The Cape Town drought: are more supply problems just around the corner?

5 October 2017

Bloom of toxic cyanobacteria in Theewaterskloof Dam(Photo: Bill Harding, 1991)

Bloom of toxic cyanobacteria in Theewaterskloof Dam(Photo: Bill Harding, 1991)

Monitoring for blue-green algae in times of drought – a need for increased vigilance:

The continuing and very troubling, ok downright scary is what it is, drought in the Western Cape should be on the minds of all those affected.  A major metropolitan area, plus numerous municipalities, are faced with the prospect of having to provide water from tankers at street corners in the not too distant future.  Millions of people will face considerable hardship, all the while having to continue to meet their daily obligations, be it family, work, education and so on.  Many work opportunities will be lost, at least temporarily.  The shortage of water problem could also, and rapidly, become a whole lot worse should what is left of the water become undrinkable.  Not everyone can afford bottled water.

The drought, seen in the main as a shortage of water – a quantity issue – also embodies a very real quality threat.  Almost all of our raw water is drawn from reservoirs (dams in local parlance) which, in aggregate, are at around 27% full in early October.  Some of these reservoirs (but not all) receive a daily load of nutrient-rich urban wastewater (sewage) effluent, continuously being added to receding volumes and increasing shallow depths.  Along with the arriving spring, water temperatures are on the rise – and potentially toxic algae may thrive in these warm, well-mixed (by wind) nutrient enriched and solar radiation replete waters = algal bloom heaven!  Theewaterskloof (the largest bulk supply for Cape Town) and Voelvlei (second largest) have long suffered from cyanobacterial (blue-green algal) blooms, and both feature increasingly in the ‘high risk’ end of the Water Research Commission developing project (EONEMP) on satellite-monitored algal blooms.  Both suffer from ‘taste and odour’ problems, produced by algae that may also and simultaneously produce potent liver and/or neurological toxins.

The upshot of all this is that a strongly-developed toxic algal bloom in such a reservoir could suddenly, and significantly, constrain the use thereof – an event that would be difficult to endure given the present volumetric constraints.  Early warning systems are thus of paramount importance.

Algal pigment profile showing metalimnetic bloom (8 m depth) of the toxin producing Planktothrix rubescens.

Algal pigment profile showing metalimnetic bloom (at > 8 m depth) of the toxin producing cyanobacterium  Planktothrix rubescens.

Technology has existed for quite a while now that allows for the real-time, in-line algal monitoring of the raw water feeding into treatment works.  This technology, known as a fluorometric technique, can discern between blue-green and other algae, and be linked to alarms to alert the operators of an impending issue.  The technology has, in the last few months, been upgraded to monitor for cyanobacterial pigments leaking from the bloom, this providing a proxy for the presence of toxins in the water.  As such, a comprehensive early warning system is available ‘off the shelf’.  This system does not require samples to be collected, taken to a lab and analysed – a process that may incur unacceptable and potentially life-threatening delays (see Figure below).  A wide range of instruments also include handheld devices that enable the detection of noxious algae in seconds at the shoreline.

Fluorometric tracking of a blue-green algal bloom (in green), overlain with manual cell counts (in red)

Fluorometric tracking of a blue-green algal bloom (in green), overlain with manual cell counts (in red)

I am not aware of any treatment works downstream of algal-impacted reservoirs in the Western Cape that have deployed this type of monitoring system – despite the fact that we are now apparently just six months off our taps running dry.   I don’t believe I am being alarmist when I say that what we face now could become a whole lot worse (of course, arguably an earthquake could disrupt the tunnel through the mountain, cutting off Theewaterskloof water to Cape Town altogether…).

Residents of Toledo queueing for water during a toxic algal bloom in their reservoir.  Source: ToledoBlade.

Residents of Toledo queueing for drinking water during a toxic algal bloom in their reservoir. Source: ToledoBlade.

(Author details: Bill Harding is an internationally-recognised algologist.  He previously held the position of Hydrobiologist and Microbiologist at the City of Cape Town [1988-1998] and in 1992 he established the first and only dedicated algal toxin lab in Africa at the City.  He correctly predicted the avoidable appearance of toxic algae in both Theewaterskloof and Voelvlei and is an authority on wastewater-driven eutrophication (nutrient-enrichment) of South African reservoirs.  He drafted the toxic algae research and management strategy for the Water Research Commission and represented the WRC on the toxic algal task group of the Global Water Research Coalition (GWRC), also drafting the risk chapter in the GWRC international manual on toxic algae.  He is, since 2008, also the South African representative for the world-leading bbe-Moldaenke range of fluorometric algal monitoring equipment.  This Blog (Droplets) contains a number of posts addressing eutrophication and toxic algae which may be of interest to readers seeking further information).




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