Hybridizing of TOPSIS multi criteria decision making technique with entropy for ranking of groundwater quality and groundwater pollution risk

In this study, the suitability of groundwater quality for drinking proposes and wells ranking have been investigated using two techniques including water quality index (WQI) and hybrid TOPSIS-entropy theory (ETOPSIS) for 36 wells located in Shahr-e-Babak aquifer. During the WQI calculating process, the weight of each parameter is usually given by expert’s experience and chemical parameters with low weights but high concentration values (or vice versa) reduces evaluation stability. So, the hybrid ETOPSIS method has been developed. In ETOPSIS method, the weight of each parameter is calculated using entropy-based weighting approach. Also, normalizing each single parameter in ETOPSIS is to consider the effect of each qualitative parameter besides their overall influence in ranking of wells. Also in this study, pollution risk-based prioritization of wells has been applied according to time consequences of pollution in seven years and probability of pollution for the first time. According to WQI results based on 1053 standard, over 8 and 17 % of wells were in the class of "excellent" and "good" quality, respectively. Using ETOPSIS technique, the well number 11 ranked as the best and first well that had lowest pollution risk (TC = 0.95), but this well achieved the forth rank and classified as well with “good” water quality by WQI method (WQI = 54.4). Investigation of chemical parameters confirmed the validity of the ETOPSIS results. Calculation of entropy weighting showed that concentration of Mg and HCO3 with same entropy weight equal to 0.17 have highest impact and pH with entropy weight equal to 0.05 has minimal influence on the ETOPSIS ranking. The low pollution risk recognized in Eastern parts of study region, but it was high and dangerous in West and Southern west. The wells risk ranks that obtained from WQI and ETOPSIS based on 50 % pollution threshold were correlate with R2 = 0.951. Based on the results indicate that application of ETOPSIS could produce useful and more reliable results than WQI.