Zandvlei Trust

SAND RIVER CATCHMENT FORUM

Speaker presentations from meeting on Monday 21st June 2010.

Westlake River: Blue-green Algae Incident (April/May 2010) update by Candice Haskins.

On or about 10th April 2010 reports were received from two Kirstenhof residents regarding very green discolouration of the Westlake River emanating from the culvert system under the Blue Route freeway (M3).

Investigations from Monday 12th April into May revealed the following:

• Laboratory analyses undertaken by the City’s Scientific Service laboratories confirmed that the colouration was due to a group of algae known as Cyanophyceae (also known as “blue-green” algae). The genus of Cyanophceae found was Microcystis.
  Since Cyanophceae are capable of producing toxins such as microcystin, further laboratory analyses (Enzyme-linked immunosorbant assay, ELISA) were undertaken to ascertain whether these toxins were present in the river.

• Due to the toxin results being above the World Health Organisation recreational guideline (10 mg/l), warning signs were erected at various strategic locations by the City’s Environmental Health officials. 

• Monitoring of toxin levels is being undertaken on a regular basis. 

• Existing signage will be removed when results indicate that the toxin levels have subsided to safe levels.

• A press release was also prepared and later appeared in local newspapers and was reported on local radio stations.

• The City’s District Stormwater staff arranged the placement of straw bales across the stream at various locations in attempt to intercept and soak up as of the algae as possible. These were replaced once they were saturated.

• Additional water quality sampling was undertaken at Zandvlei and it was determined that toxin levels were well below the WHO Guidelines (all samples had a concentration less that 1 mg/l which is the same order of magnitude of regular background levels recorded in this vlei). It is therefore unlikely that the occurrence of toxic blue-green algae in the stream will have a negative impact on the vlei ecosystem. The practice of opening the mouth of the vlei to allow ingress of sea water assists with flushing of the algae from the system. The algae are not tolerant of saline water and so will tend to die off in the vlei. The vlei is currently open and is likely to remain so for the duration of winter.

Since Cyanophyceae are not generally found in flowing rivers and being confined to still bodies of water such as lakes and dams an investigation of the upstream area was undertaken by City Water Pollution Control, Environmental Health and Environmental Resource Management staff to establish the source of the algae. It was established that the dam on the Westlake River in the Pollsmoor Prison precinct contained large quantities of this algae and overflow from the dam wall was carrying it into the river. Since proliferation of algae relies on an abundance of nutrients being available, the source of nutrients to the dam had to be determined.
The main source of nutrients in this case is derived from a blocked and overflowing sewer manhole located adjacent to the dam. This sewerline serves quite a large component of the Westlake residential areas, shopping centre and restaurants, American embassy, Reddam School and other upstream properties. All records and information gleaned thus far indicate that the sewage system serving the Pollsmoor Prison precinct does not link into the municipal sewage system upstream of the Pollsmoor Dam and would therefore not be a contributory factor to the provision of nutrients to the dam.
Typically, blockages and overflows can be caused by a structural defect in the infrastructure itself and/or by foreign material deposited into the sewer system e.g. rags, newspaper, cutlery, bricks, sand, blankets etc. In order to determine whether any structural defects exist in this section of the municipal sewer, a CCTV inspection will be carried out within the next few weeks by the District Reticulation division of Water and Sanitation. The findings of this will be reported as soon as they are available. The sewer reticulation maintenance crew in clearing this blockage established that rags, fats and coarse paper (newspaper, pages from telephone directories etc) were present in the line.

In order to prevent the future entry of unacceptable material into the sewer system, the following actions will be undertaken in the coming months:

• The Water Pollution Control division of the Water and Sanitation department are inspecting the Steenberg Village Shopping precinct to ensure all property owners comply with the City’s legislation (by-laws) regarding the discharge of sewage and the correct management of stormwater. Some notices have already been issued to non-compliant property owners.

• Environmental Health will shortly be embarking on an educational programme in the local housing area which will include issues regarding appropriate and inappropriate deposition in the sewer system.

On-site evidence at the overflow point indicates that overflows either occur very frequently or for long periods of time, and this would explain the accumulation of large quantities of nutrients in the dam over an extended period of time. Since the dam and sewer manhole are in a remote area of the prison, no-one in the City was made aware of these overflows. Prison authorities will do regular inspections of the area and will report any future blockages directly to the City’s Reticulation division. A meeting will be held later in May with authorities from Pollsmoor in order to formalize this protocol. Pollsmoor authorities have also placed straw bales in the area immediately downstream of the dam.

With the onset of winter (cold temperatures and rain) and the minimization of future entry of nutrients into the dam (due to actions mentioned above) it is anticipated that this blue-green algae incident will subside and present toxicity levels will decline to acceptable levels and that future incidents should be very rare occurrences.

Westlake River Update: May / June 2010
Samples collected from Pollsmoor Dam and two locations on the Westlake River on 27 May 2010 were analysed for the presence of blue-green algal toxin. All three samples had exceptionally low toxin levels (less than 1 mg/l), which is well within World Health Organisation guidelines. An additional sample will be analysed this week (June 21st - 25th) whereupon a decision regarding the removal of the signage will be taken.

Water Pollution Control has commenced inspections in the Westlake Industrial Area to determine whether any contraventions are taking place.

On the 8th June 2010 City of Cape Town officials met with Pollsmoor Prison authorities along with a few members of the Sand Catchment Forum, Zandvlei Trust and WESSA to inspect the locations of sewer overflows into the Pollsmoor Dam. The inspection revealed that the Pollsmoor sewerage system links into the municipal system below the dam, and that the municipal system is located in a remote area of the prison grounds where inspections seldom take place. An inspection protocol is being established with the prison staff to ensure that any overflows are reported to the Kendal Road sewage maintenance depot as soon as they are observed.

The CCTV inspection has been completed on the municipal sewerage system with the findings indicating that there are no structural defects. The overflows are therefore entirely caused by inappropriate material entering the system.