Ground water is an important component of fresh water resources. The use of ground water is of fundamental importance to human life and is also significant to economic vitality. This study attempt to evaluate ground water quality through Drinking water Quality Index (DWQI), referencing Institute of Standards and Industrial Research of Iran (ISIRI) and World Health Organization (WHO) guidelines, in Ardabil aquifer. In order to understand ground water quality, water samples were collected from 63 wells. The primary chemical parameters like pH, TDS, TH, Ca2+, Mg2+, Na+, K+, HCO3 - , Cl- , SO4 2-, NO3 - , and F- were analyzed for DWQI. Groundwater quality classification maps based on DWQIISIRI and DWQIWHO were created by kriging method in GIS environment. Results indicated that the average values of the most chemical properties were above WHO and ISIRI standards except pH, Na+, Cl- , NO3 - , and F- which were within the WHO and ISIRI standards. DWQIWHO and DWQIISIRI are strongly correlated with each other (R2 = 0.985) but in all samples the values of DWQIWHO were greater than the corresponding DWQIISIRI. TH and TDS had the greatest correlation with DWQIWHO and DWQIISIRI and were considered as the main parameters controlling drinking water quality. Spatial distribution maps of DWQIWHO and DWQIISIRI indicated that the areas occupied by different water quality classes are in the following order: Poor > Good> Very poor> Excellent. Poor water quality class has occupied 49.2 and 61.6 percent of the study area based on DWQIISIRI and DWQIWHO, respectively. Due to this quality deterioration of groundwater quality, an immediate attention should be employed in critical locations of the Ardabil Aquifer.
