Drivers and Management of Nutrient Overload in Dams: Insights from Roodeplaat Dam, South Africa
Abstract
1. Introduction
2. Materials and Methods
2.1. Study Area Description
2.2. Data Collection
- Site A captures contributions from Edendalespruit, influenced by agricultural runoff and small settlements.
- Site B reflects upstream inputs from Pienaars River, including effluent from Baviaanspoort WWTW and upstream land use impacts.
- Site C represents inputs from Hartbeesspruit and Moreleta Rivers, reflecting mixed urban and agricultural influences.
- Site D assesses direct effluent contributions from the Zeekoegat WWTW.
- Site E captures the cumulative effects of all inputs, representing the dam’s outflow and its impact on downstream users.
- (a)
- Image acquisition
- (b)
- Pre-processing
- (c)
- Classification and analysis
- (d)
- Change detection
- (e)
- Validation and mapping
2.3. Mixed Models and Principal Component Analysis
2.4. Comprehensive Pollution Index
- Category 1: CPI < 0.20 (pristine).
- Category 2: CPI from 0.21–0.40 (less polluted).
- Category 3: CPI from 0.4–1.00 (slightly polluted).
- Category 4: CPI from 1.01–2.00 (moderately polluted).
- Category 5: CPI > 2.01 (heavily polluted).
2.5. Estimation of Flows and Nutrient Loads Using Duration Curves
- Rank Flows: Daily average flow values were ranked in descending order, from the largest to the smallest, across the period of record.
- Assign Ranks: Each discharge value was assigned a rank (M), starting with 1 for the highest flow.
- Calculate Exceedance Probability: The probability (P) that a given flow would be equaled or exceeded was calculated using Equation (4).
- High flows (0–10%)
- Moist conditions (10–40%)
- Mid-range conditions (40–60%)
- Dry conditions (60–90%)
- Low flows (90–100%)
2.6. Coupled Model Inter-Comparison Project Projections
- Projected Mean: Represents expected climate conditions under specific SSPs.
- Anomaly (Change): Highlights deviations from historical baselines.
3. Results
3.1. Spatial and Temporary Variation in Nutrient Loads
3.2. Comprehensive Pollution Index for Loads
3.3. Nutrient Load Estimation Using Duration Curves
3.4. Land Cover and Land Use Change over Time
4. Discussion
4.1. Implications for Nutrient Load Reduction
4.2. Implications for Environmental Policies
4.3. Policy Implementation Challenges and Recommendations
4.4. Climate Change Scenario: Considerations for Future Research
4.5. Key Findings
4.6. Limitations and Future Research Directions
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
ANZECC | Australian and New Zealand Conservation Council and the Agricultural and Resource Management Council for Australia and New Zealand |
CCME | Canadian Council of Ministers for the Environment |
CMIP | Coupled Model Inter-comparison Project |
COD | Chemical Oxygen Demand |
CPI | Comprehensive Pollution Index |
DWS | Department of Water and Sanitation |
EMSs | Earth System Models |
FDC | Flow Duration Curve |
IPCC | Intergovernmental Panel on Climate Change |
LDC | Load Duration Curve |
LEDAPS | Landsat Ecosystem Disturbance Adaptive Processing System |
LULCC | Land Use Land Cover Change |
N | Nitrogen |
NBS | Nature-based Solution |
P | Phosphorus |
PCA | Principal Component Analysis |
PI | Pollution Index |
RDMs | Resource Directed Measures |
RMSE | root mean square error |
RQOs | Resource Quality Objectives |
SAWQ | South African Water Quality Standard |
SDCs | Source Directed Controls |
SSPs | Shared Socio-economic Pathways |
