Rietvlei Nature Reserve and Water Quality

Photograph Frieda Prinsloo (used without permission)

Rietvlei Nature Reserve

The Milnerton Aquatic Club is located along the shores of the north lake (permanent freshwater lake) within the Rietvlei Wetland Reserve. Rietvlei forms part of the Table Bay Nature Reserve and offers a range of natural and semi-natural habitats.

While you enjoy your sport on the water or while relaxing on MAC's lawn, you will not only be able to enjoy the beautiful view on Table Mountain but also the various animals. This is a birder’s paradise, especially in spring and summer, when migrant birds arrive from the northern hemisphere. Some 180 bird species are listed, including pelicans, flamingos, ducks, coots, herons, plovers, weavers and swallows. Many small mammals (e.g. Cape Grysbok, Cape Grey Mongoose, African Clawless Otters, Caracals), reptiles and insects live here, along with several frog species.

The Friends of Rietvlei provide assistance and support for the management of the Table Bay Nature Reserve, Cape Town.

SANCCOB, the coastal bird rehabilitation centre, where oiled penguins and gannets are cleaned, is adjacent to the reserve.

As one of the custodians of Rietvlei Nature Reserve, we ask all our members to protect the nature surrounding us. Please adhere to the nature conservation rules so that it can persist for future generations. Please assist in the protection and conservation of our Rietvlei Nature Reserve.

Water Quality



Data has been kindly provided by Scientific Services Labs (SCSI), City of Cape Town. Data will be updated as soon as received.

City of Cape Town conducts usually bi-monthly tests regarding E.coli and monthly tests regarding chlorophyll-a and the main blue-green algae toxin microcystin. At times of elevated levels, additional tests might be conducted.
Test results usually take several days and longer to be released. Hence, results are only to be used as an indication of the water quality. The situation on the day when the test results are released might be different to the situation on the sampling date.

Test results can be influenced by various factors, for example, storm water inflow, sewage pipe or pump overflows, bird droplets, wind speed and direction, etc. With the delayed test results it is not possible to determine the exact reason for an isolated peak or increased test value.

'Full body contact recreation' applies to swimming and diving where the full body is immersed in water for a long time. Please note that swimming and diving are not allowed at Rietvlei nor in any inland lake or dam across South Africa.

'Intermediate contact recreation' applies to sports like windsurfing, sailing, water-skiing and canoeing and assumes partial contact that would take place during paddling, splashing and brief immersion such as when a vessel capsizes.
Relevant testing points are MAC's jetty and MAC's slipway.


CoCT Targets:
Recreational SA target guideline for intermediate contact recreation is <=1000 colony-forming units per 100ml (3 month median in fortnightly samples).
Acceptable is 1001-2000 CFU (faecal coliform count incl. Ecoli)/100ml, Risk 2001-4000 CFU/100ml, Unacceptable >4000 CFU/100ml.

Adverse effects on humans:
Most E. coli strains do not cause disease, naturally living in the gut, but virulent strains can cause gastroenteritis, urinary tract infections, neonatal meningitis, hemorrhagic colitis, and Crohn's disease. Common signs and symptoms include severe abdominal cramps, diarrhea, hemorrhagic colitis, vomiting, and sometimes fever. (Source: Wikipedia)

Historical Analysis:
Faecal contamination levels in the Rietvlei water sports area is low, and generally meet the intermediate contact guideline. Local sources of contamination could include run-off from residential properties bordering the water body, overflows from pump stations and Bayside Canal. It is noted however that flow from the latter canal first passes through dense shoreline reed beds before entering the deep water lake area which assists with water quality improvement. Isolated peaks may be due to sample anomalies e.g. collection of bird faeces in the water sample due to the resident bird population near the sample collection site (jetty). (Source: Water and Sanitation Department, 'Analysis of ambient water quality in the Diep (N) Catchment within the City of Cape Town area', 9. June 2020)

Historical Test Results (Source: Water and Sanitation Department, 'Analysis of ambient water quality in the Diep (N) Catchment within the City of Cape Town area', 9. June 2020)



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The shoreline area of Rietvlei is characterised by a narrow shallowish band inhabited by pond weed and filamentous algae. The green coloured algae is harmless and a natural component of many open water bodies and wetlands.  Both pondweed and filamentous algae thrive along the shoreline. The blue-green algae (cyanobacteria) also thrive in this nutrient enriched zone and can produce a class of toxins called microcystins.  Demonstrations of toxicity of the cyanobacterial population in a given lake do not necessarily imply an environmental or human hazard as long as the cells remain thinly dispersed. Mass developments, so called 'blooms' and especially surface scums (thick, gelatinous substance) pose the risks.

