Diep River Water Quality

The results shown here are from six sampling stations from the Blaauwberg Road Bridge down to the estuary mouth. See the general notes on the Introduction page for information about the presentation.

Bacteriological monitoring For the bacteriological analyses we have charted the faecal coliform count. In the graph below the results are shown for each month from May 2002 to December 2011; please note that the vertical axis is a logarithmic scale on these graphs. Blaauwberg Road Bridge: Downstream of Potsdam Waste Water Treatment Works outfall: Theo Marais Canal downstream of the Duikersvlei outfall: Otto du Plessis Drive Bridge: Woodbridge Island at Loxton Road Bridge: Estuary mouth: 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. The graphs show that there are disturbingly high levels of faecal coliforms in the water; even down at the estuary mouth where one would expect sea water to flush the system, the levels are mostly over the 100/100ml level.

Chemical monitoring 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 are showing only the levels of Total Nitrogen; while nitrogen is essential for primary production processes, too much nitrogen can result in excess production which causes algal blooms, it also means that oxygen levels drop and oxygen dependent life forms cannot survive. These results date from April 2002 to September 2010 Blaauwberg Road Bridge: Downstream of Potsdam Waste Water Treatment Works outfall: Theo Marais Canal downstream of the Duikersvlei outfall: Otto du Plessis Drive Bridge: Woodbridge Island at Loxton Road Bridge: Estuary mouth: 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 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 - in estuaries this is a natural situation as estuaries are net exporters of nutrients. Under hypertrophic conditions (when total nitrogen exceeds 10mg/l) nutrient production gets completely out of hand, leading to severe oxygen deprivation as the system 'chokes up'. Generally the sampling results are not too bad, in most cases the total nitrogen levels are in the mesotrophic/ eutrophic range. The high levels of total nitrogen in the Theo Marais canalhas shown a considerable improvement since mid 2006. Generally speaking, the levels of total nitrogen from 2009 is lower than previous years, mostly mesotrophic or low eutrophic.

Conclusion Of major concern, and this has been the subject of further investigation, is inflow into the river system from the various stormwater drains and canals. With growing urbanisation in the immediate vicinity of the Diep River and Rietvlei, stormwater runoff is entering the system directly and not filtering through the ground or reedbeds and wetlands. This means that nutrients and pollutants are not filtered and ends up directly in the river and lake systems. However, the Eastern Stormwater Discharges study being undertaken by the City should show us the way forward in dealing with this problem.

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.