Rietvlei Water Quality

Water samples from the end of the Milnerton Aquatic Club (MAC) jetty, the lower end of the Bayside Canal (at the end of the road next to the soccer fields), and in Bayside Canal at Blaauwberg Rd is analysed. See the general notes on the Introduction page for information about the presentation.

Bacteriological monitoring In the graph below the results for faecal coliform counts are shown for each month from May 2002 to March 2010; please note that the vertical axis is a logarithmic scale. The blue line show the MAC jetty data, the pink line is the Bayside Canal data at the end of the canal, the brown line is Bayside Canal at Blaauwberg Rd. Faecal coliform counts show how polluted the water is; the counts are presented as x per 100 ml of water. In terms of the Water Quality Guidelines, a count of over 1,000/100ml means that the water is not suitable for intermediate contact (swimming). On the graph above the 1,000/100ml level is indicated by the horizontal red line. However, for human health purposes the counts should be below 100/100ml; at MAC the counts are occasionally above 100/100ml, but most monthly counts are less than 60/100ml.

Chemical monitoring Total Nitrogen If the total nitrogen in an aquatic system is below 0.5mg/l, production processes cannot occur - this is known as an oligotrophic condition (rainwater and highly acidic mountain streams are oligotrophic). Mesotrophic conditions occur when there is enough nitrogen to support primary production processes (0.5 to 2.5mg/l), while in eutrophic conditions primary production produces more nutrients than can be utilised in the system (healthy estuarine systems are naturally eutrophic as they export nutrients to the sea). Under hypertrophic conditions (when total nitrogen exceeds 10mg/l) nutrient production gets completely out of hand, leading to severe oxygen deprivation and the system 'chokes up'. In the graph above we can see that total nitrogen at the MAC jetty (blue line) was, for the most part, well within the mesotrophic range. In 2002 and mid 2003 nitrogen levels show that there were eutrophic conditions at times, but total nitrogen was not exceptionally high. A very high level of 73.19mg/l was measured in November 2004, but this could have been a sampling or a laboratory error. The results for that day show an exceptionally high pH (26.4) and very high soluble ammonia and soluble nitrate + nitrite levels, but dissolved oxygen and total phosphorus levels were within the normal range. Dissolved Oxygen This is a measurement of the oxygen dissolved in the water and which is available to organisms breathing or utilising oxygen. Dissolved oxygen is measured in mg/liter, the maximum levels can vary from about 8mg/l to about 16mg/l; temperature, dissolved solids, and a few other elements have an effect on the saturation levels of oxygen in water (eg. higher temperatures mean lower saturation levels). However, internationally it is accepted that if the dissolved oxygen level drops below 2mg/l, oxygen-dependent life cannot survive (the horizontal red line in the graph below). The graph above indicates levels of dissolved oxygen at the MAC jetty (blue), at the end of Bayside Canal (pink), and in Bayside Canal at Blaauwberg Rd (brown). It can be seen clearly that oxygen levels at the MAC jetty are quite high; but in the Bayside Canal it has dropped below 2mg/l on several occasions.

Conclusion Overall the sampling indicates that the water quality in north lake is within acceptable parameters. There has been no perceptible deterioration in water quality since 2002; it also appears that water from the Bayside Canal is adequately filtered by the reedbeds, as well as being diluted when entering the lake, so that the effect on water quality within North Lake is minimal. But this does not mean one must be complacent, the Bayside Canal is under severe pressure and one could only expect continued deterioration of the situation. The upgrading of the Bayside Canal should improve the quality of the water flowing into Rietvlei from this source. Events in 2006 and early 2007 have shown that water sampling may not necessarily pick up any potential changes that leads to severe eutrophication, fish deaths and blue-green algae. The nature of this system is such that oxygen and nutrient levels can change very rapidly; this in itself is not strange, it is simply a further indication that Rietvlei is a very viable and dynamic ecosystem in which one can expect rapid fluctuations in many of the components contributing to life in the system.

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.