History of the Cape Flats Vleis: Paardevlei

28 July 2011

After a lull, herewith more information on the vleis of the Cape Flats:

Paardevlei, partially drained (2005), view west. Photo: Bill Harding

Paardevlei is a small, impounded vlei within the property of the former AECI factory complex situated west of the Strand (see Figure 1). The vlei, the largest of a former complex of similar shallow systems on the property, received water diverted from the Lourens River via the Melcksloot since the early 1900s. Until quite recently the vlei was eutrophic (highly nutrient enriched), and experienced frequent blooms of noxious cyanobacterial (i.e. blue-green algae). In that form the vlei offered only limited habitat diversity and aquatic ecosystem services. The propensity to support development of cyanobacteria further posed a significant health risk to humans and animals, as well as imposing a negative impact on the overall biodiversity and future use potential of the vlei.

Paardevlei pre-draining (view south) Photo: Bill Harding

Pre-1899 Paardevlei was a small, shallow wetland, probably one of a complex of similar environments located on the landward side of the False Bay coastal dunes. The vlei would have been fed by groundwater, direct rainfall and local surface runoff. At that time the land use around the vlei was agricultural. Following purchase and development of the AECI (Dynamite Factory) the vlei was cleaned of aquatic vegetation and accumulated mud, and on three occasions between 1927 and 1941 the water depth of the vlei was increased by a total of 2.32 m to provide a storage capacity of approximately 1 330 000 m3. This impoundment of the vlei was achieved by constructing a rock berm with a clay core around much of the perimeter of the vlei. Water for the vlei was drawn from the Lourens River and routed to the vlei via the Melcksloot. Given the nature of the manufacturing (explosives) undertaken at AECI, and with the minor exception of angling permits issued prior to 1971, Paardevlei has been off limits to the public since 1900.

Old wooden pipe through Paardevlei, exposed during draining (Photo: Bill Harding)

The water stored in Paardevlei was used in the various manufacturing processes at AECI, as well as for recycled cooling and other process water. During the 1920s problems experienced with algal blooms, caused by the desmid Sphaerozosma aubertianum, were controlled by the application of copper sulphate. The biological diversity of the vlei appears to have been greatest during the 1930s, with excellent water quality, and a broad array of floral and faunal components. The reported benthic development of the Charophyte Nitella hyalina reflects a relatively unimpacted system with a groundwater component. Similar conditions have been observed in dams on the slopes above Paardevlei (Bill Harding, personal observation). During the 1970s, problems were experienced with low pH levels (~5.2-6.6) associated with return flows from a brushwood cooler plant (AECI Internal Memorandum dd 17 September 1976). The problem was alleviated by curtailment of the return flow to the vlei.

From 1931 Paardevlei was progressively stocked with a range of non-indigenous fish, and the original population of indigenous Cape Kurper (Sandelia capensis) disappeared. The stocking appears to have been related to the weekend angling needs of the factory manager.  From the mid- to late 1930s a decline in water quality was reported (Harrison, 1954), with the progressive loss of rooted aquatic plants. It is reported that by 1943 all of the rooted submerged aquatic macrophytes had disappeared from the vlei. These were replaced by dense blooms of algae, a condition which, to all intents and purposes, remained in existence for some 70 years. Post-1940, near continuous applications of copper sulphate were applied, with a total of some 24 metric tonnes being added between 1941 and 1946. Subsequents attempts, during the 1950s, to reestablish aquatic macrophytes in the vlei were reportedly unsuccessful, largely due to alleged interference by waterfowl. The high levels of copper sulphate present in the system at that time are more likely to have precluded plant establishment.

Toxic cyanobacteria (blue-green algae) were identified in Paardevlei during the early 1990s, and  remained the dominant alga in the vlei until it was drained. This pattern of change, namely impoundment associated with a switch from macro- to microphyte dominance was a common characteristic amongst many of the Cape Flats vleis during the first half of the 20th Century. The collapse of the previously diverse and self-sustaining ecosystem is likely to have been a consequence of the deepened vlei, regulated water levels and the introduction of alien fish, principally largemouth bass, that reportedly decimated the indigenous Cape Kurper (Sandelia capensis).

Paardevlei shoreline pre-draining (2001) Photo: Bill Harding

In summary it may be assumed that, during the first decades of the 1900s, Paardevlei existed as a clear water vlei containing a diverse array of floral and faunal components, and experiencing occasional blooms of algae commonly associated with good water quality, such as the Desmidaceae. The vlei was noted to contain indigenous fish, and obvious populations of zooplankton. Subsequently, impoundment and water level regulation, combined with the associated impacts of herbicides and contained eutrophication and pollution, resulted in a long term change to an algal dominated, diversity-poor vlei.

Press report on fish removal from Paardevlei (Source: District Mail)

DH Environmental was closely involved with pre-rehabilitation planning and surveys at Paardevlei from the mid-1990s to 2005 – at which time DHEC formulated a turnkey rehabilitation plan for the vlei  (Full report obtainable here).  Prior to that, DHEC had successfully provided specialist services to remove all the fish from the vlei so that it could be drained and the toxin-containing sediments removed (a video of the rotenone operation can be downloaded off the DHEC website Publications Page.  The intention was to drain the vlei, clean it out, re-sculpture the vlei bed and then re-connect it to the Lourens River, using an approach that would ensure good water quality.  This has not happened yet.

Mesocosm plant establishment experiments 2004. Photo: Bill Harding

Helicopter being used to dose Paardevlei with Rotenone (Photo: Bill Harding)

Dried sediment being scraped out of Paardevlei (Photo: Bill Harding)

Fish being removed following Rotenone treatment (Photo: Bill Harding)

Regrettably Paardevlei slipped through an EIA-loophole and the vlei has remained dry, only fed by rainfall, since 2005.  This severely disrupted the value of the vlei as a bird sanctuary, especially for species such as whitebreasted cormorants and the previously-resident pairs of fish eagles.  The waterfowl had to find sanctuary elsewhere, a commodity scarce in the Helderberg.  Considerable pressure was placed on smaller waters, such as the dams at Helderberg Village and the smaller dam on the Sitari Estate, mostly all inadequate due to the absence of numbers of large trees.  DHECs contract at Paardevlei was terminated in 2006, so we do not have any current information regarding planning for completing the work on the vlei.

3 Responses to History of the Cape Flats Vleis: Paardevlei

  1. Michiel Brand says:

    78 years ago I was born at Paardevlei with the help of a midwife. 22 years of my life was spent there. My father was an employee on the dairy section of the farm.

    Looking at the pictures rolled back memories through a broad spectrum of topics of what was before the rehabilitation of the entire area. A pity, though, because on the area the mall is now occupying you would find the loveliest species of wild flowers. [Also a] pity that I did not own a camera in my childhood days, then perhaps i could have relooked at the flowers through that medium, now they are gone forever!

    Michiel Brand

  2. You will be pleased to know that the rehabilitation of the Paardevlei has now been completed and the area is being managed in terms of a City approved operational environmental management plan. The water quality, vegetation and birdlife is constantly being monitored and we are happy to advise that improvements over the 2 years since this was complete is hugely satisfying.

    You are welcome to come and enjoy the revived vlei and take a walk along the pathway.

    Mark Bezencon
    Heartland Properties

    • Bill Harding says:

      Good to know but also sad to see that the blue-green algal blooms are back so soon – maybe the design measures to offset this were not included when the rehabilitation plan was implemented? It would be useful if you could make the monitoring results available so that the claimed successes can be evaluated by both interested and professional audiences? Are the data on a website somewhere perhaps?

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