Adverse effects on humans:
Health impairments from cyanobacteria in recreational waters must be differentiated between the chiefly irritative, allergic dermal symptoms caused by unknown cyanobacterial substances and the potentially more severe hazard of exposure to high concentrations of known cyanotoxins, particularly microcystins. Most documented cases of human injury through cyanotoxins involved exposure through drinking-water, and they demonstrate that humans have become ill—in some cases seriously— through ingestion or aspiration of toxic cyanobacteria. Symptoms reported include abdominal pain, nausea, vomiting, diarrhoea, sore throat, dry cough, headache, blistering of the mouth, atypical pneumonia and elevated liver enzymes in the serum, as well as hay fever symptoms, dizziness, fatigue, and skin and eye irritations (Source: WHO safe recreational water guidelines, Volume 1)

Guidelines Used by City of Cape Town
Usually, chlorophyll—a concentration is analyzed once a month. Algal cell counts and species identification are undertaken. If blue-green cell counts are > 20 000 cells per ml, a toxin test (microcystin) is undertaken. Step-up sampling (more frequent and more locations) is undertaken when toxin levels are above 10μg/l microcystin.

According to CoCT and their understanding of the WHO guidelines for safe recreational water environments (volume 1 Coastal and Freshwaters 2003), the following applies (Source: PAAC meeting 21.11.2019).
No risk < 1μg/l toxin , Low risk 2-4 (up to 10) μg/l microcystin (20.000 blue green cells or 10μg/l  chlorophyll-a where blue green dominates), Moderate risk 20μg/l (up to 50-100μg/l ) microcystin (100.000 blue green cells or 50μg/l chlorophyll a with blue green dominance), High risk from 24 μg/l microcystin and or gelatinous scums.

Please note that MAC's understanding of the WHO recreational water guideline is different to City of Cape Town's (COCT).  We are in ongoing discussions with CoCT.
The WHO guideline for drinking water considers 1 μg/l microcystin as 'safe for life long consumption of 2l of drinking water per day'. Assuming that any watersport enthusiast at MAC (intermediate water contact) takes in less than 200ml per day while exercising (i.e. 1/10th of the 2l drinking water consumption per day), the tenth fold of the drinking water quality threshold should apply. This means, 10 μg/l microcystin in recreational water is considered the 'same quality in recrational water' as 1 μg/l microcystin in 'life long consumed drinking water'.  WHO is already using a conservative approach with the assumption of an intake of 200ml water as this is usually the amount of water one swallows in a 'drowning situation'. Usually, when a sailor or windsurfer falls into the water, he/she may take a sip of 2.5 to 5ml. In addition, water sport is not done 'daily and life long' as 'drinking 2l of water'.

Lengthy full body water immersion associated with swimming and diving will have lower thresholds, but swimming and diving, both are not allowed at MAC (Rietvlei).

Hence, based on the aforesaid and the WHO guideline, the thresholds are higher for intermediate recreational sport according to MAC's interpretation:
No risk 0-10 μg/l microcystin, Relatively low risk 10-20 μg/l microcystin, Moderate risk 20-2,000  μg/l microcystin and high risk >2000 μg/l microcystin. Please note that obviously the thresholds depend on the weight of each individual. I.e. the threshold is lower and the risk is higher for a 10kg child compared to a 80kg adult. Also the skills of the watersport enthusiast, the time spent in the water, etc. will have an impact on the 'acceptable thresholds'.

We encourage all members to make themselves familiar with the WHO guidelines for safe recreational water.

Historical Test Results (Source: Water and Sanitation Department, 'Diep Catchment – a water quality overview’, presented to PAAC, 17 January 2019)


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