TMDLs | Total Maximum Daily Loads |
TN | Total nitrogen |
TP | Total phosphorus |
TWQR | Target Water Quality Range |
USGS | United States Geological Survey |
VEPAC | Variance Estimation and Precision |
WHO | World Health Organization |
WMS | Water Management System |
WWTW | Wastewater Treatment Works |
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Site ID | WMS ID | Gauging Station | Location | Coordinates |
---|---|---|---|---|
Inflow | ||||
A | A2H029Q01 | A2H029 | Edendalespruit River | 28.391944; −25.648889 |
B | A2H027Q01 | A2H027 | Pienaars River (upstream) | 28.351147; −25.663172 |
C * | A2H028Q01 | A2H055 | Moreleta River | 28.319444; −25.66317222 |
A2H054 | Hartbeesspruit River | |||
D ** | A2H124Q01 | A2H124 | Zeekoegat WWTW effluent | 28.3721; −25.6177 |
Outflow | ||||
E *** | AR2900Q01 | AR2900 | Dam Wall | 28.33873861; −25.62319861 |
A2H102Q01 | A2H102 | Pienaars River (releases and spills) | ||
A2H100 Q01 | A2H100 | left canal | ||
A2H101 Q01 | A2H101 | left canal to the right bank |
Parameters | Unit | SAWQ | WHO | ANZECC | CCME | Water Use |
---|---|---|---|---|---|---|
TP | mg/ℓ | I: ≤0.005 | 0.01 | >0.35 * | Aquatic Ecosystem | |
A: 0.005–0.025 | - | |||||
T: 0.025–0.25 | ||||||
U: >0.25 | ||||||
NO3 + NO2 | mg/ℓ | I: ≤6 | >1 | 0.35 | - | Domestic Use |
A: 6–10 | ||||||
T: 10–20 | ||||||
U: >20 |
Site ID | Monitoring Sites | Parameters | PI | CPI | Polluted |
---|---|---|---|---|---|
A | Endendalespruit River | TP | 1.569771 | 0.847748 | Slightly polluted |
NO3 + NO2 | 0.125726 | ||||
B | Pienaars River (upstream) | TP | 6.41 | 3.369788 | Heavily polluted |
NO3 + NO2 | 0.33199 | ||||
C | Hartbeesspruit River | TP | 0.34627 | 0.196499 | Less polluted |
NO3 + NO2 | 0.046728 | ||||
D | Zeekoegat WWTW effluent | TP | 4.95 | 2.621479 | Heavily polluted |
NO3 + NO2 | 0.29 | ||||
E | Pienaars River (downstream) | TP | 1.374272 | 0.729337 | Slightly polluted |
NO3 + NO2 | 0.084403 |
LULCC Type | 2011 | 2001 | 2021 | |||
---|---|---|---|---|---|---|
Shape Area | % | Shape Area | % | Shape Area | % | |
Built | 181,182,792.23 | 26.56 | 191,564,769.75 | 28.08 ⮉ | 220,475,320.57 | 32.32 ⮉ |
Soil | 69,902,204.11 | 10.25 | 57,930,723.95 | 8.49 ⮋ | 150,154,974.47 | 22.01 ⮉ |
Vegetation | 427,206,265.57 | 62.62 | 429,469,579.45 | 62.95 ⮉ | 308,134,324.87 | 45.17 ⮋ |
Water | 3,897,785.67 | 0.57 | 3,223,974.42 | 0.47 ⮋ | 3,424,427.67 | 0.50 ⮉ |
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Mnyango, S.S.; Thwala, M.; Truter, C.; Xulu, N.G.; Schoeman, Y.; Oberholster, P.J. Drivers and Management of Nutrient Overload in Dams: Insights from Roodeplaat Dam, South Africa. Hydrology 2025, 12, 57. https://doi.org/10.3390/hydrology12030057
Mnyango SS, Thwala M, Truter C, Xulu NG, Schoeman Y, Oberholster PJ. Drivers and Management of Nutrient Overload in Dams: Insights from Roodeplaat Dam, South Africa. Hydrology. 2025; 12(3):57. https://doi.org/10.3390/hydrology12030057
Chicago/Turabian StyleMnyango, Samkele Siphelele, Melusi Thwala, Christoff Truter, Nkosinathi Goodman Xulu, Yolandi Schoeman, and Paul Johan Oberholster. 2025. "Drivers and Management of Nutrient Overload in Dams: Insights from Roodeplaat Dam, South Africa" Hydrology 12, no. 3: 57. https://doi.org/10.3390/hydrology12030057
APA StyleMnyango, S. S., Thwala, M., Truter, C., Xulu, N. G., Schoeman, Y., & Oberholster, P. J. (2025). Drivers and Management of Nutrient Overload in Dams: Insights from Roodeplaat Dam, South Africa. Hydrology, 12(3), 57. https://doi.org/10.3390/hydrology12030057