Response: ‘Always safe to drink’

2008-07-28 00:00

In reference to the article, “Global warming hits Midmar Dam”, in Weekend Witness on July 19, Umgeni Water wishes to respond to the contents.

The article reaches the opposite conclusion to what is in the published paper “Toxic blooms in Midmar Dam”. This constitutes an incorrect interpretation of the paper. CSIR researcher Dr Paul Oberholster and colleague Dr Anna-Marie Botha actually conclude that the winter bloom of Microcystis spp. during 2005 occurred with a low average surface water temperature of 10,1°C and that this is “totally antithetical” to the norm. In other words, it is not temperature that seemed to be the cause of the algae bloom but “that nitrogen plays a large role in the changes to the phytoplankton community composition”. There is even reference in the publication to the fact that the sampled sites of the study “are part of the Midmar game park sanctuary, and waterfowl may be a contributory factor to the high nutrient values in the winter months”.

Midmar Dam, like other Umgeni Water (UW) dam water resources, is comprehensively monitored by UW for a wide variety of analyses, including algae down to genus level. Sampling is done near the wall and is representative of the main body of water where water is abstracted for treatment. This enables Umgeni Water to identify and react to potential problems so that treated water is always safe to drink.

There is currently no major change being seen in the behaviour of the water temperature in the Midmar impoundment. On the contrary, median annual temperatures have cooled slightly in recent years. Winter minimum temperatures have behaved similarly. While climate change is a concern and is receiving attention at Umgeni Water, it is the possibility of droughts and floods that are by far the most important problems needing attention.

The article refers to “algae flowers”. An algal “bloom” is not an algal “flower” (most are unicellular and cannot flower). A bloom is the name given to a proliferation of algae in very high numbers, usually due to a plentiful supply of nutrients. Without these nutrients, excessive growth is not possible, regardless of temperature. Temperature can have an effect on algal blooms, but different algae have different temperature needs and some algae can develop blooms at low temperatures.

Toxin-producing algae have been detected in Midmar (mostly in summer), but these algae can be found almost everywhere. Midmar has not yet experienced an algal bloom where potential toxin-producing algal numbers have developed to the point where abstracted water quality problems are likely, at any time of the year. The WHO suggests a limit of 100 000 cyanobacterial cells/mL alert level in the water resource, at which cyanotoxins could be present up to 20ug/L.

The average cyanobacterial counts are less than 20 000 cells/ml for the period 1994 to 2008.

In Midmar, small blooms of other algal types that cause related problems have been encountered, but have been minor in nature (causing slight filter blocking). A number of the minor algal blooms that have occurred in Midmar since Umgeni Water monitoring began in the late eighties have occurred in late winter, so this appears to be normal behaviour for this impoundment. These blooms have mostly been of algae called diatoms which do not produce any toxic substances.

All relevant water resource managers involved at Midmar Dam (including the Department of Water Affairs and Forestry and Umgeni Water) are fully aware of the potential problems algae can cause. If a problem is seen to be developing, a wide range of actions can then be taken.

Besides trying to prevent nutrient enrichment in the first place, spilling the dam can remove the problem, provided that there is enough water, and in dams like Midmar there are a number of abstraction depths that can be strategically selected to avoid sucking in algae. Pre-treatment of the raw water can be undertaken to remove the algae, and even more careful than normal treatment processes followed.

At other dams, such as Inanda, where far more nutrients are present and there have been significant and potentially problematic algal blooms in the past, detailed monitoring (including analysis of the algal toxins) has been undertaken. While toxins have been detected in low amounts in some samples, the work has shown that the raw water sent to treatment plants has been entirely safe, even when specifically problematic algae (mostly cyanobacteria, also called blue-green algae) have been present in large numbers. Additionally, Umgeni Water’s treatment plants, which are designed to be effective in removing algae, can be and are operated with additional and even more comprehensive treatment to ensure removal of these problems (such as the use of activated carbon). Were a problem algal bloom to affect Midmar, similar additional monitoring and treatment would be undertaken.

Problem algae (both in terms of numbers and particularly the presence of cyanobacteria of concern) are far more likely to result from nutrient enrichment in a dam than from temperature changes. These problems are more likely in summer, but would be of concern no matter what time of year they might occur. The sources of these nutrients are related to the land-use pattern around the water resource.

• Shami Harichunder is the corporate stakeholder manager at Umgeni Water, Pietermaritzburg.

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