In this papaer, the hydrogeology of Ardabil plain has been investigated. The hydrogeologic properties of the study area were investigated based on geophysical, geological, water level and pumping test data. Also 65 wells were sampled and analyzed for hydrochemical investigation. Physical and chemical characteristics of groundwater such as temperature, EC, pH, TDS, Na+, K+, Mg2+, Ca2+, HCO3 -, SO4 2-, Cl- and CO3 2- were determined. Hydrochemical studies indicated that geological units have the greatest effect on the groundwater quality in this plain. According to the results obtained, the Ardabil plain has two aquifers, which one of them is located on top of the other. The upper aquifer is multi-layer with varying thickness (20 to 40 m) across the plain and is extracted for different kind of water supply. The number of penetrating wells to the lower aquifer is very low and may not be extracted because of its poor water quality. General direction of groundwater flow is from other directions to the north-west of the plain. The estimated hydraulic parameters of transmissivity and specific yield varies spatially from 50 to 2200 (m2/day) and 0/021 to 0.14, respectively.
