Assessment of groundwater resources potential using Improved Water Quality Index (ImpWQI) and entropy-weighted TOPSIS model

The use of groundwater resources has increased in the recent decades, so groundwater quality assessment and identification of pollutants has great importance in areas where groundwater is a main water resource. The aim of this study is to assess the suitability of groundwater in Malayer basin, Iran, using Improved Water Quality Index (ImpWQI) and Technique for Order reference by Similarity to Ideal Solution (TOPSIS) model. Determining the weight of each parameter is the most important part of TOPSIS method, thus, Entropy method was employed to assign weight of hydrogeochemical parameters. Nine parameters of groundwater quality, including K⁺, Na⁺, Mg²⁺, Ca²⁺, SO₄²⁻, Cl⁻, HCO₃⁻, pH, and TDS, in 39 sampling points in September 2018 were considered as inputs. The results indicated that groundwater contains a lot of calcium carbonate (CaCO₃) and its hardness is high. The results showed that the ImpWQI values range between 32.953 and 100.556. Moreover, seven samples belong to “Excellent” category, 31 samples belong to “Good” category, and one sample belongs to “Medium” category. Generally, groundwater has higher quality in highlands and based on entropy method, and HCO₃⁻ and SO₄²⁻ have the highest and lowest weights with values of 0.132 and 0.089, respectively. The values of entropy-weighted TOPSIS range from 0.102 to 0.979, and in the upstream of the basin, the value of this index is higher than downstream, central parts and the outlet. The correlation of these two indices indicated that the rankings based on both methods are significantly correlated (p < 0.01) with a value of R² equal to 0.94. According to the findings, application of GIS and Multi-Criteria Decision Making (MCDM) models provides acceptable results and can be adopted in the management of groundwater resources.