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Dredging Zeekoevlei Given the Go-Ahead

In July this year, the province issued a positive Record of Decision (ROD) on dredging Zeekoevlei. This is the latest milestone in the long road to rehabilitating Zeekoevlei.

Zandvlei, Princess Vlei, Rondevlei, Zeekoevlei - Photo by Gordon Richardson

Zandvlei, Princess Vlei, Rondevlei, Zeekoevlei – Photo by Gordon Richardson

To date, agriculture and urbanization have destroyed or severely degraded between 35% and 60% of South Africa’s wetlands. Wetlands help purify South Africa’s water, control erosion and flooding and are home to a wide variety of fauna and flora. Under natural conditions, a large portion of the Cape Flats region was dotted with seasonal and perennial wetlands, which supported unique communities of wetland plants and animals. However, an estimated 97% of these systems have been lost, primarily through draining and infilling, largely to accommodate housing developments. Today, the Cape Flats lowlands have the highest concentration of threatened plants per area of remaining vegetation in the world.

The challenge is to maintain and reinstate the functions of wetlands like Zeekoevlei, Rondevlei and Princess Vlei, which are an interconnected unit.  The water quality of all these wetlands has been severely compromised by nutrient-rich inflows from the surrounding catchment area. Some 56% of the phosphorus loading of Rondevlei comes from Princess Vlei, whose outflow discharges directly into Rondevlei. A 2000 study on the rehabilitation of Zeekoevlei and Rondevlei recommended linking the catchment management resources of Zeekoevlei, Rondevlei and Princess Vlei.

While Rondevlei was declared a Nature Reserve in 1952, sister vlei, Zeekoevlei has been abused for over 80 years. The natural ebb and flow of the vlei was disrupted by the Zeekoevlei weir, erected in 1954. The vlei was unable to flush out the organic sediments that flow in from the catchment area. This has resulted in sludge build-up as well as a blue-green algae and water hyacinth problem. The Southern Waters 2000 study into the rehabilitation of Zeekoevlei and Rondevlei identified the main contributors to the nutrient-loading of Zeekoevlei:

  1. Seepage from the CFWWTW: 34%
  2. Big Lotus River catchment: 28,4%
  3. Nutrient-rich sludge: 25%

The balance of the phosphorus-loading of Zeekoevlei came from the Little Lotus and local catchments, septic tanks and groundwater contamination:

  • Little Lotus Catchment: 4,7%
  • Groundwater: 3,8%
  • Septic tanks: 1,5%
  • Reed beds: 1,1%
  • Local catchment: 0,6%

The Southern Waters report highlighted the importance of the primary aquifer in the vicinity of Zeekoevlei and Rondevlei and recommended that Zeekoevlei/Rondevlei be afforded protection as a special aquifer system. Since 2000, a handful of committed local environmentalists, in partnership with the City, have succeeded in implementing the following rehabilitation measures in Zeekoevlei.

  • Since 1997, the weir is opened annually allowing the vlei to be flushed out by winter rains. This has significantly reduced blue-algae blooms.
  • In 2008, the construction of a cut-off drain between Zeekoevlei and the Cape Flats Wastewater Treatment Works has reduced this primary source of nutrient-loading by 24%.
  • The Schaapkraal stormwater drains will further reduce the nutrient loading from the fertilizers leaching from Philippi farmlands.
  • With the positive ROD on dredging the vlei, should this project go ahead, dredging is estimated to restore 21% of the volume of Zeekoevlei and 40% of the surface area.

The report identified dredging the sludge build-up as the major portion of the total cost of rehabilitation and notes ‘that is likely to also be needed at Princess Vlei, Zandvlei and several other vleis and rivers within the CMA. Accordingly, the initial cost could be spread over several rehabilitation programmes…’ Given the current Princess Vlei debate over the proposed shopping centre development vs wetland rehabilitation, this would be a timely opportunity to encourage the rehabilitation of both Zeekoevlei and Princess Vlei. The rehabilitation of Princess Vlei will also help to reduce the nutrient-loading of Rondevlei.

In the last 9 years then, we have addressed two out of three of the major sources of contaminants, accounting for some 50% of the nutrient loading of the Zeekoevlei. We have come a long way to restoring some of the natural balance of this important wetland system. Princess Vlei needs to be afforded the same chance at rehabilitation.