Computing and application of water quality index in coastal waters of the southern Caspian Sea basin

Water quality index is a tool that integrates data of effective factors on water quality. The final result of the index provides simple understanding of water quality for general people. Regarding the importance and application of water quality index in decision making of active and executive units in conservation and exploitation, the present study was conducted for the first time in the southern of Caspian Sea. The aims of paper are to determine: the environmental parameters which are important in water quality, determining weight (wi) of parameters affecting water quality index and sub-indices (qi) of each important parameters of Caspian Sea water quality index (WQIcs). This research will provide the specific and necessary information and data for calculating the water quality index in this aquatic ecosystem. The provided wi and qi were applied for water quality index calculating of the Caspian Sea data in 2018-2019. Using Principal Component Analysis results on long-term data 2003-2019 years (920 water samples), 9 important parameters (turbidity, chlorophyll-a, BOD5, COD, pH, DO%, NO3, NH4 and PO4) were determined for WQIcs and indicator weighting (wi) of each parameter was determined. The sub-index (qi) of each parameter was calculated based on standards threshold, coastal water quality requirements in marine ecosystems and reference value data of Caspian Sea. Finally, by calculating the geometric mean, the value of water quality index in the Caspian coastal waters was obtained. The application of this formula in 2018-2019 showed that 74 and 26% of WQIcs were in the range of 70-91 (medium class) and 35-69 (bad class), respectively. Among the 9 parameters mentioned above, nitrate and phosphorus (the factors of trophic enrichment) had the greatest effect on decreasing water quality index.