چكيده لاتين
Abstract
Water is a fundamental resource for socio-economic development and the sustenance of ecosystems. Its treatment is difficult, transmission is costly, and replacement is impossible. Rivers are among the most important sources of freshwater for agricultural, domestic, and industrial use. At the same time, they serve as recipients for various pollutants discharged from these activities. This study investigates the allocation of pollution loads among point and non-point sources in a river using quantitative and qualitative modeling tools. The study area is a section of the Qarasu River, extending from Shahreza village to downstream of Kermanshah city (Pol-e-Kohneh station), with a total length of approximately 80 kilometers. For this purpose, using hydrometric data and results from physical-chemical analyses of river water at five sampling stations, along with estimates of pollution loads from municipal and industrial wastewater and agricultural runoff, the river’s water quality was simulated using the Qual2Kw model and calibrated with 2021 data (1400 in the Iranian calendar). For the simulation, the river was divided into 20 hydraulic reaches, and the Runge-Kutta method with a time step of 0/7 minutes was used for numerical calculations. Based on this, the fitness function value was determined to be 3/47, indicating a 28% error in the river’s qualitative-quantitative modeling. The modeling results showed that the river’s reaeration coefficient varies between 0/22 and 0/425 per day, and critical reaches in terms of self-purification capacity were identified for BOD, COD, nitrate, and phosphate. Moreover, under current conditions, the dissolved oxygen (DO) concentration is below 5 mg/L, the Water Quality Index (WQI) of the river ranges from relatively poor to very poor, and the highest accumulation of pollutants is observed in the 10kilometer downstream section near Kermanshah city. The pollution load allocation assessment indicated that a 99% reduction in pollution loads from point sources, along with a 50% reduction from non-point sources, could bring the river’s quality to a minimally acceptable condition. In this scenario, the DO concentration is expected to exceed 7 mg/L, and the water quality index would improve to a relatively good level. Sensitivity analysis further revealed that DO concentration is more strongly influenced by upstream river discharge than by the estimated reaeration coefficient resulting from flow turbulence. Therefore, ensuring minimum environmental flow, along with treatment and recycling of urban and industrial wastewater (achieving a 99% reduction in BOD pollution load), and controlling pollutant emissions from agricultural lands (through reduced fertilizer use or runoff management), is essential for achieving optimal river water quality management.
