Department of Water Affairs and Forestry
	Institute for Water Quality Studies

POLLUTION INCIDENT IN THE ROODEPLAAT DAM, May 2000

4.2 Ad hoc pollution incident in the Roodeplaat Dam

During May 2000 a pollution incident was reported and the IWQS conducted two ad hoc sampling trips to investigate the incident. Thick cyanobacterial mats were found both in the Pienaars River (Site A in Figure 1) and the Moreleta/Hartbees Spruit (Site B in Figure 1) upstream of the confluence of the streams in the Roodeplaat Dam. Results for the chemical, biological and physical variables are shown in Table 2. Both sites had extensive decomposing cyanobacterial mats consisting mostly of Microcystis aeruginosa. At Site A traces of Azolla sp. were present in between the cyanobacteria. The extremely high chlorophyll-a concentration in the Pienaars River was 7 947µg/l . The decomposing component indicated by the high phaeophytin-a, was also very high.

The extent of decomposition of the cyanobacterial scum in the Pienaars River explains the presence of cyanobacterial toxins at this site on the 25^th May 2000. On lysis (breakdown) of the cyanobacterial cells any toxins present are released. The cyanobacterial toxin present was microcystin-YR. This incidence of the toxic cyanobacterial bloom is the result of extremely high nutrient concentrations (both the nitrogen and the phosphorus) and the extent of decomposition in the phytoplankton community. The conclusion was that the pollution incident was, therefore, eutrophication related. The incident seems to be more related to the long-term increases in nutrients within the system. Yet, the high nitrogen and phosphorus concentrations can be a result of a major influx of nutrients from a point source. It can, however, also be explained by the decomposing state of the phytoplankton population. The breakdown of the algal cells can cause the release or re-circulation of nutrients into the system.

The faecal coliform data and the E. coli data (Table 3), show that there is contamination from warm-blooded animals in both the inflowing streams. To determine whether the bacteria are from human or animal origin, the ratio of faecal coliforms to faecal streptococci must be determined (Table 4). On the second sampling date microbiological samples were also tested for faecal streptococci. The ratio of the faecal coliforms : faecal streptococci in the Pienaars River (Site A) indicates human origin, but the ratio in the Moreleta/Hartbees Spruit indicates the microbiological contamination is of animal origin.

These microbiological results and the variation from the mean concentrations of PO₄-P and TDS (Tabel 3) in the Pienaars River tend to indicate a pollution incident from a WCW. In the Moreleta/Hartbees Spruit the pollution is most probably of animal origin. The exact cause and/or whether any large-scale pollution incident did take place can not be confirmed.

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