Development of a specific water quality index for the protection of aquatic life of a highly polluted urban river

Abstract A novel method is proposed for the development of ecosystem specific water quality indices in accordance with specific guidelines for the protection of aquatic life and with the capacity to demonstrate annual cycles, spatial and temporal variations, and trends in water quality. An ecosystem-specific water quality index for the Santiago-Guadalajara River (SGR-WQI) was developed using a method based entirely on multivariate statistical methods and historical data distributions. The SGR-WQI was developed using data from 13 water quality monitoring stations in the Santiago-Guadalajara River basin, collected monthly from 2009 to 2019 by the State Water Commission of Jalisco, Mexico. The behaviour of 44 water quality parameters was statistically analyzed. It was found that 17 of the parameters best represented the water quality of the River. Parameter selection also considered the legal limits, established by the Government of Mexico, for the protection of aquatic life. To consider the protection of aquatic biota in the assessment of the index, a method was proposed to incorporate these limits in the development of rating curves providing the WQI with high sensitivity to parameter values that are outside the legal limits. Furthermore, a well-structured method based on multivariate statistical techniques was used to determine the parameter weights that best distinguish between sampling points and seasons, clearly reflecting temporal and spatial variability, as well as to demonstrate the annual cycle and trends in water quality. Colour maps were generated from the calculated SGR-WQI values, showing those sections of the river with lower water quality, and thus providing a simple visual to assist in management of the River system. The methodology proposed can be used elsewhere to develop ecosystem specific water quality indices that evaluate surface water for protection of aquatic life in accordance with local limits/standards/guidelines and that adequately reflect spatial and temporal variability in agreement with the specific context.