Application of entropy weighted water quality index and physicochemical indices to evaluate groundwater quality in Damghan Plain

In this study, the Entropy Weighted Water Quality Index (EWQI) was used to assess the groundwater suitability for drinking purposes in Damghan Plain, Iran. This index has been known as the most unbiased model for assessing drinking water quality. Additionally, physicochemical indices including Sodium Adsorption Ratio (SAR), Magnesium Hazard (MH), Kelley’s Ratio (KR), Salinity Hazard (SH), Synthetic Harmful Coefficient (K), Potential Salinity (PS), Total Dissolved Solids (TDS), Chloride (Cl-), Permeability Index (PI) and Soluble Sodium Percentage (SSP) were used to evaluate the suitability of groundwater for irrigation purposes at August 2018 (dry season) and February 2019 (wet season). The results indicated that sodium (Na+) and chloride (Cl-) are exceeding the permissible limits based on WHO standards and Cl- has the highest entropy weight. EWQI maps illustrated that the groundwater has moderate quality in the western parts and poor quality in the eastern parts of the study area. The mean value of this index has decreased from 149.47 in August 2018, to 147.26 in February 2019, which reflects that the groundwater quality has been improved for drinking purposes. The values of SAR, KR, PI and SSP indices slightly increased, which indicated that the quality of groundwater has more deteriorated in terms of these indices. The mean value of MH, SH, K, PS, TDS and Cl- indices have slightly decreased during the study period. Finally, Land Use-Land Cover (LULC) map was used to show which groundwater consumption is appropriate with its quality. Groundwater in the urban areas has moderate and poor quality for drinking purpose and suitable quality in terms of SAR, K and PI and unsuitable in terms of MH, KR, SH, PS, TDS, Cl- and SPP in agricultural lands. The suitable condition in terms of SAR, K and PI is because of the high concentration of Mg2+ and Ca2+. Thus, groundwater is not suitable for irrigation in the agriculture sector.