Groundwater quality ranking using the improved entropy TOPSIS method: a case study in Azarshahr plain aquifer, east Azerbaijan, Iran

Groundwater quality is critical to ensure the safety of water supply and sustainable social and economic development. The present study introduces a framework of ranking the groundwater quality, employing the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) and entropy weight method. The water quality guidelines of the WHO and national guidelines were adopted. According to selected groundwater quality standards, groundwater can be classified into five grades, including excellent, good, moderate, poor and very poor. TDS, SO42-, Cl-, nitrate, F- and pH were selected as the assessment indicators. A final ranking map was derived based on the assessment results. The results show that geology and land use are the main governing factors of groundwater quality in the study area. Groundwater in the study area is mainly Ca-Cl type and mixed type and the anions show a great variability. The majority of the water samples are of excellent to good quality and moderate quality. The overall groundwater quality is generally good and is generally suitable for irrigation and drinking. However, precise and long term groundwater quality monitoring in this domain is essential.