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Department of Water Affairs and Forestry |
| Institute for Water Quality Studies |
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The results are discussed under chemical characteristics, biological characteristics, physical characteristics, the trophic status of the impoundment, with the reported ‘pollution incident’ being discussed lastly. The results indicate clearly that the main cause of the ‘pollution incident’ is caused by the enhanced eutrophication due to the various inputs of nutrients into the impoundment. Therefore, the historical trends in nutrient concentrations and the trophic status of the impoundment are also discussed.
The chemical results are subdivided into nutrients and salinity as these two chemical groups pose two of the major threats to the water quality of South Africa’s fresh water resources.
The historical data of the Roodeplaat Dam at Site 1 (Figure 2) has shown a gradual increase in the mean annual TP and PO4-P concentrations since 1991. The mean annual TP concentrations have also been above the total phosphorus management objective (PMO) value of 130 µg/l (DWA 1988, ANON 1988a, ANON 1988b) since 1991 (Figure 2). This places the Roodeplaat Dam in the hyper-eutrophic class. This, in view of the fact that both the WCW’s are required to comply with the 1 mg/l P Standard, should be a serious concern to the managers of the impoundment. The Zeekoegat WCW was constructed and started to operate in late 1991, while the Baviaanspoort WCW was in operation prior to 1991.
The trends in the TP and the PO4-P concentrations at Sites 1 and 7 (Figures 3) clearly show the general increases in concentrations of both variables since 1991. What is alarming about this is the fact that the TP concentration is increasingly dominated by PO4-P (Figure 4). This increase is in such a manner that PO4-P now forms the major component of the TP concentration. This phenomenon indicates that most of the phosphorus in the system is directly available for primary growth of algae and macrophytes within the impoundment and is most probably the main cause for the reported ‘pollution incident’.
The minimum, maximum and mean TP concentrations at Site 1 are 0.012, 3.207 and 0.288 mg/l respectively, for the period 1989 to 2000. The minimum, maximum and mean TP concentrations at Site 7 are 0.046, 1.046 and 0.354 mg/l respectively, for the same period. This indicates that although the maximum TP concentrations at Site 1 is higher than that at Site 7, the mean TP concentration at Site 7 is much higher. Site 7 is, therefore, more subject to eutrophication symptoms.
The minimum, maximum and mean PO4-P concentrations at Sites 1 are 0.005, 1.294 and 0.177 mg/l respectively, for the period 1989 to 2000. The minimum, maximum and mean PO4-P concentrations at Site 7 are 0.005, 0.708 and 0.226 mg/l respectively, for the same period. This indicates the same tendency as was evident for TP. There is, therefore, more inorganic phosphorus directly available for algal or macrophyte growth at Site 7 than Site 1.
This tendency of higher mean phosphorus concentrations may be due to a number of reasons, namely 1) higher loads of phosphorus through the Pienaars River and the Moreleta/Hartbees Spruit tributaries, 2) higher contributions of phosphorus through recycling of phosphorus from the sediments due to the shallowness of the site, or 3) from recycling of phosphorus from the natural die-off of the phytoplankton population.
Figure 2. Changes in mean annual TP and PO4-P concentrations in the Roodeplaat Dam (1989 – 1999). a) Site 1 b) Site 7. PMO = Phosphorus Management Objective for TP concentrations.
Figure 3. Boxplots showing the historical trends of the TP and PO4-P concentrations in the Roodeplaat Dam at Site 1 (a and c) and Site 7 (b and d). The boxplots show the minimum, maximum, 25th percentile and the 75th percentile for each hydrological year.
Figure 4 The historical trend in the PO4:TP ratios in the Roodeplaat Dam during the period 1989 to 2000 at a) Site 1 and b)Site 7.
The phosphorus concentrations in the Roodeplaat Dam are, therefore, a major concern in the incidence of decreasing water quality. The concentrations also indicate higher mean TP and PO4-P values at Site 7 than Site 1, showing that the Pienaars River and Moreleta/Hartbees Spruit region of the impoundment is more highly impacted than the dam wall site.
The mean annual TN
concentrations (calculated as the sum of the KN and
NO3+NO2) at Site 1 and 7 (Figure 5),
compared to those of TP and PO4-P, in the impoundment
did not show similar orders of magnitude increases in
concentration during the study period. There were, however,
noticeable increases in the total nitrogen (TN) and dissolved
inorganic nitrogen (DIN) concentrations at both Sites 1 and 7.
Despite this phenomenon, the nitrogen concentrations are still
within the Target Water Quality Range (TWQR) as set by DWAF for
the aquatic environment (DWAF, 1966) for dissolved inorganic
nitrogen and the concentrations even decreased slightly after
1994.
Figure 5. Mean annual concentration for TN (KN & NO3-N+NO2-N) and DIN (NO3-N+NO2-N & NH4-N) in the Roodeplaat Dam (1989 – 1999) compared to the TWQR for dissolved inorganic nitrogen (DWAF 1996c) at a) Site 1 and b) Site 7.
Figure 6. Boxplots showing the historical trends of the TN and DIN concentrations in the Roodeplaat Dam at Site 1 (a and c) and Site 7 (b and d). The boxplots show the minimum, maximum, 25th percentile and the 75th percentile for each hydrological year.
The TN and DIN nitrogen concentrations showed the highest maximum of 9.47 mg/l at Site 1. However, the TN concentrations at Site 7 were generally higher than at Site 1 (Figure 6). The same phenomenon of increased nitrogen concentrations since 1991 is shown in Figure 6, at both the sampling sites.
The minimum, maximum and mean TN concentrations at Site 1 are <0.08, 9.47, and 1.83 mg/l respectively, for the period 1989 to 2000. The minimum, maximum and mean TN concentrations at Site 7 are 0.78, 5.18, and 2.37 mg/l respectively, for the same period. Showing the same higher mean TN tendency at Site 7, compared to Site 1.
Overall, the nitrogen concentrations were within the TWQR for DIN. There were, however, outliers found during 1996 to 1997. Nitrogen does, therefore, contribute to the increased eutrophication in the Roodeplaat Dam as can be seen from the increased incidence of eutrophication symptoms such as extreme algal blooms and nuisance aquatic weed development.
The TN:TP ratios are used to determine the limiting nutrient within a system. A TN:TP ratio of greater than 15:1 signifies phosphorus limitation. From an eutrophication management perspective this is the desirable situation as P input into the system is easier to manage. Low phosphorus concentrations also favour green algae, which may be less problematic to manage than cyanobacteria that are commonly linked to the production of toxins. In contrast, a TN:TP ratio less than 10:1 signifies nitrogen limitation. From an eutrophication viewpoint this is a less desirable situation since some of the cyanobacteria are able to fix atmospheric nitrogen and are, therefore, not so dependant on dissolved nitrogen being present. Atmospheric nitrogen is not available to green algae and because of this the low TN:TP ratio indicating nitrogen limitation favours cyanobacteria development and encourages their dominance in a system.
The mean annual TN:TP ratios (Figure 7) followed a decreasing trend where nitrogen has been the limiting nutrient since 1990. This might be the effect of the extremely high input of phosphorus into the system (Figure 2 and 3), while the nitrogen trends in the Roodeplaat Dam increased only slightly. The direct input of effluent into the Roodeplaat Dam from the Zeekoegat Water Care Works (which is situated on the western side of the Roodeplaat Dam) might have contributed significantly towards the changes in nutrient ratios.
Figure 7. TN:TP ratios in the Roodeplaat Dam at a) Site 1 and b) Site 7 for the period 1989 to 2000.