Department of Water and and Sanitation
	Institute for Water Quality Studies

ASSESSMENT OF THE WATER QUALITY OF THE EBENEZER DAM

REPORT NUMBER: N/B800/02/DEQ/2699

PROJECT: TROPHIC STATUS PROJECT

STATUS OF REPORT: FINAL

DATE: AUGUST 1999

INTRODUCTION

This report was compiled following the request of the Water Control Officer (WCO) for data of the Ebenezer Dam. It is essential that samplers that have been sampling for the IWQS at different impoundments should have access to the results to allow samplers to understand the purpose of sampling and receive feedback. This is also proposed as one way of acknowledging the samplers for years of hard work. Interpretation of data was also undertaken to assist WCO with the assessment of the data.

BACKGROUND

The Ebenezer Dam was included in the Trophic Status Project. This initiated a sampling programme that was conducted by the Water Control Officer at the dam. The IWQS provided all the equipment for the sampling programme. Analysis of samples was done at IWQS for the following variables:

• Inorganic chemical constituents
• Total nutrients
• Chlorophyll a
• Algal composition
• Turbidity (Secchi disc readings)
• Suspended solids
• Phaeophytin

This data has now been assessed and this report is the result there-off.

SITE BACKGROUND

The Ebenezer Dam is situated in the upper reaches of the Groot Letaba River catchment on the confluence of the Broederstroom and Helpmekaar rivers (Figure 1). The catchment is in mountainous area with mainly natural vegetation. The small rural town of Haenartsburg is in the catchment of the Ebenezer Dam.

The dam wall was completed in 1960 and the water was mainly intended for irrigation purposes (RSA, 1964). Lately pipelines have been installed to supply water to a great number of rural communities in the Northern Province.

WATER QUALITY OF THE EBENEZER DAM

The chemical water samples received at IWQS were analysed for inorganic variables and total nutrients. The results are stored on the main database of the Department of Water and Sanitation in Pretoria. The statistics for the chemical variables measured are shown in Table 1. The variables are organised in alphabetical order.

Inorganic chemistry

The mean total dissolved solids concentration, which is an indication of the salinity in the system, is 39 mg/l. This low concentration indicates that there are no saline impacts on the impoundment. As the TDS concentration is a total of the major ions in the system the contribution of the different major ions to the TDS concentration is shown in Figure 2. The changes in time for the mean annual, major ion concentrations, total alkalinity (TAL) and electrical conductivity are shown in Figure 3a and 4. It can be seen that concentrations have remained fairly constant with the possible exception of sulphate and TAL. This also indicates the impact of activities on salinity is minimal. Figure 3b show the minimum, 5 percentile, mean, 95 percentile and maximum concentrations of the main ions found in the Ebenezer Dam for the period 1989 to 1998.

Nutrients

The main nutrients measured, include ammonium (NH4), nitrate & nitrite (NO3 &NO2), Kjeldahl nitrogen (KN), ortho-phosphorus (PO4) and total phosphorus (TP).

Figure 5 indicates that in the Ebenezer Dam, the mean annual TP and PO4 concentrations for the period 1989 to 1998, were consistently within the required Phosphorous Management Objective (PMO) of 130 µg/l P (DWA 1988, ANON 1988a, ANON 1988b). The mean annual TP and PO4 concentrations were also below the 47 µg/l that were suggested by WALMSLEY and BUTTY (1980) and there is also no increase in the concentrations during the period. The impoundment is, therefore, not very productive in terms of eutrophication and plant production.

The mean annual nitrogen concentrations in the Ebenezer Dam are reflected in Figure 6 and indicate that the nitrogen concentrations within the impoundment were continuously within the TWQR and consistently below 1 mg/l . There were no distinct increases in the nitrogen species concentrations during the study period.

The mean annual TN:TP ratios for the Ebenezer Dam are shown in Figure 7. The TN:TP ratios indicate that the water quality in the Ebenezer Dam is phosphorous limited indicating that eutrophic conditions are unlikely to occur. Ratios have also remained fairly consistent, therefore no problem is expected to occur in the near future. The nutrient contents of the impoundment indicate that the water quality of the system is of a desirable quality when eutrophication is considered.

Biological characteristics

The chlorophyll a concentration (Figure 8) is an indication of the phytoplankton production in the impoundment and is almost consistently below 10 µg/l in the Ebenezer Dam. Only during 1995 to 1996 was there an increase in mean annual chlorophyll a concentration. The concentrations however never exceeded 30 µg/l . Cyanobacteria (potential toxic) were present as a part of the algal population during 1991 to 1993 (Figure 9) and there is, therefore, little chance of toxic algal blooms. Either Chlorophyta or Chrysophyta dominated the algal population during the study period.

Figure 9. Mean annual dominance of algal groups in the Ebenezer Dam

Physical characteristics

Mean Secchi disc readings, a measurement for turbidity, are plotted in Figure 8. The readings ranged between 1.8m and 6.3m, indicating that the impoundment is a clear system. There is a slight positive correlation between the Chl a and the Secchi disc readings (r² = 0.38) in the Ebenezer Dam, indicating that the chlorophyll a might contribute to the turbidity in the system, however, chlorophyll a is not the main contributor towards turbidity in the system.

The temperature profiles shown in Figure 10 show that there is distinct stratification (temperature variation of approximately 10°C at a depth of around 8m) during the warm summer periods in the Ebenezer Dam and complete thermal mixing (temperature remains consistent at approximately 15°C throughout all depths) during the colder winter months. Surface water temperatures varied between 16.4°C and 23.7°C. Monthly oxygen profiles in the Ebenezer Dam shows a distinct hypolimnion from between 10 m and 16 m, even during the winter months.

Trophic status of the Ebenezer Dam

Table 2 indicates that the Ebenezer Dam is oligotrophic (low nutrient and plant growth) with only periodically increased nutrient concentrations, towards mesotrophic conditions (higher nutrient and plant production). The Ebenezer Dam water is of high quality in terms of its trophic status and within acceptable levels. This is supported by the low salinity in the impoundment and the chlorophyll a concentration that never exceeds 30 µg/l .

CONCLUSION

The Ebenezer Dam is oligotrophic with a tendency to being mesotrophic. There is no noticeable salinity increases. The biological symptoms coupled to eutrophication indicators in the form of nutrients also indicate that the water in the Ebenezer is of high quality.

RECOMMENDATIONS

Reports on the trophic status of impoundments should be compiled for:

• the information of Water Control Officers
• acknowledgement to samplers for the years that they contribute towards this information gathering

ACKNOWLEDGEMENTS

1. Annelise Gerber, Alfred Seloana and Joyce Maluleka, the Technical Section of the IWQS, for their commitment towards the Trophic Status Project monitoring.
2. The Water Control Officers at Ebenezer Dam for his continued effort to do the local sampling.
3. Elna Vermaak for producing the map for this report.
4. Toni Belcher for comments on the final report.

REFERENCES