Investigation of self-purification capacity and water quality of Haraz river during dry and wet season

Rivers are one of the primary water resources for agricultural, drinking, industrial and environmental applications; thus, assessment of the river water quality has a great significance. This study aims to evaluate the self-purification capacity of the Haraz river and identify critical areas with the lack of dissolved oxygen along the river. Also, the QUAL2K model, IRWQI and NSFWQI indexes were applied for the determination of water quality. In this study, primary pollutant sources including urban areas, industrial centers, farmlands and multiple cold-water fish farms that their water usage and discharge have severe consequences on the quality and quantity of the river's stream were identified and located. This model was built and calibrated for four seasons with data from six sampling stations of Haraz river. Based on this study, upstream of Haraz river has adequate self-purification capacity in comparison to its midstream and downstream, especially in the Amol city area, lack of dissolved oxygen was observed and self-purification capacity considerably reduced. The critical areas of Haraz river in spring and winter seasons are downstream and the estuary region, while in summer and autumn, critical areas are increased in Amol city. In general, as the elevation decreases, the water quality decreases. Only in upstream areas (near the Poloor village and before the Chelav station), water quality is in the average condition, but near the Caspian sea, the condition of Haraz river is worrying due to the existence of contamination sources.