Rietvlei & Diep River Water Quality

Water quality has been monitored in Rietvlei and the Diep River on a regular basis since 2002. A large number of determinands are monitored, but to most of us the tables with the results do not make much sense. To try and understand what is happening in the system, we have looked at some of the more critical indicators and transfered the data to graphs so that one can visualise the situation. Bacteriological and chemical monitoring of the system is done on a monthly basis by the City of Cape Town at several sampling stations in the Diep River and Rietvlei, and these results are used here with permission by the City. In interpreting these results for these webpages we are specifically looking at the effects of water quality on the ecological processes in the system, NOT the effect on human health and recreation use. The City's Health Department will consider these same figures in terms of the impact on human health issues, their conclusions may differ from ours, but this is not a contradiction - it is merely looking at the same figures from a different aspect. For our purposes we have only used the total faecal coliform count from the bacteriological sampling - this gives a good idea how polluted the water is at each of the stations. Chemical monitoring involves up to 19 determinands sampled on a monthly basis. Interpreting all this data is a complex task, and for easier understanding of what is happening, we have chosen a few indicators and presented the results in graphs. Our primary concern is the ecological health of the system, and two determinands in particular can be used to as indicators of ecological viability. Total nitrogen (N) is an important indicator of the ecological health of an aquatic ecosystem - too little N means the system is nutrient poor and cannot function effectively, too much means nutrient production gets out of hand and the system can 'choke up'. High total nitrogen also leads to dissolved oxygen levels dropping resulting in oxygen-dependent organisms suffering. Dissolved oxygen is another indicator of ecological health, when dissolved oxygen levels drop it indicates either hyper productivity or bacteriological or chemical pollution in the system.

Water quality results for North Lake in Rietvlei and for the Diep River from the Blaauwberg Road Bridge to the estuary mouth are presented separately. The graphs in the following pages are a bit small and not all the text is legible, but one can clearly see the data levels. All the graphs show the results taken on a monthly basis; sampling is normally done during the last week of each month. Where gaps appear, it means that either sampling was not done, or something could have gone wrong in the laboratory, but either way no data is available. Results of bacteriological and chemical sampling until the end of September 2009 is presented. » Rietvlei Water Quality » Diep River Water Quality

The water quality results are provided by the City of Cape Town and is used here with permission by the City. The interpretation of these results as presented here is done by a qualified ecologist with years of experience in physical and chemical limnology and estuarine ecology, and does not necessarily reflect the City of Cape Town's views. The results provided by the City have been used selectively to reflect the ecological health of the system. The full series of results must be interpreted to get a complete picture of the effect of the water quality of Rietvlei and the Diep River on human health and recreational activities. Drawing conclusions from the results presented here must be done with great care as they can easily be quoted out of context.