Abstarct: Ardestan aquifer with area of approximately 1369 km2 is extended at a alluvial plain with area of about 4375 km2. This plain is situated at the north of Ardestan County at 100 km North East of Esfahan. hydrogeological investigation carried out by applying data of 27 piezometers from 1984 to 2015. According to the equipotential map, general groundwater flow direction is from S and SW to N and NE. Also it can be inferred from unit hydrograph of aquifer that groundwater level has been declined about 20 meters in average at a 31 years period. In other words, total amount of reduction of aquifer storage at a 31 years period is 1384 Mm3. The amount of water level decline in whole of the aquifer is not the same and it spatially shows considerable variation. At the central part of the aquifer, where the exploitation wells have the largest concentration, the highest decline of water level can be observed. In this basis, Ardestan plain from the water level decline point of view has been classified to three groups: low, intermediate and high water level decline. To access the hydrogeochemical properties of the Ardestan aquifer 14 samples collected in from exploitation boreholes existence in the Ardestan plain February 2015.electrical conductivity map of Ardestan aquifer indicates that electrical conductivity corresponding to flow direction (from the south - southwest to north- northeast) increases from 3000 to 6000 m/cm. Comparison of electrical conductivity maps in 1995 and 2015 indicates that increase of electrical conductivity, in this period is due to intrusion of brines from Dagh-Sorkh brine to northern and central part of Ardestan aquifer. Over exploitation from aquifer and successive drought are the most important factors of increasing the electrical conductivity and intrusion of brine. Evaluation of quality diagrams of Ardestan plain indicates the groundwater type is chloride and its facies is sodic and predominant chemical process in this aquifer is dissolution of halite.