Comprehensive Hydrodynamic and Qualitative Modeling of Landfill Leachate Impact on Dam Reservoirs (Case Study: Haraz Dam Reservoir)

This study presents an integrated assessment of water quality in the Haraz Dam reservoir, considering both external pollutant inflows and internal contributions from a nearby unregulated landfill. The CE‐QUAL‐W2 hydrodynamic model was employed to simulate monthly variations of nitrate, phosphate, ammonium, dissolved oxygen, and pH across four water withdrawal levels, capturing the effects of thermal stratification and anaerobic conditions. Nitrate and phosphate were identified as the main drivers of water quality degradation, with the baseline Iran Water Quality Index for Surface Water Resources‐Conventional Parameters (IRWQIsc) values classifying the reservoir as “relatively poor.” Four management scenarios were evaluated: Scenario 1, reducing nitrate and phosphate inflows by 50%, improved IRWQIsc by 9.5 points, and lowering nitrate and phosphate concentrations by 46.8% and 41.6%, respectively, and shifting water quality to “average.” Scenario 2, maintaining current pollutant loads over 5 years, predicted IRWQIsc values consistently below 40 at lower withdrawal levels, indicating ongoing poor water quality. Scenario 3, applying a sustained 50% reduction over 5 years, further improved IRWQIsc by 10 points, with nitrate and phosphate reductions of 52.7% and 51.6%. Scenario 4, removing the nitrate‐contaminated leachate spring, reduced nitrate in the agricultural withdrawal level by only 4.9%, highlighting the limited impact of intrareservoir interventions alone. These results demonstrate that watershed‐level management of pollutant inflows is essential for achieving long‐term improvements in reservoir water quality, offering a replicable framework for hydrodynamic and water quality modeling in dam reservoirs.