892 ISSN 0097-8078, Water Resources, 2020, Vol. 47, No. 5, pp. 892–903. © Pleiades Publishing, Ltd., 2020. Application of Water Quality Index and Water Quality Model QUAL2K for Evaluation of Pollutants in Dez River, Iran Samaneh Abdeveis a, *, Hossein Sedghi a, **, Houshang Hassonizadeh b, ***, and Hossein Babazadeh a, **** a Department of Water Engineering, Science and Research Branch, Islamic Azad University, Tehran, 1477893855 Iran b Department of Civil Engineering, Islamic Azad University, Shoshtar, 6451741117 Iran *e-mail: abdovissamaneh@gmail.com **e-mail: hsedgh@yahoo.com ***e-mail: hassoni44@hotmail.com ****e-mail: h_babazadeh@srbiau.ac.ir Received October 1, 2016; revised October 23, 2016; accepted October 23, 2016 Abstract—Dez River in Iran is a long river and has generally good-quality water. Unfortunately, in this river water quality has decreased over recent years due to drought, industrial and agricultural developments. Dez River plays a significant role in moderating the quality of Karun River After adjoining it. So, maintaining good water quality of Dez River is very important. In this case study, water quality index was evaluated in an 8-year period with regard to the growth of industry and drought occurrence. The calculated Iran Water Qual- ity Index showed values of 70.1–85. This index reduced from extremely good to good for DEZ River. Simu- lation of some water quality parameters (electrical conductivity, dissolved oxygen, carbonaceous biochemical oxygen demand, NH 4 -N, NO 3 -N, and pH) were evaluated using Water Quality Model (Qual2k). The observed data in July 2012 and March 2013 were used for calibration and validation of the Qual2k model respectively. In conclusion, the Qual2k model was proposed as a suitable tool for water quality assessment and forecast in the future. Keywords: water quality, Dez River, IRWQI, Qual2k model DOI: 10.1134/S0097807820050188 INTRODUCTION Dez River in Iran is a source of water supply for agriculture, drinking water and industry. Given to industrial development, agriculture activities, urban- ization, drought, inter-basin water transfers and changing cropping patterns, declining quantity and quality of the river water has become evident in recent years. For example, decreased dissolved oxygen (DO) concentrations due to assimilation some pollutants by microorganisms, chemical oxidations of reduced pol- lutants and respiration of plants, algae and phyto- plankton often represent water quality impairments [9]. One of the simplest analytical methods for evalu- ation of water resources quality is water quality index (WQI), which is considered as a robust decision-mak- ing tool in water quality managements. The Iran Water Quality Index for Surface Water Resource-Conven- tional Parameters (IRWQIsc index) is a modification of NSFWQI based on the local condition in Iran [1]. Hamedi et al. [10] investigated the pollutants influ- ence upon the water quality of an urban watercourse in southeast of Tehran, Iran (Sorkhe-Hessar Water- course) by IRWQI. The index value was equivalent to 15−29.9, demonstrated the poor qualitative condi- tions for the watercourse. Water health and quality monitoring and assessing in the Tajan River, Iran was performed using several water quality indices includ- ing IRWQI by Aazami et al. [1]. The results of this study revealed that ecological condition and water quality were reduced from river up- to down-stream. Mehrasbi and Farahmand Kia [13], applied Qual2k model for evaluation of nutrient control strategies in the small rivers. Results showed the Kine-Vars River is saturated with N and P and was classified as eutrophic. Zhang et al. [25] simulated the water quality in the Hongqi River, in China. The aim of this case study was to facilitate optimal program selection. Based on the results obtained, the optimal scenario comprised a bio-contact oxidation system upstream, followed by an ecological floating bed, and a vertical moveable eco-bed downstream. Kannel et al. [12] used Qual2k model in the river Bagmati, Nepal. The influence of various water quality management strategies on DO concentrations were examined considering: (i) modi- fications in pollution loads (ii) flow augmentation (iii) local oxygenation. Results showed the impression HYDROCHEMISTRY, HYDROBIOLOGY: ENVIRONMENTAL ASPECTS