Evaluation of water quality of Tajan River using IRWQIsc index

River water is used for various purposes more than water from any other source. This study was designed and implemented to evaluate the water quality of the Tajan River by the Iran Surface Water Quality Index (IRWQIsc). Considering the establishment of polluting sources on the banks of the Tajan River (various agricultural, industrial, urban and domestic wastewaters), sampling was done at 9 stations with 3 repetitions in two seasons (January 2021 and June 2021), and 11 water quality parameters including; BOD5, COD, ammonium, nitrate, phosphate, turbidity, pH, fecal coliform, electrical conductivity, dissolved oxygen and total hardness were measured using standard methods in the laboratory. In the present study, the average amount of BOD5, COD, ammonium, nitrate, phosphate, turbidity, pH, fecal coliform, and electrical conductivity increased in both seasons from upstream to downstream stations, dissolved oxygen in both seasons from above It decreased downstream and the total hardness did not have a regular trend from upstream to downstream. The results of the study of the IRWQIsc index showed that this index was between 23.7 and 70.08 in the cold season and between 64.21 and 16.35 in the hot season for all the stations of the Tajan River. In general, the water of the Tajan River was placed in the descriptive-qualitative class of medium and bad, and the closer the agricultural lands were to the studied stations, the more the water pollution was. The most important source of pollution in the study area; Indiscriminate harvesting of sand and paper mills in Mazandaran, fish breeding farms, animal husbandry, sewage of Sari city, surrounding villages, effluents of paddy fields, and agricultural fields. In this regard, attention to the comprehensive management of water resources and assessment of environmental effects should be prioritized.