steeper hydraulic gradient is created near the coast, which may help to reduce the outflow of good quality groundwater, as well as reduce the risk of seawater intrusion into the Witzand unit. Ponds are generally 1-4 m deep, which is enough to prevent excessive growth of algae or water plants, and shallow enough to prevent anaerobic conditions developing except at the bottom.

7. Capacity:
Discharges during storm events can reach up to 72 000 m3/day at Atlantis, while summer baseflow averages 2160 m3/day (Wright, 1994). The baseflow is mostly groundwater entering the stormwater pipelines in areas where these are below the water table.

• Stormwater and wastewater infiltration augments the natural recharge of the groundwater in the Witzand unit by 1.5 to 2.5 Mm3/year.
• Infiltration of treated industrial wastewater, softening plant regenerant brine, and industrial area stormwater, is together exceeding 2 Mm3/year.

The accepted abstraction capacity for the Witzand wellfield is 5 Mm3/year, and for the Silwerstroom wellfield is 1.8 Mm3/year. This means that about 30% of the abstracted water from the Witzand wellfield derives from artificial recharge.

8. Experiences with Operation and Maintenance:
Maintenance of the recharge structure is important. The bottom of the pond must be inspected and treated regularly in order to minimize clogging to maintain infiltration rates and keep evaporation from open water to a minimum.

Iron-related clogging of abstraction boreholes due to overpumping of the boreholes has proven to be an extensive and serious problem. From 1999 to 2002, boreholes were examined and rehabilitated using special treatment techniques.

9. Experiences with Monitoring and Evaluation:
Managing water quality and, in particular, salinity has been one of the greatest challenges for the Atlantis Water Scheme. Management actions to control salinity in the Atlantis water supply have included the launching of a detailed chemical investigation of the salinity sources. Regular monitoring takes place around the recharge and abstraction areas and at potential pollution sources. This also provides an early warning system against any potential uncontrolled spills and illegal discharges of harmful contaminants.

Improved environmental practices have been initiated by some of the industries. These have resulted in the commissioning of site assessments and in some cases the initiation of groundwater monitoring programmes. Increased understanding of their contamination threats allows these industries to improve their operating procedures to protect their water resource.

10. Experiences with related subjects (e.a. erosion prevention, quality of drinking water, ..):
Water quality: As water migrates through porous soil and rock, pollutant attenuation mechanisms include precipitation, sorption, physical filtration, and bacterial degradation. If functioning properly, this approach is presumed to have high removal efficiencies for particulate pollutants and moderate removal of soluble pollutants. In Atlantis, the Witzand unit is separated from areas with poorer groundwater potential (where also the noxious industries of Atlantis are located) by high bedrock, which acts as a groundwater divide.

Financial information

11. Experiences with management:
The artificial recharge system at Atlantis is a complex, large schale scheme. The system has been managed by the Water Department of the City of Cape Town and its predecessors, the Cape Metropolitan Council, the Western Cape Regional Services Council and the Cape Divisional Council.

12. Benefits:
The inhabitants of Atlantis (> 100 000) have access to good quality drinking water in a semi-arid region with little surface water resources.

General conclusions

13. Generic factors of success and traps ('do's and don'ts'):

• A detailed analysis of rainfall pattern, number of rainy days, dry spells, and evaporation rate and detailed hydrogeological studies should be undertaken to demarcate suitable percolation tank sites.
• During construction of the basin, driving heavy equipment over the infiltration surface should be avoided if possible.
• Scheduling basin "rest" periods of sufficient duration between flooding periods allows drying and biodegradation of clogged layers. Periodic deep ponding increases basin heads to overcome surface clogging.

14. What can be used elsewhere, under which conditions:
Infiltration ponds recharge the groundwater in an aquifer by infiltration of water stored in the pond through the bottom of the pond. Therefore percolation ponds can be applied almost anywhere, provided that there is a supply of clean fresh water available at least part of the year to fill the pond, the bottom of the pond is permeable, and the aquifer to be recharged is at, or near the ground surface. Infiltration ponds require a relatively large surface area for spreading. Therefore ponds are only suitable where there is ample room for installation. Because infiltration ponds are very vulnerable to contamination, they should be located in protected areas.

15. Advantages and disadvantages:

16. Links to detailed information of this project / technique:
Dr. Gideon Tredoux
Council for Scientific and Industrial Research (CSIR)
gtredoux@csir.ca.za
Tel: 021 888 2591
Cell: 082 940 6147

17. References

• Asano T. 1985. Artificial Recharge of Groundwater. Butterworth Publishers. Stoneham, USA.
• Gale I. 2005. Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas. IAH - MAR, UNESCO IHP. Paris, France.
• Tredoux G., and Cavé L.C. 2002. Atlantis Aquifer: A status report on 20 years of groundwater management at Atlantis. ENV-S-C 2002-069. CSIR
• Tredoux G., and Cavé L.C. 2002. Long-term stormwater and wastewater infiltration into a sandy aquifer, South Africa. Management of Aquifer Recharge for Sustainability. Proceedings of ISAR-4, Adelaide, South Australia.
• Wright, A. 1994. Artificial recharge of urban wastewater, the key component in the development of an industrial town on the arid west coast of South Africa. Water Down Under '94. Proceedings of the IAH Congress: Vol.2, Part A, 39-41. Adelaide: International Association of Hydrogeologists.
• Technologies for water harvesting and soil moisture conservation in small watersheds for small-scale irrigation (FAO)

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